Graduate Thesis Or Dissertation
 

Modelling and flow measurement of shallow geothermal systems with downhole heat exchangers

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  • The low temperature geothermal deposits in the Klamath Falls area are commonly used for heating buildings. The system often used to accomplish this consists of a heat exchanger that extends the full length of the casing within the well. This casing has slots cut in it to allow geothermal water from the aquifer to circulate through the casing and past the heat exchanger. A mathematical model for the thermosyphoning in this system was developed. The model was extended to include: a partial-length heat exchanger, the effects of the mixing of the fluid from inside the casing with that from the aquifer, and the effects on heat transfer coefficients caused by the scaling of the casing and heat exchanger. A parametric study of the system showed that the energy extraction rate from the heat exchanger is largely dependent upon: the temperature of the reservoir, the surface area of the heat exchanger, the ratio of the flow area inside the casing to the area of the annulus between the casing and the well bore, and the degree of mixing between the water from the aquifer and the cooler water from inside the casing. Flow measurements were made to verify the mathematical model. A hot-film anemometer was used to measure the convective flows within the casing but its use is greatly limited because the coating that protects the hot-film does not hold up at the temperatures encountered in a geothermal well. Construction of an energy balance around the heat exchanger in the well made it possible to estimate the convective flows within the well. Alternative designs for heat exchangers were examined. A short multi-loop heat exchanger was designed, built, and tested. Initial results indicate that this type of heat exchanger could be used to replace the full-length heat exchangers.presently in use for residences.
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