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Calorimetry of ground source heat pump using direct expansion method : measurement of flow rate using PIV method

The GSHP that use a direct expansion method is expected to have higher performance of energy-saving than the conventional type of the GSHP. We evaluated the quantity of heat by measuring temperature, humidity and flow rate at the indoor unit of the GSHP system. The purpose of this study was to improve the accuracy of the flow rate measured by hot wire anemometers using PIV method. The average of difference between the flow rate obtained by the hot wire anemometers and the PIV method was 23.6% in the heating operation and was 24.0% in the cooling operation. We obtained the compensation formula which corrects the result of the hot wire anemometers using the results of PIV method. The average of difference between the compensated flow rate by the hot wire anemometers and the flow rate by the PIV method was reduced to 5.2% in the heating operation and 1.9% in the cooling operation. The compensation formula is used performance evaluation of the GSHP system. The COP obtained by conventional method was 3.4 in the heating operation. The compensated COP was 2.8. INTRODUCTION Recently, ground source attracts attention as renewable natural energy. The underground temperature is stable throughout a year and is lower than the air temperature in summer and higher than that in winter. [1] Thus, a ground source heat pump (GSHP) has superior performance of energy-saving. A GSHP that uses an indirect heat exchange method has already been commercialized so far. In the conventional type of the GSHP, water or antifreeze are used as a heat exchange medium flowing into the underground. The depth of a borehole is about 100 m. This system does not spread widely in Japan, because initial investment cost (particularly, excavation cost of borehole) is high. [2] On the other hand, the other type of the GSHP that use a direct expansion method is expected to have higher performance of energy-saving than the conventional type of the GSHP. The GSHP using direct expansion method uses a copper tube in which the refrigerant flows through between the indoor unit and the underground heat exchanger. This system performs direct heat exchange with underground. Therefore, the depth of a borehole can be less than 100m. It is difficult to obtain the amount of heat exchanged of the GSHP using the direct expansion method because the state of refrigerant in the copper tube is not stable. [3] We evaluated the quantity of heat by measuring the enthalpy difference of air at the indoor unit of the GSHP system using direct expansion method. The enthalpy was obtained by measuring temperature, flow rate and humidity at the inlet and outlet of the indoor unit. Four units of hot wire anemometers were used for flow rate measurement. [4] However, as the distribution of the flow velocity in the outlet of the indoor unit was not uniform, it was difficult to obtain the average flow rate from only 4 points of velocity. This paper describes the improvement to the accuracy of the flow rate using the PIV method. The PIV method has higher accuracy than hot wire anemometers in measuring flow rate. The result of measuring the flow rate by PIV method compensates for the flow rate measured by the hot wire anemometers. EXPERIMENT Experimental Apparatus Figure 1 shows a schematic diagram (heating operation) of the experimental apparatus, which is GSHP system using a direct expansion method set up at University of Yamanashi. The GSHP system consists of the heat pump that the air/refrigerant heat exchanger replaced by the borehole-type underground heat exchanger made by copper tubes. The four way valve changes the circulating direction of a refrigerant when operation mode is changed. The underground heat exchanger is two long pipes connected by a U-shaped fitting at the bottom of a hole bored. The size of a borehole is 165.2 mm in diameter and 30m in depth. 10.0kg of R410A was used as a refrigerant and was mixed with compressor oil. Power of the indoor unit is 2.8kW in the cooling operation / 4.0kW in the heating operation. Heat transfer area of air conditioner is 17.4 m 2 . As Figure 2 shows, a rectangular duct is set at the outlet of the indoor unit. Cross sectional area of air in outlet of the duct is 0.0495m 2 . Two thermometers (Pt100: class A), two hygrometers, and four hot wire anemometers are set to the indoor unit. Temperature and humidity are measured at inlet and outlet of the indoor unit. Flow velocity is measured at outlet the duct. We used the smoke generator and the diode laser (532nm, 1000mW) for visualization of air flow. The smoke generator supplies smoke with inlet of indoor unite. The diode laser visualizes particle of smoke at outlet of indoor unite. The high speed camera (1000fps, 640x480 pixels) takes pictures of visualized particles. The diode laser and the high speed camera set on the traversing device. The position of the diode laser and 12th International Conference on Heat Transfer, Fluid Mechanics and Thermodynamics