Thermal energy storage strategies for effective closed greenhouse design

Applied Energy xxx (2013) xxx–xxx

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Applied Energy

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Thermal energy storage strategies for effective closed greenhouse design

0306-2619/$ - see front matter � 2012 Elsevier Ltd. All rights reserved.http://dx.doi.org/10.1016/j.apenergy.2012.12.065

⇑ Corresponding author. Tel.: +46 (0)87907476.E-mail addresses: [email protected] (A. Vadiee), viktoria.martin@ener-

gy.kth.se (V. Martin).1 Tel.: +46 (0)87907484.

Please cite this article in press as: Vadiee A, Martin V. Thermal energy storage strategies for effective closed greenhouse design. Appl Energy (2013)dx.doi.org/10.1016/j.apenergy.2012.12.065

Amir Vadiee ⇑, Viktoria Martin 1

Royal Institute of Technology, Energy Department, Heat and power Technology, 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." Three thermal energy storage systems have been studied in closed greenhouse concept." A sensitivity analysis has been considered in order to distinguish the main parameters in cost study." The peak load has the main impact on the Payback time." The SCW could be an economical option in closed greenhouse concept.

a r t i c l e i n f o

Article history:Received 3 August 2012Received in revised form 22 December 2012Accepted 27 December 2012Available online xxxx

Keywords:Heat transferEnergy conservationClosed greenhouseSolar commercial buildingSustainable energy management systemThermal energy storage system

a b s t r a c t

The closed greenhouse is an innovative concept in sustainable energy management. In principle, it isdesigned to maximize the utilization of solar energy through the seasonal storage. In a fully closed green-house, there is not any ventilation window. Therefore, the excess sensible and latent heat must beremoved, and can be stored using seasonal and/or daily thermal storage technology. This stored excessheat can then be utilized later in order to satisfy the thermal load of the greenhouse. Thermal energy stor-age (TES) system should be designed based on the heating and cooling load in each specific case. Under-ground thermal energy storage (UTES) is most commonly chosen as seasonal storage. In addition, astratified chilled water (SCW) storage or a phase change material (PCM) storage could be utilized as shortterm storage system in order to cover the daily demands and peak loads. In this paper, a qualitative eco-nomical assessment of the concept is presented. Here, a borehole thermal energy storage (BTES) system isconsidered as the seasonal storage, with a PCM or a SCW daily storage system to manage the peak load. ABTES primarily stores low temperature heat such that a heat pump would be needed to supply the heat ata suitable temperature.

A theoretical model has been developed using TRNSYS to carry out the energy analysis. From the eco-nomical feasibility assessment, the results show that the concept has the potential of becoming costeffective. The major investment for the closed greenhouse concept could be paid within 7–8 years withthe savings in auxiliary fossil fuel considering the seasonal TES systems. However, the payback time maybe reduced to 5 years if the base load is chosen as the design load instead of the peak load. In this case, ashort-term TES needs to be added in order to cover the hourly peak loads.

� 2012 Elsevier Ltd. All rights reserved.

1. Introduction order to cut down the total annual operating cost. The average EU-

Sustainability has been at the centre of attention for many dec-ades. One of the most challenging areas toward sustainability is theagriculture sector. Population growth necessitates higher produc-tion yield while the high cultivation output requires a considerablecapital investment cost, as well as direct and indirect energy in-puts. Considering the continuously increasing cost of energy, espe-cially fossil fuels, the external energy demand must be reduced in

27 energy usage in the agricultural industry is 188 W h/m2 and inthe Nordic countries, such as Sweden, it is even higher, at 299 W h/m2 [1]. About 80% of the total energy demand in commercialgreenhouses is for heating, and the electricity is mainly used forthe artificial lighting and also running the electrical devices, [2–4]. Based on the statistical data analysis, fossil fuel provides almost2/3 of the heating demand resulting in considerable annual costsand also environmental impact due to CO2 emissions [1]. To con-serve energy, the idea of using a closed greenhouse was formed[5–7].

In the conventional greenhouses, the open ventilation systemcauses significant heat loss. Therefore it leads to high energydemand, especially in the cold weather conditions. The closed

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Nomenclature

A surface areaAc frontal areaCp specific heat in constant pressure (kJ kg�1 K�1)P power (W)T temperature (K)h heat transfer coefficient (kJ kg�1 K�1)

Symbolsq density (kg m�3)m wind velocity (ms�1)DH enthalpy difference (kJ kg�1)DP pressure difference (kPa)DT temperature difference (K)

Subscriptair aircool cooling

in insidemean refer to mean temperature differenceout outdoorwater water vapour

AbbreviationsBTES borehole thermal energy storageSCW stratified chilled water storageEU-27 European Union 27 countriesFiWihex fine wire Heat exchangerkgOE kilogram oil equivalentPCM phase change materialSt Stanton numberTES thermal energy storageUTES underground thermal energy storage

2 A. Vadiee, V. Martin / Applied Energy xxx (2013) xxx–xxx

greenhouse can be independent of fossil fuel and the outside cli-mate as long as it is integrated with TES [8–11]. The principle ofthe closed greenhouse is shown in Fig. 1 [5–7]. The principle be-hind it is that the excess sensible and latent heat is removed in acontrolled way, and can be stored using seasonal and/or daily ther-mal storage technology. This stored excess heat can then be uti-lized to meet the thermal demand of the greenhouse.

As shown, in the heating mode (Fig. 1a) the greenhouse will beheated using a heat pump. Warm water is extracted from the TESand delivers low temperature heat to the heat pump while beingcooled. Thereafter, the cooled water is returned to the TES-systemand thus charges the cold side of the TES. The heat pump uses thedelivered heat to heat up the water which will charge the short-term buffer storage used to level out the daily/hourly load in theclosed greenhouse. In the cooling mode (Fig. 1b), cold water fromthe cold TES is pumped directly into the greenhouse and removesheat via a heat exchanger system. Then, the warm water is brought

Fig. 1. Conceptual features of a closed greenhouse (based on [6, 7, 15]): (a) heatingmode; (b) cooling mode (omitting heat pump cycle).

Please cite this article in press as: Vadiee A, Martin V. Thermal energy storage stdx.doi.org/10.1016/j.apenergy.2012.12.065

to the warm TES to charge it for the winter. The temperature can becontrolled by the heating and cooling system while the humiditywill be the most challenging issue which has to be regulated bythe humidification and the dehumidification systems. For this pur-pose, the FiWihex technology, (Fine Wire Heat Exchanger), is anefficient heat exchanger capable of transferring a large amount ofheat between water to air (heating) or from air to water (cooling)with very low temperature difference [16]. Since heat exchangersused in closed greenhouses application have to operate with lowtemperature difference, this type of heat exchanger is advanta-geous [16].

In general, a thermal storage system is designed based on theheating and cooling load for each specific case. Therefore, thechoice of TES is quite dependent on the greenhouse conditionsand horticultural application since different kind of plants needdifferent climate conditions, and heating and cooling demandsvary in each case. Underground thermal energy storage (UTES) isthe most commonly used seasonal storage method [4,9–13]. How-ever, a stratified chilled water (SCW) tank or a phase change mate-rial (PCM) storage can be used as the short term storage system tocover the daily or peak demands [14].

The main aim of this study is to present a thermo-economicalassessment of the closed greenhouse concept integrated with theTES system. Here, the borehole thermal energy storage (BTES) hasbeen considered as the long term storage method, with the PCM orSCW storage concepts as the alternative short term storage methods.

2. Methodology and assumptions

This work combine theoretical modeling for an energy analysis,with a thermoeconomical assessment to carry out a qualitative de-sign and feasibility study of the closed greenhouse concept com-bined with integrated TES. The details of the analysis, along witha description of assumptions made, are presented below.

2.1. Greenhouse energy modeling and analysis

For the purpose of this study, a model has been developed usingTRNSYS 16. The TRNSYS software [17] is made up of several of sub-routines for components present in energy conversion systems,buildings, controls and weather data. The model is used for sizingand evaluating the 24 h peak cooling load and heating load of anideal closed greenhouse located in Stockholm; with a total number

rategies for effective closed greenhouse design. Appl Energy (2013), http://



A. Vadiee, V. Martin / Applied Energy xxx (2013) xxx–xxx 3

of air changes per hour equal to 0.5 h�1. As there is no specific stan-dard type for greenhouses in the TRNSYS library, the greenhouse ismodelled using the type 56 building project multi-zone mode. To fitthe conditions of a greenhouse, some specific conditions have beenconsidered: (1) the roof has been modelled by two single gableroofs with a tilt angle equal to 26 degrees; (2) the walls are coveredby 99% double glazed windows with a total thermal conductivityequal to 5.68 W m�2 K�1; and the construction material has beenselected from the TRNSYS library for a conventional type of a com-mercial greenhouse [2]. The desirable thermal condition is depen-dent on the type of cultivation, crops and environmentalsituation. Here, the target temperatures for controlling the heatingmode and cooling mode were assumed to be 18 �C and 25 �C,respectively. The target values for relative humidity are set to 75%(humidification) and 85% (dehumidification). These assumed val-ues are valid for e.g. tomato growing [5].

For the system energy analysis, this paper has considered threecases of integrated storage design:

(1) A seasonal Borehole TES (BTES) will completely supply theheating and cooling power demand.

(2) A short term TES based on phase change materials (PCM)will cover a portion of the peak load and then BTES will sup-ply the remaining thermal demand.

(3) Stratified chilled water (SCW) storage will cover a portion ofthe peak load and then BTES will supply the remaining ther-mal demand.

The TRNSYS model provides initial data on peak load for thestorage design: about 1.2 kW h m�2 for heating and 1.7 kW h m�2

for cooling. The heating and cooling load profiles for the summerand winter peak days, as obtained from an annual load simulation,are presented in Fig. 2.

As shown in Fig. 2, the maximum peak cooling load (b) is113 W m�2 while the heating peak load (a) is 59 W m�2. Thereforethe cooling demand will determine the short-term TES sizing in or-der to cover a portion of the peak load. In this study, a thermo-eco-nomical study has been done; varying the amount of cooling powersupplied by the short term TES from 10% to 50% of the cooling peakload. For the comparative study on TES sizing for the three men-tioned cases, a 10,000 m2 commercial greenhouse has beenconsidered.

2.2. Thermoeconomical feasibility study

Using the results from the energy analysis presenting above, afeasibility study has been carried out in order to compare the ther-moeconomical performance of utilizing the BTES with and withoutconsidering the SCW and PCM as the short term TES system in theclosed greenhouse concept. This assessment consists of three ma-jor modules which are:

Fig. 2. Heating and Cooling load profiles for t


(1) Sizing the heat exchanger (FiWihex) in order to meet thethermal power demand.

(2) Assessing the long and short term storage volume and thetanks size due to the required energy capacity.

(3) Economic feasibility and cost performance comparison forseveral storage design strategies.

In this study, the Pay Back Period (PBP) method has been con-sidered in order to assess the cost feasibility of each case. A simplePBP is defined by Eq. (1):

PBP ¼ ðtotal investmentÞ=ðTotal annual savingÞ ð1Þ

In this method, the total annual saving includes the net savingsfrom replacing of fuel oil by electricity. The average annual rise inelectricity and fuel oil price has been considered in the cost analy-sis in order to render it more realistic. In addition, a sensitivityanalysis has been done for both electricity price and oil price vari-ations in order to obtain more accurate annual cost and PBPassessment.

A sensitivity analysis has been carried out to also assess the ef-fect of controlled ventilation in the closed greenhouse (semi-closedgreenhouse concept) on the thermoeconomical performance. Thissensitivity analysis includes a study on the following keyparameters:

� Peak load.� Mean temperature difference for heat exchanger design

(Dtmean).� PCM latent heat.� PCM price.� BTES investment cost.

In the semi-closed greenhouse concept some controlled ventila-tion has been used (e.g. via the ventilation windows) primarily inorder to cover some of the cooling demand. Higher controlledventilation can cover a larger portion of the cooling and dehumid-ification demand but on the other hand it leads to the loss of a con-siderable amount of excess heat which could otherwise be stored.The heating demand will also be increased based on the ventilationratio. In the ideal closed greenhouse (without any controlled ven-tilation) the peak heating load is 59 W m�2 while, by consideringsome controlled ventilation the peak heating demand during thewinter time will increase to 81 W m�2. In this study, it has been as-sumed that the BTES will always supply the heating load in allthree cases (with and without considering PCM and SCW). There-fore, by increasing the peak heating load the percentage of maxi-mum cooling load covered by the short-term (daily) thermalstorage system will be reduced. However, the size of the long termseasonal storage must be increased.

In order to do the sensitivity analysis for the mentioned keyparameters, it has been assumed that 30% of the peak load is cov-ered by short-term (daily) TES. The sensitivity analysis results for

he winter and summer chosen peak day.





the PBP have been chosen to be presented in this paper since it isinfluenced by both the investment cost and the annual operatingcost.

2.3. Heat exchanger design

As previously mentioned the FiWihex has been selected in orderto control the greenhouse indoor climate via heat transfer in termsof latent and sensible heat using a heat source/sink which is theTES system in this study [16]. The FiWihex is known as a countercurrent water-to-air heat exchanger [16]. The FiWihex sizing ishighly dependent on the operating temperatures which are givenby the greenhouse climate condition, as well as the storage system.As a rule of thumb, the temperature inside the greenhouse shouldnot be higher than 30 �C or lower than 18 �C. Although thedesirable temperature will vary e.g. according to the crop [15], hereit has been assumed that the desirable temperature is 20 �C. How-ever, the set-point temperatures are here taken as 18 �C and 25 �Cfor the heating and cooling system, respectively. Therefore the air-side temperature of the FiWihex is quite limited and can be consid-ered as a fixed temperature. On the water-side, the temperaturesare defined based on the storage temperatures. In this study thePCM melting temperature has been considered to be the dominantdesign temperature. The other design parameters for the FiWihexhave been summarized into Table 1. These parameters have beencalculated based on the optimal design conditions for the FiWihex[16]. The optimal air speed for the FiWihex has been calculated tobe 7.45 ms�1 where a cross flow fan has been used with a rotordiameter of 150 mm and 1264 mm length [16]. Since the FiWihexis considered being a counter-current type, it will be characterizedusing the number of transfer units (NTU). The NTU has been de-fined as the ratio of the air-side temperature reduction dividedby the mean temperature difference (Dtmean) between the air andwater [16].

NTU ¼ 2Tair;in � Tair;out

Tair;in � Tair;out þ Tair;in � Tair;outð2Þ

By applying the heat balance over the heat exchanger (Eq. (3))and combining it with the definition of the St (Stanton) (Eq. (4)),NTU can be expressed in another form which is presented in Eq.(5).

h � AAc

� �� Tair;out � Twater;in þ Tair;in � Tair;out

2

¼ Cp � q � ½Tair;in � Tair;out� ð3Þ

St ¼ h=½Cp � m� ð4Þ

NTU ¼ ðA=AcÞSt ð5Þ

Ac is the frontal surface. Therefore, the cooling power can be definedas in Eq. (6).

Table 1The FiWihex design parameters.

Parameter Unit Heating Cooling

BTES BTES + STES BTES BTES + STES

NTU N.D 0.16 0.16 0.83 1.5q Kg m�3 1.20 1.20 1.17 1.17DH kJ kg�1 16.66 16.66 41.54 41.54Capacity kW 28.3 28.3 68.6 68.6Nr FiWihex Nr.

per10,000 m2 21 21

17 17A m2 1.5 1.5 7.9 14DP kPa 5.3 5.3 27 49


Pcool ¼ DH � qmAc ð6Þ

with DH being the enthalpy change of air over the heat exchanger.

2.4. Seasonal and short term TES design and conditions

The schematic of the studied cases in this assessment have beendemonstrated in Fig. 3.

In layout (a), the short-term daily TES was not considered andthe BTES covers all of the cooling demand; however, in layout(b), the short-term daily TES (PCM or SCW) will supply 10–50%of the cooling demand, to cover some of the peak demand fromthe BTES.

With a system designed to meet the peak cooling demand, theheating demand can be fully covered by the seasonal storage system(layout c) therefore the short-term daily storage is not needed forheating purpose.

A BTES primarily stores low temperature heat such that aheat pump is needed to supply the heat at a suitable tempera-ture [18]. In this study, a heat pump with COP 5 has been as-sumed. However, from the sensitivity analysis results it wasfound that the COP does not have a major impact on the pay-back period and the assumed constant COP value will be suffi-ciently accurate for this qualitative cost analysis. The depth ofeach borehole has been assumed to be 200 m and also the ther-mal conductivity k of the rock has been assumed to be3.0 W m�1 K�1, which is considered the same thermal conductiv-ity of granite [19]. The achievable power of the borehole can beestimated by 25 � k W m�1 for the heating purpose, while theachievable power for the cooling purpose is strongly dependenton the input and output temperatures [19]. In this study, anaverage achievable borehole power of about 40 W m�1 in bothof heating and cooling demand is assumed [20].

For the proper sizing of PCM and the storage tank, some designcriteria needs to be considered. One of the main design criteria isworking temperatures defining the type of PCM. Here based onthe given temperatures, a commercial salt hydrate PCM (S19) hasbeen considered. The properties of this PCM are presented inTable 2.

The size of the PCM storage tank can be determined either byvolumetric heat capacity of the PCM or the PCM extractionpower per unit volume of the storage, whichever will requirethe largest volume. The average thermal extraction power capac-ity is considered to be 28.5 kW m�3, which has been calculatedbased on the prototype studied by Chiu et al. [22]. The storagecapacity is there assumed to be 68 kW h m�3 and a 95% packingfactor (percent PCM in the storage, by volume) is obtained inthis prototype [21,22]. According to the extraction power, stor-age capacity and the packing factor the required storage volumehas been calculated for fraction of the cooling power coveredand the energy needed to be stored, considering 10–50% peakshaving. The results are presented in Table 3. It can be concludedthat the volumetric heat capacity is the dominating designparameter in this study.

A short term SCW TES is the alternative to PCM. The SCW isone of the most common storage systems for the cooling of com-mercial buildings [23]. In this system, a shell and tube heat ex-changer with a shell and two tubes has been chosen for SCWstorage. According to the temperatures in the layout (b) inFig. 4, the corrected LMTD is calculated as 4.68 �C based onthe correction factor for the mentioned shell and tube configura-tion [24]. The volume size of the water tank has been calculatedbased on the obtained corrected LMTD. To represent a SCW inthis study, data for a commercial 5000 L storage tank has beenchosen [25].




Fig. 3. TES system layout for cooling (a,b) and heating (c) demand in the closed greenhouse concept.

Table 2The thermal properties of chosen PCM (S19- a commercial salt hydrate PCM) [21].

Phase change temperature (�C) Density (kg m�3) Latent heat capacity (kJ kg�1) Volumetric heat capacity (MJm�3) Thermal conductivity (Wm�1 K�1)

19 1520 160 243 0.43

Table 3PCM sizing based on the PCM extraction power and volumetric heat capacity for various peak shaving.

Percentage of maximum load covered by short-term daily TES (%) 10% 20% 30% 40% 50%

Energy needs to be stored (W h m�2) 1296 1296 1089 928 743Power needs to be covered (Wm�2) 54 48 37 26 15Volume requirement based on the storage capacity (m3 m�2) 1.9E�2 1.9E�2 1.6E�2 1.4E�2 1.1E�2

Volume requirement based on the extraction Power (m3 m�2) 1.9E�3 1.7E�3 1.3E�3 1E�3 0.5E�3

Fig. 4. Sensitivity analysis for payback period time in case 1 where the total heatingand cooling demand is supported by BTES as seasonal thermal storage.


2.5. Thermoeconomical assumptions

In terms of the thermoeconomical feasibility assessment, theconsidered costs include the capital investment cost of heatexchangers, ventilation system (fan), BTES, heat pumps, PCM, stor-age tanks, installation, as well as maintenance and operation costs.Since the closed greenhouse has been designed to replace the fossilfuels with the renewable energy to supply the energy demand, themajor operation cost, the fossil fuel cost, will be shifted to the cost


of electricity for running the heat pumps and other electrically dri-ven devices.

Since a commercial PCM has been chosen in this study, the priceof the PCM has been assumed based on the current market pricewhich is 2 €/kg [21]. Both of the PCM material’s price and itsthermal properties, which determine the TES sizing, could haveconsiderable impact on the PBP. Therefore, the sensitivity analysisgives an indication on the results’ dependency on the assumptionsabove. The price of the storage tank for the selected storage type istaken as 458 €/m3 [25]. The electricity price has been assumed to be0.1 €/kW h [26] with an average annual rise equal to 0.7 cent/kW h[1], while an amount of 26 €/m2 [27] has been considered for thefuel oil cost with a 5.5 cent/L as an average annual rise [28]. In addi-tion, the annual fuel oil consumption for a typical commercialgreenhouse in Sweden has been taken as 435 m3 in one hectaregreenhouse [29].

3. Results and discussion

Three cases of a commercial closed greenhouse with integratedTES have been studied here, with and without the short-term dailyTES integrated with the seasonal BTES. According to the results, itcan be concluded that 141 boreholes are needed in case that thepeak load will be handled by the BTES system only. Table 4 showsa comparison of design parameters for BTES as the seasonal TES incombination with the PCM or SCW.

From the thermoeconomical analysis, the PBP has been ana-lysed for all cases and the results show that it is about 7 years incase 1 where there is not any short term storage. However, the




Table 4a comparison for design parameters for BTES integrated with PCM and SCW for 10,000 m2 closed greenhouse.

Percentage of peak load covered by short-term TES (%) Corresponding cooling capacity of BTES (W h m�2) Num. of borehole Weight of required PCM (kg m�2)

50 1300 74 2940 1300 82 2930 1090 95 2520 930 108 2110 740 122 17

Fig. 6. Sensitivity analysis for payback period time in case 1 where 30% of peakcooling demand is covered by SCW storage as short term storage and the remainingthermal demand (including the heating demand totally) is supported by BTES asseasonal thermal storage.


PBP can reduced to 5 years by using SCW storage as the short termstorage in order to cover 50% of the peak cooling demand whileintegrated with the BTES. In addition, the PCM may not be a verycost effective short term storage system in this concept due to itshigh investment and annual costs. The high annual cost of thePCM case comes from the increased electricity cost due to theoperation of the fan. Although the electricity cost connected tooperating the heat pump will be reduced by considering theshort-term daily TES system, the ventilation (fan) operation costbecomes higher. The ventilation cost is directly dependent on thedesigned FiWihex condition. The FiWihex design performance var-ies regardless of the short-term daily TES, and is only based on theinlet and outlet temperature. Therefore, the operating cost for thecombined system BTES plus short-term TES is higher, as comparedto BTES only because of the different operating conditions for theFiWihex design. Naturally, the desirable set point temperature in-side the greenhouse will vary with crop and other specificconditions in each application [15]. However in this study, theminimum and maximum allowable operating temperature hasbeen set to 18 �C and 25 �C for heating and cooling, respectively.These set-points are quite reasonable operating temperatures fora wide range of crops [15].

The total investment cost for considering seasonal TES incombination with the SCW could be considered as the most eco-nomical method when compared with considering BTES or, BTESin combination with PCM. The total investment for the BTES incombination with PCM is higher than for the SCW, because ofthe PCM material cost. It has to be noted also that space limita-tion has not been considered in this study even though it mayaffect the investment issue since larger water storage tankswould be required as compared to the PCM storage option.

The impact of various key parameters, which have been men-tioned earlier in Section 3, has been studied using a sensitivityanalysis. The results, presented in Figs. 4–6, show that the peak

Fig. 5. Sensitivity analysis for payback period time in case 1 where 30% of peakcooling demand is covered by PCM storage as short term storage and the remainingthermal demand (including the heating demand totally) is supported by BTES asseasonal thermal storage.


load is the most decisive parameter in all three cases (BTES,BTES + PCM and BTES + SCW) which means that by reducingthe peak load by 50% the PBP could be almost reduced to thehalf. The reduction in the peak load leads to the reducing thenumber of required borehole which cut down the investmentcosts. The BTES investment cost turns out to be the second mostsignificant parameter which affects the PBP. The PBP can be re-duced 15–20% by cutting the BTES investment cost into halfsince it has a considerable impact on the total investment cost.Although the Dtmean for heat transfer has a considerable impacton the annual operation cost because of the operating cost of thefan, the results show that it does not have any significant effecton the investment cost and PBP. The PCM latent heat and thePCM price are two other parameters which have fairly highimpact on the investment costs, annual cost and PBP. The PBPcan be reduced 12% by considering a PCM material with1 € kg�1.Also, by considering a PCM with the same meltingtemperature (17 �C) but with double latent heat (320 kJ kg�1)the PBP will be reduced by 24%.

4. Conclusions

Here, a qualitative analysis has been carried out on integrating aclosed greenhouse with a TES system. The results point towardsthe feasibility of the closed greenhouse concept to fulfil the sys-tem’s seasonal heating and cooling load. It can be concluded alsothat the closed greenhouse concept has the potential of becomingcost effective. The cost analysis presented herein shows that themajor investment for the closed greenhouse concept could bepaid-off within 7 years based on the annual savings on auxiliaryfossil fuel, using only the seasonal TES to cover the closed green-house energy demand. However, the payback time may be reducedto 5 years with a short-term SCW TES integrated to cover 50% ofthe maximum cooling demand while the rest will be supplied by





the BTES. According to the cost analysis in terms of total invest-ment cost, the SCW could be an economical option in comparisonto the PCM as the short-term daily TES.

In the case that the seasonal TES will cover the whole energydemand, the operation and maintenance cost is smaller than whenusing intermediate short term storage. However, the total invest-ment cost is the smallest for the case considering the SCW as theshort-term daily TES.

A sensitivity analysis was carried out on the system key designparameters. The effects of peak load, Dtmean for the FiWihex heatexchanger, the borehole investment cost, PCM material cost andthe PCM latent heat on the payback period time as well as invest-ment and operation and maintenance costs have been studied inthis analysis. It can be concluded from the obtained results thatthe peak load is the main influencing parameter on the PBP as wellas investment cost and annual operation cost. To sum-up, the totalinvestment cost highly depends on the peak load, number ofboreholes and also PCM material cost, while the annual saving isstrongly dependent on the FiWihex performance and operatingcost which is effected by Dtmean.

Acknowledgements

The authors would like to acknowledge the Stiftelsen Lant-bruksforskning for providing funding to this research work andalso the Polygeneration as a part of Explore energy operated bythe KTH. Special acknowledgement goes to the reference groupconsisting of Slottsträdgården Ulriksdal, SLU, Svegro, GustafslundHandelsträdgård.

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http://www.zonneterp.nl/english/flyer_greenhouse_village.pdf

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Thermal energy storage strategies for effective closed greenhouse design