Hot Dry Rock - Summary

Hot Dry Rock adds a new flexibility to the utilization of geothermal energy. Almost always the approach has been to limit that utilization to places where there is a natural source of water associated with a source of heat. Actually, the result was that steam was mined. Clearly there are much larger heat resources available which lack natural water to transport that energy to the surface. Also, as is found in hydrothermal fields being mined for steam, the water supply finally gets used up. There is a strong motive in the existing capital investment to revitalize those resources. Techniques for introducing, recovering and utilizing the water necessary to recover the heat from below the surface of the earth is the subject of this session. Implicit in that utilization is the ability to forecast with reasonable accuracy the busbar cost of that energy to the utility industry. The added element of supplying the water introduces costs which must be recovered while still supplying energy which is competitive. Hot Dry Rock technology can supply energy. That has been proved long since. The basic barrier to its use by the utility industry has been and remains proof to the financial interests that the long term cost is competitive enough to warrant investment in a technology that is new to utility on-grid operations. As the opening speaker for this session states, the test that is underway will ''simulate the operations of a commercial facility in some ways, but it will not show that energy from HDR can be produced at a variety of locations with different geological settings''. Further, the Fenton Hill system is a research facility not designed for commercial production purposes, but it can give indications of how the system must be changed to provide economic HDR operations. And so it is that we must look beyond the long term flow test, at the opportunities and challenges. Proving that the huge HDR resources can be accessed on a worldwide scale must involve the construction of additional sites, preferably to the specifications of the now Federal geothermal community. These facilities will have to be engineered to produce and market energy at competitive prices. At the same time, we must not rest on our technological laurels, though they be many. Design and operational techniques have been conceived which could lead to improved economics and operations for HDR. These must be pursued and where merit is found, vigorously pursued. Accelerated research and development ought to include revolutionary drilling techniques, reservoir interrogation, and system modeling to assure the competitiveness and geographical diversity of applications of HDR. Much of this work will be applicable to the geothermal industry in general. More advanced research ought to include such innovations as the utilization of other operating fluids. Supercritical carbon dioxide and the ammonia/water (Kalina) cycle have been mentioned. But even as the near and more distant outlook is examined, today's work was reported in the HDR session. The start-up operations for the current test series at the Fenton Hill HDR Pilot Plant were described. The surface plant is complete and initial operations have begun. While some minor modifications to the system have been required, nothing of consequence has been found to impede operations. Reliability, together with the flexibility and control required for a research system were shown in the system design, and demonstrated by the preliminary results of the plant operations and equipment performance. Fundamental to the overall success of the HDR energy resource utilization is the ability to optimize the pressure/flow impedance/time relationships as the reservoir is worked. Significant new insights are still being developed out of the data which will substantially affect the operational techniques applied to new systems. However, again, these will have to be proved to be general and not solely specific to the Fenton Hill site. Nevertheless, high efficiency use of the reservoir without unintended reservoir growth or water retention or life degradation depends on detailed Understanding of the hydraulic behavior of this reservoir and the degree to which that understanding applies to HDR reservoirs in general. In summary, it would seem that the nation and its utility system has a vital, practicable, and economical source of energy on the brink of availability. The research and development needed to assure the optimization of that resource application continues