Verification of Optimum Operation Method by Simulation for the HVAC System with a Thermal Storage Tank in an Actual Building

An optimum operation scheme was applied to the thermal storage system in an actual building and evaluated using measurement data. First, the accuracy of the load prediction was verified. In a trial operation, the expected error percentage (EEP) of the sum of the daily cooling load was 9.8%, which shows the cooling load prediction is sufficiently accurate for practical use. Second, the accuracy of the system simulation of the HVAC system was verified. The simulation error of the total energy consumption was 0.6% on average, and the root mean square error (RMSE) was 4.9 kWh, which indicates that the simulation is sufficiently accurate for estimating the performance of the HVAC system. Finally, the effect of optimum operation was verified by measurement data. The coefficient of performance (COP) of the heat source system was improved by 3.5% and the COP of the entire system was improved by approximately 10.9% with optimum operation. INTRODUCTION The thermal storage system in a building’s HVAC system theoretically exemplifies the technological advances of energy conservation as well as reduction of CO2 emissions. It has been reported, however, that energy conservation is not achieved in actual operation due to non-optimal system operation. To solve this problem, for example, investigation of operational procedures which intend to minimize energy consumption in a stratified chilled water heat storage tank was reported using measured data of a real system (Bahnfleth, W.P. et al. 1994.) or a method to optimize the set points of HVAC system was discussed (Lu Lu et al. 2005.). However, simulation has not been applied to obtain a total optimal operation strategy of an air-conditioning system with a thermal storage tank. A part of the authors previously proposed a basic methodology for optimum operation of a thermal storage water system based on simulation technology and thermal load prediction (Yoshida et al., 1997). Also, the effect of optimum operation by simulation (Yoshida et al., 1999, 2001, Yamaguchi et al., 2005) was shown in simulation basis. In the present paper, the optimum operation scheme is applied to the heat source system with a thermal storage tank in an actual building and validated using measured real data. PROFILE OF THE BUILDING The building in which the optimum operation was applied is shown in Figure 1. It is an office building built in 1999 in Fukui Prefecture, which is located in the central northern part of Japan. The building has five stories above ground and one below. The total floor area is 10,724 m. This building has two HVAC systems. One is a system with a thermal storage tank for the offices and the other is a system without a thermal storage tank for the machine rooms. The optimum operation scheme was applied to the system for the offices. Figure 2 shows the diagram of the HVAC system for the offices. It includes one air source heat pump and one reciprocating chiller. The air-conditioning system is an all-air, single-duct system which can supply a variable air volume. Figure 1 Appearance of the building