Advanced Binary Cycles: Optimum Working Fluids

A computer model (Cycle Analysis Simulation Tool, CAST) and a methodology have been developed to perform value analysis for small, low- to moderate-temperature binary geothermal power plants. The value analysis method allows for incremental changes in the levelized electricity cost (LEC) to be determined between a baseline plant and a modified plant. Thermodynamic cycle analyses and componentsizing are carried out in the model followed by economic analysis which provides LEC results. The; emphasis of the present work is on evaluating the effect of mixed working fluids instead of pure fluids on the LEC of a geothermal binary plant that uses a simple Organic Rankine Cycle. Four resources were studied spanning the range of 265 degrees F to 375 degrees F. A variety of isobutane andpropane based mixtures, in addition to pure fluids, were used as working fluids. This study shows that the use of propane mixtures at a 265 degrees F resource can reduce the LEC by 24% when compared to a base case value that utilizes commercial isobutane as its working fluid. The cost savings drop to 6% for a 375 degrees F resource, where an isobutane mixture is favored. Supercritical cycleswere found to have the lowest cost at all resources.