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RESEARCH PRODUCT

Ground Coupled Heat Pumps in Mixed Climate Areas: Design, Characterization and Optimization

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This contribution reviews the research work developed by the authors on the design, characterization and optimization of ground coupled heat pumps in mixed climates areas. The design of a ground coupled heat pump HVAC system starts with the estimation of the thermal loads that the air-conditioned area demands. The capacity of the ground source airconditioning system is determined from this thermal load estimation. With this value and a proper estimation of the ground thermal properties, the characteristics of the water to water heat pump and the required length and layout of the borehole heat exchangers are estimated. Determining ground thermal properties is crucial for an accurate design of the air conditioning system. In situ thermal response tests are carried out to have a measurement at site of ground properties. These tests are based on the Kelvin infinite line source model of heat transfer by thermal conduction. Improvements of this technique can be pursued in different ways. One approach consists in refining the model describing the borehole heat exchanger to include effects not taken into account. Finite length effects can be incorporated in the analysis procedure of test in situ outputs as shown in Bandos et al (2008a), Bandos et al (2008b) and Bandos et al (2009a). A filtering technique of the undesired effect produced in temperature measurements by the ambient temperature can be used to improve the estimating of ground thermal properties (Bandos et al (2009b), Bandos et al (2009c), Bandos et al (2009d)). Another approach to improve the in situ estimation of ground thermal properties is the development of new devices able to measure relevant quantities for the correct characterization of heat transfer between the fluid and the ground. This characterization could be done if the evolution of the fluid temperature along the heat exchanger is known. A sensor probe including a temperature sensor, an acquisition system, temporary storage and wireless communication has been developed to obtain these measurements (Martos et al (2008), Martos et al (2009), Martos et al (2010a), Martos et al (2010b), Martos et al (2010c)). With this new information it will be possible to infer some properties about the ground structure relevant for the design of the ground coupled system.