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Optimization of a solar cooling system with interior energy storage
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Solar Energy 84 (2010) 1244–1254 www.elsevier.com/locate/solener
Optimization of a solar cooling system with interior energy storage C. Sanjuan *, S. Soutullo, M.R. Heras Department of Energy, Energy Efficiency in Buildings Unit, CIEMAT, Madrid E-28040, Spain Received 27 May 2009; received in revised form 31 March 2010; accepted 1 April 2010 Available online 4 May 2010 Communicated by: Associate Editor Ruzhu Wang
Abstract This paper focuses on the optimization of the performance of a solar absorption cooling system composed by four units with interior energy storage. A full dynamic simulation model that includes the solar collector field, the absorption heat pump system and the building load calculation has been developed. It has been applied to optimize the coupling of a system based on this new technology of solar powered absorption heat pump, to a bioclimatic building recently constructed in the Plataforma Solar de Almeria (PSA) in Spain. The absorption heat pump system considered is composed by four heat pumps that store energy in the form of crystallized salts so that no external storage capacity is required. Each heat pump is composed of two separate barrels that can charge (store energy from the solar field) and discharge (deliver heat or cold to the building) independently. Different configurations of the four units have been analysed taking into account the storage possibilities of the system and its capacity to respond to the building loads. It has been shown how strong the influence of the control strategies in the overall performance is, and the importance of using hourly simulations models when looking for highly efficient buildings. Ó 2010 Elsevier Ltd. All rights reserved. Keywords: Solar cooling; Absorption; Simulation; Control
1. Introduction Solar absorption systems are suitable to meet cooling energy load in buildings. Nevertheless, there are some characteristics of these systems that still have to be taken into account. The COP is in the range of 0.6–0.7 for single effect absorption machines, and in the range of 1.2–1.5 for double effect absorption machines. Additionally, absorption cooling systems require thermal energy supply, which means a solar collector field if looking for renewable energy systems that can also provide daily hot water and heating energy. It is thus very important to assure elevated solar contribution in the performance of these systems (Mendes et al., 1998) because electrical compressors which are nowadays the alternative to absorption systems reach COP values in the range of 3–3.5. *
Corresponding author. Tel.: +34 913466344. E-mail address: [email protected] (C. Sanjuan).
0038-092X/$ - see front matter Ó 2010 Elsevier Ltd. All rights reserved. doi:10.1016/j.solener.2010.04.001
The efficiency of the solar absorption 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 looses at the distribution system, and the profiles of the cooling load. The performance of the system is thus highly dependant on the coupling between all these components, and consequently on the control strategies. Solar absorption systems operate mainly under partial load conditions because of the disadjustment between solar resource on the one side and the energy load of the building on the other side. It is very important to have detailed knowledge of the overall operation of the system during the whole year in order to optimize its design. Many authors have worked on the absorption systems performance. There are many experimental and theoretical studies about the efficiency of the solar collectors and the COP of the absorption unit (Izquierdo Milla´n et al., 1997). A more accurate approximation is to calculate the overall efficiency as the multiplication of both, the solar
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collector efficiency and the absorption machine COP (Mittal et al., 2005). All of this combined with the big amount of published works that analyse the influence of the temperature levels and flow rates in the performance of the absorption units (Mittal et al., 2006), can give some initial ideas about the size of the system. The main disadvantages of these analysis is that they resolve steady state energy balances in the machines for nominal conditions and derive the performance of the chillers from the entering temperatures. They do not take into account the performance of the systems out of nominal conditions, the influence of the storage system and its inertial affects in the coupling and decoupling between solar energy generation and cooling energy deliverance to the building, or the influence of the control system as a possibility to optimize overall performance depending on the loads profile. There are some papers which modelize 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 (Wilbur and Mitchell, 1975; Eicker and Pietruschka, 2009). IEA Task 25 “Solar Assisted Air Conditioning of Buildings” published results of several cases studies of hotels and office buildings in different cities of Europe and developed a decision scheme (Henning and Albers, 2004) and a simulation tool that performed system simulation for solar assisted air conditioning systems (IEA Task 25) and (Henning, 2004). The document “Design of solar assisted air conditioning systems (2): selection of collector and buffer storage” 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 fraction (IEA Task 25). IEA Task 38 “Solar Air-Conditioning and Refrigeration” have set the complete system evaluation (simulation and monitoring) as one of their main objectives, to make possible the comparison of available simulation tools and their applicability for planning and system analysis (IEA Task 38). Almost the totality of the studies that have been published to the date have centred their investigations in conventional liquid absorption technologies. The cases studied are systems mainly composed of a solar field, a liquid absorption heat pump that converts energy in a continuous cycle, and exterior storage. Unlike previous works, the objective of this article is to propose an alternative system based on several absorption heat pump units that store energy the form of crystallized salts. The main properties of this system are the power fractioning into small units and the storage capacity of each absorption heat pump so that no exterior storage is required.
and Development aimed to reduce the energy consumption in buildings, particularly the conventional energy used for heating and cooling. The project, called ARFRISOL (Bioclimatic Architecture and Solar Cooling, in Spanish), intends to demonstrate that it is possible to reach high levels of savings of conventional energy, by taking advantage of the solar energy in five different offices buildings, using different passive techniques for heating and cooling, as well as by means of solar active systems (website Arfrisol). The goal of this investigation was derived from the necessity of taking a decision on the optimal operation of the solar absorption systems used in the buildings. The big novel point of the new technology used in this project was to replace the traditional solar liquid absorption machine by four heat pump units that store energy in the form of crystallized salts, and thus to replace the outside storage tank with the interior storage barrels of the units. Except some minor performance restrictions, many possibilities of connecting and controlling the performance of the absorption units were still open during the design process. The analysis looked for to optimize the solar fraction to the total cooling energy demanded by the buildings. To that effect, a dynamic model which couple the solar heating and cooling system with the building hourly loads was developed using the dynamic simulation tool TRNSYS (website TRNSYS). Within this scope, this paper deals with the necessity of analysing the solar heating and cooling systems coupled to the buildings, whenever high levels of energy savings are the objective. It is necessary to attend simultaneously to hourly values of solar energy production in the collector field, to the conversion and storage of that energy in heating and cooling delivered to the building, and to the loads distribution of the building. 3. Case study The case of study is an office building recently constructed and based in the Plataforma Solar de Almeria (PSA), with 1114 m2 of built area but only is conditioned a surface of 860 m2. See Fig. 1. The building consists in a ground level building, developed around an east–west axis. It has been projected following the principles of Bioclimatic
2. Scope and motivation of the work Currently, the Spanish Ministry of Science and Innovation is promoting a Singular Strategic Project of Research
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Fig. 1. Office building in Plataforma Solar de Almeria (PSA).
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Architecture. The offices are facing south to collect the solar radiation in winter in order to warm up naturally these rooms. The South facßade is protected by a 2 m projection (acting as a porch) to block the direct radiation during the summer. The basic project of the construction comprises the use of massive walls to prevent the interior from extreme temperatures and to reduce the effects of the daily variation of external temperatures. This wall with high thermal inertia assures a good insulation of the building and mitigates the wave amplitude of the interior temperature. A collector double wing structure put over the roof has been designed with fixed slopes to work as a shading device during the summer time and to let solar radiation reach the building during the winter time. On the south facing wing, solar collectors will supply the heating and cooling energy to satisfy the demand. The collector field has a total aperture area of 170 m2 and it has 90 high efficiency flat plate collectors with transparent insulation material, from Unisolar (Model UNISOL CP-1) (website Grupo Unisolar). The solar collector has an optical efficiency of 0.7 and a loss coefficient of 3.1 W/m2 K. The absorption system is composed of four absorption units (model CW10 of Climatewell) with a cooling power peak of 20 kW and a storage capacity of 60 kWh each (website Climatewell). The distribution of the heat and cold to the system is through radiant floor and inductors. See Figs. 2 and 3. The performance of the absorption units considered in this study differs from the conventional liquid absorption pumps in some important aspects. Each unit is composed of two barrels that can work independently wether charging energy into the salts, only as a storage tank, or discharging the energy stored in the salts in form of cooling energy. By means of the storage in form of crystallized salts the trithermal absorption heat pump (generator, evaporator and condensator) has been converted in two bithermal pumps: generator heat pump and refrigeration heat pump. This design implies that the absorption unit does not to work in a closed loop, converting energy in a continuous process as conventional fluid absorption pumps. Instead the production of cold in the evaporator is decoupled form the generator fed by solar energy. In the CW10 units heat conversion and heat storage is combined in one, so that the heat from the solar collectors can be stored and delivered to the building at any time (see Fig. 4).
Resuming, the case study installation is composed of: – – – – –
170 m2 of solar aperture area (UNISOL CP1-TIM), four absorption pumps (CLIMATEWELL CW10), 80 kW of cooling peak power (20 kW/unit CW10), 240 kWh of storage capacity (60 kWh/unit CW10), 860 m2 of conditioned building surface.
4. Methodology This paper focuses on the analysis of the energetic performance of a solar powered absorption system composed by four independent units which have a storage capacity of 60 kWh each. The main goal is to find out, by means of dynamical simulations, which is the optimal configuration of the four units considering that they are coupled to a solar collector field and to the building. The methodology that has been followed is: – Building loads pre-calculation. – Modelization of the absorption units, formed by two barrels which can charge (store energy in the form of salts) and discharge (deliver the stored energy to the building). – Development of a simulation environment that couples the solar cooling system with the building hourly loads. – Development of a model that finds the optimal operation of the absorption units, by optimizing the sequential decision of charging and discharging depending on the solar energy production and the building energy demand. – Optimization of the overall performance of the system. 5. Simulation 5.1. Building loads Dynamic simulation tool TRNSYS has been used for the pre-calculation of the building loads. The model created includes all passive techniques implemented in the building (thermal mass walls, ventilation, shading strategies, ventilation through solar chimneys and nocturnal radiative cooling) as well as detailed internal gains (especially occupancy) (SEL et al., 2004). The input climate file is a TMY created by long series of measured data in the
Fig. 2. Configuration of the system.
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Building Loads
Solar Collector Field
Fig. 3. Connection of the four absorption chillers CW10.
Plataforma Solar de Almeria (Zarzalejo et al., 1995). The integration of the precalculated loads into the complete model has been done using a model of a fluid pipe that allows to injecting heat into the fluid flow when cooling the building. The precalculated building loads have been used as hourly input in the cooling system simulation in all cases analyzed except for results of Section 6.2. For this section two fictitious years of building loads have been created to analyse the influence of daily load distributions (see Section 6.2). The annual profile of the building loads show 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. For the total conditioned surface of 860 m2, this means a cooling peak load of 70 kW. 5.2. Modelization of the absorption heat pump The simulation of the absorption chillers can be approached in two ways. One way is by resolving the whole set of energy balances in all the components of the machine and the other, is by modelling the absorption cooling unit based on curves of operating experience with the machine. In this project, the second approach has been used.
Charge Capacity Curves CW10 Charge Capacity(kW)
Fig. 4. Composition of the CW10.
Each absorption heat pump CW10 is composed of two barrels. Each barrel is composed of 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 – LiCl. During the discharging process, the bowl containing the salts absorbs the water from the evaporator until it cannot 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 discharging cooling energy to the building or as a storage tank. Fig. 4 shows the schematic performance of CW10 with one barrel charging energy from the solar collectors (drying the salts), and the other barrel working in the discharge mode. It has been created a new simulation model in TRNSYS that characterise each barrel of the absorption pump CW10. The modelization of each barrel has been based on experimental curves of efficiency for charging and discharging which have been provided by the manufacturer (see Figs. 5a and 5b). In the process of charge, the model estimates the charging power rate for different temperatures of the water flow from collectors and the cooling tower operating temperature. In the process of discharge, the model estimates the cooling rate versus the temperature of the chilled water flow returning from the building and the cooling tower operating temperature. The three curves correspond to the temperature of operation of the cooling tower. The basis of the performance of the salt bowl is the same as a storage tank and has a maximum storage capacity of
20 15 10 40ºC 30ºC 20ºC
5 0 60
70
80
90
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Return temperature from solar field (ºC)
Fig. 5a. Charge capacity curves.
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20ºC 40ºC 30ºC
10 8 6 4 2 0 3
5
7
9
11
13
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Chilled water return temperature (ºC)
Fig. 5b. Cooling discharge capacity curves.
30 kWh which is measured with the storage level. The model contains an internal variable that represents the storage level (accumulated energy in crystallized salts) which is stored at each time step during the charging process, until it reaches the maximum storage capacity. In the same way, during the discharge process, this variable represents the descending level until it reaches the minimum storage capacity. Each barrel operates in charge or discharge, depending on the energy stored in the CW10 and when it reaches the maximum or minimum capacity switches its function to the opposite. The barrel is totally charged when the salts are dry and all the water is back in the evaporator, and totally discharged when the salts cannot absorb more water. The dissipation circuit (cooling tower) has been modelled as an ideal system at the temperature of 20 °C for cooling. 5.3. Solar cooling system performance model The cooling system is a solar absorption system of four units CW10, which have a cooling power of 10 kW each working in a continuous cycle (one barrel charging and one barrel discharging), but that can deliver a peak cooling power of 20 kW each when both barrels discharge simulta-
neously. This way, it is expected that a good cooling demand prediction combined with a proper control strategy allow the system to meet all the demand, even the peaks. For each absorption heat pump CW10 there are many possible situations: both barrels charging or discharging simultaneously, alternatively or only one barrel working. 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 load of the building and the energy level of each barrel, the model decides the sequences of charging and discharging. The model can consider restrictions on the flow rate of the circuits entering the system which constrains the maximum and minimum number of machines that can be charged or discharged at every time step. Fig. 6 shows the optimized operation of four absorption machines during 2 days. In this configuration eight barrels work independently. There are two axes; the right axis represents the modes of charging and discharging of each barrel while the left axis shows the corresponding storage level (named in the figure as C). When mode is equal to 1 the barrels are on a charging process, which means an increase of the energy stored in the form of crystallized salts (storage level) until it reaches its maximum capacity (30 kWh). Mode equal to 3 means discharging process and the reduction of the storage capacity. In this figure the barrels which have the same storage level and the same mode are represented together. 6. Analysis and results The influence of the following parameters in the overall performance of the system has been studied:
Operation of 4 absorption pumps (8 barrels charging and discharging) 60
50
2
Energy KW
40
30 -2 20
10
0
-6
C 1,6
C 2,4,5
C3
C7
C8
Fig. 6. Operation of four absorption machines. Sequences of charging and discharging.
Absorption units modes
Discharge Capacity (kW)
Discharge Capacity Cooling CW10 12
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– Influence of the total collector’s surface in relation to the building load and the cooling power installed. – Influence of the daily profile of the building loads. – Influence of the control strategies that rule the interconnection and operation of the four cooling units.
6.1. Influence of the installed collector surface Table 1 shows the percentage of solar fraction of the cooling loads as a function of the total surface of installed solar collectors. As it can be seen, the solar fraction presents an asymptotical tendency to the 100%. It stabilizes with a collector aperture surface of 170 m2 with a value of the 91%. From this point, high increases in the total collector surface are required to obtain small increases in the solar fraction percentage. For a total cooling power of 80 kW and a collector aperture area of 170 m2, with a solar fraction of 91% the relation between the installed collector surface and the nominal cooling power is 2.125 m2/kW. In the work of Eicker and Pietruschka (Eicker and Pietruschka, 2009) for a conventional solar absorption cooling system in Madrid, the relation to achieve a solar fraction of 80% was 2–4 m2/kW. Another difference between both studies is the type of solar collector used: high efficiency flat plate collector and vacuum tube collector (Eicker and Pietruschka, 2009). Comparing both works, it is interesting to note that although the use of flat plate collectors, the optimization of the performance of the absorption system composed of four units with interior storage (see Section 6.3) permits to obtain higher solar fraction for a similar ratio between solar field area and nominal cooling power (m2/kW).
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month. The values have been taken from the dynamical simulations (see Section 5.1) with a continuous occupancy schedule from 7 am to 18 pm. The second year of building loads has the same daily accumulated energy, but with a different hourly distribution. It presents two peaks of consumption: one in the morning and one in the afternoon because it supposes an almost empty building during lunch time. As it can be seen in Figs. 7 and 8, the solar fraction is lower in the second case (case with two peaks). Fig. 7 shows the monthly distribution of the solar fraction during the summer period in both cases. Fig. 8 shows the total cooling solar fraction for different solar collector surface. Although in both cases the solar fraction is lower when the daily profile presents two peaks, the differences are never higher than 5%. This indicates that the absorption system has a high adaptability to the load profile, as long as the load profiles show moderate peaks typical from bioclimatic buildings (see Section 3). 6.3. Influence of the control strategy The influence of the control strategy has been evaluated and the results show how an improved control strategy allows a power fractioning performance without a loose of efficiency. The following figures show the results of three configurations that have been studied: Case 1: four CW10 units working as one big heat pump (two barrels of 40 kW cooling power each). Fig. 9a. Case 2: two blocks of two CW10 units as two heat pumps (four barrels of 20 kW cooling power each). Fig. 9b. Case 3: four CW10 units connected in parallel (eight barrels of 10 kW cooling power each). Fig. 9c.
6.2. Influence of the building cooling loads It has been analysed the influence of the building cooling loads on the percentage of solar fraction to the annual loads. For the particular purpose of this section, two fictitious years of building loads have been created. In the first case, the building loads are the same for every working day of each month, and correspond to the typical day of each
Table 1 Percentage of solar contribution to cooling demands as a function of the solar field size. Collectors aperture area (m2)
Solar fraction (%)
120 150 160 170 180 190 200 220 240
78 86 87 91 91 91 91 94 94
Figs. 11, 13 and 15 show the energy balances in the system for the three configurations (Cases 1–3). The energies represented in the left axis are the thermal energy produced in the solar field, the charging energy of the absorption pumps, the discharging energy of the absorption pumps to the building (converting the thermal energy stored in the crystallized salts into cooling energy), and the building cooling loads. Depending on the configuration studied, the whole system is composed of a number of independent blocks, that charge or discharge energy depending on their storage levels, the cooling demands of the building and the solar energy available at each time. To provide more information on the charging and discharging processes of each of the independent blocks, storage levels (energy accumulated in crystallized salts) are also included in these figures, named as C. The discharging process can be started when there is cooling demand of the building and at least one of the independent blocks has reached its maximum storage level. When this happens, the energy accumulated in form of crystallized salts decreases while producing the cooling energy necessary to cover the building loads.
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Solar Fraction (%)
95 90 85 80 75 70 65
r be em ec
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Typical day profile with peaks
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Fig. 7. Month solar fraction (%) for two daily profiles.
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Solar Fraction (%)
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60 120
140
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Solar collector surface (m ) Typical day profile
Typical day profile with peaks
Fig. 8. Cooling solar fraction (%) for different solar collector surface (m2) for two daily profiles.
40 kW
20 kW
40 kW
20 kW
Fig. 9a. Configuration Case 1. 20 kW
Simultaneously, when there is enough solar energy available, the rest of independent blocks initiate the charging process with the consequent increase of the storage level. Fig. 10 shows the monthly percentage of solar fraction to the total cooling energy load of the building in the Case 1. This configuration achieves a monthly energy contribution to the building between 40% and 60%. As mentioned before, Fig. 11 represents the energy balances of the whole
20 kW
Fig. 9b. Configuration Case 2.
system. At the selected days, the system can only supply the energy to cover the morning and afternoon peaks. The rest of the day is needed to recharge all the barrels. The absorp-
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solar field and the cooling loads of the building at each time step. As in the previous cases, the lowest percentages (less than 70%) are obtained in the months with higher energy loads, July and August. Fig. 15 shows 2 days when the system is able to supply the entire cooling load due to the correct distribution and operation of the barrels. Each tank charges or discharges energy according to the needs of the building loads. The higher fractioning of the power into eight units improves the performance of the system increasing even more its adaptability to the load profile.
10 kW
10 kW
10 kW
10 kW
10 kW
6.4. Comparison with liquid absorption cooling systems 10 kW
10 kW
10 kW
Fig. 9c. Configuration Case 3.
tion units work always under constant mass flow rate, and this means that the system has only two levels of power. Fig. 12 shows the monthly percentage of solar fraction to the total cooling energy load of the building for the Case 2. The solar fraction of this configuration rises to 80%. Fig. 13 shows the energy balances of the whole system during two summer days. The number of hours in which the system cannot provide cooling energy to the building is reduced in comparison to Case 1. The fractioning of the power into four units improves the performance of the system increasing its adaptability to the load profile. Fig. 14 shows the monthly percentage of solar fraction to the total energy load for the Case 3. The average solar fraction for the cooling period rises to 91%. In this configuration each barrel can charge or discharge independently, and the sequences depend on the energy produced by the
The objective of this section is to compare the performance of cooling system with interior energy storage and conventional liquid absorption systems with outside energy storage tank. Yazaki Absorption heat pump efficiency curves have been used for the simulations of the conventional absorption pump (website Yazaki). The building loads have been left untouched. Simulations have been done for different storage volumes between 2 m3 and 65 m3. Fig. 16 presents results of the three cases studied in this work in comparison with the performance of a conventional liquid absorption chiller with a cooling capacity of 80 kW and exterior storage tank (Case LiBr). The final column refers to the design criteria for liquid absorption solar systems resulting from Task 25 (IEA Task 25). The graph shows the relationship between the size of the collector field (m2), the size of conditioning building area (m2), and gives an expected value of the solar fraction for a storage capacity of the system of four peak load hours, which is the storage capacity of the salts (240 kWh). Fig. 16 shows that with the same surface of solar collectors and storage capacity it is possible to obtain higher solar fractions, depending on the control strategy applied to the cooling machines. Cases 1–3 show how the adaptability of the studied system increases with the number of independent blocks, even when the total power is constant.
Case 1 100
60
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Fig. 10. Cooling solar fraction (%). Case 1.
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Energy Balances
Solar Field Charging
150
Discharging Load
Energy KW
C1 C2
100
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Fig. 11. Cooling energy balances (kW). Case 1.
Case 2 100
Solar Fraction (%)
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Fig. 12. Cooling solar fraction (%). Case 2.
Energy Balances
Solar Field Charging
150
Discharging Load
Energy KW
C 1,4 C 2,3
100
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Fig. 13. Cooling energy balances (kW). Case 2.
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Case 3 100
Solar Fraction (%)
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r ov em be r D ec em be r N
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Fig. 14. Cooling solar fraction (%). Case 3.
Energy Balances
Solar Field Charging
150
Discharging Load C 1,6
Energy KW
C 2,4,5 C 3,7
100
C8
50
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Fig. 15. Cooling energy balances (kW). Case 3.
120
Cooling Solar Fraction (%)
100
80
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0 0.1
0.20
0.30
m2 absorber per m2 conditioned area Case BrLi
Case 3
Case 2
Case 1
IEA Task 25
Fig. 16. Cooling solar fractions. Comparison between present study and IEA Task 25 results.
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When comparing Cases 1–3 to a conventional liquid absorption system with outside energy storage (Case LiBr), Fig. 16 shows that similar solar fractions can be obtained in both cases. The main differences lay on the storage volume needed in each case. In Cases 1–3 the absorption heat pumps that store the energy in the form of crystallized salts do not need any additional storage volume further than the dimensions of the four heat pumps (each CW10 heat pump are 1.4 m wide 1 m depth 2 m height). The liquid absorption machine needs 16 m3 of exterior volume storage (in addition to the dimensions of the cooling machine) to reach similar solar fraction. The IEA values of solar fraction are lower than the values obtained in the case study, probably due to the fact that the building that is being analysed is a bioclimatic building and its load of energy is lower than the cases studied in Task 25.
7. Conclusions The influence of the collector’s aperture surface in the solar fraction to the energy loads provides an asymptotical tendency to the 100%, getting the stability around 170 m2 for this building, with a value of 91%. The relationship between the collector field and the cooling power remains in 2.125 m2/kW. The daily load profile with one peak produce higher solar fraction percentages than the profile with two peaks, but the differences are never higher that the 5%. The simulation results also show that depending on the control strategy the percentages of solar fraction vary from values of around 50% to values of 90%. The results of the simulations for the different configurations of the system (Cases 1–3) have been compared to a conventional system. This system is composed of an absorption cooling machine and an exterior storage tank with the same capacity as the salts. The volume of the storage tank to provide the same solar fraction is 16 m3. As seen in Fig. 16, similar solar fractions can be achieved in both systems, the big differences lays on the additional storage that is needed in the case conventional storage. The results have also been compared to the design criteria for liquid absorption solar systems resulting from Task 25 of the International Energy Agency. The IEA values of solar fraction are lower than the values obtained in the case study, probably due to the fact that the building that is being analysed is a bioclimatic building and its load of energy is lower than the cases studied in Task 25. The main advantages of the system studied in this paper, versus liquid absorption systems, is that absorption process and energy storage is combined in one so that the production of cooling energy can be decoupled from the solar gains without external storage tank. The fact of working with small units has some advantages and disadvantages. With same solar collector surface and storage capacity it is possible, depending on the control strategy, to obtain higher solar fractions. The adaptability of the system
increases with the number of independent blocks (even when the total power is constant). This type of systems cannot substitute big absorption pumps because too many units would be required, but for medium powers, it provides a big adaptability to the system and allows many possibilities of operation. Nevertheless, further studies should study the performance of big absorption systems combining a big liquid absorption heat pump for the mean load and these small units to supply the peaks of power. Acknowledgements The PSE-ARFRISOL, Reference PS-120000-2005-1, is a Strategic Singular Scientific-Technological Project accepted by the National Plan of Research and Develop 2004–2007, 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 Arfrisol.. Climatewell. . Eicker, U., Pietruschka, D., 2009. Design and performance of solar powered absorption cooling systems in office buildings. Energy and Buildings 4, 81–91. Grupo Unisolar SA. . Henning, H.M., 2004. Solar Assisted Air-Conditioning of Buildings. A Handbook for planners. Henning, H.M., Albers, J., 2004. Decision Scheme for the Selection of The Appropriate Technology Using Solar Thermal Air-Conditioning. Guideline Document, IEA Solar Heating and Cooling. Task 25: Solar-Assisted Air-Conditioning of Buildings. IEA Solar Heating and Cooling, Task 25: Solar-Assisted Air-Conditioning of Buildings. SOLAC Computer Design Tool. . 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. . Izquierdo Milla´n, M., Herna´ndez, F., Martı´n, E., 1997. Solar cooling in Madrid: energetic efficiencies. Solar Energy 60, 367–377. Mendes, L.F., Collares-Pereira, M., Ziegler, F., 1998. Supply of cooling and heating with solar assisted heat pumps: an energetic approach. International Journal of Refrigeration 21 (2), 116–125. Mittal, V., Kasana, K.S., Thakur, N.S., 2005. The study of solar absorption air-conditioning systems. Journal of Energy in Southern Africa 16 (4). Mittal, V., Kasana, K.S., Thakur, N.S., 2006. Modelling and simulation of a solar absorption cooling system for India. Journal of Energy in Southern Africa 17 (3), 1. SEL, TRNSSOLAR, CSTB, TESS, 2004. Multizone Building modelling with Type56 and TRNBuild. TRNSYS 16, vol. 6. TRNSYS (Transient Systems Simulation Program). . Zarzalejo, L.F., Te´llez, F.M., Palomo, E., Heras, M.R., 1995. Creation of typical meteorological years (TMY) for Southern Spanish cities. In: International Symposium Passive Cooling of Buildings. Athens, Greece.
Solar Energy 84 (2010) 1244–1254 www.elsevier.com/locate/solener
Optimization of a solar cooling system with interior energy storage C. Sanjuan *, S. Soutullo, M.R. Heras Department of Energy, Energy Efficiency in Buildings Unit, CIEMAT, Madrid E-28040, Spain Received 27 May 2009; received in revised form 31 March 2010; accepted 1 April 2010 Available online 4 May 2010 Communicated by: Associate Editor Ruzhu Wang
Abstract This paper focuses on the optimization of the performance of a solar absorption cooling system composed by four units with interior energy storage. A full dynamic simulation model that includes the solar collector field, the absorption heat pump system and the building load calculation has been developed. It has been applied to optimize the coupling of a system based on this new technology of solar powered absorption heat pump, to a bioclimatic building recently constructed in the Plataforma Solar de Almeria (PSA) in Spain. The absorption heat pump system considered is composed by four heat pumps that store energy in the form of crystallized salts so that no external storage capacity is required. Each heat pump is composed of two separate barrels that can charge (store energy from the solar field) and discharge (deliver heat or cold to the building) independently. Different configurations of the four units have been analysed taking into account the storage possibilities of the system and its capacity to respond to the building loads. It has been shown how strong the influence of the control strategies in the overall performance is, and the importance of using hourly simulations models when looking for highly efficient buildings. Ó 2010 Elsevier Ltd. All rights reserved. Keywords: Solar cooling; Absorption; Simulation; Control
1. Introduction Solar absorption systems are suitable to meet cooling energy load in buildings. Nevertheless, there are some characteristics of these systems that still have to be taken into account. The COP is in the range of 0.6–0.7 for single effect absorption machines, and in the range of 1.2–1.5 for double effect absorption machines. Additionally, absorption cooling systems require thermal energy supply, which means a solar collector field if looking for renewable energy systems that can also provide daily hot water and heating energy. It is thus very important to assure elevated solar contribution in the performance of these systems (Mendes et al., 1998) because electrical compressors which are nowadays the alternative to absorption systems reach COP values in the range of 3–3.5. *
Corresponding author. Tel.: +34 913466344. E-mail address: [email protected] (C. Sanjuan).
0038-092X/$ - see front matter Ó 2010 Elsevier Ltd. All rights reserved. doi:10.1016/j.solener.2010.04.001
The efficiency of the solar absorption 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 looses at the distribution system, and the profiles of the cooling load. The performance of the system is thus highly dependant on the coupling between all these components, and consequently on the control strategies. Solar absorption systems operate mainly under partial load conditions because of the disadjustment between solar resource on the one side and the energy load of the building on the other side. It is very important to have detailed knowledge of the overall operation of the system during the whole year in order to optimize its design. Many authors have worked on the absorption systems performance. There are many experimental and theoretical studies about the efficiency of the solar collectors and the COP of the absorption unit (Izquierdo Milla´n et al., 1997). A more accurate approximation is to calculate the overall efficiency as the multiplication of both, the solar
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collector efficiency and the absorption machine COP (Mittal et al., 2005). All of this combined with the big amount of published works that analyse the influence of the temperature levels and flow rates in the performance of the absorption units (Mittal et al., 2006), can give some initial ideas about the size of the system. The main disadvantages of these analysis is that they resolve steady state energy balances in the machines for nominal conditions and derive the performance of the chillers from the entering temperatures. They do not take into account the performance of the systems out of nominal conditions, the influence of the storage system and its inertial affects in the coupling and decoupling between solar energy generation and cooling energy deliverance to the building, or the influence of the control system as a possibility to optimize overall performance depending on the loads profile. There are some papers which modelize 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 (Wilbur and Mitchell, 1975; Eicker and Pietruschka, 2009). IEA Task 25 “Solar Assisted Air Conditioning of Buildings” published results of several cases studies of hotels and office buildings in different cities of Europe and developed a decision scheme (Henning and Albers, 2004) and a simulation tool that performed system simulation for solar assisted air conditioning systems (IEA Task 25) and (Henning, 2004). The document “Design of solar assisted air conditioning systems (2): selection of collector and buffer storage” 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 fraction (IEA Task 25). IEA Task 38 “Solar Air-Conditioning and Refrigeration” have set the complete system evaluation (simulation and monitoring) as one of their main objectives, to make possible the comparison of available simulation tools and their applicability for planning and system analysis (IEA Task 38). Almost the totality of the studies that have been published to the date have centred their investigations in conventional liquid absorption technologies. The cases studied are systems mainly composed of a solar field, a liquid absorption heat pump that converts energy in a continuous cycle, and exterior storage. Unlike previous works, the objective of this article is to propose an alternative system based on several absorption heat pump units that store energy the form of crystallized salts. The main properties of this system are the power fractioning into small units and the storage capacity of each absorption heat pump so that no exterior storage is required.
and Development aimed to reduce the energy consumption in buildings, particularly the conventional energy used for heating and cooling. The project, called ARFRISOL (Bioclimatic Architecture and Solar Cooling, in Spanish), intends to demonstrate that it is possible to reach high levels of savings of conventional energy, by taking advantage of the solar energy in five different offices buildings, using different passive techniques for heating and cooling, as well as by means of solar active systems (website Arfrisol). The goal of this investigation was derived from the necessity of taking a decision on the optimal operation of the solar absorption systems used in the buildings. The big novel point of the new technology used in this project was to replace the traditional solar liquid absorption machine by four heat pump units that store energy in the form of crystallized salts, and thus to replace the outside storage tank with the interior storage barrels of the units. Except some minor performance restrictions, many possibilities of connecting and controlling the performance of the absorption units were still open during the design process. The analysis looked for to optimize the solar fraction to the total cooling energy demanded by the buildings. To that effect, a dynamic model which couple the solar heating and cooling system with the building hourly loads was developed using the dynamic simulation tool TRNSYS (website TRNSYS). Within this scope, this paper deals with the necessity of analysing the solar heating and cooling systems coupled to the buildings, whenever high levels of energy savings are the objective. It is necessary to attend simultaneously to hourly values of solar energy production in the collector field, to the conversion and storage of that energy in heating and cooling delivered to the building, and to the loads distribution of the building. 3. Case study The case of study is an office building recently constructed and based in the Plataforma Solar de Almeria (PSA), with 1114 m2 of built area but only is conditioned a surface of 860 m2. See Fig. 1. The building consists in a ground level building, developed around an east–west axis. It has been projected following the principles of Bioclimatic
2. Scope and motivation of the work Currently, the Spanish Ministry of Science and Innovation is promoting a Singular Strategic Project of Research
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Fig. 1. Office building in Plataforma Solar de Almeria (PSA).
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Architecture. The offices are facing south to collect the solar radiation in winter in order to warm up naturally these rooms. The South facßade is protected by a 2 m projection (acting as a porch) to block the direct radiation during the summer. The basic project of the construction comprises the use of massive walls to prevent the interior from extreme temperatures and to reduce the effects of the daily variation of external temperatures. This wall with high thermal inertia assures a good insulation of the building and mitigates the wave amplitude of the interior temperature. A collector double wing structure put over the roof has been designed with fixed slopes to work as a shading device during the summer time and to let solar radiation reach the building during the winter time. On the south facing wing, solar collectors will supply the heating and cooling energy to satisfy the demand. The collector field has a total aperture area of 170 m2 and it has 90 high efficiency flat plate collectors with transparent insulation material, from Unisolar (Model UNISOL CP-1) (website Grupo Unisolar). The solar collector has an optical efficiency of 0.7 and a loss coefficient of 3.1 W/m2 K. The absorption system is composed of four absorption units (model CW10 of Climatewell) with a cooling power peak of 20 kW and a storage capacity of 60 kWh each (website Climatewell). The distribution of the heat and cold to the system is through radiant floor and inductors. See Figs. 2 and 3. The performance of the absorption units considered in this study differs from the conventional liquid absorption pumps in some important aspects. Each unit is composed of two barrels that can work independently wether charging energy into the salts, only as a storage tank, or discharging the energy stored in the salts in form of cooling energy. By means of the storage in form of crystallized salts the trithermal absorption heat pump (generator, evaporator and condensator) has been converted in two bithermal pumps: generator heat pump and refrigeration heat pump. This design implies that the absorption unit does not to work in a closed loop, converting energy in a continuous process as conventional fluid absorption pumps. Instead the production of cold in the evaporator is decoupled form the generator fed by solar energy. In the CW10 units heat conversion and heat storage is combined in one, so that the heat from the solar collectors can be stored and delivered to the building at any time (see Fig. 4).
Resuming, the case study installation is composed of: – – – – –
170 m2 of solar aperture area (UNISOL CP1-TIM), four absorption pumps (CLIMATEWELL CW10), 80 kW of cooling peak power (20 kW/unit CW10), 240 kWh of storage capacity (60 kWh/unit CW10), 860 m2 of conditioned building surface.
4. Methodology This paper focuses on the analysis of the energetic performance of a solar powered absorption system composed by four independent units which have a storage capacity of 60 kWh each. The main goal is to find out, by means of dynamical simulations, which is the optimal configuration of the four units considering that they are coupled to a solar collector field and to the building. The methodology that has been followed is: – Building loads pre-calculation. – Modelization of the absorption units, formed by two barrels which can charge (store energy in the form of salts) and discharge (deliver the stored energy to the building). – Development of a simulation environment that couples the solar cooling system with the building hourly loads. – Development of a model that finds the optimal operation of the absorption units, by optimizing the sequential decision of charging and discharging depending on the solar energy production and the building energy demand. – Optimization of the overall performance of the system. 5. Simulation 5.1. Building loads Dynamic simulation tool TRNSYS has been used for the pre-calculation of the building loads. The model created includes all passive techniques implemented in the building (thermal mass walls, ventilation, shading strategies, ventilation through solar chimneys and nocturnal radiative cooling) as well as detailed internal gains (especially occupancy) (SEL et al., 2004). The input climate file is a TMY created by long series of measured data in the
Fig. 2. Configuration of the system.
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Building Loads
Solar Collector Field
Fig. 3. Connection of the four absorption chillers CW10.
Plataforma Solar de Almeria (Zarzalejo et al., 1995). The integration of the precalculated loads into the complete model has been done using a model of a fluid pipe that allows to injecting heat into the fluid flow when cooling the building. The precalculated building loads have been used as hourly input in the cooling system simulation in all cases analyzed except for results of Section 6.2. For this section two fictitious years of building loads have been created to analyse the influence of daily load distributions (see Section 6.2). The annual profile of the building loads show 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. For the total conditioned surface of 860 m2, this means a cooling peak load of 70 kW. 5.2. Modelization of the absorption heat pump The simulation of the absorption chillers can be approached in two ways. One way is by resolving the whole set of energy balances in all the components of the machine and the other, is by modelling the absorption cooling unit based on curves of operating experience with the machine. In this project, the second approach has been used.
Charge Capacity Curves CW10 Charge Capacity(kW)
Fig. 4. Composition of the CW10.
Each absorption heat pump CW10 is composed of two barrels. Each barrel is composed of 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 – LiCl. During the discharging process, the bowl containing the salts absorbs the water from the evaporator until it cannot 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 discharging cooling energy to the building or as a storage tank. Fig. 4 shows the schematic performance of CW10 with one barrel charging energy from the solar collectors (drying the salts), and the other barrel working in the discharge mode. It has been created a new simulation model in TRNSYS that characterise each barrel of the absorption pump CW10. The modelization of each barrel has been based on experimental curves of efficiency for charging and discharging which have been provided by the manufacturer (see Figs. 5a and 5b). In the process of charge, the model estimates the charging power rate for different temperatures of the water flow from collectors and the cooling tower operating temperature. In the process of discharge, the model estimates the cooling rate versus the temperature of the chilled water flow returning from the building and the cooling tower operating temperature. The three curves correspond to the temperature of operation of the cooling tower. The basis of the performance of the salt bowl is the same as a storage tank and has a maximum storage capacity of
20 15 10 40ºC 30ºC 20ºC
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Fig. 5a. Charge capacity curves.
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Fig. 5b. Cooling discharge capacity curves.
30 kWh which is measured with the storage level. The model contains an internal variable that represents the storage level (accumulated energy in crystallized salts) which is stored at each time step during the charging process, until it reaches the maximum storage capacity. In the same way, during the discharge process, this variable represents the descending level until it reaches the minimum storage capacity. Each barrel operates in charge or discharge, depending on the energy stored in the CW10 and when it reaches the maximum or minimum capacity switches its function to the opposite. The barrel is totally charged when the salts are dry and all the water is back in the evaporator, and totally discharged when the salts cannot absorb more water. The dissipation circuit (cooling tower) has been modelled as an ideal system at the temperature of 20 °C for cooling. 5.3. Solar cooling system performance model The cooling system is a solar absorption system of four units CW10, which have a cooling power of 10 kW each working in a continuous cycle (one barrel charging and one barrel discharging), but that can deliver a peak cooling power of 20 kW each when both barrels discharge simulta-
neously. This way, it is expected that a good cooling demand prediction combined with a proper control strategy allow the system to meet all the demand, even the peaks. For each absorption heat pump CW10 there are many possible situations: both barrels charging or discharging simultaneously, alternatively or only one barrel working. 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 load of the building and the energy level of each barrel, the model decides the sequences of charging and discharging. The model can consider restrictions on the flow rate of the circuits entering the system which constrains the maximum and minimum number of machines that can be charged or discharged at every time step. Fig. 6 shows the optimized operation of four absorption machines during 2 days. In this configuration eight barrels work independently. There are two axes; the right axis represents the modes of charging and discharging of each barrel while the left axis shows the corresponding storage level (named in the figure as C). When mode is equal to 1 the barrels are on a charging process, which means an increase of the energy stored in the form of crystallized salts (storage level) until it reaches its maximum capacity (30 kWh). Mode equal to 3 means discharging process and the reduction of the storage capacity. In this figure the barrels which have the same storage level and the same mode are represented together. 6. Analysis and results The influence of the following parameters in the overall performance of the system has been studied:
Operation of 4 absorption pumps (8 barrels charging and discharging) 60
50
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0
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Fig. 6. Operation of four absorption machines. Sequences of charging and discharging.
Absorption units modes
Discharge Capacity (kW)
Discharge Capacity Cooling CW10 12
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– Influence of the total collector’s surface in relation to the building load and the cooling power installed. – Influence of the daily profile of the building loads. – Influence of the control strategies that rule the interconnection and operation of the four cooling units.
6.1. Influence of the installed collector surface Table 1 shows the percentage of solar fraction of the cooling loads as a function of the total surface of installed solar collectors. As it can be seen, the solar fraction presents an asymptotical tendency to the 100%. It stabilizes with a collector aperture surface of 170 m2 with a value of the 91%. From this point, high increases in the total collector surface are required to obtain small increases in the solar fraction percentage. For a total cooling power of 80 kW and a collector aperture area of 170 m2, with a solar fraction of 91% the relation between the installed collector surface and the nominal cooling power is 2.125 m2/kW. In the work of Eicker and Pietruschka (Eicker and Pietruschka, 2009) for a conventional solar absorption cooling system in Madrid, the relation to achieve a solar fraction of 80% was 2–4 m2/kW. Another difference between both studies is the type of solar collector used: high efficiency flat plate collector and vacuum tube collector (Eicker and Pietruschka, 2009). Comparing both works, it is interesting to note that although the use of flat plate collectors, the optimization of the performance of the absorption system composed of four units with interior storage (see Section 6.3) permits to obtain higher solar fraction for a similar ratio between solar field area and nominal cooling power (m2/kW).
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month. The values have been taken from the dynamical simulations (see Section 5.1) with a continuous occupancy schedule from 7 am to 18 pm. The second year of building loads has the same daily accumulated energy, but with a different hourly distribution. It presents two peaks of consumption: one in the morning and one in the afternoon because it supposes an almost empty building during lunch time. As it can be seen in Figs. 7 and 8, the solar fraction is lower in the second case (case with two peaks). Fig. 7 shows the monthly distribution of the solar fraction during the summer period in both cases. Fig. 8 shows the total cooling solar fraction for different solar collector surface. Although in both cases the solar fraction is lower when the daily profile presents two peaks, the differences are never higher than 5%. This indicates that the absorption system has a high adaptability to the load profile, as long as the load profiles show moderate peaks typical from bioclimatic buildings (see Section 3). 6.3. Influence of the control strategy The influence of the control strategy has been evaluated and the results show how an improved control strategy allows a power fractioning performance without a loose of efficiency. The following figures show the results of three configurations that have been studied: Case 1: four CW10 units working as one big heat pump (two barrels of 40 kW cooling power each). Fig. 9a. Case 2: two blocks of two CW10 units as two heat pumps (four barrels of 20 kW cooling power each). Fig. 9b. Case 3: four CW10 units connected in parallel (eight barrels of 10 kW cooling power each). Fig. 9c.
6.2. Influence of the building cooling loads It has been analysed the influence of the building cooling loads on the percentage of solar fraction to the annual loads. For the particular purpose of this section, two fictitious years of building loads have been created. In the first case, the building loads are the same for every working day of each month, and correspond to the typical day of each
Table 1 Percentage of solar contribution to cooling demands as a function of the solar field size. Collectors aperture area (m2)
Solar fraction (%)
120 150 160 170 180 190 200 220 240
78 86 87 91 91 91 91 94 94
Figs. 11, 13 and 15 show the energy balances in the system for the three configurations (Cases 1–3). The energies represented in the left axis are the thermal energy produced in the solar field, the charging energy of the absorption pumps, the discharging energy of the absorption pumps to the building (converting the thermal energy stored in the crystallized salts into cooling energy), and the building cooling loads. Depending on the configuration studied, the whole system is composed of a number of independent blocks, that charge or discharge energy depending on their storage levels, the cooling demands of the building and the solar energy available at each time. To provide more information on the charging and discharging processes of each of the independent blocks, storage levels (energy accumulated in crystallized salts) are also included in these figures, named as C. The discharging process can be started when there is cooling demand of the building and at least one of the independent blocks has reached its maximum storage level. When this happens, the energy accumulated in form of crystallized salts decreases while producing the cooling energy necessary to cover the building loads.
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Solar Fraction (%)
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Fig. 7. Month solar fraction (%) for two daily profiles.
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Fig. 8. Cooling solar fraction (%) for different solar collector surface (m2) for two daily profiles.
40 kW
20 kW
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Fig. 9a. Configuration Case 1. 20 kW
Simultaneously, when there is enough solar energy available, the rest of independent blocks initiate the charging process with the consequent increase of the storage level. Fig. 10 shows the monthly percentage of solar fraction to the total cooling energy load of the building in the Case 1. This configuration achieves a monthly energy contribution to the building between 40% and 60%. As mentioned before, Fig. 11 represents the energy balances of the whole
20 kW
Fig. 9b. Configuration Case 2.
system. At the selected days, the system can only supply the energy to cover the morning and afternoon peaks. The rest of the day is needed to recharge all the barrels. The absorp-
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solar field and the cooling loads of the building at each time step. As in the previous cases, the lowest percentages (less than 70%) are obtained in the months with higher energy loads, July and August. Fig. 15 shows 2 days when the system is able to supply the entire cooling load due to the correct distribution and operation of the barrels. Each tank charges or discharges energy according to the needs of the building loads. The higher fractioning of the power into eight units improves the performance of the system increasing even more its adaptability to the load profile.
10 kW
10 kW
10 kW
10 kW
10 kW
6.4. Comparison with liquid absorption cooling systems 10 kW
10 kW
10 kW
Fig. 9c. Configuration Case 3.
tion units work always under constant mass flow rate, and this means that the system has only two levels of power. Fig. 12 shows the monthly percentage of solar fraction to the total cooling energy load of the building for the Case 2. The solar fraction of this configuration rises to 80%. Fig. 13 shows the energy balances of the whole system during two summer days. The number of hours in which the system cannot provide cooling energy to the building is reduced in comparison to Case 1. The fractioning of the power into four units improves the performance of the system increasing its adaptability to the load profile. Fig. 14 shows the monthly percentage of solar fraction to the total energy load for the Case 3. The average solar fraction for the cooling period rises to 91%. In this configuration each barrel can charge or discharge independently, and the sequences depend on the energy produced by the
The objective of this section is to compare the performance of cooling system with interior energy storage and conventional liquid absorption systems with outside energy storage tank. Yazaki Absorption heat pump efficiency curves have been used for the simulations of the conventional absorption pump (website Yazaki). The building loads have been left untouched. Simulations have been done for different storage volumes between 2 m3 and 65 m3. Fig. 16 presents results of the three cases studied in this work in comparison with the performance of a conventional liquid absorption chiller with a cooling capacity of 80 kW and exterior storage tank (Case LiBr). The final column refers to the design criteria for liquid absorption solar systems resulting from Task 25 (IEA Task 25). The graph shows the relationship between the size of the collector field (m2), the size of conditioning building area (m2), and gives an expected value of the solar fraction for a storage capacity of the system of four peak load hours, which is the storage capacity of the salts (240 kWh). Fig. 16 shows that with the same surface of solar collectors and storage capacity it is possible to obtain higher solar fractions, depending on the control strategy applied to the cooling machines. Cases 1–3 show how the adaptability of the studied system increases with the number of independent blocks, even when the total power is constant.
Case 1 100
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Energy Balances
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Fig. 11. Cooling energy balances (kW). Case 1.
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Fig. 13. Cooling energy balances (kW). Case 2.
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Case 3 100
Solar Fraction (%)
80
60
40
20
r ov em be r D ec em be r N
be
r O ct o
em be
st
pt
gu Se
Au
Ju ly
e Ju n
M ay
ril Ap
ch M ar
ua br Fe
Ja n
ua
ry
ry
0
Fig. 14. Cooling solar fraction (%). Case 3.
Energy Balances
Solar Field Charging
150
Discharging Load C 1,6
Energy KW
C 2,4,5 C 3,7
100
C8
50
0
Fig. 15. Cooling energy balances (kW). Case 3.
120
Cooling Solar Fraction (%)
100
80
60
40
20
0 0.1
0.20
0.30
m2 absorber per m2 conditioned area Case BrLi
Case 3
Case 2
Case 1
IEA Task 25
Fig. 16. Cooling solar fractions. Comparison between present study and IEA Task 25 results.
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When comparing Cases 1–3 to a conventional liquid absorption system with outside energy storage (Case LiBr), Fig. 16 shows that similar solar fractions can be obtained in both cases. The main differences lay on the storage volume needed in each case. In Cases 1–3 the absorption heat pumps that store the energy in the form of crystallized salts do not need any additional storage volume further than the dimensions of the four heat pumps (each CW10 heat pump are 1.4 m wide 1 m depth 2 m height). The liquid absorption machine needs 16 m3 of exterior volume storage (in addition to the dimensions of the cooling machine) to reach similar solar fraction. The IEA values of solar fraction are lower than the values obtained in the case study, probably due to the fact that the building that is being analysed is a bioclimatic building and its load of energy is lower than the cases studied in Task 25.
7. Conclusions The influence of the collector’s aperture surface in the solar fraction to the energy loads provides an asymptotical tendency to the 100%, getting the stability around 170 m2 for this building, with a value of 91%. The relationship between the collector field and the cooling power remains in 2.125 m2/kW. The daily load profile with one peak produce higher solar fraction percentages than the profile with two peaks, but the differences are never higher that the 5%. The simulation results also show that depending on the control strategy the percentages of solar fraction vary from values of around 50% to values of 90%. The results of the simulations for the different configurations of the system (Cases 1–3) have been compared to a conventional system. This system is composed of an absorption cooling machine and an exterior storage tank with the same capacity as the salts. The volume of the storage tank to provide the same solar fraction is 16 m3. As seen in Fig. 16, similar solar fractions can be achieved in both systems, the big differences lays on the additional storage that is needed in the case conventional storage. The results have also been compared to the design criteria for liquid absorption solar systems resulting from Task 25 of the International Energy Agency. The IEA values of solar fraction are lower than the values obtained in the case study, probably due to the fact that the building that is being analysed is a bioclimatic building and its load of energy is lower than the cases studied in Task 25. The main advantages of the system studied in this paper, versus liquid absorption systems, is that absorption process and energy storage is combined in one so that the production of cooling energy can be decoupled from the solar gains without external storage tank. The fact of working with small units has some advantages and disadvantages. With same solar collector surface and storage capacity it is possible, depending on the control strategy, to obtain higher solar fractions. The adaptability of the system
increases with the number of independent blocks (even when the total power is constant). This type of systems cannot substitute big absorption pumps because too many units would be required, but for medium powers, it provides a big adaptability to the system and allows many possibilities of operation. Nevertheless, further studies should study the performance of big absorption systems combining a big liquid absorption heat pump for the mean load and these small units to supply the peaks of power. Acknowledgements The PSE-ARFRISOL, Reference PS-120000-2005-1, is a Strategic Singular Scientific-Technological Project accepted by the National Plan of Research and Develop 2004–2007, 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 Arfrisol.