Analysis and simulation of ice storage air-conditioning systems

Abstract

Ice?storage air?conditioning is a form of distributed load management. The system makes ice during off?peak periods and uses it to cool the building during on?peak periods. In the present work, an ice?storage air?conditioning system is modelled, taking into account the diurnal variations in cooling load. The space to be conditioned is thermally modelled, considering all the loads entering the space. A procedure for the design of all components of the refrigeration system-namely the evaporator, condenser, compressor, and brine?to?water heat exchanger-is included. A generalised computer program for the simulation and analysis of the system has been developed. The program calculates the air?temperature variations inside the conditioned space over a day and the cooling load. It also designs the evaporator, condenser, compressor, and brine?to?water heat exchanger based on the load characteristics. The program calculates the properties of the refrigerant at various conditions given any two independent properties. Simulation is carried out for both ice?making and air?conditioning modes, for systems with water?cooled and air?cooled condensers, for different places in India, for different months of the year, and for different storage strategies such as full ice storage, partial ice storage, and no ice storage. The program can also analyse different types of buildings with different load profiles, such as offices, cinema halls, auditoria, churches, etc. The input parameters for the program are ambient temperature and humidity, global and diffuse radiation data, type and orientation of the building, storage strategy (percentage), the day of the year, cooling?load factors, type of condenser and evaporator (specified as a code), inner and outer diameters of the tubes, mass?flow rates of brine and water/air, arrangement of the tubes, etc. The program was run on CYBER?992 and typically required 900 seconds of CPU time per execution. In the present analysis, the refrigerant employed is R22 and the secondary refrigerant is a 30?percent ethylene?glycol solution. Some of the conclusions obtained from the analysis are listed below:

By employing ice storage, the system size can be reduced by nearly 30 to 35 percent, resulting in reduced initial system cost. By employing partial ice?storage systems, the peak compressor?power requirement can be considerably reduced, thereby helping reduce the electric?power demand. Since the variation in dry?bulb temperature is greater than the variation in wet?bulb temperature, it is advantageous to employ ice storage for systems with air?cooled rather than water?cooled condensers. By employing partial ice storage, the on?time period of the compressor can be considerably increased, resulting in optimum utilisation of the compressor.