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Recent national focus on the value of increasing US supplies of indigenous renewable energy underscores the need for re-evaluating all alternatives, particularly those that are large and well distributed nationally. A panel was assembled in September 2005 to evaluate the technical and economic feasibility of geothermal becoming a major supplier of primary energy for US base-load generation capacity by 2050. Primary energy produced from both conventional hydrothermal and enhanced (or engineered) geothermal systems (EGS) was considered on a national scale. This paper summarizes the work of the panel which appears in complete form in a 2006 MIT report, 'The future of geothermal energy' parts 1 and 2.In the analysis, a comprehensive national assessment of US geothermal resources, evaluation of drilling and reservoir technologies and economic modelling was carried out. The methodologies employed to estimate geologic heat flow for a range of geothermal resources were utilized to provide detailed quantitative projections of the EGS resource base for the USA. Thirty years of field testing worldwide was evaluated to identify the remaining technology needs with respect to drilling and completing wells, stimulating EGS reservoirs and converting geothermal heat to electricity in surface power and energy recovery systems. Economic modelling was used to develop long-term projections of EGS in the USA for supplying electricity and thermal energy. Sensitivities to capital costs for drilling, stimulation and power plant construction, and financial factors, learning curve estimates, and uncertainties and risks were considered.

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A new approach to the topographic correction for terrestrial heat flow measurements is presented. The approach features calculation of a Fourier series fit to the surface temperature‐surface elevation data where the surface temperatures are based on a model that includes surface temperature variations due to microclimate variations. The mathematics of the terrain correction problem are similar to the upward (away from source) continuation problem in gravity and magnetics so several solutions, in addition to the Fourier series approach, are available in the literature that allow an accurate calculation of the correction provided the surface boundary condition is properly specified. However, the usual boundary condition applied, a linear relation between ground surface temperature and elevation, is shown to be inadequate for drill holes in the depth range 30–200 m no matter how low the topographic relief. Thus a model of ground surface temperature is developed that includes the effects of elevation, slope orientation, and slope angle. Because of the effects of microclimate, the classical models that have isothermal surfaces that generally parallel the topographic surface are significantly in error in many cases, and the patterns of isotherms near the topographic surface are more complicated than was previously recognized. This complexity causes gradient variations with depth in 30‐ to 200‐m holes that have not been previously recognized as being related to topographic effects. Because the temperature effects of slope orientation and inclination do not scale with respect to the magnitude of the relief, significant terrain corrections may be required even in areas of relatively low relief. The application of the technique is illustrated by application to a line dipole hill and a group of drill holes near Wilbur, Washington. In addition, several examples of two‐dimensional terrain effects and one example of three‐dimensional terrain effects are illustrated for topographic sections in the northwestern United States. In the United States, most ‘anomalous’ gradients in the upper 100–200 m of drill holes in impermeable rocks can be explained by a combination of topographic and microclimatic effects, without resorting to temporal climatic changes or unknown types of water effects. The depth of the holes necessary for reliable heat flow measurements in such settings is a signal to noise problem where the noise is the effect at depth of the microclimatically related surface temperature variations, coupled with the topographic effect, and the signal is a temperature increase at any depth due to the background geothermal gradient. Typically, the noise has decreased to a few degrees centigrade per kilometer within the depth range 100–200 m. Thus the general conclusion has been that these depths of holes are required for reliable heat flow values. In fact, when linear temperature‐depth data are observed in shallower holes or when appropriate corrections are made, reliable measure ments in impermeable rocks may be consistently ma...