- Email: [email protected]
Comparative study of internal storage and external storage absorption cooling systems
Renewable Energy 36 (2011) 1645e1651
Contents lists available at ScienceDirect
Renewable Energy journal homepage: www.elsevier.com/locate/renene
Technical Note
Comparative study of internal storage and external storage absorption cooling systems S. Soutullo*, C. San Juan, M.R. Heras Department of Energy, Energy Efficiency in Buildings Unit, CIEMAT, Madrid E-28040, Spain
a r t i c l e i n f o
a b s t r a c t
Article history: Received 22 March 2010 Accepted 11 November 2010 Available online 9 December 2010
This work presents a comparative study of the performance of absorption cooling systems with internal storage and with external storage. A full dynamic simulation model including the solar collector field, the absorption heat pump system and the building loads has been performed. The first system is composed by four heat pumps that store energy in the form of crystallized salts so that no external storage capacity is required. The second one is a conventional system composed of one liquid absorption pump and external storage in a water tank. Many batteries of simulations have been done to evaluate the performance of these cooling machines when varying solar field surface, solar collector’s efficiency curve and the storage capacity of the systems. Two different indices have been calculated to analyze the response of both systems: Solar Fraction and Primary Energy Ratio. The comparison between both absorption chillers indicates that in order to reach similar values of storage energy, conventional system has a greater room requirement than four units with internal storage working in parallel, requiring an external water tank of at least 15 m3. Ó 2010 Elsevier Ltd. All rights reserved.
Keywords: Solar thermal applications Solar cooling Absorption Storage
1. Introduction The combined use of solar energies with absorption pumps is a renewable way to supply the cooling loads demanded by the building during the warming periods. In this context, the Spanish Project ARFRISOL (Bioclimatic Architecture and Solar Cooling, in Spanish) [1] aims to demonstrate that it is possible to achieve high levels of energy savings by means of bioclimatic design and the use of renewable energies, in five offices buildings. In order to analyze the energy performance of different absorption chillers, two solar cooling systems have been evaluated under different conditions of the solar field surface, solar collector’s efficiency curve and the storage capacity of the systems. The first system analyzed is composed by four heat pump units that store energy in the form of crystallized salts, Model CW10 developed by Climatewell [2]. The second system is a conventional chiller composed of one-liquid absorption heat pump, Model SC20 developed by Yazaki [3], and external storage in a water tank. With this purpose, two dynamic simulation models, which couple the solar cooling system with the building hourly demands, have been created. The results presented
* Corresponding author. E-mail address: [email protected] (S. Soutullo). 0960-1481/$ e see front matter Ó 2010 Elsevier Ltd. All rights reserved. doi:10.1016/j.renene.2010.11.017
in this article are based on the simulation work performed to analyse the behaviour of both systems and the comparison between them. When trying to make a review on studies about solar absorption systems, it is remarkable that most of the information is related to liquid absorption systems with exterior storage. Many authors have worked on the performance of liquid absorption chillers. There are many experimental and theoretical studies about the efficiency of the solar collectors and about the performance of the absorption unit, e.g. Izquierdo Millán et al. [4]. Studies on the overall efficiency of the system composed of solar collector field and absorption machine have also been published, e.g. Mittal et al. [5] and [6]. All of this combined with works that analyse the influence of the temperature levels and flow rates in the performance of the absorption units, give some initial ideas about the size of the system. The main disadvantage of these analyses is that they resolve steady state energy balances in the machines for nominal conditions. Only few authors have modelized the performance of the whole system considering the coupling between solar thermal systems, absorption unit and building loads. In them, a dynamic simulation program has been used in order to evaluate the transient behaviour of the system [7] and [8]. The results show that the efficiency of the solar cooling systems depends not only on the COP of the chillers, but also on the efficiency of the solar collector field, the storage system capacity, the losses at the distribution system and the profiles of the cooling demand. The performance of the
1646
S. Soutullo et al. / Renewable Energy 36 (2011) 1645e1651
This is one of the five “Office Buildings Prototypes for Research and Demonstration” constructed within the Singular Strategic Project ARFRISOL. The PSA building has been projected following the principles of Bioclimatic Architecture and has a total built area of 1114 m2 but only a surface of 860 m2 is conditioned. The annual profile of the building loads shows maximum values of 80 W/m2 in both heating and cooling periods. The reason of the year stability of the loads is mainly derived of the highly inertial building design and the bioclimatic strategies.
Nomenclature SF Solar Fraction Qcooling,Abs Cooling energy provided by the absorption machine, KWh Qcooling,tot Total cooling energy demanded by the building, KWh PER Primary energy ratio Eelec,tot Total electrical energy, KWh 3elec Primary energy conversion factor for electricity
3. Simulation models
system is thus highly dependant on the coupling between all these components, so it is very important to have detailed knowledge of the overall operation of the system during the whole year in order to optimize the solar contribution [9]. IEA Task 25 “Solar Assisted Air Conditioning of Buildings” [10] published results of several case studies of hotels and office buildings in different cities of Europe. In this context, Henning et al. have developed a decision scheme [11] and a simulation tool that performed system simulation for solar assisted air conditioning systems [12]. The document “Design of solar assisted air conditioning systems (2): selection of collector and buffer storage” [13] establishes a relationship between the size (m2) of the collector field, size (m2) of conditioning building and the storage capacity of the system, and gives an expected value of the solar contribution. More recently, under the umbrella of the IEA, Task 38 [14] has developed a unified procedure and performance assessment for solar assisted heating and cooling systems [15]. This article reports how dynamical simulation can be used to evaluate and compare the overall energy performance between a conventional chiller and a new absorption machine. Almost the totally of previous works were based on liquid absorption chillers that use H2O as refrigerant and LiBr as absorbent. The innovation of this work is that an alternative system, based on the several absorption units that store energy in salts, has been proposed and modelled. 2. Case study The case of study of this article is an office building recently constructed and based in the Plataforma Solar de Almeria (PSA).
TRNSYS [16] has been used as a dynamic simulation program in order to obtain the performance of each cooling system analyzed: absorption pumps with interior storage in the form of salts and a single-effect chiller with exterior storage tank. 3.1. Building loads precalculation The model created for the PSA building includes all passive techniques implemented as: thermal mass walls, ventilation, shading strategies, ventilation through solar chimneys and nocturnal radiative cooling, as well as detailed internal gains [17]. With all of these variables, the building cooling loads have been precalculated using a weather file created from long series of measured data in the Plataforma Solar de Almeria [18]. The integration of the calculated loads into the hourly transient model has been done using a model of a fluid pipe that allows to injecting heat into the fluid flow when cooling the building. 3.2. Modelization of the absorption system with internal storage The solar absorption system with internal storage is composed of four units CW10, and each unit has two barrels with a cooling power of 10 kW and a storage capacity of 30 kW h each. Each barrel has two separate bowls, the one is filled with salts (reactor) and the other is filled with water (evaporator). The salt used for the process is Lithium Chloride e LiCl. During the discharging process, the bowl containing the salts absorbs the water from the evaporator until it can not absorb more water. In the charging process the salt in the bowl is dried with the heat coming from the solar collector field, and the water returns to the evaporator. Each barrel can work whether charging heat from the solar system to the salt, or
Climatewell CW10 unit 4
20 18 16
Barrel Level
14 12 10
2
8 6 1
4 2
0 5400
5420 Mode1
5440
5460 Mode2
5480
0 5520
5500
Barrel1
Fig. 1. Schema of the performance of the Climatewell CW10 unit.
Barrel2
Storage Capacity (kWh)
3
S. Soutullo et al. / Renewable Energy 36 (2011) 1645e1651
1647
Fig. 2. Configuration of the absorption system with internal storage.
Fig. 3. Configuration of the absorption system with internal storage in TRNSYS.
discharging cooling energy to the building. When both barrels discharge simultaneously, CW10 can deliver a peak cooling power of 20 kW. Fig. 1 shows a schema of the charging and discharging process of the Climatewell CW10 unit. The modelization of the absorption heat pump CW10 has been based on experimental curves of efficiency provided by the manufacturer (Climatewell). A new simulation model in TRNSYS, that characterize each barrel of the absorption unit, has been created [19]. The basis of the performance of each barrel is the same as a storage tank and has a maximum storage capacity of 30 kWh which it is measured with the storage level. The model contains an internal variable that represents the energy level (accumulated in crystallized salts) which is stored at each time step in charging process until it reaches the maximum capacity. In the same way, during the discharge process, this variable represents the descending level until it reaches the minimum capacity. To optimize the performance of a system composed by any number of units, a model that rules the charging and discharging sequences of the barrels has been developed. This model identifies the status of each barrel. Each time step, depending on the energy
production on the collector field, the energy demand of the building and the energy level of each barrel, the model decides the sequences of charging and discharging. Fig. 2 shows the configuration of the whole system, including all of the components. Fig. 3 shows the modelling of the system in TRNSYS. 3.3. Modelization of the absorption system with external storage The solar absorption system with external storage is composed of one single-effect absorption chiller (Model WFC-SC20 Yazaki) with a cooling power of 80 kW, and an external storage in a water tank. The WFC-SC Yazaki machine uses in the refrigerant cycle, a solution of lithium bromide (absorber) and water (refrigerant). The heat which comes from the solar field liberates the refrigerant from the solution and produces a refrigerant effect in the evaporator when the water circulated through the condenser and the absorber. A simulation model of the cooling system has been created in TRNSYS. With this purpose, types of the standard and TESS library have been used. The Yazaki absorption pump has been modelled using the single-effect hot water-fired absorption chiller. This
Fig. 4. Configuration of the absorption system with external storage.
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S. Soutullo et al. / Renewable Energy 36 (2011) 1645e1651
Fig. 5. Configuration of the absorption system with external storage in TRNSYS.
model makes use of a normalized catalogue data based on the efficiency curves of the absorption pump, in order to predict the performance of the chiller. The thermal performance of external storage has been modelled with a standard type of TRNSYS, which calculates the energy balances of the tank (TRNSYS). The performance of the system has been done through the control of the pumps, in order to maximize the solar fraction produce by the chiller directly fed by the solar field or by the external storage tank. The regulation has been done taking into account the cooling loads demanded by the building, the energy coming from the solar collector field and the storage tank. Fig. 4 shows the configuration of the whole system, including all of the components. Fig. 5 shows the modelling of the system in TRNSYS. 4. Analysis Parametrical simulations have been performed to analyse the response of both solar cooling systems when varying the following parameters: - Total surface of solar collector field. - Efficiency curve of the each collector studied. - Exterior storage capacity. To examine the performance of the systems, to main indexes have been calculated in each case: solar fraction (SF) and the primary energy ratio (PER). The solar fraction (SF) is the ratio between the cooling energy delivered by the absorption pumps and the total cooling energy demanded by the building (Eq. (1)). The primary energy ratio (PER) of the solar assisted cooling system is the ratio between the cooling energy provided by the absorption pumps and the total electricity consumed by the system (Eq. (2)).
Qcooling;Abs SF ¼ Qcooling;tot PER ¼
Qcooling;Abs Eelec;tot
(1)
(2)
3elec
The cooling and electricity fluxes comes from the simulation results while the primary energy conversion factor for electricity
has been set with the following value 3elec ¼ 0.4, being based on European Directives, Task 25 and Task 32. Table 1 shows the range of values of each parameter simulated. As it can be seen, three different collectors have been modelized in order to analyze its influence on the SF and PER. For each efficiency curve of the collector, the total surface of the solar field has been varied. These simulations have been carried out for the two analyzed absorption systems. In the case of external storage, different storage capacities of the external water tank have been analysed. 5. Results and discussion Figs. 6e8 show the solar fraction for the cooling demand obtained when using an absorption chiller with external storage. For these simulations, the solar collector field surface, the type of the collector and the capacity of the external tank have been varied. Graphs are related to the efficiency of the following collectors: flat plate collectors (FPC), high efficiency flat plate collectors (HEFPC), and evacuated tube collectors (ETC), respectively. In each image, several storage volumes are presented by different curves in order to obtain the solar fraction produced with increasing of the solar field surface. Data have been fitted to second degree polynomial curves by regression, obtaining in all cases, high correlations (higher than 80%). Data agree with the law of diminishing marginal returns, which states that as the storage volume increases, its improvement effect on the solar contribution is less than the previous storage volume. Somehow storage volumes from 15 m3 to 30 m3 produce similar solar fractions. Same effects occur when increasing the solar collector field surface. The influence of the type of collectors can also be examined: The solar fraction reached by the FPC collector,
Table 1 Range of the parametrical runs of: the surface of solar collector field, the efficiency curves of the collectors and the exterior storage capacity. Solar collector field (m2): Solar collector efficiency curve: Flat plate collector (FPC) High efficiency flat plate collector (HEFPC) Evacuated tube collector (ETC) 3
Storage capacity (m ):
88e250 m2
ho ¼ 0.709, a1 ¼ 5.562 W/mK ho ¼ 0.744, a1 ¼ 3.508 W/mK ho ¼ 0.775, a1 ¼ 1.476 W/mK, a2 ¼ 0.0075 W/mK 1e30 m3
Solar fraction as a function of solar field surface (FPC) 100
2
St 30 m3 R = 0.999 2
90
St 25 m3 R = 0.997
Solar Fraction (%)
St 20m3 R2 = 0.998 2
80 70
St 15 m3 R = 0.981 2 St 10 m3 R = 0.996
St 5 m3 R2 = 0.980 2
St 1 m3 R = 0.992
60 50 40 120
140
160
180
200
220
240
Solar collector field surface (m 2 ) Storage 1 m3 Storage 20 m3
Storage 5 m3 Storage 25 m3
Storage 10 m3 Storage 30 m3
Storage 15 m3
Fig. 6. Solar fraction obtained as a function of solar collector field surface FPC.
Solar fraction as a function of solar field surface (HEFPC) 110
Solar Fraction (%)
100 90 St 1 m3 R2 = 0.996
80
St 5 m3 R2 = 0.999 St 10 m3 R2 = 0.997
70
2 St 15 m3 R = 0.996
St 20 m3 R2 = 0.989
60
St 25 m3 R2 = 0.986 2
St 30 m3 R = 0.994
50 120
140
160
180
200
220
240
2
Solar collector field surface (m ) Storage 1 m3 Storage 20 m3
Storage 5 m3 Storage 25 m3
Storage 10 m3 Storage 30 m3
Storage 15 m3
Fig. 7. Solar fraction obtained as a function of solar collector field surface HEFPC.
Solar fraction as a function of solar field surface (ETC) 105
Solar Fraction (%)
100 95 90
St 1 m3 R2 = 0.959 St 5 m3 R2 = 0.999
85
St 10 m3 R2 = 0.996 St 15 m3 R2 = 0.999
80
2
St 20 m3 R = 0.997 St 25 m3 R2 = 0.928
75
St 30 m3 R2 = 0.833
70 120
140
160
180
200
220
240
Solar collector field surface (m 2 ) Storage 1 m3 Storage 20 m3
Storage 5 m3 Storage 25 m3
Storage 10 m3 Storage 30 m3
Storage 15 m3
Fig. 8. Solar fraction obtained as a function of solar collector field surface ETC.
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S. Soutullo et al. / Renewable Energy 36 (2011) 1645e1651
Solar Fraction for internal and exterior storage
Solar Fraction (%)
100
80
60
40
20 80
100
120
140
160
180
200
220
240
Solar collector field surface (m 2 ) FPC storage 1m3 HEFPC storage 1m3 ETC storage 1m3 FPC Interior Storage
FPC storage 15m3 HEFPC storage 15m3 ETC storage 15 m3 HEFPC Interior Storage
FPC storage 30m3 HEFPC storage 30m3 ETC storage 30 m3 ETC Interior Storage
Fig. 9. Solar Fraction achieved with interior and exterior storage for three types of collectors.
which varies between 49 and 89%. The solar fraction reached by the HEFPC collector varies between 67 and 100%, and between 84 and 100% by ETC. Fig. 9 compares the solar fraction obtained for the case of 4 units of CW10 working in parallel, and the absorption system with external storage in a water tank. Only data for 1 m3, 15 m3 and 30 m3 storage volumes curves are presented, taking into account that the data for the rest of the storage volumes lay within these three limits. In these simulations, the solar collector field surface and the type of the collector (flat plate collectors, high efficiency flat plate collectors and evacuated tube collectors) have been varied. The solar fraction reached by the FPC collector varies, with the increasing of the solar collector field, from 40% to 84%. For the HEFPC collector, the solar fraction obtained varies from 57% to 95%. For the ETC collector, the solar fraction achieved varies between 74 and 100%. The system with internal storage reaches its asymptotical limit before the system with external storage. In the case of external storage, the different cases show that the influence of the storage
volume which is almost imperceptible for small collector field increases with the size of the solar collector field (especially with FPC and HEFPC collectors). The comparison between these curves shows that the SC20 Yazaki with an external storage tank of 15 m3 reaches values similar to those obtained by the set of 4 CW10 working in parallel. Fig. 10 shows the values of PER for the case of 4 units of CW10 working in parallel, and the absorption system with external storage in a water tank. Only data for 1 m3, 15 m3 and 30 m3 storage volumes curves are presented. As it can be seen in Fig. 10, PER follow similar tendencies as SF. The maximum value of PER is obtained when the system is capable to provide all the cooling demanded by the building. This value is 1.3. Finally, data relating room surface needed by the different systems analysed is presented in Fig. 11. The room surface has been obtained summing the dimensions of all the components: 4 CW10 or 1 Yazaki SC20 with external storage tank (Table 2). Despite the surface required by 4 CW10 machines is greater than 1 SC20 Yazaki machine; the second needs an external tank
PER for internal and exterior storage 1.4 1.2
PER ()
1.0 0.8 0.6 0.4 0.2 0.0 80
100
120
140
160
180
200
220
240
Solar collector field surface (m 2 ) FPC storage 1m3 HEFPC storage 1m3 ETC storage 1m3 FPC Interior Storage
FPC storage 15m3 HEFPC storage 15m3 ETC storage 15 m3 HEFPC Interior Storage
FPC storage 30m3 HEFPC storage 30m3 ETC storage 30 m3 ETC Interior Storage
Fig. 10. Primary Energy Ratio achieved with interior and exterior storage for three types of collectors.
S. Soutullo et al. / Renewable Energy 36 (2011) 1645e1651
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Room requirement for each absorption system 24
Room Surface (m2)
20
16
12
8
4
0 Yz+1m3
Yz+5m3
Yz+10m3
Yz+15m3
Yz+20m3
Yz+25m3
Yz+30m3
4CW10
Absoption system 2
Fig. 11. Total room (m ) required for absorption systems with internal and external storage.
Table 2 Relating room surface needed by the two different systems analysed: 4 CW10 and SC20 Yazaki with external storage. Model
Width (m)
Depth (m)
Height (m)
Yazaki WFC-SC20 CW10
1.06 1.38
1.3 0.7
2.03 1.85
to store energy that comes from the solar field. As it can be seen in Fig. 11, only the case of 1 Yazaki with an external tank of 1 m3, has less surface requirements than in the case of 4 CW10. For same SF and PER the absorption system with internal storage has less room requirement. 6. Conclusions This paper aims to shed light on absorption cooling systems with internal storage (CW10). With this purpose they have been compared to a conventional system composed of an absorptioncooling machine and exterior storage tank. Parametrical simulations varying the solar collector surface, the type of collector and the storage tank capacity (in the case of exterior storage) have been performed. The comparison between both systems indicates that similar solar fractions can be achieved in both systems. The big differences lay on the additional storage (and the corresponding room surface) that is needed in the case of conventional storage. While the system based on CW10 units does not require exterior storage, the conventional system needs an exterior storage of 15 m3 to reach similar solar fractions. The primary energy ratio (PER) of the cooling system reaches a maximum value of 1.3 when the system is capable to provide the 100% of the cooling demanded by the building.
Plan of Research and Develop 2004e2007, cofinanced with FEDER Founds and supported by the Spanish Ministry of Innovation and Science. The authors would like to thank all the companies and Institutions included in PSE-ARFRISOL project.
References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10]
[11]
[12] [13]
[14] [15]
[16] [17]
Acknowledgements
[18]
The PSE-ARFRISOL, Reference PS-120000-2005-1, is an Strategic Singular Scientific-Technological Project accepted by the National
[19]
Arfrisol. www.climatewell.com Climatewell. www.climatewell.com Manual Yazaki; 2009. Izquierdo Millán M, Hernández F, Martín E. Solar cooling in Madrid: energetic efficiencies. Solar Energy 1997;60:367e77. Mittal V, Kasana KS, Thakur NS. The study of solar absorption air-conditionig systems. Journal of Energy in Southern Africa 2005;16(No. 4). Mittal V, Kasana KS, Thakur NS. Modelling and simulation of a solar absorption cooling system for India. Journal of Energy in Southern Africa 2006;17(No. 3). Wilbur PJ, Mitchell CE. Solar absorption air conditioning alternatives. Solar Energy 1975;17:193e9. Eicker U, Pietruschka D. Design and performance of solar powered absorption cooling systems in office buildings. Energy and Buildings 2009;4:81e91. Letz T. Validation and Background information, the FSC procedure. A technical report of subtask A. IEA e SHC Task 26; 2002. IEA Solar Heating and Cooling, Task 25: Solar-assisted air-conditioning of buildings. SOLAC Computer Design Tool: http://www.iea-shc-task25.org/ english/hps6/index.html Henning HM, Albers J. Decision scheme for the selection of the appropriate technology using solar thermal air-conditioning. Guideline Document. Task 25: Solar-assisted Air-conditioning of Buildings. IEA Solar Heating and Cooling; 2004. Henning HM. Solar assisted air-conditioning of buildings. A handbook for planners; 2004. IEA Solar Heating and Cooling, Task 25: Solar-assisted air-conditioning of buildings. Design of solar assisted air conditioning systems (2): selection of collector and buffer storage. IEA Task 38. Solar air-conditioning and Refrigeration. Website: http://www. iea-shc.org/task38/index.html Sparber W, Thuer A, Besana F, Streicher W, Henning HM. Unified monitoring procedure and performance assesment for solar assisted heating and cooling systems. Lisbon, Portugal: Eurosun; 2008. Transient Systems Simulation program (TRNSYS). http://sel.me.wisc.edu/trnsys/ SEL, TRNSSOLAR, CSTB, Tess, Multizone Building modeling with Type56 and TRNBuild. TRNSYS 16 6 (2004). Zarzalejo LF, Téllez FM, Palomo E, Heras MR. Creation of typical meteorological years (TMY) for Southern Spanish cities. Athens, Greece: International Symposium Passive Cooling of Buildings; 1995. Sanjuan C, Soutullo S, Heras MR. Optimization of a solar cooling system with internal energy storage. Solar Energy 2010;84(7):1244e54.
Contents lists available at ScienceDirect
Renewable Energy journal homepage: www.elsevier.com/locate/renene
Technical Note
Comparative study of internal storage and external storage absorption cooling systems S. Soutullo*, C. San Juan, M.R. Heras Department of Energy, Energy Efficiency in Buildings Unit, CIEMAT, Madrid E-28040, Spain
a r t i c l e i n f o
a b s t r a c t
Article history: Received 22 March 2010 Accepted 11 November 2010 Available online 9 December 2010
This work presents a comparative study of the performance of absorption cooling systems with internal storage and with external storage. A full dynamic simulation model including the solar collector field, the absorption heat pump system and the building loads has been performed. The first system is composed by four heat pumps that store energy in the form of crystallized salts so that no external storage capacity is required. The second one is a conventional system composed of one liquid absorption pump and external storage in a water tank. Many batteries of simulations have been done to evaluate the performance of these cooling machines when varying solar field surface, solar collector’s efficiency curve and the storage capacity of the systems. Two different indices have been calculated to analyze the response of both systems: Solar Fraction and Primary Energy Ratio. The comparison between both absorption chillers indicates that in order to reach similar values of storage energy, conventional system has a greater room requirement than four units with internal storage working in parallel, requiring an external water tank of at least 15 m3. Ó 2010 Elsevier Ltd. All rights reserved.
Keywords: Solar thermal applications Solar cooling Absorption Storage
1. Introduction The combined use of solar energies with absorption pumps is a renewable way to supply the cooling loads demanded by the building during the warming periods. In this context, the Spanish Project ARFRISOL (Bioclimatic Architecture and Solar Cooling, in Spanish) [1] aims to demonstrate that it is possible to achieve high levels of energy savings by means of bioclimatic design and the use of renewable energies, in five offices buildings. In order to analyze the energy performance of different absorption chillers, two solar cooling systems have been evaluated under different conditions of the solar field surface, solar collector’s efficiency curve and the storage capacity of the systems. The first system analyzed is composed by four heat pump units that store energy in the form of crystallized salts, Model CW10 developed by Climatewell [2]. The second system is a conventional chiller composed of one-liquid absorption heat pump, Model SC20 developed by Yazaki [3], and external storage in a water tank. With this purpose, two dynamic simulation models, which couple the solar cooling system with the building hourly demands, have been created. The results presented
* Corresponding author. E-mail address: [email protected] (S. Soutullo). 0960-1481/$ e see front matter Ó 2010 Elsevier Ltd. All rights reserved. doi:10.1016/j.renene.2010.11.017
in this article are based on the simulation work performed to analyse the behaviour of both systems and the comparison between them. When trying to make a review on studies about solar absorption systems, it is remarkable that most of the information is related to liquid absorption systems with exterior storage. Many authors have worked on the performance of liquid absorption chillers. There are many experimental and theoretical studies about the efficiency of the solar collectors and about the performance of the absorption unit, e.g. Izquierdo Millán et al. [4]. Studies on the overall efficiency of the system composed of solar collector field and absorption machine have also been published, e.g. Mittal et al. [5] and [6]. All of this combined with works that analyse the influence of the temperature levels and flow rates in the performance of the absorption units, give some initial ideas about the size of the system. The main disadvantage of these analyses is that they resolve steady state energy balances in the machines for nominal conditions. Only few authors have modelized the performance of the whole system considering the coupling between solar thermal systems, absorption unit and building loads. In them, a dynamic simulation program has been used in order to evaluate the transient behaviour of the system [7] and [8]. The results show that the efficiency of the solar cooling systems depends not only on the COP of the chillers, but also on the efficiency of the solar collector field, the storage system capacity, the losses at the distribution system and the profiles of the cooling demand. The performance of the
1646
S. Soutullo et al. / Renewable Energy 36 (2011) 1645e1651
This is one of the five “Office Buildings Prototypes for Research and Demonstration” constructed within the Singular Strategic Project ARFRISOL. The PSA building has been projected following the principles of Bioclimatic Architecture and has a total built area of 1114 m2 but only a surface of 860 m2 is conditioned. The annual profile of the building loads shows maximum values of 80 W/m2 in both heating and cooling periods. The reason of the year stability of the loads is mainly derived of the highly inertial building design and the bioclimatic strategies.
Nomenclature SF Solar Fraction Qcooling,Abs Cooling energy provided by the absorption machine, KWh Qcooling,tot Total cooling energy demanded by the building, KWh PER Primary energy ratio Eelec,tot Total electrical energy, KWh 3elec Primary energy conversion factor for electricity
3. Simulation models
system is thus highly dependant on the coupling between all these components, so it is very important to have detailed knowledge of the overall operation of the system during the whole year in order to optimize the solar contribution [9]. IEA Task 25 “Solar Assisted Air Conditioning of Buildings” [10] published results of several case studies of hotels and office buildings in different cities of Europe. In this context, Henning et al. have developed a decision scheme [11] and a simulation tool that performed system simulation for solar assisted air conditioning systems [12]. The document “Design of solar assisted air conditioning systems (2): selection of collector and buffer storage” [13] establishes a relationship between the size (m2) of the collector field, size (m2) of conditioning building and the storage capacity of the system, and gives an expected value of the solar contribution. More recently, under the umbrella of the IEA, Task 38 [14] has developed a unified procedure and performance assessment for solar assisted heating and cooling systems [15]. This article reports how dynamical simulation can be used to evaluate and compare the overall energy performance between a conventional chiller and a new absorption machine. Almost the totally of previous works were based on liquid absorption chillers that use H2O as refrigerant and LiBr as absorbent. The innovation of this work is that an alternative system, based on the several absorption units that store energy in salts, has been proposed and modelled. 2. Case study The case of study of this article is an office building recently constructed and based in the Plataforma Solar de Almeria (PSA).
TRNSYS [16] has been used as a dynamic simulation program in order to obtain the performance of each cooling system analyzed: absorption pumps with interior storage in the form of salts and a single-effect chiller with exterior storage tank. 3.1. Building loads precalculation The model created for the PSA building includes all passive techniques implemented as: thermal mass walls, ventilation, shading strategies, ventilation through solar chimneys and nocturnal radiative cooling, as well as detailed internal gains [17]. With all of these variables, the building cooling loads have been precalculated using a weather file created from long series of measured data in the Plataforma Solar de Almeria [18]. The integration of the calculated loads into the hourly transient model has been done using a model of a fluid pipe that allows to injecting heat into the fluid flow when cooling the building. 3.2. Modelization of the absorption system with internal storage The solar absorption system with internal storage is composed of four units CW10, and each unit has two barrels with a cooling power of 10 kW and a storage capacity of 30 kW h each. Each barrel has two separate bowls, the one is filled with salts (reactor) and the other is filled with water (evaporator). The salt used for the process is Lithium Chloride e LiCl. During the discharging process, the bowl containing the salts absorbs the water from the evaporator until it can not absorb more water. In the charging process the salt in the bowl is dried with the heat coming from the solar collector field, and the water returns to the evaporator. Each barrel can work whether charging heat from the solar system to the salt, or
Climatewell CW10 unit 4
20 18 16
Barrel Level
14 12 10
2
8 6 1
4 2
0 5400
5420 Mode1
5440
5460 Mode2
5480
0 5520
5500
Barrel1
Fig. 1. Schema of the performance of the Climatewell CW10 unit.
Barrel2
Storage Capacity (kWh)
3
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Fig. 2. Configuration of the absorption system with internal storage.
Fig. 3. Configuration of the absorption system with internal storage in TRNSYS.
discharging cooling energy to the building. When both barrels discharge simultaneously, CW10 can deliver a peak cooling power of 20 kW. Fig. 1 shows a schema of the charging and discharging process of the Climatewell CW10 unit. The modelization of the absorption heat pump CW10 has been based on experimental curves of efficiency provided by the manufacturer (Climatewell). A new simulation model in TRNSYS, that characterize each barrel of the absorption unit, has been created [19]. The basis of the performance of each barrel is the same as a storage tank and has a maximum storage capacity of 30 kWh which it is measured with the storage level. The model contains an internal variable that represents the energy level (accumulated in crystallized salts) which is stored at each time step in charging process until it reaches the maximum capacity. In the same way, during the discharge process, this variable represents the descending level until it reaches the minimum capacity. To optimize the performance of a system composed by any number of units, a model that rules the charging and discharging sequences of the barrels has been developed. This model identifies the status of each barrel. Each time step, depending on the energy
production on the collector field, the energy demand of the building and the energy level of each barrel, the model decides the sequences of charging and discharging. Fig. 2 shows the configuration of the whole system, including all of the components. Fig. 3 shows the modelling of the system in TRNSYS. 3.3. Modelization of the absorption system with external storage The solar absorption system with external storage is composed of one single-effect absorption chiller (Model WFC-SC20 Yazaki) with a cooling power of 80 kW, and an external storage in a water tank. The WFC-SC Yazaki machine uses in the refrigerant cycle, a solution of lithium bromide (absorber) and water (refrigerant). The heat which comes from the solar field liberates the refrigerant from the solution and produces a refrigerant effect in the evaporator when the water circulated through the condenser and the absorber. A simulation model of the cooling system has been created in TRNSYS. With this purpose, types of the standard and TESS library have been used. The Yazaki absorption pump has been modelled using the single-effect hot water-fired absorption chiller. This
Fig. 4. Configuration of the absorption system with external storage.
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Fig. 5. Configuration of the absorption system with external storage in TRNSYS.
model makes use of a normalized catalogue data based on the efficiency curves of the absorption pump, in order to predict the performance of the chiller. The thermal performance of external storage has been modelled with a standard type of TRNSYS, which calculates the energy balances of the tank (TRNSYS). The performance of the system has been done through the control of the pumps, in order to maximize the solar fraction produce by the chiller directly fed by the solar field or by the external storage tank. The regulation has been done taking into account the cooling loads demanded by the building, the energy coming from the solar collector field and the storage tank. Fig. 4 shows the configuration of the whole system, including all of the components. Fig. 5 shows the modelling of the system in TRNSYS. 4. Analysis Parametrical simulations have been performed to analyse the response of both solar cooling systems when varying the following parameters: - Total surface of solar collector field. - Efficiency curve of the each collector studied. - Exterior storage capacity. To examine the performance of the systems, to main indexes have been calculated in each case: solar fraction (SF) and the primary energy ratio (PER). The solar fraction (SF) is the ratio between the cooling energy delivered by the absorption pumps and the total cooling energy demanded by the building (Eq. (1)). The primary energy ratio (PER) of the solar assisted cooling system is the ratio between the cooling energy provided by the absorption pumps and the total electricity consumed by the system (Eq. (2)).
Qcooling;Abs SF ¼ Qcooling;tot PER ¼
Qcooling;Abs Eelec;tot
(1)
(2)
3elec
The cooling and electricity fluxes comes from the simulation results while the primary energy conversion factor for electricity
has been set with the following value 3elec ¼ 0.4, being based on European Directives, Task 25 and Task 32. Table 1 shows the range of values of each parameter simulated. As it can be seen, three different collectors have been modelized in order to analyze its influence on the SF and PER. For each efficiency curve of the collector, the total surface of the solar field has been varied. These simulations have been carried out for the two analyzed absorption systems. In the case of external storage, different storage capacities of the external water tank have been analysed. 5. Results and discussion Figs. 6e8 show the solar fraction for the cooling demand obtained when using an absorption chiller with external storage. For these simulations, the solar collector field surface, the type of the collector and the capacity of the external tank have been varied. Graphs are related to the efficiency of the following collectors: flat plate collectors (FPC), high efficiency flat plate collectors (HEFPC), and evacuated tube collectors (ETC), respectively. In each image, several storage volumes are presented by different curves in order to obtain the solar fraction produced with increasing of the solar field surface. Data have been fitted to second degree polynomial curves by regression, obtaining in all cases, high correlations (higher than 80%). Data agree with the law of diminishing marginal returns, which states that as the storage volume increases, its improvement effect on the solar contribution is less than the previous storage volume. Somehow storage volumes from 15 m3 to 30 m3 produce similar solar fractions. Same effects occur when increasing the solar collector field surface. The influence of the type of collectors can also be examined: The solar fraction reached by the FPC collector,
Table 1 Range of the parametrical runs of: the surface of solar collector field, the efficiency curves of the collectors and the exterior storage capacity. Solar collector field (m2): Solar collector efficiency curve: Flat plate collector (FPC) High efficiency flat plate collector (HEFPC) Evacuated tube collector (ETC) 3
Storage capacity (m ):
88e250 m2
ho ¼ 0.709, a1 ¼ 5.562 W/mK ho ¼ 0.744, a1 ¼ 3.508 W/mK ho ¼ 0.775, a1 ¼ 1.476 W/mK, a2 ¼ 0.0075 W/mK 1e30 m3
Solar fraction as a function of solar field surface (FPC) 100
2
St 30 m3 R = 0.999 2
90
St 25 m3 R = 0.997
Solar Fraction (%)
St 20m3 R2 = 0.998 2
80 70
St 15 m3 R = 0.981 2 St 10 m3 R = 0.996
St 5 m3 R2 = 0.980 2
St 1 m3 R = 0.992
60 50 40 120
140
160
180
200
220
240
Solar collector field surface (m 2 ) Storage 1 m3 Storage 20 m3
Storage 5 m3 Storage 25 m3
Storage 10 m3 Storage 30 m3
Storage 15 m3
Fig. 6. Solar fraction obtained as a function of solar collector field surface FPC.
Solar fraction as a function of solar field surface (HEFPC) 110
Solar Fraction (%)
100 90 St 1 m3 R2 = 0.996
80
St 5 m3 R2 = 0.999 St 10 m3 R2 = 0.997
70
2 St 15 m3 R = 0.996
St 20 m3 R2 = 0.989
60
St 25 m3 R2 = 0.986 2
St 30 m3 R = 0.994
50 120
140
160
180
200
220
240
2
Solar collector field surface (m ) Storage 1 m3 Storage 20 m3
Storage 5 m3 Storage 25 m3
Storage 10 m3 Storage 30 m3
Storage 15 m3
Fig. 7. Solar fraction obtained as a function of solar collector field surface HEFPC.
Solar fraction as a function of solar field surface (ETC) 105
Solar Fraction (%)
100 95 90
St 1 m3 R2 = 0.959 St 5 m3 R2 = 0.999
85
St 10 m3 R2 = 0.996 St 15 m3 R2 = 0.999
80
2
St 20 m3 R = 0.997 St 25 m3 R2 = 0.928
75
St 30 m3 R2 = 0.833
70 120
140
160
180
200
220
240
Solar collector field surface (m 2 ) Storage 1 m3 Storage 20 m3
Storage 5 m3 Storage 25 m3
Storage 10 m3 Storage 30 m3
Storage 15 m3
Fig. 8. Solar fraction obtained as a function of solar collector field surface ETC.
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Solar Fraction for internal and exterior storage
Solar Fraction (%)
100
80
60
40
20 80
100
120
140
160
180
200
220
240
Solar collector field surface (m 2 ) FPC storage 1m3 HEFPC storage 1m3 ETC storage 1m3 FPC Interior Storage
FPC storage 15m3 HEFPC storage 15m3 ETC storage 15 m3 HEFPC Interior Storage
FPC storage 30m3 HEFPC storage 30m3 ETC storage 30 m3 ETC Interior Storage
Fig. 9. Solar Fraction achieved with interior and exterior storage for three types of collectors.
which varies between 49 and 89%. The solar fraction reached by the HEFPC collector varies between 67 and 100%, and between 84 and 100% by ETC. Fig. 9 compares the solar fraction obtained for the case of 4 units of CW10 working in parallel, and the absorption system with external storage in a water tank. Only data for 1 m3, 15 m3 and 30 m3 storage volumes curves are presented, taking into account that the data for the rest of the storage volumes lay within these three limits. In these simulations, the solar collector field surface and the type of the collector (flat plate collectors, high efficiency flat plate collectors and evacuated tube collectors) have been varied. The solar fraction reached by the FPC collector varies, with the increasing of the solar collector field, from 40% to 84%. For the HEFPC collector, the solar fraction obtained varies from 57% to 95%. For the ETC collector, the solar fraction achieved varies between 74 and 100%. The system with internal storage reaches its asymptotical limit before the system with external storage. In the case of external storage, the different cases show that the influence of the storage
volume which is almost imperceptible for small collector field increases with the size of the solar collector field (especially with FPC and HEFPC collectors). The comparison between these curves shows that the SC20 Yazaki with an external storage tank of 15 m3 reaches values similar to those obtained by the set of 4 CW10 working in parallel. Fig. 10 shows the values of PER for the case of 4 units of CW10 working in parallel, and the absorption system with external storage in a water tank. Only data for 1 m3, 15 m3 and 30 m3 storage volumes curves are presented. As it can be seen in Fig. 10, PER follow similar tendencies as SF. The maximum value of PER is obtained when the system is capable to provide all the cooling demanded by the building. This value is 1.3. Finally, data relating room surface needed by the different systems analysed is presented in Fig. 11. The room surface has been obtained summing the dimensions of all the components: 4 CW10 or 1 Yazaki SC20 with external storage tank (Table 2). Despite the surface required by 4 CW10 machines is greater than 1 SC20 Yazaki machine; the second needs an external tank
PER for internal and exterior storage 1.4 1.2
PER ()
1.0 0.8 0.6 0.4 0.2 0.0 80
100
120
140
160
180
200
220
240
Solar collector field surface (m 2 ) FPC storage 1m3 HEFPC storage 1m3 ETC storage 1m3 FPC Interior Storage
FPC storage 15m3 HEFPC storage 15m3 ETC storage 15 m3 HEFPC Interior Storage
FPC storage 30m3 HEFPC storage 30m3 ETC storage 30 m3 ETC Interior Storage
Fig. 10. Primary Energy Ratio achieved with interior and exterior storage for three types of collectors.
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Room requirement for each absorption system 24
Room Surface (m2)
20
16
12
8
4
0 Yz+1m3
Yz+5m3
Yz+10m3
Yz+15m3
Yz+20m3
Yz+25m3
Yz+30m3
4CW10
Absoption system 2
Fig. 11. Total room (m ) required for absorption systems with internal and external storage.
Table 2 Relating room surface needed by the two different systems analysed: 4 CW10 and SC20 Yazaki with external storage. Model
Width (m)
Depth (m)
Height (m)
Yazaki WFC-SC20 CW10
1.06 1.38
1.3 0.7
2.03 1.85
to store energy that comes from the solar field. As it can be seen in Fig. 11, only the case of 1 Yazaki with an external tank of 1 m3, has less surface requirements than in the case of 4 CW10. For same SF and PER the absorption system with internal storage has less room requirement. 6. Conclusions This paper aims to shed light on absorption cooling systems with internal storage (CW10). With this purpose they have been compared to a conventional system composed of an absorptioncooling machine and exterior storage tank. Parametrical simulations varying the solar collector surface, the type of collector and the storage tank capacity (in the case of exterior storage) have been performed. The comparison between both systems indicates that similar solar fractions can be achieved in both systems. The big differences lay on the additional storage (and the corresponding room surface) that is needed in the case of conventional storage. While the system based on CW10 units does not require exterior storage, the conventional system needs an exterior storage of 15 m3 to reach similar solar fractions. The primary energy ratio (PER) of the cooling system reaches a maximum value of 1.3 when the system is capable to provide the 100% of the cooling demanded by the building.
Plan of Research and Develop 2004e2007, cofinanced with FEDER Founds and supported by the Spanish Ministry of Innovation and Science. The authors would like to thank all the companies and Institutions included in PSE-ARFRISOL project.
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Acknowledgements
[18]
The PSE-ARFRISOL, Reference PS-120000-2005-1, is an Strategic Singular Scientific-Technological Project accepted by the National
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