Downhole Heat Exchangers
A downhole heat exchanger, also called a borehole heat exchanger, is a heat exchanger installed inside a vertical or inclined borehole. It is used to capture or dissipate heat to or from the ground. DHT's are used for geothermal heating, sometimes with the help of a geothermal heat pump.
The down hole heat exchanger is of a design that eliminates the problems associated with disposal of geothermal water since only heat is taken from the well. These system can offer significant savings over surface heat exchangers where available heat loads are low and geologic and ground water conditions permit their use.
The use of a DHE for domestic or geothermal space and domestic water heating has several appealing features when compared to the alternative geothermal heat extraction techniques. It is essentially a passive means of exploiting the geothermal energy because in marked contrast to the alternative techniques, no water is extracted or flows from the well. Environmental and institutional restrictions generally require geothermal water to be returned to the aquifer from which it was obtained. Therefore, techniques involving removal of water from a well require a second well to dispose of the water. This can be a costly addition to a small geothermal heating project. The cost of keeping a pump operating in the sometimes corrosive geothermal fluid is usually far greater than that involved with the maintenance of a Downhole Heat Exchanger.
The principal disadvantage with the DHE technique is its dependence on the natural heat flow in the part of the hot aquifer penetrated by the well. A pumped well draws in hot water and the resultant heat output is normally many times the natural value. This limitation on the potential heat output of a DHE makes it most suitable for small to moderate sized thermal applications.
DHE output range from supplying domestic hot water for a single family at New Mexico to a High school in Oregon. The single family is supplied from a 40 ft well and the school at over one Mwt from a 560 ft, 202°F, 16 in. diameter well. The DHE's are also used in New Zealand, Austria, Turkey, USSR and others.
The most common DHE consists of a system of pipes or tubes suspended in the well through which clean water is pumped or allowed to circulate by natural convection. In a standard design, the wells are 10 or 12 in diameter drilled 20 or more ft into geothermal fluids and an 8 in. casing is installed. A packer is placed around the casing below any cold water or unconsolidated rock usually 20 to 50 ft and the well cemented from the packer to the surface. The casing is torch perforated in the live water area and just below the static water level.
Perforated sections are usually 15 to 30 ft long and the total cross sectional area of the perforations should be at least 1-1\2 to 2 times the casing cross section. Because fluid level fluctuate summer to winter the upper perforations should start below the lowest expected level. A 3\4 or 1 in. pipe welded to the outside of the casing and extending from ground surface to below the packer permits sounding and temperature measurements in the annulus and is very useful in diagnosing well problems
The space heating DHE is usually 1-1\2 or 2 in. black iron pipe with a return U-bend at the bottom. The domestic water DHE is 3\4 or 1 in. pipe. The return U bend usually has a 3 – 5 ft section of pipe welded on the bottom to act as a trap for corrosion products that otherwise could fill the, U-bend preventing free circulation. Coupling should be malleable rather than cast iron to facilitate removal.
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