Biological Control of Cooling Water in Geothermal Power Generation

Abstract

Mighty River Power (MRP) operates a diverse fleet of geothermal power plants, including two geothermal power plants with condensing steam turbines. The cooling water used to condense the exhausted steam is condensed geothermal steam, recirculated through a mechanical draft evaporative cooling tower. The use of condensed geothermal steam as cooling water presents both opportunities and challenges when compared to the use of surface water or groundwater for cooling. The condensed steam has low levels of dissolved solids, and very low levels of suspended solids reducing the likelihood of mineral scale formation and erosion within the cooling water system. The presence of hydrogen sulphide in the geothermal steam (and the condensed steam) presents several challenges to the management of the cooling water system, including the build-up of sulphur deposits, the management of sulphur metabolising bacteria and the limited choice of sulphide compatible biocides. This paper discusses the implications of these challenges to the management of cooling water systems using condensed geothermal steam including discussion of significant cooling water events resulting from these challenges.