What is Open Loop Geothermal?
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What is Open Loop Geothermal
In an open loop system , a pump draws groundwater from an abstraction borehole and passes it through a heat exchanger / heat pump – where heat is passed to or from the primary refrigerant loop – before returning it to the ground in a separate injection well. The abstraction and injection lines must be placed far enough apart to ensure thermal recharge of the source. Because groundwater comes from a naturally insulated source it is at a relatively constant temperature all year-round and forms an excellent reliable and efficient heat source.
As these systems involve the withdrawal and discharge of water from a natural source, it is important to consult a hydrologist to ensure that this process is done correctly. In a closed loop system, the refrigerant is only heated or cooled a few degrees as it runs through the ground a few inches below the surface – lower temperature differentials and variations in soil temperature result in an inherently less efficient system.
Injection well systems
This well should be drilled at the same time as the supply well, and be deep enough to receive the maximum amount of discharge water from the heat pump. It should be located at least 30 meters from the source well and should recharge directly into the same aquifer from which it came.
A major consideration is that a typical injection well will only accept 50 to 75 percent of the water from an abstraction well of the same dimensions. When water is withdrawn from a well, a cone of depression ( The shape of the water flowing into the well ) forms with the low point being at the well screen.
The opposite occurs in a injection well but, in this case, the high point of the cone of impression is at the well casing which, of course, is impervious. If there's a high static water level in the well, say 6 to 10 meters, the injection water can rise enough to bubble out of the well cap or overflow pipe. A prudent driller should take this into account when drilling the injection well and ensure that the injection well is larger than, or at least the same diameter as, the supply well and that it also has twice the screen area. Finally, the injection well recharge pipe should terminate below the static water level to minimize encrustation and other mineral precipitation.Most of the problems encountered with early installations have been overflow problems.
Since the water is to be returned to the environment and its chemistry cannot, therefore, be artificially controlled in any way, the system may need to be protected from corrosion by careful choice of materials for the heat exchanger and pump. Similarly it is not possible to dose the water to prevent the formation of lime scale or other mineral deposits which can build up over time – careful analysis of the water beforehand should foresee any such potential problems and enable a routine of periodic closed-circuit chemical purging to be formulated.