There are four major differences between standing column well (SCW) and closed loop geothermal systems.
The use of exchange fluid
Loop materials and characteristics
Use of heat exchangers
Applications of the technology
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1. The Use of Exchange Fluid.
The fundamental difference between the two systems is the exchange fluid used to affect the heat transfer. In the case of grouted loops, the working fluid is water with an additive (typ. methanol) contained in the closed HDPE pipe loop that runs from the well to the building. In the case of SCW, the working fluid is well water which fills the borehole from water bearing zones (WBZ) intersected by the bore. There are advantages to each of these design approaches.
2. Loop Materials and Characteristics
The grouted loop provides a continuous leak-tight HDPE wall that is guaranteed for at least fifty years free from defect that requires little to no maintenance. But, this comes with a heat transfer penalty, in that, the plastic wall together with the surrounding grout filler, impose a thermal impedance on the transfer of heat to the bore wall. This is further limited by the HDPE pipe size that can be comfortably be manipulated down the bore, representing approximately 40% of the bore wall surface area. Moreover, the installed loop upcomer and downcomer are not thermally isolated, allowing heat to be shunted, rather than transferred to the bore rock for storage. The net effect of these limitations to heat transfer is to require twice the drilled footage for a given project than SCW design. This is predicated on achieving the same working fluid temperature.