If I understand it correctly the current state of the art GSHP system design is based upon infinite thermal stability of arrays of adequately spaced, U-Tube assemblies in vertical bore holes. Aside from the low thermal conductivity of the plastic assemblies further exacerbating the inherent losses of the U-tube design their intrinsic low conductive area/working fluid volume ratio require significant flow rates to maintain turbulence resulting in large capacity distribution valves and manifolds, a related increase in first cost and higher operation costs due to pumping losses and maintenance.
With the availability of hydro-excavation technologies it is feasible to excavate narrow slots which reach depths of 15-20 feet where the average soil temperature lag relative to ambient conditions is adequate for efficient ground source heat pump use. The comparatively low cost and large resultant heat transfer areas of such narrow trenches in conjunction with the use of (2 sided) flat plate heat exchangers would seem a potential improvement over current designs. The large area/volume ratios of such HX's coupled with high thermal conductivity of (coated) aluminum alloy plates would allow for compact thermal arrays which could be cycled seasonally based on thermal saturation.
Further, since most GSHP systems are cooling dominated and several studies have demonstrated the improved overall efficacy of hybrid systems using chillers or sky-radiant cooling to offset thermal saturation it seems possible that high density, shallow heat transfer arrays could be conditioned thru direct moisture convection / conduction using local grey water outputs. Systems could be designed to cycle between arrays depending upon conditions against diurnal and seasonal loads.
I'm curious if such system designs have been explored relative to Life Cycle Assessments of system costs of vertical bore hole designs. Any input is appreciated.
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