The World's Largest Single Cylinder Geothermal Power Generation Unit--Nga Awa Purua Geothermal Power Station, New Zealand

Abstract

The Nga Awa Purua (NAP) Geothermal Power Station, jointly owned by Mighty River Power Limited (MRP) and the Tauhara North No.2 Trust, became successfully operational in April 2010. It was constructed in the Rotokawa geothermal field near the city of Taupo, New Zealand A prominent feature of the Rotokawa geothermal field is its exceptionally high energy two- phase fluid which persuaded MRP to take advantage of its generation possibilities and select a singlecylinder steam turbine with a triple flash separation system. The resulting turbine design includes 31.5” last stage moving blades producing an output of 147 MW at the maximum and means the NAP turbine is now the largest single-cylinder geothermal steam turbine in the world. This paper introduces and outlines the technical features of the NAP triple flash Steam Separation System (SSS), the steam washing system, brine pH modification, the reinjection system, the single cylinder geothermal steam turbine, other power generation facilities and the control functions. The NAP project is MRP’s second new geothermal project built over the past four years and is part of their corporate strategy to substantially expand their geothermal generation capacity. A key philosophy in MRP’s original geothermal expansion plan was to ideally achieve a high level of standardization of design of the layout, operational processes and plant across all future new plants. In reality, steamfield differences between the first two projects meant the respective selection of differing dual-flash and tripleflash systems. Nevertheless, central design, plant and operational features were common across both projects and significant synergies achieved. Moreover, these same standardized features have now been proven and will likely be the basis for other future MRP projects.