In general, the cooling and heating load of a building fluctuates with time. Accordingly, the heat source equipment of an air-conditioning system without heat storage equipment requires a capacity which can treat maximum load, and moreover, must be operated by partial load at times, while following load fluctuation. On the other hand, in the air-conditioning system with heat storage equipment, the heat source equipment whose capacity is smaller than maximum load is operated with full load neglecting load fluctuation, and surplus energy (chilled or warm water) is stored. The stored heat can be used to cover the shortage in heat source equipment on maximum load. Consequently, in the system where heat storage equipment is employed, capacity of heat source equipment, electric-receiving equipment, other incidental equipment, and machine room area can be minimized if operating time of heat source equipment can be made longer than air-conditioning time zone.

At present, urban pollution constitutes a public problem; atmospheric pollution is produced by exhaust gas when oil or gas is used for the operation of air-conditioning heat source equipment. Regarding this problem, an air-conditioning system whioh is operated only by electric energy is attracting attention. For such a system, a heat pump system with heat storage equipment utilizing nighttime electric power with discounted rate is well known.

Furthermore, the following are advantages of an air-conditioning system with heat storage equipment: the efficiency of heat source equipment is reduced with partial load operation; however, high efficiency can be maintained with heat storage operation, and it is possible to air condition a multipurpose building that has a great variety of air-conditioning time zones with one heat source equipment.

In addition, utilization of heat storage equipment is also effective when energy supply is restricted by time, and is unstable. For instance, when solar energy is used as a heat source of an air-conditioning system, the collection of the energy can be done only during daytime and under fine weather; and changes greatly when affected by clouds and other adverse weather conditions. In order to air condition a building using such an unstable heat source, the energy obtained must be stored in some way. For a similar reason, heat storage equipment becomes necessary to recover waste heat of a building or industrial facility.

By installation of heat storage, several merits can be expected. However, problems such as cost of heat storage equipment arise, and the total quantity of energy for air-conditioning increases.