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Ground-Coupled Heat Pump (GCHP) systems are an attractive choice of system for heating and cooling buildings (Genchi, 2002; Sanner, 2003; Omer, 2008; Urchueguia, 2008). By comparison with standard technologies, these heat pumps offer competitive levels of comfort, reduced noise levels, lower greenhouse gas emissions, and reasonable environmental safety. Furthermore, their electrical consumption and maintenance requirements are lower than those required by conventional systems and, consequently, they have a lower annual operating cost (Lund, 2000). Ground source systems are recognized by the U.S. Environmental Protection Agency as being among the most efficient and comfortable heating and cooling systems available today (US EPA, 2008). The European Community and other international agencies, such as the DOE or the American International Energy Agency, are considering GCHP in the field of "heat production from renewable sources". In 2002, the growth in the number of air conditioning systems driven by ground coupled (geothermal) heat pumps was estimated in the range from 10% to 30% each year (Bose 2002). The number of installed units worldwide, around 1.1 million (Spitler, 2005), illustrates the high acceptance of this emerging technology in the Heating, Ventilation & Air Conditioning (HVAC) market. A Ground Coupled Heat Pump is a heat pump that uses soil as source or sink of heat. A GCHP exchanges heat with the ground through a buried U-tube loop. Since this exchange strongly depends on the thermal properties of the ground, it is very important to have knowledge of these properties when designing GCHP air-conditioning systems. The length of Borehole Heat Exchangers (BHE) needed for a given output power greatly depends on soil characteristics, such as temperature, particle size and shape, moisture content, and heat transfer coefficients. Correct sizing of the BHEs is a cause for design concern. Key points are building load, borehole spacing, borehole fill material, and site characterization. Over-sizing carries a much higher penalty than in conventional applications. Methods to estimate ground properties include literature searches, conducting laboratory experiments on soil/rock samples and/or performing field tests. Due to these factors, the completion of a 3 |
