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 . Solar energy is accessed easily and has enormous potential; thus, the utilization of solar energy has been given more attention by scientists .
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 . 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  and Ward  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 . Best et al.  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.  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.  based on TRNSYS.
There are also many investigators focusing on research for the components of system. Ghaddar  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.  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.  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.  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.  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.  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.  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.  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. . In terms of the design of storage tank in the air-conditioning system, Lu et al.  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.  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.