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On the penalty temperature in the design of borehole heat exchangers for ground-source heat pump systems

pubblicazioni - Articolo ISI

On the penalty temperature in the design of borehole heat exchangers for ground-source heat pump systems

In winter the geothermal heat pumps can lead to a substantial lowering of the temperature of the soil in the neighborhood of the heat exchangers in the ground, producing a phenomenon of progressive depletion of the geothermal source. The method developed by RSE for the evaluation of the temperature penalty was applied to a real case of residential heating.

Better energy efficiency of ground coupled heat pump systems in comparison with traditional applications leads to continued growth in the number of installations for space conditioning. The solution with vertical heat exchangers is the most widespread. The design of the borehole heat exchangers (BHEs) is a nodal point, both for the energy efficiency and the economic point of view. In literature, several methods to design these systems are available. However, to promote their application, easy-to-use procedures for designers are required. Among the literature models, the ASHRAE method is surely the simplest procedure and, as a consequence, it is suitable for this goal. In this approach, sizing of the BHE length is strongly affected by the parameter named penalty temperature, which is an index to evaluate the long-term behaviour of the borehole field. In this paper, a review of this index is reported and in addition a new approach for its evaluation is presented. Furthermore, a detailed analysis of the proposed method is performed on a real case-study building with only heating conditioning.

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