Proceedings 36th New Zealand Geothermal Workshop 24 - 26 November 2014 Auckland, New Zealand ENERGY AND EXERGY ANALYSIS OF AN AIR-COOLED GEOTHERMAL POWER PLANT WITH FIXED NOZZLE TURBINE IN SUBSONIC EXPANSION AND SUPERSONIC EXPANSION VIA CFD ANALYSIS Choon Seng Wong 1 and Susan Krumdieck 2 1,2 Department of Mechanical Engineering, University of Canterbury, Private Bag 4800, Christchurch, 8041, New Zealand 1 choon.wong@pg.canterbury.ac.nz 2 susan.krumdieck@canterbury.ac.nz Keywords: Air-cooled geothermal plant, Organic Rankine Cycle, Supersonic Expansion, Fixed geometry turbine, CFD Analysis. ABSTRACT The performance of air-cooled geothermal power plants is susceptible to the changes in ambient temperature especially in New Zealand due to the large difference in the atmospheric temperature for summer and winter seasons. The turbine is typically designed for a specific set of operating conditions, but the operating conditions also change with the resource enthalpy and the heat sink temperature. The effect of the fluctuation in the condenser temperature due to the changes in ambient temperature affects the pressure ratio across the turbine, thus reducing the efficiency of the turbine. This paper investigates the potential for adapting a 100 kW gas turbine with fixed nozzle vanes into an ORC turbine. An air-cooled ORC system was designed using the geothermal source conditions and the mean daily temperature in Rotokawa, New Zealand. The effect of the air temperature on the condenser temperature, and thus on the operation of the turbine was studied. The performance of the turbine was studied via computational fluid dynamic (CFD) tools with different pressure ratio to take into account of subsonic expansion and supersonic expansion across the selected turbine in a single stage configuration. The turbine performance curve was then incorporated into the ORC system and the efficiency map of the ORC system using the common fluids, namely isobutane, R245fa and R134a, was generated. The model results were used to develop a correlation between the ambient temperature and the performance of the ORC system. 1. INTRODUCTION The demand for Geothermal Energy is increasing continuously. Twenty four countries generated electricity from geothermal resources with total installed capacity of 8,930 MWe in year 2005 (Bertani, 2005). The total installed capacity increased up to 10,898 MWe in year 2010 (Bertani, 2012). Geothermal plant can be classified into a number of different categories: single flash, double flash, binary plant, hybrid or combined cycle (DiPippo, 2012). The type of the geothermal plant is dependent on the quality of the brine, temperature of the heat source and the condition of the geothermal reservoir. Single flash plant is by far the most popular geothermal plant with the highest installed capacity of 41% of the total capacity of the geothermal plant based on the data taken from WGC2010 (Bertani, 2012). Binary plant has the least installed capacity at 11% but the number of units is twice the number of units of single flash plant and quadruple of units of double flash plants (Bertani, 2012). Binary plant can be cooled via water-cooling and/or air- cooling. Chena Geothermal ORC plant is water-cooled during summer and air-cooled during winter to maximize the cycle performance (Holdmann, 2007). Of course, water-cooling is favored over air-cooling due to the much higher heat capacity and heat transfer coefficient of water compared to dry air (Mendrinos, Kontoleontos, & Karytsas, 2012). Dry cooling induces an extra installation cost and operational cost, up to ten times more than the water cooling system in a similar size to the air cooling system (Li, Pei, Li, Wang, & Ji, 2012). The air-cooling is still commonly used in New Zealand geothermal plants in Rotokawa (Legmann & Sullivan, 2003), Wairakei (Thain & Carey, 2009), Mokai, and Ngawha (Dunstall, 1999) due to the geographical location and lack of access to a water body. Seasonal ambient temperature variation has been shown to reduce the plant net power output by up to 40% from winter to summer in northern Nevada (Kanolu & engel, 1999). The exergy efficiency reduces with increasing ambient temperature, indicating the degradation in plant performance (Mines, 2002). Operation of geothermal plant away from the design point ambient temperature changes performance of the evaporator, pump and condenser (Wendt & Mines, 2011). Several researchers have investigated the off-design effects of the ORC on the turbine. The change in ambient temperature contributes to off-design performance of the turbo-generator. Greg (Mines, 2002) has modelled the turbine off-design performance as a deviation from the velocity ratio using the corrections chart from Balje (Balje, 1981). The turbine correlation chart developed by Balje, however, is limited to incompressible fluids (Balje, 1981). The turbine-generator efficiency was assumed to be constant by Cengel (Kanolu & engel, 1999) and Marco (Astolfi, Romano, Bombarda, & Macchi, 2014) in modelling the binary geothermal plant. Mitsos modeled the turbine performance with isobutane as a function of the design isentropic efficiency, ratio of enthalpy drop and ratio of volumetric flow rate (Ghasemi, Paci, Tizzanini, & Mitsos, 2013). This study aims to investigate the performance of the turbine with different working fluids at different pressure ratio using CFD modeling. An existing 100 kW gas turbine with published geometry data (Sauret, 2012) was adapted for the ORC unit in this study. The performance of the turbine using different fluids was conducted using ANSYS-CFX. A binary plant model was developed in Engineering Equation Solver (EES). The analysis was performed using the actual heat source conditions from the Rotokawa binary plant (Legmann & Sullivan, 2003) and the ambient temperature profile in Taupo in the year 2012 (NIWA, 2014) to determine the pressure ratio across the turbine. Four different working fluids are studied, namely n-Pentane, isobutane, R245fa, and R134a. The fluids are chosen as they are commonly used in