Abstract
In ground-source heat-pump systems, the heat exchange rate is influenced by various design and operational parameters that condition the thermal performance of the heat pump and the running costs during exploitation. One less-studied area is the relationship between the pumping costs in a given system and the heat exchange rate. This work analyzes the investment and operating costs of representative borehole heat-exchanger configurations with varying circulating flow rate by means of a combination of analytical formulas and case study simulations to allow a precise quantification of the capital and operational costs in typical scenario. As a conclusion, an optimal flow rate minimizing either of both costs can be determined. Furthermore, it is concluded that in terms of operating costs, there is an operational pumping rate above which performance of geothermal systems is energetically strongly penalized.
Highlights
The thermal efficiency of a borehole heat exchanger is characterized by its thermal resistance, i.e., the thermal resistance between the circulating fluid and the borehole wall
The procedure used for the analysis of the Thermal Response Test (TRT) is explained in detail in [37] and the parameters thermal conductivity of the ground (λ), borehole thermal resistance (Rb ), undisturbed ground temperature (T0 ) and ground thermal diffusivity (α) are drawn by means of a method of adjustment to the main models
An extensive theoretical and numerical tool was developed to filter, refine and select optimal borehole configuration meeting the required criteria arising from the installation and conductivity material and evaluating the correlation between borehole thermal efficiency (Rbe f f ) and borehole pressure losses depending on the fluid flow rate
Summary
The thermal efficiency of a borehole heat exchanger is characterized by its thermal resistance, i.e., the thermal resistance between the circulating fluid and the borehole wall. This parameter originally was defined by Mogenson [1] and widely analyzed by Eskilson [2] and Hellström [3]. (iv) the thermal conductivity of the ground (λ), the thermal resistance of the borehole heat exchanger (Rb ), the undisturbed ground temperature (T0 ), and, the injection (or extraction) of heat ratio (thermal power input) (q) (that depends on flow rate and temperature gap of working fluid). By improving the thermal efficiency of the borehole, either the number of drilling meters can be reduced (while maintaining the thermal efficiency of the heat pump) or the average working fluid temperature of the borehole can be improved (while improving the thermal efficiency of the heat pump, maximizing the system efficiency)
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