Binary-type energy conversion systems are typically used to exploit low-temperature geothermal resources. There are many different technical variations of binary plants, including those known as Organic Rankine Cycles (ORC) and proprietary systems known as Kalina cycles. Recent articles in the technical literature claim dramatic efficiency advantages for Kalina cycles over ORCs, thereby providing the motivation for this study. Claims of remarkable superiority for one type of technology relative to another ought to be verifiable by recourse to actual performance data. Since there is now an actual Kalina cycle in operation, it ought to be possible to make a comparison between it and ORC plants that have been in operation for some time. Comparisons between power plants must use an appropriate and consistent thermodynamic basis. It has been shown that the Second Law of thermodynamics is the best basis for such comparisons. This principle, employing the concept of exergy, is used here to shed light on these claims. Furthermore, we introduce a methodology to render the comparison of plant efficiencies on common input and environmental conditions, even though the plants being compared operate with somewhat different fluid inlet and ambient temperatures.

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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