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
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.
3. Use of Heat Exchangers
The SCW design uses a PVC separator or sleeve, which stands as an internal column thermal barrier, isolating thermal water transferred from the building in the annular space, from the interior supply water. SCW use a submersible pump located inside the sleeve and set perhaps 100’ below the static water level. At the base of the PVC separator, there is a perforated section that connects the annular space to the interior space of the sleeve. The forced circulation at the bore wall insures excellent heat transfer directly to the full bore diameter. Since SCW use ground water as their working fluid, there is a need for a heat exchanger located in the building to create separation between ground water and the building heat pump loop. Ground water drawn from the well is returned totally or in part back to the well after building heat exchange. To extend operation late in a heating or cooling season, a small percentage, e.g. 10%, is commonly bled by not returning that portion to the well. When a bleed strategy is employed, the effect is to reduce the heat flux on the bore wall, while at the same time drawing-in fresh ambient water as make-up, thus diluting the water temperature in the bore. Since SCW design requires a submersible pump to pump ground water with variable water chemistry, the installation will require careful selection of pipe and heat exchanger materials; perhaps, filtration equipment, and occasional maintenance, eventually the pump. But, such maintenance may be no more onerous than standard potable water well maintenance.
4. Applications of the Technology
Both geothermal design approaches have value in serving different markets. The residential market, in particular, is 95% served by closed loop installations. To the degree that space allocation (footprint) for the geothermal field is a major limitation, SCW has served that niche quite well over the last 25 yrs. Debatably, larger projects, such as L.E.E.D. commercial buildings, schools and federal facilities, where there is likely to be found a dedicated, or subcontracted, engineer for facilities management, are more likely to consider SCW geothermal designs for their HVAC needs.