In recent years, considering the actual state of final energy consumption in Japan, energy conservation measures in the civilian sector, especially in the air conditioning field are urgent cases. In general, the air conditioning system is said to account for about 70% or more of the operation time with a load factor of 50% or less, and it is required to improve the partial load efficiency during low load factor operation. Also, looking at stocks of existing buildings by size, small buildings of less than 1,000 square meters account for about 90% of the total, but due to economic restrictions greatly, the energy management of the system is almost none.
In this research, we aim to improve the operation efficiency of the system and reduce energy consumption, and also propose an improvement method of the system for the centralize air conditioning system of the existing small and medium-sized building. We focused on the fact that the required heat capacity of the thermal storage tank can be drastically reduced by adopting a period shorter than one hour, the heat cycle of charge and discharge instead of heat cycle of day which is adopted mostly by conventional heat storage technology. As a result, it was possible to construct a heat storage system using a small thermal storage tank, and to propose an improvement method of a system that can be easily introduced into existing small and medium - sized buildings.
The TES tank is added on a bypass pipe and charges heat when the volume of chilled water from the heat-source equipment exceeds that required by the secondary side. Simultaneously, the heat-source equipment improves the operational load factor as heat is charged in the TES tank. Once charge process done, chilled water is supplied to the secondary side from the tank. Also, the heat-source equipment and the chilled-water pumps can be stopped their operation. This system is considered acceptable at the heating season, because the daily average of the operational load ratio is relatively low.
The unit size of the TES system is 1 m3, and charge and discharge process is induced by temperature stratification inside the tank. There are 56 phase change materials (PCMs) arranged horizontally, determined by CFD analysis.
Two-dimensional TES simulation model was designed and verified by experimental equipment. The difference of charge-discharge rate between experiment and simulation was within 5% in cases of every flow rate, therefore it can be said that the simulation model has enough accuracy.
We estimate the effect of the operating efficiency improvement and energy saving of the HVAC/TES system caused by introducing the proposed system by using a simulation model. The model building is a ten-story office building, located in Tokyo. The simulation period is four months from December to March where the air conditioning system is in heating operation, and the calculation interval is 2 seconds. As a result of the simulation, the heat source equipment energy consumption in the heating period is reduced by 30% by improving the operation efficiency of the heat source equipment during the heat charge/discharge cycle, confirming the effectiveness of the proposed system.
