Applied Energy 114 (2014) 880–888

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Applied Energy

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Energy management strategies for commercial greenhouses

0306-2619/$ - see front matter � 2013 Elsevier Ltd. All rights reserved.http://dx.doi.org/10.1016/j.apenergy.2013.08.089

Abbreviations: EP, Energy Productivity; ER, Energy Ratio; IRAC, Infrared Anti-Condensate; SCB, statistiska centralbyrån (statistic administration office in Swe-den); TES, thermal energy storage; UN, United Nation.⇑ Corresponding author. Tel.: +46 704644073.

E-mail address: Amir.Vadiee@energy.kth.se (A. Vadiee).

Amir Vadiee ⇑, Viktoria MartinRoyal Institute of Technology (KTH), Energy Department, Heat and Power Division, Stockholm, Sweden

h i g h l i g h t s

� An energy analysis in the greenhouse has been assessed using the TRNSYS tool.� The opportunities to improve the energy conservation in greenhouses are assessed.� A thermo-economic assessment has been performed for selected opportunities.� The overall energy performance in the commercial greenhouses is assessed.� The influence on four climatic conditions is discussed.

a r t i c l e i n f o

Article history:Received 28 February 2013Received in revised form 23 August 2013Accepted 30 August 2013Available online 9 October 2013

Keywords:Energy managementEnergy performanceCommercial greenhouseClosed greenhouse

a b s t r a c t

Growth in population and the ever-increasing development of new production technology leading to ris-ing energy use in the agricultural industry. Although the greenhouse is one of the most energy intensivesectors in the agricultural industry, it is important because of its ability to intensify production. Thispaper has assessed energy management strategies (including single and combined energy conservationopportunities), with special emphasis on Nordic climates, where fossil fuel-based heating is still signifi-cant, despite a recent conversion to biomass boilers.

The results show that the ‘‘Double thermal screen’’ and ‘‘Double glazing’’ with 60% reduction in energydemand are the most effective single opportunity for energy conservation. However, the highestimprovement (80%) is obtainable using the closed greenhouse concept, with a potential payback of5–6 years under favorable conditions. It can be concluded that some of the single opportunities can bemore practical in terms of their PBP in comparison to a complex concept, requiring a combination ofmeasures, such as the closed greenhouse.

� 2013 Elsevier Ltd. All rights reserved.

1. Introduction

The UN predicts the world population to reach 9 billion by 2030[1]. Therefore, innovative methods must be proposed in order toutilize the available arable lands in an effective way for providingfood for this foreseen population. Here, the commercial green-house has been proposed as one promising solution [2]. Commer-cial greenhouses are used to grow plants in order to reach betterquality and protect them against natural environmental effectssuch as wind or rain. Another benefit is giving the ability for outof season cultivation.

The operation of greenhouses makes use of the greenhouse ef-fect. The short wavelengths of solar irradiation which are visiblelight can pass through a transparent medium and are absorbed

by the objects on the other side. The heated objects will re-radiatelonger wavelengths which are infrared and cannot pass throughthe transparent medium. The temperature will increase due tothe accumulation of long wavelength radiation. Higher CO2 con-centration level can stimulate this phenomena because carbondioxide is fairly good infrared radiation absorbent, thus retainingthe heat in the greenhouse. There are many criteria in order to de-sign a greenhouse such as [3]:

� High transmissivity.� Heat retention in the cold period of time.� Heat dissipation in the warm period of time.� Having the optimal CO2 level of concentration in the

greenhouse.� High level of durability.� Cost consciousness.

Some of these criteria can be improved by using special modifi-cations such as better type of glazing with lower reflectivity,

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absorptivity and heat loss coefficient. Adequate ventilation systemcombined with CO2 enrichment is also used [4–6]. However, withthe most commonly used open ventilation system, the above crite-ria are never completely achieved [6]. In order to solve this prob-lem another concept was proposed, the closed greenhouse, e.g. aspresented in 1997 by a European research institute (Ecofys) [5].A closed greenhouse does not make use of ventilation windows,so it gets very hot in the sun and climate control becomes crucial.Thus a complex climate control system is needed to control tem-perature, humidity and the CO2 concentration [7].

Although a higher production yield can be obtained in commer-cial greenhouse, as compared to free land cultivation, it has to betaken into consideration that the greenhouse is one of the mostenergy consuming sectors in the horticultural industry [8]. Thispaper considers Sweden as the case study, where the total energyconsumption in the agricultural sector was about 4400 GW h in2008 [9]. Of this amount, 2300 GW h was supplied by diesel forthe horticultural machineries, 890 GW h was covered by electricityand 1220 GW h was supplied mostly by fuel oil and biomass forheating purpose [10]. In the same year the total energy used tocover the heating demand in the Swedish greenhouse sector was694 GW h which is about 15% of the total energy demand in theagricultural industry in Sweden [9]. However, for Northern climate,the heating energy demand in a commercial greenhouse represents65–85% of total greenhouse energy demand [11]. The energy mixin the commercial greenhouses in Sweden is presented in Fig. 1.This figure shows how the total energy supply for heating has in-creased by 50% from 2008 to 2011. However, the heating oil con-sumption has recently been reduced by 18% while at the sametime the biomass utilization has increased by 45% [8]. Electricityis used mainly for the artificial lighting in the greenhouse. How-ever, electricity is also needed to operate various devices such asventilation fans, pumps, and control system. [12]. The contributionof fossil fuel for covering the heating demand in the greenhouse inSweden has been measured using statistical data [8]. It has beenobserved that until 2008 the fossil fuel was still dominating withregards to covering the heating demand while it reduced to 25%by 2011 [9]. However by considering the energy demand

Fig. 1. Comparison in energy mix for the commercial

increment in the greenhouse sectors, (50% as it mentioned earlier),fossil fuel still makes up a significant part of the supply towardsheating. Therefore, any reduction in the energy demand leads toa considerable impact towards improving sustainability in green-house operation. Additionally, energy is typically the largest over-head cost in the production of greenhouse crops. Therefore energyconservation in the commercial greenhouse has been emphasizedin recent years in order to sustain cost efficient crop productions.

The commercial greenhouses normally have a very high output.In the modern and well controlled greenhouses, the productionyield can be 10–20 times more than any outdoor horticulturalsection [13]. However, high input of direct and indirect energy isrequired in order to produce a large amount of high quality prod-ucts. Fossil fuel or any kind of renewable energy sources which hasbeen utilized for satisfying the heating and cooling demand of thegreenhouse are considered as direct energy input. Direct energyinputs for the heating demand of the greenhouse have the highestimpact on the overall greenhouse performance and the finalmarket price of the product [14]. Considered as indirect energyinput are: various fixed equipment and any type of materials suchas fertilizer and chemical biocides as well as labor, transportationand irrigation used in production procedures [14]. Irrigation andfixed equipment have the lowest portion in indirect energy useof the commercial greenhouses, about 1–2% of total indirect energyuse. However the fertilizer has the highest portion in the indirectenergy use with 21% of indirect energy input. The portion of directand indirect energy inputs to a greenhouse is illustrated in Fig. 2,and is based on data from the literature [15–17].

An energy analysis is required for further assessment on green-house direct energy and corresponding energy performanceimprovement opportunities. Therefore, a commercial greenhouseneeds to be modeled and various energy conservation opportuni-ties must be studied.

The overall objective of this paper is to explore the impact onenergy efficiency improvements in the commercial greenhousesector, comparing a number of technological pathways. The influ-ence on climatic conditions is also discussed with a special focusgiven on Nordic conditions using Sweden as the case study. For

greenhouse in the Sweden in 2008 and 2011 [8].

Fig. 2. The portion of each energy inputs in the commercial greenhouses based on the literature survey [15–17].

Table 1Monthly average of day-light length and global radiation in the Sweden [20].

Month Average sunny hours per day(hours)

Global radiation(kW h m�2)

January 1 15February 3 25March 6 65April 6 110May 10 140June 8 170July 8 150August 7 120September 5 75October 4 35November 1 20December 1 6

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Nordic climates, the significant heating demand in the commercialgreenhouse sector, as well as the potential of greenhouse growingto allow for out-of-season production of crops in this climate makethe greenhouse of particular interest.

2. Methodology

In order to evaluate the impact of various opportunities forimproving the energy performance in the greenhouse, an energyanalysis is needed to understand the needs for typical greenhousein the Swedish climate. Here, values from a previously publishedstudy on the Ulriksdal greenhouse in the Stockholm are used[18]. The results obtained by theoretical modeling were validatedwith real measured data obtained from the greenhouse [18]. Thenthe total average use of energy for heating purpose for a conven-tional greenhouse in Sweden was obtained as 320 kW h m�2 [18].Electricity consumption in the commercial greenhouse mainlyconsists on the artificial supplementary lighting [19]. Sweden, asa case study, has an annual global radiation averaging between750 and 1100 kW h m�2 with 1100 to 2000 h sunshine [20]. Mostcrops need a daily 12–24 h lighting [19]. Therefore, assuming18 h daily, light the artificial lighting requirement can be estimatedto 4650 h annually. This is done be considering the average dailyhours of sunshine in Sweden as summarized in Table 1 [20]. Thisnumber can be compared with Mediterranean climate conditionswhere it is reduced to 2500 h [19].

To identify technological pathways for energy efficiencyimprovements in a greenhouse, a literature survey has beenperformed [11,14,19,21–31]. For example, Nederhoff and Houter[13] have separately evaluated some solutions recommended forimprovement of the energy performance in the commercialgreenhouses. The conclusion of their evaluations are summarizedin Table 2 [13,21] and categorized into three groups. The alterna-

tives given in the first group promise 1–2% improvement in the en-ergy performance of the greenhouse which is directly calculatedfrom the potential energy saving. However the energy performancecan be improved up to 5% by applying the medium impact (secondgroup) category and it can be improved more 5% by any alterna-tives belong to the high impact (third group) category. Althoughsome single improvement solutions have a low and mediumimpact, they still may lead to a considerable energy saving. Forexample by closing the ventilation windows properly, the heatingpower demand can reduce by 7 kW m�2 which is about 10% ofpeak heating demand in a commercial greenhouse [21]. As anotherexample, although the foundation and sidewall insulation isconsidered as a medium impact solution it leads to a 15 kW h m�2

saving in annual heating demand [21]. In the medium impactcategory (second group), the use of an efficient climate controlsystem, including the heating and cooling system utilities, isprobably the most effective alternative. The energy demand canbe reduced by 18 kW h m�2 with e.g. accurate measurement andcontrol systems [21].

High impact solutions have a considerable influence on the en-ergy performance. However, they may come with a high cost aswell. For this reason, energy performance data of a commercialgreenhouse in Sweden obtained through TRNSYS modeling [18]was used to qualitatively assess the high energy improvement im-pact solutions, for energy savings as well as economic feasibility.

The input data for the reference greenhouse modeled is pre-sented in Table 3. In addition to the energy savings, a simplepay-back for the investment was performed, taking into accountthe capital investment need per measure using data from the liter-ature, as well as the value of any fuel and electricity savings.

The overall energy performance in the commercial greenhousesis then proposed to be assessed using the following parameters,with equations defined in the literature [14,17,32].

The Energy Ratio (ER) is a ratio between the system energy out-put and input. In the commercial greenhouses the energy value ofthe products is considered as energy output while the total green-house energy input includes both direct and indirect energy inputs.Then the ER is a measure of the energy utilization efficiency in thegreenhouses [14,17,32] and is represented by the followingequation.

Energy RatioðERÞ ¼ energy output½MJ=m2�Total greenhouse energy inputs½MJ=m2� ð1Þ

The Energy Productivity (EP) (Eq. (2)). This parameter is used tocompare the productivity of commercial greenhouse in eachproposed energy management scenarios [14,17,32].

Energy ProductivityðEPÞ ¼greenhouse production yeild kg

m2

h iTotal greenhouse energy inputs MJ

m2

� �ð2Þ

Table 2Alternative means for energy performance improvement in a greenhouse (based onNederhoff and Bartok evaluation [13,21]).

Low impact (1–2%) Pass the heating pipes along the greenhouse wall withsufficient space between themUsing insulation into the boiler house’s wallsUsing variable speed or frequency controlled pumps

Medium impact(3–5%)

Repair the gaps and cracks in the greenhouse structureKeep the ventilation windows and fans closed/shut offwhile they are not usedInstall the heating pipes as close to the cultivated areaas much as possibleInsulate the heating and cooling pipes and ductsEnsure regular system maintenanceInsulate the greenhouse foundation and sidewallMinimize the light blocking object above the cropCalibrate and regulate the ventilation windowshydraulic system

High impact (morethan 5%)

Using the heating system with high efficiency (higherperformance)Using highly accurate climate control systemUsing high efficient artificial lighting systemRegular maintenance of the weather stationUsing short and long term thermal storage systemOptimizing the CO2 enrichment control systemUsing thermal screenUsing better insulation in the greenhouse constructionHigh performance glazing system

Table 3Summary of model details based on the case study Ulriksdal greenhouse.

Reference greenhouse

Type Conventional single gable greenhouseLocation StockholmOrientation East_WestWeather data Meteonorm library (Type 109 in TRNSYS)Area 4600 m2

Volume 13,750 m3

Glazing Single ply polyethylene U = 5.68; R = 0.18; s = 0.87Walls U-value = 0.7 W m�2 K�1; solar absorptance = 0.6;

thickness = 8 cmGround U-value = 0.3 W m�2 K�1; solar absorptance = 0.8;

thickness = 42 cmRoof tilt angle 26�Occupancy Max 25 person (in working hour between 8:00 and

18:00Infiltration ratio 0.5–1.5 h�1 (ACH)Lighting system High output T8 Fluorescent;1860 Lumen per m2;

170 kW h m�2; light equipment efficiency = 0.9;room lighting efficiency = 0.6

Minimum allowabletemperature

18 �C

Maximum allowabletemperature

25 �C

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The third parameter is the Net Energy (NE) which is the Net En-ergy output (output minus input energy) in the commercialgreenhouse.

Net EnergyðNEÞ ¼ Total greenhouse energy output

� Total greenhouse energy inputsMJm2

� �ð3Þ

Based on the official statistical reports published StatisticsSweden about the production of the horticultural products in2010, it can be understood that the main products of the Swedishcommercial greenhouse are cucumber, tomato, aromatic plantsand lettuce [33]. Therefore, this paper presents an assessment onthe performance of Swedish commercial greenhouses using datafor cucumber [33]. The total production yield of cucumber from

Swedish commercial greenhouses is about 45 kg m�2 and the totalenergy output for cucumber production is 36 MJ m�2 [34]. For thiscase of cucumber production in the Swedish climate, a variety ofefficiency improvement measures have been assessed in terms oftheir effect on the ER and EP values. The total energy input for eachcase has been calculated using the obtained results from the previ-ously mentioned TRNSYS model [18]. The average total energy in-put in a typical commercial greenhouse in Sweden is around490 kW h m�2 [9]. The total energy input achieved by the TRNSYSmodel applied to the conventional commercial greenhouse is470 kW h m�2. There is thus a good agreement between all givenenergy input values.

Based on the obtained outcomes from the energy performanceanalysis and the literature reviews, the energy management path-ways are finally discussed for four climatic regions includingnorthern (cold), temperate, hot arid and tropic climate conditions.The most promising alternatives for each climate condition havebeen proposed in a pathway for effective management of thegreenhouse using a morphological chart.

3. Results and discussions

An energy pathway needs be formed in order to improve thegreenhouse performance by reducing the direct energy inputsand conserve energy throughout the system. This paper has as-sessed such pathways including single and combined opportuni-ties to improve energy performance in the commercialgreenhouses, with special emphasis on Nordic climates. The pres-ent situation show that the total energy use in Swedish commer-cial greenhouse has increased by 50% on average between 2008and 2011. However the heating oil consumption has been reducedby 18%, while the biomass usage has been increased by 45%.

3.1. Opportunities in energy performance improvement forcommercial greenhouses

Table 4 shows a thermoeconomic assessment for the proposedopportunities which was previously introduced in Table 2.

One especially interesting holistic approach to managing theheating, as well as cooling demand in a greenhouse is the closedgreenhouse concept. As shown in Table 4 the greenhouse energyperformance can be improved about 80% by applying the closedgreenhouse concept. Of course, this concept comes with a highinvestment cost, the highest incremental cost ($/m2) of all themeasures considered. Although the pay-back time is reasonable(5–6 years) this upfront capital investment can make it compli-cated to implement. Considering other reported benefits of theclosed greenhouse concept, such as improved water conservationand higher production yield due to effective climate control maymake the concept even more interesting [5,6,35–38], and shouldtherefore be included in real implementation case evaluations forthe concept.

Besides the closed greenhouse concept, it is worth mentioningsome solutions showing promise in terms of very short pay-backtimes, less than two years. These solutions include alternative(double) covering materials, and alternative lighting in the green-house (Table 4).

3.2. Overall energy performance assessment

The overall energy performance has been assessed by introduc-ing three parameters: Energy Ratio (ER), Energy Productivity (EP)and Net Energy (NE). The results shows (Fig. 3) that the ER andEP in the closed greenhouse concept are almost twice as high as

Table 4Summary of energy performance improvement opportunities for a commercial greenhouse based on Swedish climate conditions [19,22–26,29,30,48,49].

Energy conservationopportunity

EnergysavingkW h m�2

Energyperformanceimprovement %

Energy savingequivalentcosta $ m�2

Incrementequip cost$ m�2

SimplePBP year

Other specification

Heat distribution system Reference type: atmospherically vented with a 65% seasonal efficiencyPower vented 77 25 2.5 3b 1 Eff. 78%Direct vented 86 28 3 16b 5 Eff. 80%Cooling methods In the conventional greenhouse the cooling demand will meet by free cooling using the ventilation windows and an opportunity to

store and later utilization of a 105 kW h m�2 will be lost.Single layer thermal screen 120 40 4 16c 4 40% energy saving factor; R-value = 1.67

[26]Double layer thermal screen 180 60 6 28c 5 60% energy saving factor; R-value = 4.17

[26]Covering material Reference type: single ply polyethylene U = 5.68; R = 0.18; s = 0.87Double polyethylene 118 40 4 2d 0.5 U = 2.3; R = 0.43; s = 0.78Double polyethylene IRAC

(Infrared Anti-Condensate)

175 58 6 7d �1 U = 1.4; R = 2; s = 0.76

Wall insulation Reference type: net structure R-value = 0.25R-5 46 15 1 10e 10 Net R-value = 0.44R-10 54 18 2 15e 7.5 Net R-value = 0.62Lighting Reference type: high output T8 Fluorescent, total input 40 W, Efficiency (lumen per watt) = 85; life = 12,000 hHigh pressure sodium 47f 30 5g 10h 2 Total input = 400 W efficiency (lumen per

watt) = 117; life = 15,000-24,000 hPulse start metal halide 10f 6 1g 3h 3 Total input = 320 W; efficiency (lumen per

watt) = 90; life = 80,000–20,000 hClosed Greenhouse concept Reference type: single glazed, atmospherically vented heaters where using fuel oil, free cooling using ventilation windows (Case Study:

Ulriksdal greenhouse). Closed greenhouse type: double ply IRAC, FiWihex, active cooling using heat pumpi R-10, wall insulation, highpressure sodium lighting.245 80 8 40–50j 5–6j Described in 3.a

a The energy saving equivalent cost is calculated based on cost benefit due to reduce the fuel oil consumption. The fuel oil price is assumed 1.4 $/gal [48].b The prices have been obtained from Eugene A. scales & Associates, Inc. [25].c The thermal screen cost is obtained from Pacific Gas & Electric Company [26].d The glazing material cost obtained from EnviroCept. Greenhouse & Supply website [50].e The wall insulation material cost obtained from Allcostdata website [51].f Energy saving due to the reduction in electrical energy demand by improving the lighting system [19].g The assumed electricity price is 0.1 $/kW h [52].h The mentioned prices are obtained from ARGUS control. [19].i In the closed greenhouse the ground source heat exchanger or pad and fan cooling system generally will be used based on the geographical climatic situation.j The closed greenhouse total estimated increment cost and its payback period is obtained from the former study by the authors [18].

Fig. 3. Changes in Energy Ratio and Energy Productivity due to energy efficiency measures in a commercial greenhouse.

Table 5The equivalent required production yield improvement for the proposed energy savings measures.

Energy savings measure Energy input reduction (MJ/m2) Equivalent required production yield improvement (kg/m2)

Direct vented 310 10Double layer thermal screen 648 28Double glazing (IRAC) 630 27R-10 wall insulation 195 6High pressure sodium lighting 169 5Closed greenhouse concept 882 49

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Fig. 4. Net Energy reduction due to the proposed energy efficiency measures in a commercial greenhouse.

Fig. 5. The priority rank of the main criteria in the closed greenhouse concept usingthe rank graph analysis.

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in a conventional greenhouse. ER and EP can be improved by twomechanisms:

� Energy input reduction.� Increasing the annual production yield.

Some of the greenhouse performance improvement opportuni-ties affect both of these mechanisms. For example, the closedgreenhouse concept promises a considerable reduction in the heat-ing demand and at the same time the production yield is expectedto increase due to better climate control. A number of parameterssuch as temperature, irradiation, artificial lighting, human power,machinery, pesticides, fertilizer has direct and indirect effect onthe production yield [32]. Usually, an empirical correlation is usedin order to find the impact factor for each of these parameters onthe production yield [32,39]. However, such a correlation can onlybe used for the same type of greenhouse and production with thesimilar regional climate condition. Therefore these impact factorswere not considered in this study and the ER and EP have been cal-culated only based on the energy input reduction. However, to stillget a clear link between the value of energy and productivity, anassessment is presented with the required equivalent productionyield improvement for obtaining the same ER and EP value for eachrespective opportunity. For example, in the closed greenhouse con-cept, the energy input will be reduced by 882 MJ/m2 and it causes a109% increasing of the ER and EP. Then, in a conventional green-house, the production yield needs to be increased by 49 kg/m2

(base case production yield has been considered 45 kg/m2) in orderto causes the same ER and EP improvement. Therefore the equiva-lent production yield improvement for the closed greenhouse con-cept is equal to 49 kg/m2 based on the ER and EP analysis. Adetailed assessment for equivalent production yield improvementsfor the proposed opportunities is given through Table 5.

The NE as defined in Eq. (3) can be obtained from the total en-ergy input and total greenhouse energy output. In the commercialgreenhouses the NE is a negative value due to the big difference be-tween the energy input and output generally. Therefore the NetEnergy is highly dependent on the energy input changing. TheNet Energy reduction due to energy performance improvementopportunities is presented in Fig. 4. As shown, the closed green-house concept has the highest impact (50% reduction) with doubleglazing, and double thermal screen measures being almost as good(40% reduction).

Finally, to shed some insights on the importance of energy per-formance for the commercial greenhouse sector in Sweden, a ques-tionnaire has been used for a survey among the five largestcommercial greenhouses. The evaluation of this survey shows thatthe heating demand is the second major expense for the grower,following labor costs. Also, the electricity consumption is consider-able due to the need for artificial lighting. These greenhouses useconventional cooling and dehumidification using ventilation. Noneof the target greenhouses utilize TES in order to manage their en-ergy demand. Thus, there is a potential Swedish commercial green-house to improve the system energy performance. Here, the closedgreenhouse concept can be of particular interest. Thus, one ques-tion in the questionnaire was formulated in order to evaluate theimportance of potential benefits from using the closed greenhouseconcept, ranking the benefit from 1 to 4. Here, 1 means the mostimportant benefit and 4 the least important one. The highlightedbenefits were, based on what has been previously claimed[7,11,21]:

(A) Water conservation improvement.(B) Production rate improvement.(C) Indoor climate control system improvement.(D) Energy conservation improvement.

The result has been evaluated using the rank graph method[40]. This method can be used in order to analyze the ranked obser-vation in a questionnaire. In the rank graph the results are pre-sented in the semicircle with a radius of unity. The resultsappear in terms of the vectors inside the proposed semicircle.The length of obtained vectors are representing of the degree ofconcordance between the objects of the survey, while the direction

Fig. 6. Morphological chart of different optional technology used in closed greenhouse concept based on Van Ooster study [44,45].

886 A. Vadiee, V. Martin / Applied Energy 114 (2014) 880–888

angle is corresponding to the average ranking of the each items[40].

The results from the evaluation are presented in Fig. 5. Asshown, the energy conservation improvement (D) is the mostimportant issue, and also the concordance is great. The productionyield improvement (B) is the second prioritized criteria, and indeedit is expected to have a most economical benefit for the grower. Itshould be noted that the improvement in the production yield canalso be achieved by indoor climate control improvements (C) ingeneral. A more accurate climate control system leads to highercontrol on planting and growing conditions. However, the moreaccurate climate control system can at the same time significantlyimprove energy conservation. The water conservation improve-

ment (A) might be more important in hot and arid regions of theworld, however from the survey (Fig. 5) it is the benefit of leastinterest to Swedish growers.

4. Assessment of energy management strategies

By combining alternatives to improve the energy performanceof the commercial greenhouses, several alternative energy man-agement scenarios appear. However, no specific optimal solutionhas so far been defined. An optimization becomes multi-objective,aiming for minimum CO2 emissions and maximum energy and costefficiencies. Thus, finding an optimized scenario is a task of balanc-

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ing these criteria and an ‘‘optimum’’ strategy will be found consid-ering the priority for each particular case.

In this study, the proposed energy management scenarios areonly based on the closed greenhouse concept which is a promisingsolution for all optimizing criteria [4–6,41–43]. In order to proposethis energy management pathway, the following procedure needsto be considered. First, the design objectives need to be defined.Here the main design objective is minimizing the fossil energyand maximizing the solar energy utilization as well as reducingthe greenhouse gas emission by improving the energy performancein the commercial greenhouse. In this step, the main design con-straints need to be defined as well. For example, the temperatureshould be maintained in the range 18–30 �C and the relativehumidity must be kept between 75% and 85% [12]. The geologicaland climatic condition need to also be considered in this step.

In the second step, the criteria needs to be prioritized, whichcan be energy performance, costs, and emission level. Here the en-ergy performance has been considered as the most important de-sign criteria while the cost issue is the second most important.Thereafter, based on the design objectives and the criteria require-ments, some functional alternatives can be suggested in form of amorphological chart as proposed in Fig. 6.

A conceptual design can be obtained by combining functionalalternatives. For example the energy source for heating purposemay be supplied by geothermal or solar energy based on the designconstrains and geological conditions. The proposed conceptual de-sign does not necessarily contain one single alternative for eachfunction, but may include a combination of alternatives. For exam-ple, for the heating system a combination of boiler and heat pumpmay be selected at the same time for a proposed conceptual design.In the very last step the proposed conceptual designs need to beevaluated based on the prioritized designing criteria and the de-sign objectives.

Fig. 6 Presents a chart of five greenhouse energy managementscenarios: for Nordic (black line), temperate (blue line), tropic(green line), hot arid climates (red line), and the optimum scenariosuggested by Van’t Ooster by choosing Netherland as the casestudy [45]. Here, input for the Nordic climate is based on the pres-ent study, whereas the temperate, tropic and hot arid regions havebeen discussed in other separate studies by Hemming et al. [44]but are interpreted here. The proposal of the concept of graphicalscenarios was originally made by Van’t Ooster [45]. In that study,there was presented one scenario for minimizing the fossil fuelusage in the Netherlands. This scenario is also included in Fig. 6(dashed1 purple line). Technologies included in the figure are thosethat can be implemented in the closed greenhouse concept.

The main differences between the suggested scenarios are withregards to the energy source, as well as the cooling and dehumidifi-cation strategies, which depend strongly on the climatic conditions.

The solar collector can be utilized in the closed greenhouse con-cept in the form of solar shielding in order to maximize the solarenergy utilization and at the same time providing a cooling strat-egy [41]. It can also be utilized directly for a solar hot water sys-tem, as well as through photovoltaic panels for electricitygeneration. However the solar hot water system may be recom-mended only in hot arid and tropical conditions when consideringthe cost.

Although the free cooling via direct ventilation may be consid-ered as the easiest and the cheapest method for cooling and dehu-midification of the greenhouses, it is the major source of heat lossand leads to higher heating demand specifically in Nordic and tem-perate climate condition. Therefore it is not considered in the

1 For interpretation of color in Fig. 6, the reader is referred to the web version othis article.

f

closed greenhouse concept. However by applying controlled venti-lation with recovery system, i.e., the semi-closed concept [18], thecooling cost may be reduced considerably. However the ventilationwith recovery system in the semi closed concept may be suggestedas an efficient cooling and dehumidification method but also it rec-ommends integrating the system with active cooling using the al-ready installed heat pump. Actually, the closed greenhouse will actas a large solar collector itself. Therefore by choosing the right con-struction material for covering and also insulation and by fully (orpartly) closing the ventilation windows the accumulated excessheat inside the greenhouse can be stored using a seasonal andshort term thermal storage for further utilization.

Based on former studies this amount of stored heat can be suf-ficient for the heating demand inside the greenhouse [18,46]. Inthis pathway the greenhouse will be heated using stored solar en-ergy in combination with a heat pump. Warm water is then ex-tracted from e.g. the aquifer storage and delivers lowtemperature heat to the heat pump while being cooled. Then, thecooled water is returned and thus charges the cold side of the stor-age. The heat pump provides the hot water. The hot water willcharge a short-term buffer storage which is used to level out thedaily/hourly load in the closed greenhouse. For cooling purpose,cold water from the cold side storage is pumped directly into thegreenhouse and removes heat via a heat exchanger system. Then,the warm water is brought to the warm side storage charging itfor the winter. The humidity will be the most challenging issuewhich has to be regulated by the humidification and the dehumid-ification systems. For this purpose, an efficient heat exchangermust be used with capability of transferring a large amount of heatfrom water to air (heating) or from air to water (cooling) while thetemperature difference is very low [47].

5. Conclusions

A number of opportunities to improve the energy conservationin greenhouses have been assessed and categorized based on anestimated efficiency impact factor for each improvement measure.Here, high impact improvement opportunities for a commercialgreenhouse in Nordic climate conditions were identified as: the‘‘Double thermal screen’’ (60%) and the ‘‘Double glazing’’ (60%).Two less effective opportunities, still considered as high impactsolutions are: the use of additional insulation (R-5 and R-10) with15–18% improvement; and to replace the heating system unit withpower vented and direct vented heater (25–28% improvement).The most expensive opportunity can be achieved by applying thedouble layer thermal screen with 5 years PBP. The low cost oppor-tunities were identified as: the ‘‘Double polyethylene’’ (6 monthsPBP) and ‘‘Double ply IRAC’’ (10 months PBP).

The highest improvement (80%) is obtainable using the closedgreenhouse concept, with a potential payback of 5–6 years underfavorable conditions. In this concept, the ER (efficiency in energyuse) and EP (amount of crop obtained per unit amount of energyinput) were improved by 130% and 160%, respectively.

Scenarios have been discussed for improving the energy perfor-mance in a commercial closed greenhouse. However, all proposedscenarios need to be analyzed further in order to find the mostoptimized scenario. Here, four scenarios have been proposed basedon the appropriate and available technologies for different climaticsituations. Utilizing the solar energy, e.g. through a solar shieldingfor cooling purpose and supplying energy to a storage in parallel,for sorption technology and active cooling using heat pump, andusing double glazed covering, are solutions seemingly feasible forall climate condition analyzed. However, it is also concluded thatthe climate condition will still be a certain parameter in order tochoose the most practical and efficient opportunity regarding tosuggested scenarios.

888 A. Vadiee, V. Martin / Applied Energy 114 (2014) 880–888

By considering the cost effectiveness for individual improve-ment measures, it may be concluded that some of the single oppor-tunities can be more practical in terms of their PBP in comparisonto a complex concept, requiring a combination of measures, suchas the closed greenhouse. Although the closed greenhouse can benominated as the most thermo economical optimized solution ascompared to the conventional greenhouses, there are some othersingle opportunities which may be applied in order to improvethe energy performance in the greenhouse with a fairly reasonableupfront cost.

Finally, it has to be highlighted that the parameters which havebeen studied here are proposed based on energy performanceimprovement which also affect the cost. However, the cost effec-tiveness of the closed greenhouse concept is highly dependent alsoon the yield production rate in addition to the fuel consumption.Therefore, solutions enabling increasing the yield, achievable bywell-designed indoor climate control system and desirable CO2

concentration, are of as much interest as reducing the energy de-mand. In order to attain the most optimized and applicable sce-nario, considering the cost benefits of increased yield should bedone in combine with analyzing energy conservationimprovements.

Acknowledgements

The authors would like to acknowledge the financial support forthis work from the Swedish Foundation for Agricultural Science, aswell as the knowledge contribution of commercial greenhousegrowers: Slottsträdgården Ulriksdal, Svegro, Gustafslund, RydelssOdling AB, and Alskog.

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