Reducing the Impacts of Resource Temperature Degradation and Varying Ambient Temperature On Air-Cooled Binary Plant Performance

Monday, November 8, 2010: 3:15 PM
Deer Valley I (Marriott Downtown)
Daniel S. Wendt, Advanced Process & Decision Systems, Idaho National Laboratory, Idaho Falls, ID and Gregory L. Mines, Biofuels & Renewable Energy Technology, Idaho National Laboratory, Idaho Falls, ID

As the geothermal industry moves to use geothermal resources that are more expensive to develop, there will be increased incentive to use more efficient power plants. Because of increasing demand on finite supplies of water, this next generation of more efficient plants will likely need to reject heat sensibly to the ambient (air-cooling). Though they use no water, air-cooled systems have higher capital costs, reduced power output (rejection at a higher temperature), lower power sales due to higher parasitics (fan power), and have greater variability in power output (variation in the dry-bulb temperature). These effects are compounded as the resource temperature decreases.

Several currently available technologies have been studied to evaluate their potential to cost-effectively increase plant overall efficiency and total power generated during the plant operational life. These technologies include recuperation, in which turbine exhaust superheat is used to preheat the condensed working fluid prior to heat exchange with the geothermal fluid; reheat, where an intermediate heat exchange between the working fluid and geothermal fluid takes place during the expansion of the working fluid in the turbine; non-consumptive use of EGS makeup water to supplement the heat rejection; and substitution of working fluids at an intermediate point in the geothermal plant operational life.

Aspen Plus process models were developed to calculate optimal equipment sizing for maximizing the net power output of various process configurations in which geothermal resource temperature, geothermal fluid flow rate, ambient temperature, working fluid composition, heat exchanger pinch points, turbine and pump efficiencies, and process equipment and piping pressure drops were specified. A minimum geothermal fluid outlet temperature constraint was set to mitigate concerns of mineral precipitation on the heat exchanger surfaces. Two scenarios modeled were a baseline geothermal process employing the Organic Rankine Cycle (ORC) and a modified geothermal process design employing recuperation, reheat, and makeup water heat rejection in the Organic Rankine Cycle.

Following completion of the “plant design” modeling described above, a process modeling analysis was performed to evaluate the effect of hourly ambient temperature variation and declining resource temperature over the operational life of the two power plant designs. The equipment specifications from the baseline and modified geothermal plant process designs were fixed and correlations were incorporated into the process models to adjust heat transfer coefficients, pump and turbine efficiencies, and pressure drops in order to evaluate process performance as geothermal resource and ambient temperatures varied. The capability to vary turbine nozzle geometry was included in all simulations to provide an additional degree of freedom in establishing optimal process conditions. In all cases evaluated, the geothermal fluid flow rate was assumed constant.

A comparison of the baseline geothermal plant and modified geothermal plant capital costs and total power generated during the plant operational life was then performed for a scenario with a defined resource temperature decline and annual temperature profile. The methodologies used to select the plant design conditions, including ambient design temperature, working fluid selection, and turbine nozzle area for a declining resource temperature are discussed.

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