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International Heat Transfer Conference 15

ISSN: 2377-424X (online)
ISSN: 2377-4371 (flashdrive)

Experimental study on ground source heat pump by direct expansion method

DOI: 10.1615/IHTC15.acr.008940
pages 949-962

Tetsuaki Takeda
Graduate School of Medicine and Engineering, University of Yamanashi, Takeda 4-3-11, Kofu, Yamanashi 400-8511, Japan

Daiki Yokoyama
University of Yamanashi

Akio Ohashi
University of Yamanashi

Shuhei Ishiguro
University of Yamanashi

Shumpei Funatani
Graduate School of Medicine and Engineering, University of Yamanashi, Takeda 4-3-11, Kofu, Yamanashi 400-8511, Japan

Koichi Ichimiya
Department of Research Interdisciplinary Graduate School of Medical and Engineering, Department of Mechanical System Engineering, University of Yamanashi, Japan


KEY WORDS: Air conditioning and refrigeration, Geothermal energy, Ground source heat pump

Abstract

The ground source heat pump (GSHP) uses pipes which are buried in the ground to extract heat from the ground. This heat can be used to heat radiators, under-floor or warm air heating systems and hot water in the house. The GSHP are adopted a closed loop and vertical system. The ground stays at a constant temperature under the surface, thus the heat pump can be used throughout the year. In a conventional system which is called an indirect heat exchange method, the vertical systems use two long pieces of pipe with a U-shape at the bottom in a hole bored in the ground. The GSHP circulates a mixture of water and antifreeze around a loop of pipe, called a ground loop, which is buried in the ground. On the other hand, the GSHP by a direct expansion method circulates a mixed refrigerant in the ground loop. The length of the ground loop depends on the size of the house and the amount of heat we need. Longer loops can draw more heat from the ground, but need more space to be buried in. The depth of the boreholes of the conventional GSHP will need 100m or more. However, it is possible to reduce the depth of the boreholes in the GSHP by the direct expansion method. In this experiment, the depth of the boreholes was 30m and R410A was used as the refrigerant. The heat exchanger of the ready-made air-conditioner was replaced by the underground heat exchanger. The underground heat exchanger consisted of the narrow copper tubes inserted into the bottom end long pipe which was filled with water. Resistance thermometer and thermocouple were adopted as a measuring device of temperature. The temperatures of the refrigerant and the copper tube were measured at various points of the heat pump and the ground loop. The temperature and the flow velocity of air in the air-conditioner were measured. The flow rate and the pressure of the refrigerant in the primary loop were also measured. The results of the experiment under the cooling condition were obtained as follows. We operated 4kW air-conditioner using one 30m borehole. In the preliminary experiment the coefficient of performance (COP) was about 4.7 or more. The COP of the GSHP by the direct expansion method was obtained and the amount of heat transferred by the underground heat exchanger was discussed.

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