Sufficient work has been done to date to establish geothermal energy as a significant source for meeting our future energy needs. At the present time there are only a few geothermal power plants in operation; in the United States, The Geysers field is the only location of commercial importance. The Geysers field is a vapor dominated reservoir, producing steam that is used directly in steam turbines. Such reservoirs are comparatively rare, and represent only a small portion of the available geothermal energy. Dry rock or magma sources are a potentially much larger source of geothermal energy, but the technology for utilizing these resources has not been developed. Liquid dominated reservoirs are a major source of geothermal energy and technology is available for exploiting them. Thus, liquid dominated reservoirs are the geothermal energy resource of major interest at this time. The principal power generation processes being considered for use with liquid dominated geothermal reservoirs are the binary, the flashed steam, and the direct contact; although many variations within the general classifications are possible. The flashed steam process is probably the simplest and is suitable for use with heavy scaling brines. The capital cost for the plant is probably lowest of the three, but brine usage for unit of power produced tends to be high. The binary process appears to be more efficient, but depends on the use of shell and tube heat exchangers for transferring heat from the brine to the working fluid. The use of such exchangers is impractical for brines with heavy scaling characteristics, and in any event the cost of the exchangers is a large part of the total plant cost. The direct contact binary process offers the possibility of reduced plant cost, good efficiency, and suitability for use with scaling brines. The process is, however, the least developed of the three and presents areas of technological uncertainty. The work reported here is a preliminary investigation of some of the technical and economic aspects of the direct contact process, and was undertaken in order to evaluate the commercial potential of the process. The work consisted of laboratory tests, a conceptual design, a 50 MWe plant design, an economic study, and a pilot plant program. The laboratory tests were made to obtain enough information to develop a realistic preliminary plant design. A glass direct contact heat exchange column was designed, constructed, and operated to get information on general performance characteristics, tray efficiencies, sizing correlations, and temperature approaches. A hot water-normal hexane system was used to permit operation in a glass column at atmospheric pressure. The physical characteristics of this system are close to those proposed for the commercial plant so the answers obtained are applicable to the commercial system. Hydrocarbon stripping tests were made using a glass column and nitrogen as stripping gas in order to establish the feasibility of recovering hydrocarbon from spent hot water. The tests showed that more work is needed in this area but that hydrocarbon stripping is feasible. Solubility tests were conducted to determine the effect of temperature, and brine composition on the solubility of three hydrocarbons. This information was needed to evaluate the impact of hydrocarbon losses on the cost of power production. A computer program was developed to aid in the evaluation of various possible direct contact thermodynamic cycles. The effect of operating variables on power production was evaluated; including the effect of brine inlet temperature, operating pressure, condensing temperature and choice of working fluid. After a general investigation, an optimum cycle was found for a specific site, the Heber field. The Heber field, located in the Imperial Valley of Southern California, was chosen as representative of a medium temperature low salinity reservoir with excellent commercial potential. As part of the conceptual design, studies were made on the prevention of working fluid loss in rejected hot water and in noncondensable gases, and the elimination of particulates in expander inlet gas. The studies were used to establish practical solutions to these important design problems, at least for the preliminary design. The results of the conceptual design were used to develop a preliminary design for a 50 MWe (delivered) plant. The plant was sized for 50MWe since preliminary studies made as part of another project had established this as an economic size. An estimated capital cost was developed for the 50 MWe plant, for use in the economic study. The economic study made use of a considerable body of information developed as part of a project done for the Electric Power Research Institute by Holt/Procon, a joint venture of The Ben Holt Co. and Procon Incorporated. This information included plant and operating costs for a closed loop binary and a two-stage flashed steam process as well as costs for brine production and power transmission. The same methods and factors were used for the direct contact process. Thus, it was possible to obtain a comparison among the three processes on a consistent basis. The relative values obtained should be relatively accurate. Since the process shows promise, a pilot plant program was prepared outlining areas for future work