Model organic Rankine cycle for brine at Olkaria geothermal field, Kenya

Abstract

A mix of conventional and wellhead power plants is used to generate geothermal power at Olkaria. All these plants utilize only the steam phase of the geothermal fluid flashing process. As a result, huge volumes of brine are left behind which if utilized can extend the exploitable potential of the field. This is possible using organic Rankine cycle (ORC) power plants that have been used successfully all over the world as bottoming units for conventional geothermal power plants. Every installation of organic Rankine cycle power plant requires a custom design tailored to suit the particular conditions of the source fluid and ambient climatic conditions. This paper models a suitable power plant for installation at the combined brine stream from separator stations SD2 and SD3 in Olkaria IV field bound for reinjection to well OW-911. The scope covered here is the design and sizing of major plant components from which it has been estimated that an ORC plant running on npentane as the working fluid installed on this brine stream will have a gross capacity of 4.6 MW and cost about 2,202 USD/kW. The cost is low because the installation of a bottoming ORC plant usually comes at such a time when the major geothermal development cost has been assigned to the conventional power plants. Therefore, it is the key finding of this paper that in a well-developed geothermal field like Olkaria, bottoming ORC plants can generate extra power at an economically competitive cost.