engleski

Impact of borehole cement-bentonite grout thermal conductivities on a long-term ground-source heat pump efficiency

Qualitative cementation of a bore heat exchanger is one of the most important parameters when analyzing long-term efficiency of the ground-source heat pump system. Depending on the geological setting and techno-economic aspects, numerous combination of grouts are used today in practice, with different content of bentonite, cement, sand, water and some additives. Using low conductivity grouts in the ground with good thermal conductivity, leads to increase of equivalent borehole thermal resistance. Since most of the heat pumps operating today have start/stop compressors, there is frequent occurrence of unsteady-state type of heat transfer which leads to rapid sub-cooling of the working fluid inside the pipes. If grout thermal properties are significantly lower than ground conductivity, appearance of high borehole thermal resistance will lead to lower average monthly entering source temperature to the heat pump. This will lead to worsen seasonal performance factor of the system, especially in long-term. This paper will show how much certain grouts mixes affects efficiency and economy of the ground-source heat pump, using simulation analysis of the borehole field for 30 year operation.

thermogeology; borehole heat exchanger; borehole resistance; thermal conductivity; Thermal Response Test (TRT)

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