SUSTAINABILITY OF LOW-ENTHALPY GEOTHERMAL SYSTEMS – INSIGHTS FROM HANTUSH'S LEAKY AQUIFER

The Earth's ability to store and transmit thermal energy will be a key factor in the Nation's ability to reduce its reliance on fossil fuels and emissions of greenhouse gases. In the United States, direct-use and heat-pump applications of geothermal energy are the only form of renewable energy that has sustained increases in the commercial and residential sectors over the past 15 years. With the primary application being for space conditioning of residential and commercial buildings, the ubiquitous nature of low-enthalpy reservoirs has significant benefits over the distribution systems required for fossil fuels and electricity.

In the Northeast U.S., there is considerable interest in ground source heat pumps for residential heating. Used in the heating (heat extraction) mode only, the sustainability of these systems will require a mechanism for drawing heat towards the well. Satisfying the typical residential heating load in the Northeast (10kW) requires a combination of capturing the natural geothermal heat flux(40mW/m2), inducing recharge from the ground surface, or relying on the advective transport of heat with the groundwater.

Hydrologists have historically capitalized on the mathematical similarities between heat flow and groundwater movement. Here, we use advances in well hydraulics, pioneered by M.S. Hantush, to evaluate the expected distribution of heat flow in low-enthalpy geothermal systems in crystalline bedrock. Because of the very low thermal diffusivity of rock (10E-6 m2/s), the ability to capture the natural geothermal gradient and induce heat flow (leakage) from the surface requires significant thermal drawdowns in the well that potentially jeopardize the efficiency of ground source heat pumps. In the absence of significant advective heat fluxes, sustained efficiency of ground source heat pumps requires injection of thermal energy during the non-heating seasons.