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Combined Cycle Strategies for High Enthalpy, High Non-Condensable Gas Resources

By Tim Dunford, Bill Lewis, Kevin Wallace, POWER Engineers, Inc. and William Harvey, Reykjavik University

ABSTRACT

Geothermal fluids in certain locations contain high proportions of non-condensable gases. At locations where the geofluid is sufficiently energetic, flash cycles are often preferred. However, the cost and parasitic power consumption of non-condensable gas (NCG) removal systems can be considerable, especially if they must be oversized to account for uncertainty in resource conditions. Binary plants do not require extensive NCG removal equipment when harnessing such a resource, however the effect of NCGs on exchanger heat transfer coefficients must be analyzed, water cooling may not be an option, and the cycles may have lower resource utilization efficiency and higher cost per installed kW. As alternatives, this study presents several concepts for using combined cycles that fuse the benefits of water-cooled flash cycles and binary technology. We compare and contrast cycle operating parameters for conventional dual flash, binary and two combined cycle configurations, using a base case reservoir with resource enthalpy of 1000 kJ/kg and high NCG in geofluid content (2%). The combined cycle is shown to have several appealing features for developers to consider when choosing between these cycle alternatives. A case study of a new combined cycle plant analogous to those presented is discussed.

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