Exergy Analysis of Southern Negros Geothermal Field, Philippines

Abstract

Exergy analysis is important for quantifying the performance, identifying the sources/causes of inefficiency, and for determining options for optimising the output of a thermal process (Szargut et al., 1988). In a geothermal power plant, the performance is commonly measured through the second law (exergy/utilisation) efficiency (surface) of the process. It is usually obtained from the ratio between the net generated electrical power and the exergetic power from the produced geothermal fluids at the wellheads. This means that exergy analysis is only conducted at the surface level (ground) of the system and does not take into account the sub-surface reservoir, making the optimisation options limited. This work aims to perform a holistic (surface and sub-surface) exergy analysis of the Southern Negros Geothermal Production (SNGP) field. Exergy at the surface was calculated using the identified fluid properties at different components of the power plant. Meanwhile, exergy at the sub-surface was analysed using the modified AUTOUGH2 numerical reservoir model. Based on the results, the standard utilisation efficiency (surface) of SNGP field is 40.5%. This is comparable to 39.9 % (Aqui et al., 2005), which was referred to the exergy of the fluid at the production wellhead. However, it was more appropriate to measure the power plant performance through the overall utilisation efficiency (surface and reservoir), i.e. 34.51%. This is because the reservoir is an integral part of the energy conversion process for steam/power production. Any changes at the surface parameters will consider the impact to the reservoir, which is the heat source for producing work. An optimisation assessment was carried out to determine the field’s optimum overall utilisation efficiency at varying separator pressure. Results produced the optimum brine reinjection temperature of 140 ºC. This value was nearly the same as the results at the existing brine reinjection temperature of 165 ºC. Thus, if the existing separator pressure will be decreased to the optimal setting, the performance of the geothermal system is most favourable. Considering that reservoir pressure reduces as mass extraction continues over time, decreasing the separator pressure allows more low-pressure highii enthalpy operating production wells to sustain production and maintain the steam supply to the power plant.