IN DEGREE PROJECT TECHNOLOGY,FIRST CYCLE, 15 CREDITS

, STOCKHOLM SWEDEN 2017

Small-scale biogas and greenhouse system

ROBERT ALEXANDERSSON

STEPHAN TRAN

KTH ROYAL INSTITUTE OF TECHNOLOGYSCHOOL OF ARCHITECTURE AND THE BUILT ENVIRONMENT

TRITA -IM-KAND 2017:15

www.kth.se

Small-scale biogas and greenhouse system

Robert Alexandersson Stephan Tran

Supervisors: Gunnar Bech

Anders Malmquist

AL125X Bachelor degree project in Energy and Environment Stockholm 2017

Abstract Greenhouse cultivation is a growing industry, especially in mild climates, much due to the ability to adjust the growing conditions and increased water utilization efficiency. The most important aspect on the cultivation is the indoor temperature. The variation in temperature is due to the Swedish climate where the highest and lowest outdoor temperature respectively varies greatly during the year. To enable optimal indoor climate additional heating is required during colder periods. Presently, most of the existing greenhouses utilizes combustion of fossil fuels for heating, which contributes to the climate change through the emissions of greenhouse gases. One way to circumvent this impact is to exchange the fossil fuels with biogas. Combining biogas production and greenhouse cultivation enables synergies and a more closed cycle of material flow can be achieved. However, this combination is rather immature due to lacking previous research, giving this report its main purpose, to examine the synergies and sustainability of combining a greenhouse with small-scale biogas production. Initially, an extensive literature study was carried out followed by a simulation based on the obtained knowledge. The simulation was comprised of two greenhouses with different geometries, one with the shape of an arch with polyethylene-film cladding and the other with a sawtooth roof with glass cladding, both with two layers. The other properties such as internal area and volume are more or less the same for the simulated greenhouses. Useful data such as outdoor temperature, rainfall and solar irradiation etc. was obtained for the city of Enköping, Sweden. The calculations for the models were carried out in the program Microsoft Excel. In order to evaluate the feasibility of these models a reference greenhouse was studied, which had similar properties and conditions. The optimal temperature for tomato cultivation is 20° C, and to maintain this level over the entire year it was found that the heat requirements were 89 500 kWh for the arched greenhouse and 94 400 kWh for the sawtooth greenhouse. In comparison with the reference greenhouse, the heat requirement was around 200 kWh per m2 and year less in the simulated greenhouses. Furthermore, it was found that around 31 800 kWh of cooling is required over the year (249 kWh per m2 and year) for the arched greenhouse and 30 900 kWh per year (241 kWh per m2 and year) for the sawtooth greenhouse, to keep the indoor temperature at 20 °C. Moreover, two to three possible harvests annually gives the yield of 3456-5184 kg tomatoes per year. Both the simulated greenhouses are feasible concepts, however the sawtooth greenhouse is a better option due to its increased longevity and lower contribution of greenhouse gas emissions over time. Furthermore, more research needs to obtain a fully closed cycle. Keywords: biogas, small-scale, greenhouse, cultivation, temperature, energy balance, material balance, closed cycle, hydroponics.

Foreword This report is the result of a bachelor thesis comprising 15 ECTS credits, conducted at Royal Institute of Technology (KTH) in spring 2017, within the field of sustainable energy technology. The report constitutes a part of the Degree Programme in Energy and Environment at KTH. The content of the report was mainly based on an idea by Gunnar Bech, chairman for Innovationsverket in Gamleby. Acknowledgment and gratitude is addressed towards both Gunnar Bech and Anders Malmquist, Associate professor at the department for Energy Technology at KTH, for inspiration and advice during the work process. Stockholm, May 2017 Robert Alexandersson and Stephan Tran

Tableofcontents

1 Introduction...................................................................................................................11.1 Background..........................................................................................................................11.2 Purpose and goal..................................................................................................................1

2 Literature study.............................................................................................................22.1 Biogas....................................................................................................................................2

2.1.1 Biogas production on smaller scale........................................................................................22.1.2 Utilization of small scale biogas.............................................................................................42.1.3 Current trend and potential of biogas in Sweden....................................................................5

2.2 Greenhouse...........................................................................................................................62.2.1 Shape and orientation.............................................................................................................72.2.2 Greenhouse materials.............................................................................................................82.2.3 Climate control in greenhouses............................................................................................102.2.4 Sustainability aspect of the greenhouse................................................................................11

2.3 Horticulture/plant growth..................................................................................................132.3.1 Temperature requirement.....................................................................................................132.3.2 Growth medium....................................................................................................................142.3.3 Nutrient requirement............................................................................................................172.3.4 Water requirement................................................................................................................212.3.5 Humidity requirement..........................................................................................................212.3.6 Carbon dioxide requirement.................................................................................................212.3.7 Light requirement.................................................................................................................23

3 Method and model........................................................................................................253.1 Planning the cycle of the system.........................................................................................253.2 Dimensions of the greenhouse.............................................................................................263.3 System energy balance........................................................................................................28

3.3.1 Thermal mass.......................................................................................................................283.3.2 Biogas energy content..........................................................................................................293.3.3 Energy balance.....................................................................................................................29

3.4 System material balance.....................................................................................................323.4.1 Water....................................................................................................................................323.4.2 Biogas slurry and nutrients...................................................................................................323.4.3 Carbon dioxide.....................................................................................................................333.4.4 Crops....................................................................................................................................33

3.5 Location climate data.........................................................................................................343.6 Sustainability aspect...........................................................................................................343.7 Reference greenhouse for comparison................................................................................35

4 Result and discussion...................................................................................................364.1 Greenhouse conditions without climate control.................................................................364.2 Greenhouse energy balance................................................................................................37

4.2.1 Heating.................................................................................................................................374.2.2 Cooling.................................................................................................................................384.2.3 Electricity.............................................................................................................................394.2.4 Combustion engine power required for greenhouse operation.............................................404.2.5 Biogas required for energy...................................................................................................404.2.6 Reference greenhouse system...............................................................................................41

4.3 Greenhouse material balance.............................................................................................424.3.1 Crops....................................................................................................................................424.3.2 Water....................................................................................................................................424.3.3 Biogas slurry........................................................................................................................434.3.4 Carbon dioxide.....................................................................................................................44

4.4 Sustainability .....................................................................................................................45

5 Conclusion....................................................................................................................48

6 Future study.................................................................................................................50

7 References....................................................................................................................51

9 Appendix......................................................................................................................57

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Nomenclature 1 - Abbreviations Abbreviation Expansion BS Biogas slurry CBS Concentrated biogas slurry DRAPS Double recirculating aquaponics system GHG Greenhouse gas emission HPS High pressure sodium IPP Integrated production and protection IR Infrared radiation LCA Life cycle analysis LED Light emitting diode MSW Municipal solid waste NFT Nutrient film technique PAR Photosynthetically active radiation PC Polycarbonate PE Polyethylene PMMA Polymethyl methacrylate PV Photovoltaic RAS Recirculating aquaculture system SEK Swedish krona SMHI The Swedish meteorological and hydrologic institute SRAPS Single recirculating aquaponics system UV Ultraviolet VAT Value added tax

Nomenclature 2 - Variables Variable Representation Unit

A Area m2

cp Specific heat capacity J/(kg ·°C) ɛ Emissivity coefficient - E Energy J h Heat convection coefficient W/(m2·°C) k Heat conduction coefficient W/(m ·°C) L Characteristic length m Λ Transmission and ventilation losses W/°C m Mass kg mg Gas mass g M Molar mass g/mol µ Dynamic viscosity kg/(m · s) n Moles mol Nu Nusselt number - P Pressure Pa Pr Prandtl number - Q̇in Influx of heat power W Q̇out Outflux of heat power W Q̇rad Radiation heat power W R Gas constant: 8.314 (Pa · m3)/(mol · K) Re Reynolds number - ρ Density kg/m3 σ Stefan-Boltzmann constant: 5.67 · 10-8 W/(m2 · K4) T Temperature °C Tamb Ambient temperature °C Tg Gas temperature K Tinf Reference temperature °C Tsurf Surface temperature K Tsurr Surronding temperature K τ Timestep h τB Time constant of structure s u Velocity m/s U Overall heat transfer coefficient W/( m2·°C) V Volume m3

V̇ Ventilation rate m3/s x Material thickness m

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1 Introduction

1.1 Background Greenhouse cultivation is a growing industry, especially in mild climates, much due to the ability to manipulate growing conditions and increased water utilization efficiency (von Zabeltitz 2011, 1). There are many aspects to consider in greenhouse cultivation, and one of the foremost important aspect is additional heating during cold seasons, to enable an optimal indoor climate. Today a large amount of heated commercial greenhouses utilizes combustion of fossil fuels for heating, which adds to the environmental impact through greenhouse gas emissions contributing to climate change. One way to circumvent this impact is to exchange the fossil fuels with biogas. A further benefit of having biogas generation and greenhouse cultivation in conjunction is that resources from biogas generation and combustion have the potential to enhance crop growth, through the use of biogas slurry and carbon dioxide. This way a more closed cycle of materials flow could be achieved. Presently, the technology for biogas production conducted at larger scales, commercial scale are more mature compared to small-scale biogas systems. In Sweden on average one new small-scale biogas facility is constructed annually. However, the technology is not unfamiliar since in Germany on average 1500 new small-scale biogas facilities are constructed annually. The difference is the existing policy, mostly the economical aspect, where the farmers in Germany are guaranteed a fix price for each produced kWh that sells to the local grid. This does not apply to the farmers in Sweden, additionally, the price for biogas has been lower in Sweden compared to Germany (Edström & Nordberg, 2004). With low price for biogas there is no direct incentives to sell the produced energy, thus giving the conclusion that it instead should be reused in the local area and its facilities.

1.2 Purpose and goal This report was created in joint collaboration with Gunnar Bech from Innovationsverket and the faculty of sustainable energy technology in KTH, in order to explore utilizations of biogas in a rural area. The main purpose of this report is to examine the synergies and sustainability of combining a greenhouse with a biogas facility in a rural area. In addition, the lack of previous research of this kind of system makes this interesting to examine. To fulfil the purpose of this report the following goals were set up:

● To find synergies between small-scale biogas production and greenhouse operation ● To strive towards a closed loop system (recirculation) by utilizing the ascertained synergies ● To identify the contribution of greenhouse gas emissions of greenhouse materials and means

of lowering the them ● To acquire indications of the feasibility of the concept of combined small-scale biogas

production and greenhouse operation

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2 Literature study

2.1 Biogas Biogas is a renewable and high methane content combustible gas. It can be produced through the decomposition of organic material by microorganisms under anaerobic conditions, in a process called anaerobic digestion. This production method can be utilized to obtain biogas from landfills, wastewater treatment and other organic materials, such as manure and crop residues from agricultural activity (Energigas Sverige, 2014). Biogas mainly contains methane (CH4) and carbon dioxide (CO2), with the following constituents in considerably smaller concentrations: nitrogen (N2), hydrogen (H2), hydrogen sulfide (H2S) and oxygen (O2). Furthermore, raw biogas is usually saturated with water vapor (Avfall Sverige, 2015). The volume percentages of the compounds in biogas are presented in table 2.1. Table 2.1 Estimated volume percentages of various molecular compounds in newly produced biogas from organic waste (Christensson et al. 2009, 13; Jørgensen 2009, 4):

Molecular compound Volume % in biogas

Methane (CH4) 55-80

Carbon dioxide (CO2) 20-45

Nitrogen (N2) 0-1

Oxygen (O2) Trace

Hydrogen (H2) Trace

Hydrogen sulfide (H2S) 0-2000 [ppm]

The CH4 is the compound that contributes to the energy value of the biogas, where 1 normal cubic meter (Nm3) of CH4 has an energy value of 9.97 kWh. The unit of Nm3 is given by the volume of a gas at 0 ℃ and atmospheric pressure. Given that the amount of CH4 may vary, the energy density of raw biogas varies between 4.5 and 8.5 kWh/Nm3 (Ibid). CO2, H2S and water content are corrosive factors and the amount thus determines the corrosiveness of the biogas. For distribution of gas in grids it is important to lower the water vapor content in the biogas to reduce the risk of grid blockage - the blocking of biogas distribution pipes. This can be achieved by condensing it through a pressure increase or temperature decrease and may be desirable for the use of biogas in apparatuses, since condensed water may cause corrosion damage to them (Christensson et al. 2009, 14). The chemical reaction for biogas complete combustion of methane is presented in reaction 2.1.

(2.1) CH4 + 2O2 → 2H2O + CO2

2.1.1 Biogas production on smaller scale The small-scale biogas system contains several steps and elements that transforms organic material to biogas and its residue (biogas slurry), as is illustrated in figure 2.1. First the organic material, mostly consisting of agricultural residues but that may be mixed with other organic contents, is collected and

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undergoes pre-treatment, where undesired or harmful components are separated and the material is ground to increase the digestion rate. The treated digestate then enters the digester, the chamber where anaerobic decomposition takes place and biogas is generated. The digester is commonly heated to temperatures of around 37 ℃ (for mesophilic conditions) or 55 ℃ (for thermophilic conditions) and has a continuously running stirring device in order to blend the digestate and ensure that decomposition and heat distribution is uniform. After the main digestion process the biogas slurry may be placed in a post digester if the digestion is not fully complete. Both the main and post digestion chambers produce biogas which is lead through pipes to end usage or refinement instances (Christensson et al. 2009, 21-33).

Figure 2.1. A schematic diagram of small-scale farmyard biogas production system. Biogas can be stored in a gasometer, but the use of it is limited in small-scale applications. Instead the digester may hold the same function through the use of a flexible membrane for biogas collection inside the chamber, however, this method yields a storage capability of a considerably shorter term. Regardless of storage capability a flaring stack is connected to the digester output tubes so that the biogas may be flared when its production exceeds storage and use. When biogas is flared the CH4 is converted to CO2 and H2O and thus the global warming potential can be reduced as opposed to release of biogas into the atmosphere, due to CH4 being a much more potent GHG than CO2 (Ibid).

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All organic material may be digested with varying time requirements. Table 2.2 illustrates biogas yield and methane content of three different organic materials: cellulose, protein and fat. As can be seen, fat yields roughly double the amount of biogas per unit weight and also attains the highest energy value, defined by the methane content (Jørgensen 2009, 19). Table 2.2 Biogas yield at STP1 and methane content, upon complete digestion of three various compounds: cellulose (carbohydrate), protein and fat. The general chemical processes are also shown (Jørgensen 2009, 19).

Organic material

Process ml biogas/g

ml CH4/g

CH4 (%)

Cellulose C6H10O5 + H2O → 3CH4 + 3CO2 830 415 50.0

Protein 2C5H7NO2 + 8H2O → 5CH4 + 3CO2 + 2(NH4)(HCO3)

793 504 63.6

Fat2 C57H104O6 + 28H2O → 40CH4 + 17CO2 1 444 1 014 70.2 1 STP = Standard Temperature and Pressure (0 ℃ and 1 atm) 2 Fat in the form of glycerol trioleic acid

2.1.2 Utilization of small scale biogas Among small-scale biogas facilities in Sweden the most common utilization of the biogas is heating of directly nearby farmyard buildings with the use of a combustion or gas engine. However, some facilities also use the energy from combustion of biogas to generate electricity, with 30-35 % efficiency, and it may then be used within the farmyard or alternatively sold and distributed out through the grid (Naturvårdsverket 2012, 77; Energimyndigheten 2016, 9). In Sweden micro producers of renewable electricity, i.e. individuals such as homeowners with small power generation abilities, may sell electricity to electricity trading companies or companies that own the grid. The possible revenue varies from company to company, but it is often 0.1-0.5 SEK/kWh (Energimyndigheten, 2017a). From January 1st 2015 the micro producer can apply for a tax reduction of 0.6 SEK/kWh, but no more than 18 000 SEK, corresponding to 30 000 kWh produced annually. Also, electricity sold must not exceed electricity purchased. If revenues from electricity sales are beneath 30 000 SEK/year the micro producer may also be exempted from value-added tax (VAT) (Skatteverket, 2017). The two of the largest energy companies in Sweden, Fortum and Vattenfall, currently both pay the spot price of electricity at the Nordic electricity market Nord Pool for micro produced electricity (Fortum, 2016; Vattenfall, 2017). Furthermore, producers of renewable electricity may be granted one green certificate from the state for every MWh of electricity produced. The certificates can then be sold on the open market to other electricity providers (Energimyndigheten, 2017a). The average price per certificate between March of 2016 and March of 2017 was 146.89 SEK (Energimyndigheten, 2017b). The methane content in raw biogas can be increased through a process known as conditioning or upgrading. This is common for the production of high quality vehicle fuel. In Sweden, vehicle biogas fuel is required to have a methane content of 95 - 99 %, comparable to conventional natural gas. The large amount of carbon dioxide in raw biogas must therefore be removed. That is commonly achieved through a process known as water scrubbing, where the carbon dioxide of the raw biogas is dissolved in a water stream. However, upgrading of raw biogas is uncommon in small-scale biogas production

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systems, with few exceptions, due to economic reasons. It is instead an application more commonly in conjunction with larger operations. One method for which upgrading of biogas from small-scale operators could be realized is the implementation of local gas grids. With gas grids small biogas producers may have their gas distributed to centralized upgrading facilities (Christensson et al. 2009, 40). In 2013 an exploratory project was initiated to investigate the market conditions for expanding an existing local biogas grid in the Mälardalen region in Sweden. It was found that the production of biogas in the region (337 GWh) must increase as well as the biogas demand. Demand was found to increase in the transportation sector by 1 TWh to 2020 and 1 TWh demand was further concluded possible within local industry. Meanwhile the biogas production potential in the region was estimated to 4 TWh (Forsberg, 2014). The exhaust from boilers or combustion engines for heat and electricity generation could be utilized for CO2 enrichment in greenhouses. This could aid in boosting crop growth and reduce or eliminate the need for bought CO2 from another source (Christensson et al. 2009, 40). Another residue from the biogas system, biogas slurry, is rich in compounds with high nutritional value for plants and could thus be used for fertilization. Dependence on bought fertilizers can then be reduced and a more closed loop of resources may be achieved. In 2015 all of the biogas slurry produced in farmyard facilities was used as bio fertilizer (Energimyndigheten 2016, 19). In a recent study it was concluded that biogas production on organic farmyards come with the potential for farms to gain positive energy balance, reduce GHG emissions and offer self-sufficiency regarding organic fertilizer nitrogen (Pugesgaard et al., 2014).

2.1.3 Current trend and potential of biogas in Sweden In Sweden 2015 a total of 1 947 GWh biogas was produced. The different ways of producing biogas is presented in table 2.3. The production of biogas in Sweden has had a continuous growth since 2006 to 2015, as shown in table 2.4. Compared to the year 2014 the biogas production for small scale farm increased with 14 % (Energimyndigheten 2016, 12). Table 2.3 Different ways biogas was produced in 2015 (Energimyndigheten 2016, 7).

Different applications of biogas Percentage of total production of biogas [%]

Digestion facilities 44

Waste water treatment 36

Landfills 9

Industrial facilities 6

Small scale farm production 3

Gasification 2

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Table 2.4 Different ways biogas was produced in 2015 (Energimyndigheten 2016, 7).

Year 2006 2007 2008 2009 2010 2011 2012 2013 2014 2015

GWh 1 213 1 258 1 359 1 363 1 387 1 473 1 589 1 686 1 784 1 947

During 2015 biogas produced from small scale farms only consisted of 50 GWh, and of this 34 % was converted to heat, 16 % was converted to electricity and the remaining gas went to other applications, e.g. upgrading of biogas (Energimyndigheten 2016, 15). Furthermore, according to the report “EU Reference Scenario 2016 - Energy, transport and GHG emissions trends to 2050” by the European Commission through an analysis of future trends, shows that the quantity of GWh in terms of electricity will increase until 2050, as shown in table 2.4 (European Commission, 194). Table 2.5 Different ways biogas was produced in 2015 (Energimyndigheten 2016, 7).

Year 2015 2020 2025 2030 2035 2040 2045 2050

Electricity generated by source

(GWh)

14 846 17 307 16 195 20 890 23 299 22 662 26 440 27 121

Fuel inputs to thermal

power generation

(GWh)

4 886 4 556 4 703 5 437 5 713 5 689 6 209 6 392

2.2 Greenhouse A greenhouse is a facility for cultivating plants and crops where the growth environment is protected from the outside environment in addition to where growth and climatic conditions may be manipulated. This means that greenhouse cultivation can be made largely independent of outdoor environmental conditions (Reddy 2016, 13). The greenhouse is constructed with transparent materials for its walls and roof. This allows for high transmittance of sunlight while simultaneously trapping heat inside of it, with its structure having low transmittance for infrared radiation (IR), which lowers heat transfer from crop environment to the outside environment. As such greenhouses may be used to cultivate all year round in regions where the growing period is limited to few months of the year. For high yield and quality of greenhouse crop production the following is required (von Zabeltitz 2011, 1-2):

● Appropriate greenhouse structure ● Good mounting, installation and maintenance of the system ● Knowledge and skill of workers ● Efficient management of production ● Efficient climate control during summer/winter ● Procedures of integrated production and protection (IPP)

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2.2.1 Shape and orientation Greenhouses can be made in several different shapes and both their shape and orientation have an impact on the amount of solar irradiation reaching the greenhouse crops. The sunlight that is transmitted through the greenhouse walls varies with the time of day as well as the season. Table 2.6 presents the mean transmittance for five different greenhouse types in north-south and east-west orientation: (a) saddle roof with 15° inclination, (b) saddle roof with 25° inclination, (c) saddle roof with 35° inclination, (d) saw tooth or shed roof with 25° and 65° inclination and (e) round arch with vertical side walls. As can be seen from it light transmittance increases with the inclination of saddle-type roofs and the saw tooth and curved roof types have the highest transmittance, although the saw tooth type has a larger surface and is thus subject of greater heat loss. A north-south orientation yields high transmittance in summer, whereas an east-west orientation offers high transmittance during winter (von Zabeltitz 2011, 138). Table 2.6 Mean transmittance (in %) of different greenhouse types with North-South and East-West orientation in the months of December and June (von Zabeltitz 2011, 138).

Greenhouse type December June

North-South East-West North-South East-West

(a)

36 % 37 % 58 % 56 %

(b)

43 % 47 % 62 % 57 %

(c)

52 % 62 % 67 % 61 %

(d)

52 % 70 % 69 % 78 %

(e)

52 % 65 % 72 % 70 %

For greenhouse cultivation in northern mild climates it is necessary to weigh the structure type, insulating properties and optical characteristics of the greenhouse for optimal growth temperatures and economy. It is desired to minimize the heat requirement during cold months as well as to minimize cooling requirements during warm months (Maslak 2015, 26, 28). The width of arched or tunnel greenhouse types is suggested to be between 8 and 9 metres These can be arranged as singular units or be connected together in what is called a multi span greenhouse (von Zabeltitz 2011, 66). On the topic of greenhouse height there are different parameters to consider. Most importantly the height of the intended crop must be determined and that will set the minimum greenhouse height. Greater height of the structure, and thus greater greenhouse volume, provides better

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climatic conditions of the indoor environment. However, that brings trade-offs in the form of larger surface area resulting in increased heat transfer and higher greenhouse build costs (von Zabeltitz 2011, 60). For numerous varieties of tomato crops, most fruits develop in the first 2 meters of the crop (Schwarz, 1995) and most new greenhouses for tomato cultivation have heights of 3.5 - 6 metres to accommodate for wiring support of plants and artificial lighting above them (Peet & Welles 2005, 259-260).

2.2.2 Greenhouse materials The greenhouse cladding is the part of the structure consisting of walls and roofs and that transmits sunlight into the area enclosed by the greenhouse. The character of the cladding material determines the transmittance as well as other aspects of the material, such as adequate strength and thermal resistance. Cladding materials should have a high transmittance of PAR (photosynthetically active radiation), low transmittance of IR (infrared radiation), low ageing of UV (ultraviolet) radiation, low accumulation of dust and dirt, no dropwise condensation and have high endurance against the wind. The avoidance of dropwise condensation is desired due to the increased danger of diseases and lowered light transmission. This can be realized by using cladding with film condensation properties. For arched roof greenhouses one solution is also to create an upward pointing edge on the top the arch structure, which is commonly known as a venlo type roof structure. This will make water drops run down along the cladding material, instead of potentially dripping down from the mid-section of the roof structure (von Zabeltitz 2011, 145, 162). The light transmittance is one of the most limiting factors for yield maximization in almost all regions. In a study in North-America in 1998 it was found that a 3 % decrease of transmittance resulted in 1 % lower tomato yield, whereas a study in the Netherlands 1984 reported 1 % lower yield when transmittance decreased by 1 % (Peet & Welles 2005, 290-291). There are three main types of cladding materials used for greenhouse operations: glass, rigid plastic and plastic film. Of these, very common rigid plastics are polymethyl methacrylate (PMMA) and polycarbonate (PC) sheets and the most common plastic film consists of polyethylene (PE). These all come with different advantages and disadvantages, respectively (Maslak 2015, 27; von Zabeltitz 2011, 147):

● Glass: Glass has the highest light transmittance of all above mentioned greenhouse cladding materials, usually more than 90 % for single panes and above 80 % for double panes. The material does not deteriorate over time. It is however rather heavy, so expensive and adequately supportive structures and frames are necessary. It is also very brittle, making it susceptible to impacts. Greenhouses can have double glass panes for increased thermal resistance. The heat conduction coefficient for glass is 1 W/(m*K) (The Engineering Toolbox, n.d.).

● Acrylic plastic: This material has a light transmittance of approximately 80 % for double wall panels. It has high thermal resistance and like glass it does not deteriorate. A disadvantage it shares with glass is its brittleness and susceptibility to scratching. Furthermore, acrylic plastic is flammable. The material has a thermal conductivity of 0.2 W/(m*K) (The Engineering Toolbox, n.d.).

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● PC-sheets: Polycarbonate double sheets may have up to 83 % light transmittance. They are flexible and lightweight which make them attain desired shapes easily and lowers the need for strong supportive structures. The material is vulnerable to degradation from ultraviolet radiation, which makes it turn yellowish over time, but this process can be slowed down with special treatment to suppress deterioration from UV-radiation. PC-sheets have a thermal conductivity of 0.19 W/(m*K) (The Engineering Toolbox, n.d.).

● PE-film: Polyethylene film may have almost as high transmittance of light as glass, around 90 % for single layers and around 80 % for double layers. It is the least expensive of the mentioned materials, come in wide widths and is flexible and lightweight, making it easy to install. Like PC it degrades over time due to exposure of UV-light, which may yield a short lifetime of only 2 years. PE can however be UV-treated as well and can then have lifetimes of up to 10 years (SolaWrap, 2016). The material has a thermal conductivity of 0.33 W/(m*K) (The Engineering Toolbox, n.d.). The cost of greenhouse components and materials vary depending on level of technology. With highly technological properties the controlling capability is extensive and crop cultivation can be made very efficient, but then the initial cost of components is usually high. Measures can be taken to lower initial cost through downgrading of technological level, however control and further growth efficiency may be negatively impacted (table 2.7) (Ponce et al. 2014, 64). Table 2.7 Materials and components for low to high tech greenhouses that last for at least 10 years and their approximate costs (Ponce et al. 2014, 65).

Technology level of greenhouse

Low Medium High

Structure Wood or steel Steel Steel or aluminium

Cladding Single layer PE film Double layer PE film or rigid plastic

Glass, PC or PE

Heating No Yes, air heating Yes, hot water pipes

Cooling Yes, passive Yes, passive and/or active Yes, evaporative cooling

Growth medium Soil Soil or soil-less substrate Soil-less substrate

Irrigation Manually controlled drip irrigation

Partly automatically controlled drip irrigation or hydroponics

Fully automatically controlled drip irrigation or hydroponics

Cost [SEK/m2] 220-260 260-870 870-1 740+

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2.2.3 Climate control in greenhouses The greenhouse climate is a complex and nonlinear system, and is the main factor for optimal growth conditions for the plants. Factors with great significance of the internal climate of a greenhouse is light, temperature, humidity, CO2, water, fertilizer, etc. However, light and temperature are of significant importance for the cultivation system climate (Mahdavian, M et al. 2017, 835). The greenhouse is a complex thermodynamic system where the internal temperature may become high due to trapped incident solar radiation (Maher, A et al. 2016, 1243). The internal temperature and humidity respectively vary greatly during the day, thus regulating systems are essential to obtain optimal cultivation conditions within the greenhouse. External disturbances such as weather conditions affect the indoor climate of the greenhouse greatly, which however can be easily adjusted (Ibid, 1247).

There are different methods of regulating the heating and cooling demands in a greenhouse to obtain desired indoor temperature, both active and passive technologies. Using conventional photovoltaic (PV) modules are convenient because of their multifunctionality, e.g. the converted electricity from solar radiation can be stored in batteries and subsequently used in different appliances to regulate the indoor climate of the greenhouse, such as lamps, coolers, heaters etc. In addition, it is possible to enable both thermal and energy generation in photovoltaic/thermal modules, thus also increase the efficiency. Water is a common media that is used as energy carrier in these systems, but air can also be used. In temperate climates the air based system is more cost effective, but also enables a more simple system than water based (Cuce et al. 2016, 38-39).

Furthermore, thermal energy storage is also a common method used to stabilize the indoor temperature. The main factor of this technology is the storage material and the used container. The most common material used to store heat is gaseous materials such as air, liquids such as water and solid material such as phase change material (PCM). In addition, soil is also considered in thermal energy storage as it has similar characteristic properties as solid materials. Soil is usually used as a heat sink to reduce internal temperatures in greenhouses. In climate with higher demand for cooling, soil is a good option as a passive cooling application (Ibid, 43, 45).

Moreover, another technology with many utilities is heat pumps which can be used both for heating and cooling the greenhouse whenever required. Heat pumps can also be used to control the relative humidity of air in the greenhouse. The heat pumps are usually preferably implemented with a ground source with the heat exchanger pipes horizontally in greenhouses (Ibid, 46-48).

A common passive cooling technology that is used in the greenhouses to regulate the temperature is through wind catchers. Which has the function to lower the temperature inside by flowing fresh air through natural convection from the roof of the building. The pressure difference is an affect due to difference in temperature with outdoor and indoor, resulting in warm air inside is pushed upward and out of the building replaced with cooler air thus the temperature drops. However, since the cooling technology using wind catchers is passive and is both open and in direct contact with the greenhouse, there is an increased risk of admittance of pests to the interior of the greenhouse (Ibid, 51).

In order to enhance photosynthesis and to increase crop yield a common procedure is to enrich the greenhouse with CO2. With CO2 enrichment in greenhouses an increased yield of 30 to 40 % is possible.

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However, the CO2 enrichment affects the air temperature in the greenhouse since ventilation decreases the temperature but simultaneously also decreases the CO2 concentration in the air. In warmer climates this can be a problem, and a more precise balance between ventilation and desired CO2 level is required. There are various ways to obtain optimal climate considering the ventilation and the CO2 level in the greenhouse (Amir, R et al. 2005, 619-620).

A common way is through forced ventilation in combination with injection of CO2. In a study a few tests were made with different modes of ventilation and all with the same rate of injected CO2. The modes consisted of an injection rate of CO2 of about 1.8kg/h to a concentration of 1000 ppm and four different ventilation modes: high ventilation, low ventilation, alternating high and low ventilation and lastly, adjusting the ventilation to the heat load. The last ventilation mode gave the best results, with high levels of CO2 concentration and low electricity requirements considering the usage of ventilation (Ibid, 621-623).

2.2.4 Sustainability aspect of the greenhouse Some aspects of greenhouse cultivation have more significance for the environmental footprint than others. The choice of building materials has large significance for the contribution of greenhouse gas emissions (GHG). The building materials consist of greenhouse anchoring or foundation, greenhouse frame or structure material and cladding material, and their contribution to GHG emissions are presented in table 2.8. Furthermore, the impact of continuous greenhouse operation and maintenance must be assessed. These aspects consist of utilization of heat, electricity, water, pesticide and transport services (von Zabeltitz 2011, 118-120). Table 2.8 Greenhouse gas emissions in kilograms carbon dioxide equivalents (CO2e) per kilogram of various greenhouse structure material (Ruuska 2013, 24; Al-Amin et al. 2011, 367; Boustead 2005a, 9; Boustead 2005b, 9).

Building material CO2e [kg/kg]

Concrete 0.442

Aluminium 9.964

Steel 3.778

Glass 1.230

Polyethylene 2.130

Acrylic plastic 5.900

Polycarbonate 6.000

As can be seen in Table 2.8 polyethylene yields higher GHG-emissions than glass per weight unit, however for greenhouse construction less polyethylene film by weight is required than glass by weight, meaning that the production of polyethylene cover for a greenhouse yields lower GHG-emissions than for similar application of glass. Concerning energy balance during greenhouse operation a study conducted on three greenhouses with plastic film and two with glass cover, all with similar indoor areas,

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showed that the glass houses had up to 64 % higher heating requirements than the plastic film ones (von Zabeltitz 2011, 129, 131, 135). In a study from 2013 the environmental impact of tomato cultivation in a glass greenhouse in Central Europe was studied using the LCA-methodology, in order to create a user friendly environmental impact calculator for greenhouse production systems. Parameters included were greenhouse structure, auxiliary equipment (for example growth medium), climate control system, fertilizers, pesticides, waste management and transport. Environmental impact categories consisted of air acidification, eutrophication, global warming, photochemical oxidation (e.g. ozone depletion) and water use. The greenhouse having an expected life time of 15 years and being heated by natural gas it was concluded that the operation of the climate control system was the main burden in all impact categories and overwhelmingly so in global warming potential (table 2.9). The next significant impacts were from structure and fertilizers (Torrellas et al. 2013, 188, 190-192). Table 2.9 Impact from different greenhouse parameters on four impact categories per tonne tomato produced in a glass greenhouse in Central European climate conditions.

Impact categories 1

Air acidification [kg SO2 eq]

Eutrophication [kg PO4

-3 eq] Global warming [kg CO2 eq]

Photochemical oxidation [kg C2H4 eq]

Structure 0.282 0.087 47.19 0.013

Climate control 2.624 0.0698 1 823 0.196

Aux. equipment 0.063 0.014 8.482 0.003

Fertilizers 0.280 0.054 47.64 0.002

Pesticides 0.001 0.001 0.238 0.000

Waste 0.003 0.001 0.706 0.000

Total 3.252 0.8550 1 928 0.2150 1 Water use values were not given separately for each greenhouse parameter, but instead the total value of 14.06 m3 per tonne tomato yield was stated. The energy for the greenhouse system can be supplied by local cogeneration of heat and electricity from biogas combustion. Biogas releases GHG emissions in the form of CO2 upon combustion, but the carbon comes from plant matter that previously fixed it from atmospheric CO2

during growth. Biogas can thus be considered a renewable energy resource and yields zero net GHG emissions (this is excluding material and energy requirements for the biogas system, i.e. biomaterial collection and digester operation). Each kWh (kilowatt hour) of generated heat or electricity would in this case have no net contribution of GHG emissions (University of Florida, 2015).

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Electricity may be alternatively or supplementary supplied to the above process by the electric grid. GHG emissions per kWh may then vary depending on the source of generation. Sweden is in the Nordic electricity market and emissions can thus be derived from its electricity mix, which currently is 100 g CO2e per kWh on average (Svensk Energi, 2016).

2.3 Horticulture/plant growth

2.3.1 Temperature requirement Crops such as tomatoes are warm-season species and require greenhouses for year round cultivation in mild climates, due to the commonly cold temperatures over the year. Specific temperature requirements are illustrated in table 2.10 (von Zabeltitz 2011, 29-30). In a study on how temperature affected tomato growth, tomatoes were grown in constant temperatures of 14 ℃, 18 ℃, 22 ℃ and 26 ℃. It was found that the crops grown at 18 ℃ and 22 ℃ produced normal fruits, whereas at 14 ℃ growth was severely reduced, resulting in tomatoes of no marketable value. Crops grown at 26 ℃ had a poor appearance and suffered from abnormal vegetative trusses (Adams et al. 2001, 871).

Table 2.10 Recommendations on greenhouse temperature settings for plant growth (von Zabeltitz 2011, 29-30).

Condition Temperature of occurrence [℃]

Comment

Frost 0 Plants may succumb to frost. Risk of temperatures lower than 0 ℃ can be neglected when minimum daily outside temperature is 7 ℃.

Cold temperatures <12 Growth, quality and yield of vegetables in greenhouses are negatively affected. For a tomato crop the temperature should not be lower than 15 ℃.

Optimal daytime temperature 22-28

Optimal night time temperature 15-20

Overall optimal temperature 17-27 When only relying on heating of the greenhouse from solar radiation, suitable daily outside average temperatures are 12-22 ℃. The optimal temperature range for tomato growth can be set to 18-22 ℃.1

Hot temperatures >30 Growth, quality and yield of vegetables in greenhouses are negatively affected.

Mean maximum temperature 35-40 The maximum temperature for a tomato crop is 35 ℃.

1 From: Adams et al., 2001.

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For cultivation of tomatoes the optimal greenhouse temperature will fall within the range 18-22 ℃, accordingly. Since a daily average temperature of 12-22℃ may indicate greenhouse temperatures of 17-27 ℃, daily average temperatures of at least 12 ℃ will be required for optimal tomato growth without using biogas for additional heating. According to Swedish Meteorological and Hydrological Institute (SMHI) the average temperatures in Mälardalen, Sweden, in June, July and August of year 2016 were 15 ℃, 17 ℃ and 17 ℃, respectively, while the average temperature over the year was 7-8 ℃ (SMHI, 2017). The necessity of additional heating will thus be unlikely during the summer period and instead needs to be increased during winter and parts of spring and fall. During warm and sunny summer days the greenhouse temperature may exceed optimal cultivation temperature and thus some method of cooling is required to be adopted.

2.3.2 Growth medium There are various methods to grow plants, either in soil or without soil. Soilless horticulture is also known as hydroponics, where roots are submerged in a solution enriched with nutrients. All the necessary minerals are dissolved in the solution (Orsini et al. 2013, 714). The solution can be used with or without an external artificial medium, e.g. sand, gravel, mineral wool, peat moss or sawdust etc. with the purpose of providing a more solid support. Hydroponics are suitable for indoor horticulture and the system of hydroponics uses as little as 1/20 of the amount of water compared to regular outdoor horticulture based on soil, mostly due to evapotranspiration. In hydroponic horticulture the soil borne pest and other pathogens can be neglected. Thus, an increase in crop yield and quality can be expected (Lakkireddy et al. 2012, 29, 31). A common method in hydroponics is nutrient film technique (NFT), and is when the roots of the plant are suspended in a channel where a solution of nutrients is able to flow pass. The channel is usually inclined so the nutrient solution can flow through the whole system without external forces. This method requires less nutrient solution compared to other and in addition, also has a relative low cost when constructing (Gunning et al., 2016). An example of trough dimensions for tomato cultivation is shown in figure 2.2.

Figure 2.2. A two-dimensional bird view schematic of a trough used for soilless cultivation of tomato crops. All values are in centimeters (cm) (Rutledge 1998, 9). An application of the hydroponic technique is aeroponics, which refers to that the growth is achieved in air culture. The root of the plants is kept suspended mid-air in a room with high humidity level, around 100% humidity (Lakkireddy et al. 2012, 30). The roots are continuously or discontinuously exposed to a saturated environment containing a mist of nutrient solution (Komosa et al. 2014, 164). Due to the suspension mid-air the roots are well aerated and able to absorb the necessary amount of

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oxygen and thereby increase both the metabolic process and the growth-rate up to 10 times. In aeroponics the growing occurs without a growing medium unlike hydroponics which uses water (Lakkireddy et al. 2012, 30). The yield for tomatoes in aeroponics is, as in hydroponics, much higher than for tomatoes grown in soil. The root aeration in aeroponics are a major factor leading to a higher yield compared to a system of hydroponics (Ritter et al. 2000, 132). Moreover, in hydroponic culture the root aeration is rather poor and is only able to solve 8.7 mg O2 in one liter of water. For tomato horticulture intermittent injections of nutrient solution has an advantageous effect by decreasing the temperature around the root, especially for horticulture in greenhouses during summer. During the day the frequency of injecting nutrient solution is higher compared to the night, due to the absorption trends. The excess of the unused nutrient solution can be reused after a process of filtration and disinfection with UV irradiation. The unused nutrient mixes with the rest of the nutrient solution and can then be reused. With the use of aeroponics a saving of around 18% of nutrient solution can be made compared to hydroponic culture in mineral wool with recirculating system (Komosa et al. 2014, 164, 166, 174). The same amount of nutrients was used in both the aeroponic and the hydroponic (mineral wool culture) system and is presented in table 2.11 (Ibid, 165). Table 2.11 Nutrients used in both aeroponics and hydroponic system.

Nutrient Aeroponic/hydroponic [mg/l]

N-NH4 <14

N-NO3 210.0

P 70.0

K 351.0

Ca 170.0

Mg 84.0

S-SO4 132.0

Na 22.7

Cl 42.2

Fe 1.68

Mn 0.54

Aquaponics is a sustainable system for producing food, which maximize the usage of fresh water and nutrients while producing food. The system combines recirculating aquaculture systems (RAS) with hydroponic systems, where the hydroponic system usually uses the nutrient film technique (NFT). The system contains of two separate systems to produce e.g. fish and tomatoes. The fish is grown in the fish tanks/aquariums and the tomatoes are grown in a hydroponic system with NFT in a greenhouse. The main idea is to reuse the waste water from the fish tanks in the hydroponic system. However, an addition of nutrients may be required (Reyes et al. 2016, 30-31). Nevertheless, RAS still need a certain amount

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of fresh water but significantly less than a traditional system (both for fish production and tomato production). There are two concepts of RAS, single recirculating aquaponic system (SRAPS) and double recirculating aquaponic system (DRAPS). SRAPS is a basic concept where the waste water from the fish tank goes directly to the plants without any external adjustments or refinement. The root zone of the plant with its bacteria cleans the waste water and subsequently flows back to fish tank to be reused. In this system the main input of nutrients is fish feed, and therefore, also indirectly the nutrients for the plants as well. Which gives a deficiency of nutrients for the hydroponic system and impedes growth (Suhl et al. 2016, 335-336). The more complex system, DRAPS, which combines three biological systems, fish, plants and nitrifying bacteria. The three systems require different conditions, e.g. pH-value. The optimum range for both fish and nitrifying bacteria is between 7 and 9, while for the hydroponic system the pH varies between 5.5 and 6.5. There is a correlation between increased pH and decreased nutrients such as phosphorus, zinc, iron and manganese. In DRAPS the fish production and plant production are kept separated but connected through a 3-chamber-pit. With this combination optimal conditions regarding nutrients and pH-value can be applied for both systems. The system contains of a mechanical filter where the waste water from the fish tank flows, and subsequently goes through a bio filter with the purpose of nitrification by converting ammonium into nitrate. The water containing nitrate is recirculated back to the fish tank. Intermittently the effluent from the mechanical filter goes into the 3-chamber-pit and kept there until its use for the hydroponic system. However, before implementing the nutrition to the plant's optimal concentrations of mineral nutrients are added to obtain optimal plant growth (Suhl et al. 2016, 336-337). The required nutrients is presented in table 2.12 (Suhl, J. et al, 2016, 338). Table 2.12 Optimal concentration of mineral nutrients for tomato cultivation.

Nutrient Quantity (Mg/l)

N 151

P 37

K 234

Ca 128

Mg 24

S 110

Fe 2.0

B 0.3

Cu 0.2

Mn 1.2

Mo 0.05

Zn 0.4

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The process between the mechanical filter, bio filter and the 3-chamber-pit is where the optimization can be made, to either favour the fish production, or the plant production. With one cubic meter of fresh water 1.55 kg fish (Tilapia) and 46.1 kg tomato fruit can be produced. Furthermore, with one kilogram of fertilizer about 10 kg more of tomato can be produced in aquaponics compared to a hydroponic system. The utilization efficiency of fertilizer is improved with around 23.6 % in aquaponics compared to hydroponics (Suhl et al. 2016, 336, 340-341).

Figure 2.3. Scheme of a double recirculating aquaponics system (Suhl et al. 2016, 337).

2.3.3 Nutrient requirement Every plant needs a combination of nutrients to thrive, and for tomatoes there are at least twelve essential nutrients for a normal growth and reproduction. The nutrients can be separated into two groups, macro- and micronutrients, where macronutrients are needed in larger quantities compared to micronutrients where the requirement is far less. Excessive usage of nutrients that exceeds the requirements of the plant can reduce the tomato yield, degrade the surrounding environment and decrease the fertilizer-use inefficiency. The macronutrients consist of nitrogen (N), phosphorus (P), potassium (K), calcium (Ca), magnesium (Mg) and sulphur (S), and the micronutrients of boron (B), iron (Fe), manganese (Mn), copper (Cu), zinc (Zn), and molybdenum (Mo). The tomato production can be either soil based or based in water, so called hydroponics. The soil usually consists of some micronutrients and some calcium and magnesium, while hydroponic system needs to be provided with all the essential nutrients (Sainju et al. 2003, 1). Nitrogen, N: Nitrogen is one of the most important nutrients since it is a component of protein and amino acids. Without nitrogen plant growth and reproduction would not be possible. For the tomato production a large amount of nitrogen is needed, mostly because tomato plants use a lot of nitrogen, to not prevent growth and also have an adequate amount of nitrogen available in the soil to produce sufficient foliage to shield the plant from too much exposure of the sun. However, excessive usage of nitrogen can inflict negative consequences, e.g. reducing the tomato production by promoting an excessive vegetative growth which delays the maturity of tomato fruits or since the nitrogen is soluble in water it increases the chance of being leached from the soil and contaminate groundwater (Ibid, 2).

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Phosphorus, P: Phosphorus is an important nutrient that helps to initiate root growth of tomato and therefore, need to be in high concentration in the early stage of the process. The nutrient is also a constituent of nucleic acid. With improved growth stimulated by phosphorus the utilization of water and other nutrients in the soil are improved, and also promotes substantial growth of stem and healthy foliage. There is usually an abundant amount of phosphorus in the soil, because the nutrient absorbs easily in the soil and hardly leaches from the soil. The tomato takes up small amounts of phosphorus compared to nitrogen and potassium (Ibid, 2).

Potassium, K: Potassium is a nutrient that the plant needs in large amount and its main function is to activate enzymes and regulate pH of the tomato fruit. The nutrient also has a key part in regulating the quantity of the production of tomatoes per plant, since the nutrient stimulates early flowering and setting of fruit. Excessive amount of potassium reduces the availability of magnesium in the soil and as nitrogen, potassium is also soluble in water and can therefore leak into the groundwater (Ibid, 3-4).

Calcium, Ca: Calcium is as well amongst the nutrients that the tomato plant needs in large amount. Most of the soil contains sufficient amount of calcium for tomato growth, but deficiency occurs when the soil pH is below 4.5. The optimal range of pH in the soil is from 5.5 to 7.0. With excess of calcium the effects can be deficiency in micronutrients, such as iron and manganese (Ibid, 4).

Magnesium, Mg: The main function of magnesium is that it is a constituent of chlorophyll. Moreover, the mineral also contributes in fruit production. Deficiency of magnesium is common for greenhouse-grown tomatoes. However, the deficiency might not affect the fruit production unless the issue is severe (Ibid, 4).

Sulphur, S: Sulphur is an element of protein and amino acid. The mineral is usually applied in combination with N, P and K fertilizers, and therefore deficiency of this nutrient is rare. Through the atmosphere the tomato can absorb sulphur as sulphur dioxide (SO2) (Ibid, 4).

Boron, B: Boron can influence on the production of tomato flowers and fruits, by having an important role in the insemination and reproductive growth. Deficiency of boron can cause reduction of root growth and the deficiency triggers by increased value of pH in the soil and dryness around the root zone (Ibid, 4).

Iron, Fe: Iron is an element in many enzymes of the tomato. Deficiency of iron often occurs in soils with high pH and soilless medium. The solubility of iron in the soil decreases as the amount of excess of phosphorus increases (Ibid, 4).

Manganese, Mn: Manganese main function is to activate enzymes. The deficiency is as iron induced by high soil pH (Ibid, 5).

Copper, Cu: Copper is a constituent of oxidizing enzymes. Deficiency is more common in greenhouse-grown tomatoes or in soilless medium than field grown tomatoes (Ibid, 5).

Zinc, Zn: Zinc is essential for metabolism of nutrients in tomatoes. Moreover, in similarity with copper, zinc also show more tendency for deficiency in soilless medium and in greenhouse (Ibid, 5).

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Molybdenum, Mo: Molybdenum is a constituent of the utilization of nitrogen. Deficiency can occur when soils with low pH, peats, and soilless compost (Ibid, 5). However, since the amount of nutrients varies with the soil type and environmental conditions the desirable levels of nutrients in table 2.13 is not definitive, only a guideline (Ibid, 8). Table 2.13 Desirable levels of nutrients for tomatoes per kg of soil and plant, respectively (Sainju et al. 2003, 6).

Nutrients Soil [mg*kg-1] Plant [mg*kg-1]

P 60-70 4000

K 600-700 60000

Mg 350-700 5000

Ca 1000 12500

N 50-100 30000-50000

B 1.5-2.5 40-60

Mn 5-20 30

pH [no unit] 6.5-7.5 -

Salt [mmho/cm] 80-100 -

The nutrients that plants need can come from different sources, usually from artificial fertilizers or more organic fertilizers, e.g. municipal solid waste (MSW). The utilization of MSW which means composting household waste and other waste that is similar to household waste that is capable of undergoing aerobic or anaerobic decomposition results in less total amount of landfill waste and a nutritious product that can be used for horticulture. Moreover, the outcome of MSW might contain a different composition compared to artificial fertilizers, by containing toxicity of heavy metals, increased salt content and different ratio between the essential nutrients such as N, K and P. However, the quality of the composition can be adjusted by proper sorting in the origin (Martinez-Blanco et al. 2009, 340). The composting process can be divided in four main phases.

● The first stage is the pre-treatment. The material, both MSW and pruning waste aggregates and grinds to a maximum size of 80 mm.

● The second stage is composting or decomposition. The grinded matter puts in a tunnel with forced aeration and irrigation system for at least a minimum of two weeks. To maintain a good hygienization of the substance the temperature in the tunnel needs to exceed 65°C for two or three days.

● The third stage is curing. The decomposed material is placed in piles and occasionally turned to enable ventilation. To increase the humidity, the material is moisturized. The process takes approximately eight weeks.

● Lastly, the final stage is refining. The material that has not decomposed properly is put back in the previous stage. The size of mature compost is often less than 15 mm in size (Ibid, 343).

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The total time for the whole process until the compost is able to be applied on the soil without any problems is estimated to around 10 weeks. Decomposing biodegradable materials induces exhaust gases, however since the carbon cycle is short term it results in a neutral CO2 emission (Ibid, 344). Utilization of compost as a fertilizer for tomato plants does not affect the harvest nor the quality of the tomato. Biogas slurry, BS, are a by-product from the biogas production where organic compound has decomposed and induced a usable organic fertilizer. The utilization is simple. It is either sprayed directly or the seed is submerged in the substance to stimulate growth and germination (Yu F-B et al. 2009, 262). The fertilizer is rich in nitrogen, phosphorus and potassium, and is in the form of available nutrients. The BS also contains a lot of other necessary nutrients, e.g. sodium and calcium, and other essential elements for the plants such as various kinds of amino acids, vitamins and proteins. Improved soil fertility, and reduced emissions and fertilizer cost is an outcome of long-term utilization of biogas slurry (Yang 2011, 1 959-1 960). Furthermore, utilization of slurry results in better germination of the seed, inhibits diseases, and increases the fruit quality and yield (Yu F-B et al. 2009, 262). However, with conventional evaporation technology a refined product is possible to obtain, a concentrated biogas slurry (CBS) product that has enhanced nutrient content. With CBS about 10 times the concentration of the main nutrients, such as N, P and K, can be achieved (Ibid, 263). Nutrient content of BS and CBS are presented in table 2.14. Table 2.14 Concentration of nutrients in BS and CBS (Yu F-B et al. 2009, 263).

Nutrient/mineral Biogas slurry Concentrated biogas slurry

Total N [g/L] 0.069 0.71

Total P [g/L] 0.048 15.5

Total K [g/L] 0.34 2.1

NH+4-N [mg/L] 159 1250

Na [mg/kg] 110 1074

Mg [mg/kg] 36 434

Ca [mg/kg] 25 526

Mn [mg/kg] <1 10

Fe [mg/kg] <1 23

Zn [mg/kg] <1 8.5

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2.3.4 Water requirement The main factors that affect the water demand for the plants is transpiration, evaporation and the amount of remaining water in the plant (Schwarz, D et al., 2014, 11). In the paper “Advanced aquaponics: Evaluation of intensive tomato production in aquaponics vs. conventional hydroponics” written by Suhl, J et al. (2016) an experimental attempt to reuse wastewater from fish tanks in tomato cultivation was made. The experiment calculated that the plant density would be 2.3 plants per square meter (Suhl, J et al., 2016). However, in another paper “PRODUCTIVE PERFORMANCE OF F-1 TOMATO HYBRIDS AND THEIR F-2 POPULATIONS” (2013) written by Magaña-Lira, N et al. the plant density when cultivating tomatoes in a greenhouse with hydroponic technique is between 2.5-3 plants per square meter (Magaña-Lira, N et al., 2013, 372). Furthermore, in the paper “Population density and nitrogen fertility effects on tomato growth and yield” written by Whale, EA & Masiunas, JB (2003) a plant density of 3.2 per square meter average a fruit yield of 5.29 and 4.24 kg/plant, while a plant density of 4.2 averaged 4.17 and 3.31 kg/plant (Whale, EA & Masiunas, JB, 2003). Moreover, a study made in Ontario, Canada found that the water consumption where 28 liters per kg produced tomato for cultivation in a greenhouse, while the water consumption for tomatoes cultivated in the field corresponded to 50 liters per kg produced tomato (Dias et al. 2016, 838).

2.3.5 Humidity requirement The optimal relative humidity of the air for cultivating tomato plants is between 65 and 75 %, which is the optimal level for the whole cultivation process. During colder season the relative humidity tends to drop to low values due to the condensation of water vapour on the colder cover. When the relative humidity drops below 30% the plants will still grow, but not at optimal conditions. However, with too high relative humidity e.g. more than 85% the fruit growth may be inhibited (Schwarz et al. 2014, 9).

2.3.6 Carbon dioxide requirement One of the components required for photosynthesis is carbon dioxide (CO2). The average atmospheric carbon level was measured to be 405 ppm (parts per million) in February of 2017 (NASA, 2017). Plants and vegetation growing in an open environment are affected by the atmospheric condition and thus its CO2 level, however the concentration of CO2 differs locally above vegetation depending on photosynthesis activity. Plants are autotrophic, meaning that they are able to convert simple molecular compounds and minerals to complex molecules such as carbohydrates, lipids and proteins. They have two fundamental means of supplying energy to themselves; through the processes of respiration and photosynthesis. Respiration is constantly active in plants and is commonly summarized as the process of converting internal glucose (C6H12O6) and atmospheric oxygen (O2) into CO2 and water (H2O) (reaction 2.2). The process of photosynthesis requires light, CO2 and H2O, resulting in the production of O2 and C6H12O6 (reaction 2.3) (Holding & Streich 2013, 5-6):

(2.2) C6H12O6 + 6O2 → 6CO2 + 6H2O

(2.3) 6CO2 + 6H2O + light → C6H12O6 + 6O2

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The greenhouse is a structure that protects indoor vegetation from the outside environment and with minimal atmospheric exchange between indoor and outdoor air issues may arise concerning the carbon dioxide level indoors. When there is light the CO2 level can decrease to levels beneath 300 ppm inside the greenhouse, prompting a growth rate reduction of 40 % or more. It is thus desirable to artificially enrich the air with CO2 during times of light exposure, in order to achieve efficient growth conditions (Bergstrand et al., 2015). A growth environment with CO2 levels of 800 ppm results in increased yields of around 30 % (Ibid; Peet & Welles 2005, 289). It is further suggested that with an increased yield of 30 % due to CO2 enrichment, the lighting can be reduced by 30 % accordingly. This way CO2 enrichment could be considered for making energy savings by reducing electricity need for lamps (Bergstrand et al., 2015). CO2 requirements for various plant types, including tomato, are presented in table 2.15. Table 2.15 Absorption of CO2 in grams per plant, for four different plant types (Carvajal, 2008).

Plant Absorbed CO2 [g/plant]

Tomato 1 590

Pepper 1 029

Romaine salad 129.8

Cauliflower 342.5

With the use of natural ventilation, through openings in the greenhouse structure, even low wind velocities often enable sufficient exchange of CO2 between the greenhouse and the outside environment. During such conditions it is inefficient to supply CO2 artificially, due to the high exchange rate of air. Instead the indoor CO2 level will become similar to the outdoor level of around 400 ppm, which suffice for crop growth. For more enclosed conditions, however, the cost efficient level of CO2 inside the greenhouse is suggested to be slightly above 400 ppm, but no more than 1 000 ppm, since economic effectiveness flattens beyond that concentration level (AGA, n.d.). However, if there is little economical restraint for CO2 enrichment, as with biogas combustion, the concentration level may thus be set to exceed 1 000 ppm. CO2 concentrations of around 2 000 ppm in tomato cultivation have shown increased starch content in tomato plant leaves and deteriorated photosynthesis as a result, and should thus be avoided (Madsen, 1974). One study on a greenhouse operation utilizing landfill biogas concluded that higher crop yield from CO2 supplementation (from biogas combustion) was the major contribution to the greenhouse economy - more so than reduction of heating costs brought by combusting biogas (Jaffrin et al., 2003). Estimated CO2 supply rates for two different target greenhouse CO2 concentrations are shown in table 2.16, where a modern greenhouse is estimated to have an infiltration rate of approximately half the greenhouse volume per hour (Nederhoff 2004, 53).

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Table 2.16 Estimated CO2 supply rates in g/(m2*h) for two target concentrations of CO2, with values for non-vented and sparsely vented greenhouses, respectively (Nederhoff 2004, 53).

Target CO2 concentration [ppm]

Supply rate without venting (only infiltration) [g/(m2*h)]

Supply rate with sparse venting [g/(m2*h)]

500 5-9 7-13

900 13-31 19-58+

One further aspect of CO2 concentration is the one concerning human safety. A typical threshold for CO2 concentrations in structures is 1 000 ppm, which may be temporarily exceeded. The hygienic long term threshold for CO2 concentration lies around 5 000 ppm, a level that will cause damage to human beings (Claesson, 2016; Arbetsmiljöverket, 2015).

2.3.7 Light requirement For a plant to engage in the process of photosynthesis and thereby grow it requires exposure to light. Light is thus an important parameter to include in the planning of horticulture. The unit used for measurement of light in plant related sciences is micromoles per square meter and second (µmol/(m2*s)), which accounts for the photosynthetically active radiation (PAR) spectra of wavelength 400-700 nm. Values for conditions of positive net growth begin at 10-30 µmol/(m2*s) and typical levels during a sunny summer day are around 2 000 µmol/(m2*s). In total, most plants require 12-25 moles of this light per day (Bergstrand 2015, 4-5). For tomatoes and most other plants the optimal wavelength for growth is 630 nm, which is the wavelength of orange-red light (table 2.17) (Bergstrand et al., 2015). Tomato is an example of a photoperiod sensible crop. That means that it may enter a state of chlorosis, defined by insufficient chlorophyll production, upon intense light exposure for over 18 hours per day (Maslak 2015, 28). Table 2.17 Photosynthetic response of tomato plant. The photosynthetic response is defined as the uptake of CO2 in µmol/m2/s while exposed to light at 100 µmol/(m2*s) (Bergstrand et al., 2015).

Light, wavelength

Blue light, 450 [nm]

Green light, 530 [nm]

Orange light, 630 [nm]

Red light, 660 [nm]

Photosynthetic response [µmol/(m2*s)]

2.75

3

4

3.2

During winter months the influx of PAR in Scandinavia is only 1-5 moles/day, meaning that for year round growth additional light sources are required. These are fundamentally in form of lamps and their effectivity in this regard can be measured by the amount of micromoles emitted per watt (µmol/W). This value is usually around 1.5-2 µmol/W (Bergstrand 2015, 5). In order to maximize yield and allow year round cultivation artificial lighting may be a viable option. Two currently common types of lamps for horticulture applications are HPS (high pressure sodium) and LED (light emitting diode). HPS lamps contain xenon, mercury and sodium. When power is supplied to them the xenon is first ionized and emits a bright white light, but after approximately one minute the mercury and sodium evaporate, resulting in the emission of a yellow-orange light. They come in several different wattages, ranging from 250 W to 1 000 W. The hitherto most common power

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type has been 400 W, but recently 600 W has become more implemented in newer installations. The HPS lamp has an estimated life span of 12 000 - 14 000 hours, however the reflectors usually become partly covered in burnt dust, meaning that occasional cleaning is required for maintained efficient delivery of light (Ibid, 10-11). LED lights became efficient enough to use for artificial lighting after the introduction of brighter diodes in the 21th century. The diode consists of semiconductor elements with varying energy levels. When a current flows through the diode from a higher to a lower energy level the difference in energy levels is gives off as light. This means that LEDs can be designed to specifically emit desired wavelengths of light, by adjusting the energy levels of semiconductor elements accordingly (Ibid, 14). LED-lights can have a lifetime of up to 100 000 h, so if a daily use of 14 h is assumed, LEDs can be expected to be used for 19 years (Cuce et al. 2016, 50). For LEDs in horticulture it is suggested that a lighting system contains lights of different wavelengths. There should be a large amount of light in the red spectrum, 630-660 nm, and with that some 10 % blue light and 10 % green light is recommended. One alternative is to use white LEDs with a warm color tone, which contains red light to a large amount (Bergstrand et al., 2015). A comparison of HPS and LED lamps are shown in table 2.18. Table 2.18 A comparison of HPS (High Pressure Sodium) lamps and LED (Light Emitting Diode) lamps for greenhouse horticulture. Three HPS and four LED lamps were evaluated for their efficiency in µmol/W and their special benefits (Bergstrand 2015, 10-11, 14-15, 18).

Type of lamp Efficiency [µmol/W]

Special benefits Heat power generation

HPS 1.5 - 2.0 - Emits light that is of a highly beneficial spectrum for growth

- Well tested method for artificial horticulture lighting

~ 70 % of lamp wattage

LED 1.6 - 2.4 - Ability to manipulate emission spectra

- High efficiency in µmol/W

~ 50 % of lamp wattage

Values for solar irradiation in heat applications are often featured in W/m2, whereas values for it in horticultural applications are in µmol/(m2*s). Thus the ability to convert solar irradiation between these two units is beneficial. For PAR in natural daylight conditions equation 2.1 is suggested for approximate conversion, where solar radiation is in W/m2, the constant 4.57 in µmol/(W*s) and PAR in µmol/(m2*s) (Environmental Growth Chambers, 2017):

Solar radiation x 4.57 = PAR (2.1)

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3 Method and model Initially in the work process a model of the system is made with a material balance calculation aimed to reuse the products in the cycle and reduce the utilization of resources. Furthermore, the purpose of the model is to give an illustrative view of what is supposed to be achieved, as expressed in section “Purpose and Goals”. The output energy from the biogas facility was set to a fixed value that was obtained from Gunnar Bech, and the facility is dimensioned for small-scale production. With the knowledge of the biogas production and data about the climate where the system is located the energy and mass balance of the system can be defined, using knowledge acquired from the degree program Energy and Environment at KTH. Primarily, the minimum requirements are calculated to keep the system at a sustainable level, and then further optimizations are made. Both the material and energy balances are made with aid from the data obtained from the literature study. Further, the energy balance is calculated by using an analytic mathematical model.

3.1 Planning the cycle of the system By demarcating the most essential components a closed cycle of the system is able to be achieved and additionally fulfil the pre-set goals. The system consists of a biogas facility (including a biogas slurry tank), greenhouse and a farm, as seen in figure 3.1, which are included to enable energy and mass flow from one point to another.

Figure 3.1. An illustration of the system energy and material balances of the system.

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3.2 Dimensions of the greenhouse As for the type of greenhouse and orientation, there are two types of greenhouse structures that stand out as particularly beneficial: arched roof type and saw tooth roof type. Both types are well suited due to their high level of transmittance in summer, but especially as well during winter (table 2.6), which can be very cold in the set geographical context and thus can aid in reducing the need for additional heating and lighting. The arch type greenhouse has good endurance against wind, satisfying ventilation possibilities and low complexity of construction due to the availability of prefabricated parts and materials (Ponce et al. 2014, 54), thus making it an attractive choice for this model. However, it does not offer as high level of transmittance as the glass covered sawtooth greenhouse. A comparison of the environmental impact of PE-film and glass as greenhouse claddings is desirable to investigate. It is thus seen of interest to simulate energy and material balances for a PE-film covered arched greenhouse and a glass covered sawtooth greenhouse, respectively. Normally, one-span greenhouses have widths of around 8 meters and lengths of around 16 meters (von Zabeltitz 2011, 221) and thus those are the base dimensions for both greenhouse types analyzed in this model. The height is set to 2.5 meters for the bottom rectangular compartment and 1.5 meters for the arched roof and sawtooth roof compartment, respectively, totalling a maximum height of 4 meters, to accommodate for the greenhouse tomato crops. Illustrations of the arched and sawtooth greenhouse structures are presented in figures 3.2 and 3.3. Both greenhouse types have four troughs for hydroponic cultivation, each of them 14 m long and 1 m wide, with 0.8 m spacing between them. The plant spacing in the troughs is the same as in figure 2.2 (section 2.3.2), meaning that a total of 216 tomato plants are supported. Further greenhouse specifications are presented in tables 3.1 and 3.2.

Figure 3.2. An illustration of the arched greenhouse structure.

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Table 3.1 Arched greenhouse structural properties.

Base area: 128 m2

Surface area: 270 m2

Internal volume: 402 m3

PE-film thickness (2 layers): 3 mm per layer

Thickness of air layer between PE-films: 4 mm

Figure 3.3. An illustration of the sawtooth greenhouse structure. Table 3.2 Sawtooth greenhouse structural properties.

Base area: 128 m2

Surface area: 275 m2

Internal volume: 416 m3

Glass thickness (2 layers): 3 mm per layer

Thickness of air layer between glass layers: 4 mm

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3.3 System energy balance

3.3.1 Thermal mass The thermal mass of a structure is considered as the internal mass if it is not in contact with the outdoor temperature conditions. It has an impact on the structure’s capacity to store heat and thus the ability to retard temperature fluctuations and is essential for the energy balance model presented beneath the next heading. In this greenhouse model the thermal mass consists of the water, troughs, plants and air inside of the greenhouse. The inner layer of the cladding material (PE-film or glass) is not part of the thermal mass in this model. The greenhouse hydroponic troughs are set to have the dimensions 1 m x 14 m. The water in them reaches a height of 1 cm, which results in the circulation of 0.14 m3 water per trough, 0.56 m3 in total for all four troughs. The roots that are submerged in the water are assumed to be part of the water and thus are assigned the same attributes as water. The specific heat capacity of water is 4 200 J/(kg*K) and density of water is around 1 000 kg/m3 (Cengel & Ghajar 2015, 918), meaning that the total mass of water in the greenhouse hydroponic system is 560 kg, as received from the following equation bla:

m = ⍴ * V (3.1) where m is the mass in kg, ⍴ is the density in kg/m3 and V the volume in m3. No value for the weight of tomato plants was found. For this reason, one plant is set by the authors to weigh 4 kg. Another assumption is that one plant may yield 8 kg (“3.4.4 Crops”) and as such an average tomato weight per plant is set to 4 kg. This means that on average one plant is assigned the weight of 8 kg. Plant leaves commonly have a heat capacity comparable to that of water (Jayalakshmy & Philip 2010, 2300), so to adjust for the cellulose content of the entire plant a heat capacity of 3 500 J/(kg*K) is set to it, slightly lower than for water. The 216 plants thus have a total mass of 1 728 kg. The density of air is 1.2 kg/m3, which yields an air mass of 482 kg for the arched greenhouse and 499 kg for the sawtooth greenhouse. The specific heat capacity of air is 1 006 J/(kg*K) (Cengel & Ghajar 2015, 924). As for mineral wool, its density and specific heat capacity is 90 kg/m3 and 840 J/(kg*K), respectively (Siper, n.d.). The mineral wool is placed in the troughs to a height of 10 centimeters. The volume will thus be 5.6 m3 and the total mass of the rockwool is 504 kg. The troughs are made out of 3 millimeter thick steel sheets and with 15 centimeters high walls this yields a steel volume of 0.223 m3. The density and specific heat capacity of stainless steel is 7 900 kg/m3 and 477 J/(kg*K), respectively (Cengel & Ghajar 2015, 909). The total mass of the troughs are thus found to be 1 762 kg. The values for mass and specific heat capacities of the materials mentioned, and the resulting thermal mass for the model, are summarized in table 3.3.

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Table 3.3 Mass, specific heat capacity and thermal mass of materials within the greenhouse.

Material Mass, m [kg] Specific heat capacity, cp [J/(kg*K)]

Thermal mass, m*cp [J/K]

Water 560 4 200 2 352 000

Plants 1 728 3 500 6 048 000

Steel troughs 1 762 477 840 474

Mineral wool 504 840 423 360

Air (arched greenhouse)

482 1 006 484 892

Air (saw-tooth greenhouse)

499 1 006 502 195

Sum of thermal mass (arched greenhouse)

- - 10 148 726

Sum of thermal mass (saw-tooth greenhouse)

- - 10 166 029

3.3.2 Biogas energy content Pure methane has an energy content of 9.97 kWh/Nm3, but the biogas obtained from a digester will have a methane volume percentage of between 4.5 and 8.5 kWh/Nm3, depending on which digestate is used. The energy content of 5.5 kWh/Nm3 that is chosen for this model roughly equals 55 volume % methane in biogas. Further, the amount of raw biogas produced daily is 1 Nm3 per digester m3 (Bech, 2017).

3.3.3 Energy balance In order to quantify the heat balance for the greenhouse system an analytical method was used in Microsoft Excel to obtain temperature values for each time step, here chosen to be 1 hour, over an entire year. The main expression used is equation 3.8, which is deduced from the expressions that follow below (an example of the calculation method in Microsoft Excel is illustrated in Appendix). Equation 3.2 illustrates the energy balance for a structure with a uniform internal temperature distribution, where E is the internal energy, τ is the time step and Q̇in and Q̇out are the inflow and outflow of heat power, respectively (Claesson, 2016).

If applying the internal energy on a body with given mass and specific heat capacity, it can be expressed as:

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where Tinf is a reference temperature, a constant. Applying equation (3.3) into equation (3.2) the following expression is received:

Loss of heat from a structure is exerted by the difference between the internal temperature of the structure and the outdoor ambient temperature and may be summarized into two main constituents: transmission and ventilation. This is presented in the following equation:

where ⍴ air represents the density of air, cp,air the specific heat capacity of air, Tamb the ambient temperature, V̇vent is the venting rate, A the total surface area of the structure and U the overall heat transfer coefficient for the structure. Lambda is the same as the sum of the two products within brackets in the equation. In this model only infiltration is considered, since it exchanges the entire greenhouse air volume in 2 hours - a rate that exceeds the minimum venting requirement of 0.35 liters per m2 and second for structures in general, set by Boverket (Boverket, 2017). Substituting equation (3.5) into equation (3.4) yields equation (3.6):

which may be rewritten as:

If the right hand side of equation (3.7) is considered constant during a certain time step, then for each time step the solution may be expressed as:

where T0 is the initial temperature, Tinf is the steady state solution (the solution to equation (3.7) when the derivative is equal to zero) and τB is the structures time constant (which equals the thermal mass divided by Λ. Tinf may then be retrieved from the following equation:

For simulations of energy balances of buildings throughout an entire year time steps of 1 day, 1 hour or 30 minutes are commonly used, since longer time increments lower the quality of temperature change

31

predictions of buildings (Claesson, 2016). For the simulation of the greenhouse a time step of 1 hour was chosen, much due to the climate data acquired from SMHI having hourly intervals. The overall heat transfer coefficient U, embedded in Lambda in equation (3.9) above, may be represented as in the following equation:

where heat conduction coefficient, k, may be assigned a constant value for a given material. A represents area, x the thickness of the assessed material and h the heat convection coefficient. The convection coefficient can vary considerably with varying wind velocity and it is thus relevant to make it dependent of it instead of assigning to it a constant value. In this model, the convection coefficient is derived from the expressions for flow over a flat plate, regarding external forced convection (Çengel & Ghajar 2015, 451): Laminar case:

Turbulent case:

where L is the characteristic length of the surface that experiences convection. The Reynolds number is given by:

where µ is the dynamic viscosity of air, u is the wind velocity and L is the characteristic length (here taken to be the greatest length of the greenhouse). The wind velocity for turbulent flow can be found by rearranging equation (3.13) to find u for a Reynolds number of 5*105. For a characteristic length of 16 m, turbulent flow may be considered at wind velocities of 0.47 m/s and higher and laminar cases at wind velocities under 0.47 m/s, but above 0 m/s as that means there is no wind. The wind data from SMHI does however only have whole number integers for the velocity, meaning that the laminar case is never used in the model. When the wind velocity is 0 m/s there is no forced convection and instead a small convection coefficient of 2.5 W/(m2*K) is set during this condition. Radiation losses are given by equation (3.14) (Ibid, 29):

where ε is the emissivity coefficient of the surface material, σ is the Stefan-Boltzmann constant [5.67*10-8 W/(m2*K4)], A the surface area, Tsurf the surface temperature in degrees Kelvin and Tsurr the surrounding temperature in degrees Kelvin.

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3.4 System material balance The general material balance over the greenhouse can be described with the following condition:

𝐼𝑛𝑝𝑢𝑡 + 𝐺𝑒𝑛𝑒𝑟𝑎𝑡𝑖𝑜𝑛 = 𝑂𝑢𝑡𝑝𝑢𝑡 + 𝐴𝑐𝑐𝑢𝑚𝑢𝑙𝑎𝑡𝑖𝑜𝑛 + 𝐶𝑜𝑛𝑠𝑢𝑚𝑝𝑡𝑖𝑜𝑛 The material balance will be the same for both of the different shapes/forms of greenhouses, since the inside contents and their dimensions will be similar. Thus, only one material balance for each category is made. The cladding and the cover of the greenhouses is not included in the calculations.

3.4.1 Water Both of the greenhouses support a total of 216 plants, with the total mass of 1 728 kg per harvest and according to the study “Life cycle perspectives on the sustainability of Ontario greenhouse tomato production: Benchmarking and improvement opportunities” by Dias G.M et al. (2016), 28 liters of water is needed for each kg of tomato produced. The hydroponic system in the greenhouse needs at least 560 liters of water, which was defined in the previous part. However, the system will require more water because of losses due to evaporation and transpiration (so called evapotranspiration). The evapotranspiration determines the amount of input water in the system. Closed system such as hydroponic system in greenhouses have a lower evapotranspiration compared to cultivation outdoors in the open field. Furthermore, the input of water also depends on the rainfall. With greater rainfall less fresh water needs to be used. In Enköping where the greenhouse will be placed the total amount of rainfall corresponded to 584.8 mm. One millimeter of rainfall corresponds to one liter per square meter (SMHI, 2017).

3.4.2 Biogas slurry and nutrients The main nutrients the plants need are the macronutrients, or more specifically nitrogen, phosphorus and potassium. The tomato plant requires a specific amount of each nutrient and the biogas slurry contains a specific amount of nutrients. Thus, with knowledge of the total yield in one year the required input of biogas slurry to the system can be calculated. The total mass of the plant used in these calculations was a plant mass of 1 728 kg yield per harvest. The requirements of nutrients for the tomato plant and the nutrient content in the biogas slurry and in the concentrated biogas slurry is presented in table 3.4. Table 3.4. Macronutrient requirements per kg of plant, as well as in g per liter BS and g per liter CBS.

Nutrient Plant requirements (g/kg)

Content in BS (g/l)

Content in CBS (g/l)

N 40 0.069 0.71

P 4 0.048 15.5

K 60 0.34 2.1

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3.4.3 Carbon dioxide One normal cubic meter (Nm3) of biogas with 55 % methane content and 40 % carbon dioxide content by volume will have a mass of carbon dioxide that may be deduced from equation (3.15), known as the ideal gas law (Felder & Rousseau 2005, 192): mg = (P*V*M)/(R*Tg) (3.15) where mg is the mass in grams, P the pressure in pascals, V the volume in m3, M the molar mass in grams/mol, Tg the gas temperature in Kelvin and R the gas constant [8.314 (m3*Pa)/(mol*K)]. Upon complete combustion the methane will react with oxygen to form carbon dioxide and water (reaction 2.1, section 2.1). The reaction scheme shows that moles CH4 is equal to moles CO2 and thus the moles of CH4 may be calculated with an alternative formula of the ideal gas law (equation 3.16) in order to find the mass of CO2 from combustion (equation 3.17 - where n represents moles). The mass of CO2 from complete combustion is presented in table 3.5. This value may then be used together with data from table 2.16 to acquire the necessary CO2 input for the investigated greenhouse types. n = (P*V)/(R*Tg) (3.16) mg = M*n (3.17) Table 3.5. Mass of CO2 in 1 Nm3 of raw biogas and of complete combustion of CH4 (55 % in 1 Nm3).

Mass CO2 per Nm3 [g/Nm3]

Raw non-combusted biogas 785

Combustion reaction 1 080

Total mass of CO2 from biogas and combustion reaction

1 865

3.4.4 Crops In the United States a tomato yield of 18 kg per plant and harvest represents an excellent yield, while yields of 9-10 kg per plant and harvest are more common. Annually, 2-3 tomato harvests may be achieved in greenhouse cultivation (Peet & Welles 2005, 300). In this model each plant is set to yield an average of 8 kg of tomatoes for every harvest.

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3.5 Location climate data The purpose of this report is to examine the possibilities of combining a biogas facility with a greenhouse to obtain a closed cycle, by obtaining low losses in energy and mass transfers, in a rural area. Thus, the place Enköping with the coordinates (latitude: 59.64 & longitude: 17.05) in Mälardalen was chosen, the location of which is illustrated in figure 3.4. Key data and climate values for Enköping during a whole year for each hour was obtained from Sveriges meteorologiska och hydrologiska institut (SMHI), which is the Swedish institute for meteorology and hydrology. Due to incomplete results during the whole year the closest year with as complete results as possible was chosen, which was 2014. The types of data that were obtained from SMHI were air temperature, rainfall, global radiation and lastly, wind speed and angle of the wind direction.

Figure 3.4. Location of Enköping (59.64, 17.05) (Google maps).

3.6 Sustainability aspect A sustainability analysis of the two greenhouses are made, and the main aspect are to conclude and compare the amount of GHG-emissions the two greenhouse types contribute with. The areas of focus is mainly the energy and mass transfers, and the main material to construct a greenhouse. The greenhouses consist of different cladding materials, either polyethylene-film or glass, which is the main factor that can differentiate the greenhouses apart considering the emissions.

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3.7 Reference greenhouse for comparison In the bachelor thesis “Effektivare energianvändning i växthus - Förslag till miljöförbättrande åtgärder för ett hållbart odlande” by Hugosson & Schön (2014) an existing greenhouse around 200 km north of Enköping was analysed. The base area of the greenhouse is 218 m2 with an internal volume of 818.4 m3. The cover consisted of 4 mm one-layer glass with an insulation of 10 cm. The main plant cultivated in this greenhouse was decorative plants or plants with recreational purposes. Furthermore, the heating system consisted solely of a modern 115 kW oil burner from 2004. The desired temperature in this greenhouse is between 15°C and 20°C. Moreover, the heat source is almost sufficient enough to maintain the desired temperature through the whole year, however with deviation on a colder period in December where the outdoor temperature drops to -25 °C. During one of the colder periods the heating requirements reached 99 000 kWh, which correspond to 773.4 kWh per m2. The greenhouse cools naturally without any external ventilation, only through opening windows and let the pressure difference drive the airflow. The indoor temperature reaches upwards 38 °C at e few times during the year, and generally during the summer period reach high temperatures significantly higher than the desired level.

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4 Result and discussion

4.1 Greenhouse conditions without climate control Figures 4.1 and 4.2 illustrate the indoor temperature for each hour of the year, in the arched and sawtooth greenhouse structures, in the hypothetical case where they are closed and do not have climate control systems. The graphs look very similar and the difference between arched and sawtooth greenhouse maximum and minimum temperature is small. The minimum temperature occurred on the 28th of December (09:00), where both the arched and the sawtooth greenhouse had an indoor temperature of -17.0 ℃. The maximum temperature occurred on the 4th of August (13:00), where the arched greenhouse had an indoor temperature of 69.9 ℃ and the sawtooth greenhouse 69.5 ℃. These temperatures are not suitable for cultivation, which demonstrates the importance of climate management in greenhouses. The graphs show a highly fluctuating temperature pattern and during summer days temperatures may alter by 45 ℃.

Figure 4.1. Graph illustrating the temperature inside the arched and PE-film covered greenhouse through the year, when it is closed and does not have any climate control. Air exchange occurs solely by infiltration.

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Figure 4.2. Graph illustrating the temperature inside the sawtooth and glass covered greenhouse through the year, when it is closed and does not have any climate control. Air exchange occurs solely by infiltration.

4.2 Greenhouse energy balance

4.2.1 Heating It was found that 89 500 kWh of heat for the arched greenhouse and 94 400 kWh for the sawtooth greenhouse are required to keep the indoor temperature at 20 ℃, over the entire year. However, degree hours do not take the heat provided from solar radiation into consideration. With the heat contribution from the sun the heat requirement from internal heating systems was estimated to be 68 900 kWh per year (539 kWh per m2 and year) for the arched and 73 300 kWh per year (573 kWh per m2 and year) for the sawtooth greenhouse, respectively. The method of degree hours was also relied upon in order to determine the necessary power of the heating system. At 23:00 on the 12th of January the ambient temperature was -12.1 ℃ and the weather was windy. With no solar contribution the necessary heat power for that hour was 33.7 kW for the arched and 35.9 kW for the sawtooth greenhouse. A summary of heat requirements for both greenhouse types is presented in table 4.1. When HPS-lamps are on, they contribute around 6.7 kW of heat. When emitting the same amount of light, LED-lamps contribute around 4.2 kW of heat. The power of the heating system must thus be evaluated with this information in hand, as well as the fact that thermal radiation losses from the greenhouse through the ground has not been investigated in this model. The model does however imply that the heating system should have a power output of around 34-36 kW in order to keep the indoor temperature at 20 ℃ all year round.

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Artificial lighting is heavily required during the winter season, when light levels from the sun are low. The longest the lamps must be on for one day is 20 hours (on the 24th of December 2014) for both greenhouses. The heat they contribute with over the year is around 10 000 kWh from HPS or around 6 200 kWh from LED, depending on the greenhouse type, as seen in table 4.2. Table 4.1. Summary of greenhouse heating requirement.

Greenhouse type Power of greenhouse heating system [kW]

Power of greenhouse heating system per m2 [kW/m2]

Heating requirement per year [kWh/year]

Heating requirement per m2 and year [kWh/(m2*year)]

Arched (PE-film) 33.7 0.26 68 900 539

Sawtooth (glass) 35.9 0.28 73 300 573

Table 4.2. Summary of heat provided by lights that emit the same amount of growth stimulating light.

Greenhouse type

Light type Heat power [kW]

Heat power per m2 [W/m2]

Heat per year [kWh/year]

Heat per m2 and year [kWh/(m2*year)]

Arched (PE-film)

HPS 6.7 52.3 10 000 78.1

LED 4.2 32.8 6 250 48.8

Sawtooth (glass)

HPS 6.7 52.3 9 770 76.3

LED 4.2 32.8 6 110 47.7

4.2.2 Cooling To evaluate the cooling load of the greenhouse the same method was used as for determining the heat load. It was found that around 31 800 kWh of cooling is required over the year (249 kWh per m2 and year) for the arched greenhouse and 30 900 kWh per year (241 kWh per m2 and year) for the sawtooth greenhouse, to keep the indoor temperature at 20 ℃. The largest cooling load occurred at 11:00 on the 4th of August, during intense sunlight and where the ambient temperature was 31.6 ℃. The cooling requirement was then 91.7 kW for the arched and 89.7 kW for the sawtooth greenhouse, respectively. Much of the cooling load would however be negated with the utilization of sunscreens and additional venting. At this hour, a sunscreen with a light transmittance of 0.4 would lower the heat load from the sun with 46 kW, which would cut the cooling requirement of 91.7 kW in half. Various levels of venting rates may be achieved by creating openings in the greenhouse structure (the greenhouse cladding can be made to have sections that can be opened and closed), and with large open sections in the cladding the temperature could be held close to the same as the ambient temperature. If CO2 enrichment would be desirable at the time, an electric cooling system could be required. Otherwise a high venting rate

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would keep the CO2 concentration at the same level as the outdoor one and thus be sufficient but not optimal for crop growth. The energy requirement data obtained are summarized in table 4.3. Table 4.3. Summary of cooling requirement for the greenhouse.1

Greenhouse type Maximum cooling power required [kW]

Cooling requirement per year [kWh/year]

Cooling requirement per m2 and year [kWh/(m2*year)]

Arched (PE-film) 91.7 31 800 249

Sawtooth (glass) 89.7 30 900 241 1 Note that these are values that need not be covered by energy intensive equipment. Reduction in cooling requirement will be substantially lowered with the use of sunscreens and/or additional venting.

4.2.3 Electricity By using 16 lights of HPS type at 600 W each, the minimum amount of light for the plants can be emitted in reasonable uptime hours for each day in both greenhouses. If LED-lights are used instead, 21 lamps at 400 W each are required to emit the same amount of light. Altogether, the HPS-lights require 9.6 kW of electric power, whereas the LED-lights require 8.4 kW. The annual electricity required for the HPS-lights and LED-lights are 14 290 kWh and 12 510 kWh in the arched greenhouse, whereas it is 13 960 kWh and 12 220 kWh in the sawtooth greenhouse. These results are summarized in tables 4.4 and 4.5. Table 4.4. Summary of electric power and energy consumption by HPS-lights or LED-lights, alternatively, in the arched greenhouse.

Lights Electric power [kW]

Electric power per m2 [W/m2]

Electric energy per year

[kWh/year]

Electric energy per m2 and year

[kWh/(m2*year)]

HPS (16 á 600W) 9.6 75.0 14 290 112

LED (21 á 400W) 8.4 65.6 12 510 97.7

Table 4.5. Summary of electric power and energy consumption by HPS-lights or LED-lights, alternatively, in the sawtooth greenhouse.

Lights Electric power [kW]

Electric power per m2 [W/m2]

Electric energy per year

[kWh/year]

Electric energy per m2 and year

[kWh/(m2*year)]

HPS (16 á 600W) 9.6 75.0 13 960 109

LED (21 á 400W) 8.4 65.6 12 220 95.5

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4.2.4 Combustion engine power required for greenhouse operation With 33.7 kW needed for heating and heat power from lights being 6.7 kW for HPS and 4.2 kW for LED, respectively, a biogas combustion system for heating will need to generate 27 kW of heat when HPS-lights are switched on and 29.5 kW if LED-lights are used, during the hour of highest heat loss. For an engine with combined electric and heat power generation, the electric power generation efficiency is around 33 %. Thus, for a HPS-lighted greenhouse the engine power is 40.5 kW, with 27 kW heat output and 13.5 kW electric output. For a LED-lighted greenhouse the engine power is 44.3 kW, with 29.5 kW heat output and 14.8 kW electric output. A summary of these values are presented in Table 4.6. The lights need to be on for long durations of time in the winter, where ambient temperatures are the coldest, but in order to make sure the temperature stays at 20 ℃ at these instances without lighting, an engine of 50-55 kW could be desirable for the 33.7 kW and 35.9 kW required for the respective greenhouse as calculated in section 4.2.1 (such an engine power is denoted as “Safe engine wattage” in table 4.6). Table 4.6. Summary of the cogeneration engines needed for the greenhouse with LED and HPS-lights, respectively. The safe engine wattage for each greenhouse is also shown (by safe it is meant that the indoor temperature can be further guaranteed to stay at minimum 20 ℃ throughout the year).

Lights Heat power [kW]

Electric power [kW]

Engine power [kW]

Engine power per m2 [kW/m2]

Arched greenhouse

with HPS 27 13.5 40.5 0.32

with LED 29.5 14.8 44.3 0.35

Sawtooth greenhouse

with HPS 30.2 15.1 45.3 0.35

with LED 31.7 15.9 47.6 0.37

Arched - Safe engine wattage

Either

34

17

51

0.40

Sawtooth - Safe engine

wattage

Either

36

18

54

0.42

4.2.5 Biogas required for energy Summarized in table 4.7 are the yearly amounts of biogas needed for the operation of both greenhouse types, with either HPS-light or LED-light systems, without engine heat losses. The maximum amount of biogas required in one hour to maintain the greenhouse temperature at 20 ℃ is shown in table 4.8, for each greenhouse type (this hour occurs during the highest rate of heat transfer). Based on those values the minimum volume of the digester for biogas production, seen in table 4.9, was calculated.

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Table 4.7. Annual requirement of biogas in Nm3 for heat and electricity. Values are shown for both greenhouses, with either HPS or LED light systems.1

Arched greenhouse (PE-film) Sawtooth greenhouse (glass)

With HPS-lights [Nm3/year]

With LED-lights [Nm3/year]

With HPS-lights [Nm3/year]

With LED-lights [Nm3/year]

10 713 11 395 11 552 12 218 1 These values are for combustion of biogas without heat losses. Table 4.8. Maximum hourly biogas requirement in Nm3 for greenhouse operation (on the hour of greatest heat transfer).1

Arched greenhouse (PE-film) - biogas [Nm3] Sawtooth greenhouse (glass) - biogas [Nm3]

6.12 6.52

1 These values are for combustion of biogas without heat losses. Table 4.9. Minimum digester volumes, in m3, for greenhouse operation.

Arched greenhouse (PE-film) - digester volume [m3]

Sawtooth greenhouse (glass) - digester volume [m3]

147 157

4.2.6 Reference greenhouse system One difference between the reference greenhouse, which is presented in section 3.7, and the simulated greenhouses is the cultivated plants and their different requirements for optimal climate conditions. The tomato crop is rather sensitive to significant temperature differences while the decorative plants have a greater resilience towards temperature differences. Both the sawtooth greenhouse and the reference greenhouse are covered with glass. However, the sawtooth one has two layers of 3 mm glass while the reference greenhouse has only one layer with a thickness of 4 mm. In addition, the arched greenhouse, with PE-film as cover, has a lower transmittance compared to glass, but it also has a higher thermal resistance and thus has less variation in the indoor temperature. Nonetheless, both simulated greenhouses reach harmful conditions without climate control, peaking at a maximum of 69.9°C for the arched and 69.5°C for the sawtooth. Furthermore, another difference between the greenhouses is the base area and the internal volume, where the reference greenhouse has almost double the size in area and volume. The internal heating requirement is around 200 kWh per m2 less in the simulated greenhouses compared to the reference greenhouse. Most likely due to higher thermal conductivity and colder outdoor temperatures, since the reference greenhouse is located further north than the simulated ones. The base area of the reference greenhouse is 218 m2, thus giving a saving of 43.6 MWh if the reference greenhouse would utilize the same system and endure the same conditions as the simulated greenhouses. Lastly, from the perspective of a sustainable cultivation the reference greenhouse has a lot of potential with improvements, where one of the main areas is its heating system which uses oil. A substitute for

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oil could for example be biogas. Other improvements that affect the choice of heating system is the thermal resistance of the cladding material.

4.3 Greenhouse material balance

4.3.1 Crops With 216 plants and each capable of yielding on average 8 kg of tomatoes per harvest results in 1 728 kg per harvest of the entire greenhouse. Given that 2-3 harvests can be made each year in year round tomato cultivation in greenhouses, the annual tomato yield thus ranges from 3 456 kg to 5 184 kg, or 13.5-27 kg/m2 (table 4.10). As a comparison, the annual yield of greenhouse tomatoes in the Netherlands ranges from 47-53 kg/m2 (Peet & Welles 2005, 301). Table 4.10. Tomato yields from both greenhouse types in kg and kg/m2 for 1-3 harvests per year. Typically 2-3 harvests can be made each year.

Harvests per year: Tomato yield [kg] Tomato yield [kg/m2]

1 1 728 13.5

2 3 456 27

3 5 184 40.5

4.3.2 Water The water usage is in direct correlation with the amount of produced tomatoes, with 28 liters of water for each kg produced tomato (table 4.11). The amount of water required for the hydroponic system needs to be continuously maintained at 560 liters. Table 4.11. Water required per harvest.

Harvests per year: Tomato yield [kg] Water required for cultivation [m3]

1 1 728 48.4

2 3 456 96.8

3 5 184 145.2

Furthermore, water can be supplied from different sources e.g. rainfall. For the area “Enköping” the rainfall during the year 2014 amounted to 584.8 mm. With consideration to the base area of the greenhouses, which is 128 m2, gives a total amount of water of 74.85 m3 if the collection is equivalent to the base area. Since the potential collection of water from rainfall correlates to the collection area, a greater amount of water can be collected if the area increases. The amount of water from rainfall would be sufficient to cover one and almost two harvests. Further, rain may be collected from roofs of the buildings residing on the farmyard. The distribution of the rainfall during the whole year is presented in figure 4.3.

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Figure 4.3. Rainfall in millimeters for every hour of the year. For the optimal yield the total amount of water in the system is 145 712 liters. Which is the total amount of tomatoes from three harvest multiplied to the consumption for each kg produced tomato, and includes the amount of water continuously needed in the hydroponic system. The material balance is presented below. Input of freshwater + 74.85 m3 (rainfall) + Recirculation = Evapotranspiration + 145.15 m3 (cultivation) + 0.56 m3 (hydroponic system) The required input of freshwater is dependent on the evapotranspiration and the potential of reusing the water, if assumptions of non-perfect cycle where the input of water exceeds the requirements for cultivation is made.

4.3.3 Biogas slurry The total quantity of biogas slurry required as input for the greenhouse is aligned with the total yield of tomatoes for the whole year. Only the main macronutrients required for cultivating tomatoes have been analyzed. To fulfil the nutrient requirements for the plant using only biogas slurry an amount of 2 003.5 m3 and 3 005.2 m3 is needed respectively for two and three harvest, which is presented in table 4.12. The nutrient content of nitrogen is low in the slurry in comparison to the requirement, therefore nitrogen is the determinant nutrient. To fulfil the requirements for phosphorus and potassium a far lower quantity is required. If utilizing concentrated biogas slurry about a tenth of the amount slurry of is needed, with the exception of potassium which deviates from the other two nutrients.

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Table 4.12. Amount of nutrients required for the tomato plant and the the volume required of biogas slurry and concentrated biogas slurry, respectively.

Nutrient Tomato yield (kg)

Nutrients required (kg)

Required BS (m3)

Required CBS (m3)

N

3 4561 138.2 2003.5 194.7

5 1842 207.4 3005.2 292.1

P

3 456 13.8 288 0.90

5 184 20.7 432 1.34

K

3 456 207.4 609.9 98.7

5 184 311.0 914.8 148.1

1 Two harvests 2 Three harvests

If harvesting at maximum potential, which is three times during a year and only using biogas slurry as a fertilizer an excessive amount of phosphorus and potassium will be accumulated. The excess of two of the major macronutrients are summarized in table 4.13. The used volume of biogas slurry is 3 005.2 m3, which is the amount needed to supply sufficient nitrogen. Thus, an external input of fertilizer e.g. artificial fertilizer or other concentrated fertilizer should be used to optimize the nutrient balance since an excessive use of potassium can be harmful because of its solubility in water and potential leakage. Excess of potassium can also reduce the availability of other nutrients, such as magnesium. Table 4.13. Excess nutrients of phosphorus and potassium when inserting the maximum amount of biogas slurry (three harvest per year).

Nutrients Excess nutrient (kg) Excess nutrient (%)

P 123.5 595.6

K 710.7 228.5

4.3.4 Carbon dioxide Since both greenhouse types had the same base area and almost the same volume, the CO2 influx rates presented in table 4.14 are values that suit them both. The concentration of 500 ppm lies around 100 ppm above the ambient outdoor level, whereas 900 ppm is 100 ppm below the level where further CO2 enrichment is considered to not be as efficient. However, if biogas combustion provides excess CO2 it may be just as well to utilize the excess CO2 in the greenhouse to levels above 1 000 ppm, especially since there will not be an extra cost related to that. During the colder periods of the year around 5-6 Nm3 of biogas is commonly needed for heating each hour, which in turn yields 9 000 - 11 000 g of CO2. If the biogas system is dimensioned according to the energy requirement and biogas is burnt constantly to, for instance, generate electricity for profit, this amount of CO2 will be available through the entire

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year and may then be used in the greenhouse during sunlight or artificial lighting. These amounts of CO2 will result in CO2 concentrations well above 1 000 ppm in both non-vented and sparsely vented greenhouses of the simulated types. Preferably, human presence inside the greenhouses should be avoided during these conditions as a safety measure. Table 4.15 shows the amount of biogas needed per hour for the CO2 enrichment levels presented in table 4.14. Table 4.14. Influx of CO2 in grams per hour and liters per hour1 in both greenhouse types in order to maintain the CO2 level at 500 and 900 ppm, respectively, during no ventilation (only infiltration) and sparse ventilation.

Target CO2 concentration [ppm]

Supply rate without venting (only infiltration) [g/h]

Supply rate without venting (only infiltration) [l/h]

Supply rate with sparse venting [g/h]

Supply rate with sparse venting [l/h]

500 896 0.49 1 280 0.70

900 2 816 1.54 4 928 2.69 1 Volume influx rates for NTP (normal temperature and pressure conditions: 20 ℃ and 1 atm). Table 4.15. Biogas in Nm3 per hour required for CO2 enrichment in both greenhouse types, to keep the CO2-level at 500 and 900 ppm, respectively, during no ventilation (only infiltration) and sparse ventilation.

Target CO2 concentration [ppm]

Biogas for CO2 enrichment - only infiltration [Nm3/h]

Biogas for CO2 enrichment - sparse venting [Nm3/h]

500 0.48 0.69

900 1.51 2.64

At very high temperatures, when substantial venting may be necessary, CO2 enrichment may not be effective since the high rate of air exchange will maintain the CO2 concentration in the greenhouse at the same level as the outside ambient level. During conditions of lower cooling requirement it may however be beneficial to utilize the biogas produced to operate a cooling system, so that CO2 from biogas combustion can be simultaneously used and increase growth rate.

4.4 Sustainability The two greenhouses investigated in this study have similar dimensions but one important difference, namely the cladding material. The claddings, PE-film and glass respectively, have distinct thermal and transmissive properties, but there exists a difference in environmental impact as well. The polyethylene film that was used as cladding material in this model contributes more GHG-emissions per kg of material than glass, however, a glass greenhouse with similar dimensions would result in larger initial GHG-emissions, due to glass having a much larger density than polyethylene (a certain volume of glass for a greenhouse structure will weigh more than the same volume of PE-film for a similar greenhouse structure). In order to obtain a better view of the environmental impact of the cladding material the life cycle must be studied as well. For instance, today PE-films for greenhouse applications may have life expectancies that exceed 10 years, but glass can be used for decades without deteriorating, which in turn suggests that out of the two materials glass is the superior choice over time, when it comes to GHG-emissions, as seen in table 4.16. Glass does however have lower thermal resistance than PE-film, which leads to larger temperature variations inside the greenhouse and higher

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energy demand for heating. This in turn has an impact on biogas required for greenhouse operation and thus the scale of the biogas production system, an aspect that should be included in an environmental impact study. Table 4.16. GHG-emissions in CO2-equivalents for glass and polyethylene film for the studied greenhouse, with different life expectancies of PE. The mass of the claddings were calculated and multiplied with respective values from table 2.8 to obtain their contribution of GHG-emissions.

Time span PE-film for the greenhouse, CO2e emissions [kg]

Glass for the greenhouse, CO2e emissions [kg]

Finished construction - start of greenhouse operation

4 600 7 000

After 30 years of operation, PE changed 2 times (PE life expectancy of 15 years)

9 200 7 000

After 30 years of operation, PE changed 3 times (PE life expectancy of 10 years)

13 800 7 000

After 30 years of operation, PE changed 6 times (PE life expectancy of 5 years)

27 600 7 000

On the topic of greenhouse lighting HPS has been a widely used technology and still is to date. However, it requires more electric power than LED and has a significantly lower life expectancy (12 000 - 14 000 hours) than LED (up to 100 000 hours). These are important parameters since lower power consumption in general means lower GHG emissions and higher life expectancy correlates with less material use over a life cycle - key aspects regarding system sustainability. Especially the use of LED over HPS in the investigated greenhouse models could be of great value regarding sustainability, due to the higher life expectancy. The production of the 1 762 kg of steel needed for the troughs contribute with 6 657 kg of CO2e. To reduce the total GHG emissions of the greenhouse operation it would be of interest to investigate other materials to hold the hydroponic nutrient solution. With reduced freshwater availability ways to optimize the usage in greenhouses is highly interesting. Cultivating crops in hydroponic systems utilizes less water compared to cultivating in an open field, since the evaporation is lower in a greenhouse due to higher humidity. To further increase the efficiency other methods can be implemented, e.g. aquaponics systems. Furthermore, different methods of irrigation also affects the usage of water, where the most efficient is through drip-irrigation but with this technology among other things sensors are required and thus gives a more complex system. The system studied consisted of NFT, where the water flows through the roots naturally and therefore much easier to implement in reality. Some nutrients are more soluble in water, and may contaminate nearby groundwater. The consequence of solely use biogas slurry as a fertilizer is an excess of other nutrient as shown in table 4.13 and thus increased risk of leakage to the groundwater and its effects.

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As for economic sustainability, being able to control numerous climate factors with high precision can lead to better crop yield and quality, but it can become very expensive to set up such a cultivation system. As presented in table 2.7 a greenhouse incorporated with a high level of technology can cost more than 900 SEK/m2. For the greenhouses investigated in this report that would mean a cost of at least 115 000 SEK, only for materials and components. For this operation to be able to go around financially the benefit (profit) of the greenhouse cultivation must exceed this and other eventual costs, i.e. labor costs.

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5 Conclusion This report has covered different types of cladding materials, both plastic and glass, and energy balances have been made for both of them, respectively. Synergies in the systems have been identified, such as using the nutrients from biogas slurry, using CO2 in the greenhouses from the biogas combustion, and lastly energy from the biogas combustion, providing the possibility to generate both heat and electricity for greenhouse cultivation purposes. To provide adequate heat and electricity for the investigated greenhouses the following parameters are required: Arched greenhouse with PE-film

● Heat: 68 900 kWh/year ● Electricity: 12 500 kWh/year (with LED) or 14 300 kWh/year (with HPS) ● Cogeneration engine power: 51 kW ● Biogas: 10 700 Nm3/year (with LED) or 11 400 Nm3/year (with HPS) ● Minimum digester volume: 147 m3

Sawtooth greenhouse with glass

● Heat: 73 300 kWh ● Electricity: 12 200 kWh/year (with LED) or 14 000 kWh/year (with HPS) ● Cogeneration engine power: 54 kW ● Biogas: 11 600 Nm3/year (with LED) or 12 200 Nm3/year (with HPS) ● Minimum digester volume: 157 m3

This study indicates that a system with combined biogas production and greenhouse cultivation is feasible. The biogas produced may be used to generate heat and electricity for year round crop growth. In addition almost 1.9 kg of CO2 is generated for every Nm3 of biogas combusted, meaning that the CO2 from biogas combustion will suffice to reach concentrations of more than 1 000 ppm, concentrations that are significantly higher than the ordinary background concentration of 400 ppm and thus increase growth rate. Furthermore, also the supply of water from rainfall is feasible since the water from rain almost corresponds to half of the need for a whole year with three harvests, if a collection area similar to the greenhouse base area is utilized. Rain collection area may for example be extended to roofs of other nearby buildings. It is not optimal or recommended to solely utilize nutrients from biogas slurry, since the excess of nutrients would do more harm than good for both the plants and the nearby environment through leakage. If solely dependent of nutrients from biogas slurry a volume of 3 000 m3 annually would be needed to provide sufficient nitrogen for three harvests, but would result in 596 % excess in phosphorus and 229 % excess in potassium. One solution is to combine fertilization from biogas slurry with an alternative source of nutrient, to optimize the nutrient balance and thus the cultivation. The simulated greenhouses are both more efficient than the existing reference greenhouse 200 km north of Enköping, mainly in terms of heat requirements, using around 200 kWh per m2 less. The two different

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cladding materials used in the simulated greenhouses have different life expectancies. Glass does not deteriorate over time compared to PE-film and also has a much longer life expectancy than PE-film. Even though the initial GHG-emission of constructing greenhouses is higher for the one with glass as cladding material, the emissions are higher for the PE-film covered greenhouse due to material deterioration and thus periodic film replacement. This concludes that the simulated greenhouse with glass as cladding material is the better option in the longer perspective, in terms of ecologic sustainability, due to lower GHG-emissions.

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6 Future study Recently a scientific breakthrough was made at the Royal Institute of Technology where a science team successfully produced transparent wood. Numerous applications, especially in the construction sector, have been suggested for the material and as of March 2017 the science team behind the product received a grant from the European Research council to further support the research (KTH, 2017). Due to the transparency and sustainability of the material it could arguably be of interest to study the feasibility of utilizing it as a greenhouse cladding material. With the use of renewable energy for heating and electricity, transparent wood could further reduce the environmental footprint of the greenhouse. Furthermore, on the topic of more sustainably manufactured cladding materials, it could be of interest to study the feasibility of bioplastic cladding materials. Greenhouse heat loss through the ground has not been considered in this report and is a subject of interest for future studies regarding greenhouse energy balance. Furthermore, obtaining a fully closed cycle of energy and mass has also not been considered, e.g. the life cycle of each component within the system. The input of greenhouse cultivation residues into a digester can be a study of interest, in order to see how much they may contribute to biogas and nutrient production for a more closed cycle. Improved integration of elements such as nutrients or water could make the system more efficient, and also more accurate. Better irrigation systems with sensor controlled drip-irrigation would provide a more accurate water supply and reduce losses mainly due to evaporation, and can thus prove to be a valuable future study. Moreover, studies shows that use of aquaponics system would decrease the evapotranspiration and thus give a more efficient system with less losses. The value of evapotranspiration for the cultivating tomatoes inside a greenhouse using a hydroponic system has not been included in this report, due to its complexity. However, including this factor would give a better accuracy of the study.

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9 Appendix The analytic temperature equation with 1 hour time stepping (using equation 3.8) that was utilized in Microsoft Excel for the greenhouse simulations:

Greenhouse indoor temperature after 1 hour: where Tinf is the steady state solution (expressed in equation 3.9), T0 is the temperature of the previous time step (here, the initial temperature), is the time step (here it is 1 hour) and B is the time constant of the structure (equal to thermal mass divided by lambda, as seen in section 3.3.3). When the temperature after 1 hour is obtained (T1) the temperature after the next hour can be calculated using the same procedure as follows:

Temperature after 2 hours: and

Temperature after 3 hours: and so on. Energy balance was made in Microsoft Excel by using equation 3.2:

where energy transfer through the greenhouse claddings and incoming solar radiation was taken into account, in order to find the amount of biogas derived heat necessary to keep the greenhouse indoor temperature at 20 ℃.