Analysis of ground source heat pumps with horizontal ground heat exchangers for northern Japan
Introduction
Energy resources of Japan are very limited and for that reason about 97% of oil and natural gas has to be imported; about half of these primary energy sources is converted to the electric power; the commercial and residential sector accounts for about 27% of the total energy consumption; of this, space heating and air conditioning account for 24.5% of the total household electricity consumption [1]. In rural areas of northern Japan, continuous heating (oil-fuelled furnaces) is needed in winter period for numerous greenhouses packed with a large variety of vegetables, while in the mainland Japan huge energy demand for air conditioning arises in summer time. Due to these reasons Japan may face in the near future serious uncertainty regarding the economic growth, keeping up a high living standard, and maintaining international competitiveness. These concerns have stimulated an interest in ground source heat pumps (GSHP) as this technology offers considerable savings of the primary non-renewable energy resources while keeping the surrounding environment nearly intact. Besides, it also offers lower cost of heating/cooling and higher operating efficiency (COP > 3) than electric resistance heating or air-source heat pumps. Potential applications of this technology are in heating/cooling buildings, growing vegetation in greenhouses, drying crops, heating water at fish farms, pasteurizing milk, etc. Moreover, the farming sector offers lesser restriction on the ground availability which is beneficial to the use of horizontal GHE; this in turn could reduce problems related to unstable geology and high installation costs of vertical GHE. The impacts of low thermal conductivity (λ) of volcanic soils on the length of the GHE and long term use of combined heating and cooling operation on the ground environment (degradation of ground thermal and moisture storage capacity) remain however unknown. Therefore, elucidation of these issues is the principal objective of this paper. In addition, this paper intends to examine the potential use of GSHP with a horizontal GHE for residential space heating and cooling in northern Japan.
Section snippets
Ground source heat pump basics
A GSHP unit is an assembly of an electrically driven compressor, two heat exchangers (refrigerant-air and refrigerant-water), and an expansion valve (throttle). Its primary aim is utilization of the ground as a heat source in winter and as a heat sink in a summer period. In winter/summer, natural heat of the ground is absorbed/rejected by an antifreeze solution flowing in the GHE, which can be a series of plastic pipes installed below the ground surface (Fig. 1) or submersed in a water
Energy analysis of ground source heat pumps
Evaluation of the GSHP thermodynamic performance is based on the following energy related characteristics. The total energy consumption (Wo) of the GSHP system includes all system components:
The seasonal heating COP:
The seasonal cooling COP:
Finally, the annual overall COP is defined as follows.where is electrical energy supplied to a compressor; is electrical energy supplied to an indoor circulation fan; is
Meteorological and geological considerations
Computer simulation of a GSHP system is based on comprehensive daily average meteorological data input (e.g. ambient temperature, solar radiation, wind speed, precipitation, cloudiness, etc.). The data used was obtained from National Agricultural Research Center for Hokkaido Region (NARC) in Hitsujigaoka, Sapporo, covering a one-year period from September 1998 to August 1999. The ground geological information (Table 1) was also provided by the NARC from one of its experimental site in Sapporo.
Results and discussion
Preliminary results from several computer simulations indicate that after the third year of GSHP operation, the ground thermal regime remains practically unchanged. Therefore, in order to save computational time and data storage, all simulations were carried up to three years, and the results from the third year of operation were considered for the analysis. For simplicity, desuperheater option was not considered in this study.
Conclusions
Computer simulation and analysis of GSHP system with horizontal GHEs was applied to geological and weather conditions of Sapporo and a typical residential house of about 200 m2 of living space, 5.5 kW of design heating load and 3.3 kW of design cooling load (requiring about 59.6 and 3.80 GJ/year for heating and cooling, respectively). For these conditions, a 5.5-kW GSHP unit installed with a 300 m single-layer GHE buried at 0.5 m depth in a serpentine configuration separated horizontally by 0.5 m (a
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