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 evaluation 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 degree 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 degree F resource, where an isobutane mixture is favored. Supercritical cycles werefound to have the lowest cost at all resources.