Article 正式发布 Versions 1 Vol 28 (6) : 1195-1204 2019
Cold Storage Capacity for Solar Air-Conditioning in Office Buildings in Different Climates;不同气候区公共建筑太阳能空调系统蓄冷量方法研究
: 2019 - 11 - 05
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Abstract & Keywords
Abstract: The building sector accounts for more than 40% of the global energy consumption. This consumption may be lowered by reducing building energy requirements and using renewable energy in building energy supply systems. Solar air-conditioning systems (SACS) are a promising solution for the reduction of conventional energy in buildings. The storage, especially the cold storage, plays an important role in SACS for unstable solar irradiation. In this paper, we took the absorption refrigerating unit as an example, and the solar air-conditioning system of an office building in Beijing was simulated. The accuracy of this model was verified by comparing with the SACS operation data. Moreover, based on the simulation data, the cold storage capacity of the solar air-conditioning system in different climatic regions was studied. The cold storage capacities of SACS in 20 cities distributed in different climate regions were studied systematically. The results simulated by our proposed model will be beneficial to the SACS design, and will enlarge the application of SACS. 建筑业能耗占全球总能耗的40%以上。通过减少建筑本身的负荷需求和在建筑供能系统中使用可再生能源的方式可以降低这部分能耗。太阳能空调系统(Solar air-conditioning system 以下简称SACS)是一种用于减少建筑行业常规能源应用的有效方法。为了解决太阳能的不稳定性,SACS蓄能部分,尤其是蓄冷装置是十分重要的。在本文中,作者以北京某办公建筑的太阳能空调系统为例,对系统进行了模拟分析。通过与实测数据对比,模型的准确性得到了很好地验证。基于模拟模型,对不同气候区太阳能空调蓄冷装置的蓄冷量进行了分析,并系统的对不同气候区的20个城市SACS系统的制冷能力进行了详细地研究。我们的计算结果将有利于辅助SACS在中国各地设计与推广。
Keywords: office building, solar air-conditioning system, cold storage capacity, TRNSYS
1. Introduction
Solar air-conditioning systems (SACS) are one of the efficient methods of saving energy in office buildings. They can be divided into absorption type, adsorption type and injection type, to name a few. Among them, the absorption type was the first developed refrigeration method for solar air-conditioning systems [1]. Solar energy is accessed easily and has enormous potential; thus, the utilization of solar energy has been given more attention by scientists [2].
However, the quality of solar energy is affected by the irradiation, temperature and other meteorological conditions. Its instability makes it difficult to match the construction load, which restricts the development of solar air-conditioning system [3–6]. For example, in office buildings, although the solar air-conditioning system can work normally during the weekend, there is no cooling load. Therefore, a cold storage method is necessary for the efficient utilization of solar air- conditioning systems in office buildings. In order to alleviate the difference between energy supply and demand, solar air-conditioning systems with an energy storage device were proposed [7]. Through storing the cooling capacity produced by the solar air-conditioning system, it can assure the continuous and stable energy supply, which could meet the requirement of the various periodic variations of the load in office buildings.
The research into solar absorption refrigeration machines started early in many countries. Wilbur and Mitchell [8] and Ward [9] contrasted the characteristic parameter (COP) of absorption refrigeration systems with different working mediums. They indicated that the higher COP could be achieved by using LiBr-H2O as the medium, and the COP using NH3-H2O was lower than that using LiBr-H2O, which had higher equipment requirements. A solar absorption refrigeration system was installed in the Kuwaiti Ministry of Defense office building in 1983, and it operated well until 1995 [10]. Best et al. [11] analyzed several characteristics (i.e. solar collection efficiency, solar energy utilization ratio and refrigerating machine efficiency) of a solar absorption refrigeration system built in Mexicali city, Mexico. Florides et al. [12] modeled one LiBr-H2O system with TRNSYS, according to the weather parameters of Nicosia during a typical meteorological year. They determined the optimal size of the storage tank, collector slope and area, and the optimal thermostat setting of the auxiliary boiler through optimization. The solar absorption air- conditioning system using evacuated tube solar collectors and LiBr absorption unit was presented and simulated by Assilzadeh et al. [13] based on TRNSYS.
There are also many investigators focusing on research for the components of system. Ghaddar [14] studied the effect of two different conditions on the utilization ratio of water tank, including the fluid temperature with well stratified temperatures, and the cold and hot water mixed completely. Their results indicated that the utilization ratio of a water tank with well stratified temperatures was about 6% higher than when mixed completely, and the operation efficiency of total system improved about 20% with a well stratified of water tank. Lavan et al. [15] explored the effect of the ratio of height to diameter for a water tank and the form of the cold- water inlet on the temperature stratification. They found that the temperature difference between the supply and return water was larger, and the fluid in the water tank was stratified more easily, as the ratio of height to diameter for the water tank increased. Mather et al. [16] proposed a large energy storage method formed by a series of water tanks. It was verified that each tank could maintain a stratified temperature well through simulation, which resulted in the fact that the water tank storage system could reach a stabilized state, when the solar radiation varied with day and night. Rosiek et al. [17] built the solar cooling storage air-conditioning system containing two chilled water storage units, and analyzed the energy consumption and efficiency of the system through experiments. Helm et al. [18] presented a solar absorption air-conditioning system which adopted the chilled water storage tank, replacing the cooling tower to eliminate condensing heat.
The research and development of solar absorption air-conditioning systems started relatively unbalanced around the world. Wang et al. [19] combined the solar absorption refrigeration technology and novel potential energy storage technology, and took one air system of a building as an example to research the operation performance. It indicated that the storage/refrigeration system using solar energy based on water cooling mode can meet the requirements of building cooling load in the whole cooling season, on the condition of continuous sunny days. Moreover, the system had an automatic regulating function; and during the non-design days, the energy collected and consumed reached the balance through a change in the initial parameters of the solution in the tank. A series of theoretical and experimental studies were carried out by Wang et al. [20] about phase change materials, cold storage tanks and solar air- conditioning systems integrated with cold storage. The cold storage process in solar air-conditioning systems under the unsteady-state working conditions was simulated, based on the experimental curve of the chilled water temperature of a solar absorption chiller and a solar adsorption chiller. Through the analyses of the temperature distribution and temperature variations within the capsule, some useful references were acquired for optimizing the control and operation of the solar air-conditioning system.
Bu et al. [21] introduced a kind of multi-stage tank heat storage system for solar energy air-conditioning systems. On the basis of theoretical analysis, the system of multistage tanks was built to study the performance of the multistage tanks system with different series. According to the characteristics of the novel mixed solar absorption cycle, an energy storage system with high efficiency was presented by Wan et al. [22]. In terms of the design of storage tank in the air-conditioning system, Lu et al. [23] performed the comprehensive analyses by hourly solar radiation and practical projects. The principles for the size selection of storage tank and the design methods of its structure and insulation were obtained by calculations. Meng et al. [24] studied an integrated system combined solar energy and conventional energy to supply heating, hot water and air- conditioning. The heating and refrigeration potential storage technologies were utilized in the system. Further, they proposed the working process and calculation model of winter heat pump and summer refrigerating.
These studies play an important role in the development of SACS. However, the mismatch between energy supply and demand is still a problem for SACS. As can be seen in Fig. 1, SACS can work normally during the weekend (20170617–18), but there is no cooling load for office buildings. At present, energy storage technology has been widely used in solar air-conditioning technology as a means of adjusting and alleviating the mismatch between energy supply and demand. Energy storage technology mainly includes sensible heat energy storage, phase change energy storage and thermochemistry energy storage. The approaches that are widely applied in cooling storage are through the water tank with thermal preservation and phase change materials in solar absorption refrigeration systems.

Fig. 1 The solar irradiation, dry-bulb temperature and cooling capacities of water chilling units for SACS of an office building in Beijing China from 06/14/2017 (Wednesday) to 06/20/2017 (Tuesday)
In this paper, we took the absorption refrigerating unit as an example, and simulated the solar air-conditioning system of a nearly zero energy office building in Beijing. Moreover, based on the simulation data, the cold storage capacity of the solar air-conditioning system in different climatic regions was studied.
2. Solar air-conditioning storage system
2.1   System design
In the office building, there were 40 offices and 5 conference rooms, and the total floor area was 4025 m2 [25]. In this paper, TRNSYS was used to simulate the operation of the solar air-conditioning system [26], and the schematic of the solar air-conditioning system of this office building is shown in Fig. 2. As can be seen from Fig. 2, the system was mainly composed of a vacuum tube solar heating collected panel, hot and cold- water storage tank, vertical u-shaped tube, single-effect LiBr absorption refrigerator, two ground source heat pump units, and so on [27]. In addition, performance parameters of main components are shown in Table 1.
This system could satisfy the winter heating and summer cooling requirements. The systems were composed of three water chilling units, which included one absorption refrigerator and two ground source heat pumps. The absorption refrigerator and ground source heat pump 1# were on manual standby to undertake the new wind load of the whole building, while ground source heat pump 2# was used to undertake the other load. The water system of the ground source heat pump provided cooling water for all the units, and the solar heat collecting system provided the hot water for absorption refrigerator.
2.2   Control strategy for solar system in summer
The control strategy for the solar air-conditioning system in summer is shown in Fig. 3.

Fig. 2 Schematic of solar air-conditioning system in office building used for this study and the photograph of vacuum tube solar heating collected panel
Table 1 Performance parameters of solar equipment
Vacuum tube solar heating collected panel320 m2
Heat water
storage tank
HWT110 m3
HWT230 m3
Cold water
storage tank
CWS115 m3
CWS215 m3
rated refrigerating35.2 kW
rated power0.21 kW
Ground source
heat pump 1#
rated refrigerating50 kW
rated power of refrigeration9.3 kW
rated heating51.7 kW
rated power of heating13.1 kW
Ground source
heat pump 2#
rated refrigerating99.5 kW
rated power of refrigeration19 kW
rated heating115 kW
rated power of heating24.7 kW

Fig. 3 The control strategy for the solar air-conditioning system in summer
Model 1: The load was provided only using cold water storage tank.
Model 2: Only operate absorption refrigerator.
Model 3: Only operate ground source heat pump 1#.
Model 4: Only operate ground source heat pump 2#.
Model 5: Simultaneously operate absorption refrige- rator and ground source heat pump 2#.
Model 6: Simultaneously operate ground source heat pump 1# and 2#.
Model 7: Operating absorption refrigerator storing cooling to storage tank.
3. Parameters
3.1   Solar energy collector model
A vacuum tube solar heating panel was utilized in this system [28]. The heat collecting efficiency of the flat plate collector was determined by the intensity of solar radiation on the slope, inlet and outlet fluid temperature of the flat plate collector and ambient temperature [29]. The efficiency of the flat plate collector can be calculated by Equation (1):
where a0 represents interception (maximum) collector efficiency; a1 is the negative of the first order coefficient in the collector efficiency equation, in kJ/(h·m2·K); a2 is the negative of the second order coefficient, kJ/(h·m2·K); IT is the total radiation of solar energy collector (inclined plane), kJ/(h·m2); Ta is the ambient temperature, °C; Tav is average temperature of fluid in collector, °C; T0 and Ti are outlet and inlet temperature of fluid in collector, respectively, °C.
3.2   Heat storage water tank model
The capacity of the heat storage water tank was determined by the maximum heat storage of the system. The amount of heat storage was influenced by the effective heat collected in the solar energy collector and the dynamic rule of heat demand of the system [30]. Thus, the capacity of the water tank can be calculated with the estimation method, presented by Equation (3):
where V is capacity of water tank, m3; Qz is effective heat collected by solar energy collector, kJ; Qy is heat demand with sunshine condition, kJ; (ρχ)w is the volumetric specific heat of heat storage fluid, kJ/(m3·°C); Δt is the temperature difference of heat storage fluid, °C; and ηx is heat deficit rate of heat storage fluid.
3.3 Single-effect LiBr absorption refrigerator model
The energy balance formula in the equipment is presented as Equation (4):
where is the removed energy from the cooling water, kJ/h; is the removed energy from the chilled water, kJ/h; is the removed energy from the hot water, kJ/h; is the power consumption of the pump, kJ/h.
The return temperature of hot water can be presented as follows:
where Thw,in is the inlet water temperature on the side of the hot medium, °C; and Thw,out is the return water temperature on the side of the hot medium, °C. Cphw is the specific heat capacity of hot medium, kJ/(kg·K).
The return temperature of cooling water can be presented as follows:
where Tcw,in is the inlet water temperature on the side of the cooling medium, °C; Tcw,out is the return water temperature on the side of the cooling medium, °C. Cpcw is the specific heat capacity of cooling medium, kJ/(kg·K). The refrigeration efficiency of refrigerator can be calculated by Equation (7):
4. Results and Discussion
4.1   Model performance
Table 2 Average daily temperature and total irradiation in June
ItemsJune 14June 18June 21
Average daily temperature /°C28.1128.3927.08
Average total irradiation /MJ·m−226.7117.8614.73

Fig. 4 Hourly temperature and solar radiation change

Fig. 5 Simulated and experimental results of outlet temperature of chilled water for refrigerator
4.2 Cold storage capacity of different climate regions

Fig. 6 Simulated and experimental results of inlet temperature of hot medium for refrigerator
Table 3 Results of simulation and experimental values
average temperaturemaximum relative error/
relative error/
relative error/
simulation result/°Cexperimental result/°C
0614chilled water7:45-16:3013.4913.6119.870.190.94
heating medium water78.4577.345.070.171.44
0618chilled water8:15-16:3014.2915.0213.60.014.86
heating medium water74.9174.693.970.010.29
0621chilled water8:15-14:009.249.584.780.374.51
Table 4 Cold storage capacity of the SACS. (20 cities, 65 typical days)
Climatic regionsCities (latitude)MonthRefrigerating capacity/
Refrigerating capacity per power of refrigerator/MJ·kW−1Daily average temperature/
Daily average COP value of unitRadiation (8:00–17:00)/ MJ·m−2
Cold regionsBeijing (40°)61136.4316.1424.600.6013.73
Dalian (39°)71070.0615.2023.100.6113.49
Xi’an (33°)61800.3125.5724.600.6221.77
Yinchuan (38°)61851.4226.3021.100.6220.10
Urumqi (44°)71547.2121.9826.800.6019.49
Mild regionsKunming (25°)61020.9614.5020.300.6013.40
Guiyang (26°)61303.9318.5222.700.6015.32
Hot in
summer and
warm in winter
Guangzhou (23°)51118.2915.8826.100.6212.85
Haikou (20°)51416.1820.1229.200.6016.40
Nanning (23°)51171.6416.6426.400.6312.92
Hot in
summer and cold in
Chengdu (30°)6905.1012.8625.500.6011.94
Guilin (25°)51641.3623.3126.500.6219.55
Continued Table 4
Climatic regionsCities (latitude)MonthRefrigerating capacity/
Refrigerating capacity per power of refrigerator/MJ·kW−1Daily average temperature/
Daily average COP value of unitRadiation (8:00–17:00)/
Hot in
summer and cold in
Shanghai (31°)61394.8719.8124.900.6316.28
Wuhan (30°)61180.6716.7724.800.6113.51
Hefei (32°)61387.1719.7025.500.6315.10
Server cold regionsHailar (49°)71564.2122.2220.500.6218.17
Tsitsiha (47°)71057.6815.0223.500.6213.19
Shenyang (41°)71163.9916.5325.700.6313.34
Siping (43°)7906.6612.8824.800.6011.58
Changchun (44°)71074.3615.2622.800.6013.67

Fig. 7 The relationship between the refrigerating capacity and the intensity of solar radiation during the operation time
5. Conclusions
The method for the SACS cold storage capacity in office buildings was performed. A SACS for an office building in Beijing was simulated with TRNSYS. The SACS operation data for this building during the summer season were used to verify the accuracy of the model. The chilled outlet and hot inlet temperature to and from the absorption refrigerator were selected. The maximum and minim relative error of outlet chilled water temperature for the refrigerator between the simulated and experimental results were 4.86% and 0.94% respectively. For the inlet temperature, they were 1.44% and 0.29%, respectively. The reliability of this method can be trusted with similar trends between the simulation and experimental results.
The cold storage capacity of SACS in 20 cities, which belong to different climate regions, was calculated, during operating time of SACS for 65 typical days. The volume of phase change material was 1/8 to the volume of water for a weekend in Beijing. The simulation results revealed that the cold capacity of the absorption refrigerator will increase with the increase of solar radiation.
This method can be used to design a reasonable SACS in different cities, and will enlarge the application of SACS. Further work on the SACS in an office building will focus on the match of the cold storage and heat storage. The phase change material will be selected.
This work was funded by the National Key R&D Program of China (No. 2017YFC0702600).
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Article and author information
SUN Zhifeng1,2
ZHAO Yaohua1
XU Wei2
WANG Dongxu2*
LI Huiyong3
ZHANG Xinyu2
LI Huai2
LI Yang4
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Published: Nov. 5, 2019 (Versions1
Journal of Thermal Science