Thermal diffusivity is a material property which describes the rate at which heat flows through a material, typically measured in mm²/s or in²/hr.
Using a thermal wave to model the ground loop. Say we look at the lateral boreholes as being hypothetically square instead of round. The sides would not provide heat after a relatively short period of time, due to a limited amount of heat stored between the boreholes. However if the upper reservoir of the geothermal leaf takes 2,000 years to draw down 30% then in 200 years it would be down 3%. If the lower reservoir (core of the Earth) does not cool down, then the combined reservoirs would have only a net reduction of 1.5% production capacity over 200 years. The presence of radio active isotopes in the upper reservoir would also reduce the 1.5% decline over 200 years.
If we take the circumference of a 6 inch diameter borehole (18 in) and divide it in half (= 9 in) then a borehole 1,500 ft long would have (1500x12x9=162,000 square in) on the top and an equal amount on the bottom for a total of 324,000 sq in. If our quartzite has a thermal diffusivity of 146 BTU/hr square inch then each borehole would produce 47,304,000 BTU an hour with a decline of 1.5% over 200 years.
If 4,231 MBTU are require for producing 160 Mw of electricity then (4,231,000,000/47,304,000= 89) then around 90 boreholes would be require.
J. David Reynolds
Founder & President
Atlantic Geothermal
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