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Influence of Undisturbed Ground Temperature and Geothermal Gradient on the Sizing of Borehole Heat Exchangers

Undisturbed ground temperature is one of the most crucial thermogeological parameters needed for shallow geothermal resources assessment. Energy considered to be geothermal is energy stored in the ground at depths where solar radiation has no effect. At depth where undisturbed ground temperature occurs there is no influence of seasonal variations in air temperature from surface. Exact temperature value, and depth where it occurs, is functionally dependent on surface climate parameters and thermogeologic properties of ground. After abovementioned depth, increase of ground temperature is solely dependent on geothermal gradient. Accurately determined values of undisturbed ground temperature, and depth of occurrence, are beneficial for proper sizing of borehole heat exchangers and ground source heat pump system as a whole. On practical example of building being heated and cooled with shallow geothermal resource, via heat pump system, influence of undisturbed ground temperature and geothermal gradient on size of borehole heat exchanger is going to be presented. Sizing of borehole heat exchanger was calculated with commercial software Ground Loop Designer (GLD), which uses modified line source and cylinder source solutions of heat conduction in solids.

Borehole heat exchanger; shallow geothermal energy; geothermal heat pump; undisturbed ground temperature; geothermal gradient

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