MODELLING AND SIMULATION OF AN ORGANIC RANKINE CYCLE GEOTHERMAL POWER PLANT

MODELLING AND SIMULATION OF AN ORGANIC RANKINE CYCLE GEOTHERMAL POWER PLANT

Abstract

Popularity of small-scale binary cycle geothermal power plant is noticeable in the new fields of Mokai and Rotokawa, New Zealand. Steady state model of similar design is often noticed in the literature. However, literature of dynamic model of such kind of plant is limited. A dynamic model is very useful to the plant's operators to predict plant performance, as they need to commit their power output to buyers in advance. It also gives them the ability to analyse any room for improvement. In this project, a steady state model of Mokai 1 geothermal power plant of Taupo, New Zealand, was developed. Dynamic components are added to the system to convert it from a steady state model to a dynamic model. The dynamic model takes into account effect of various internal and external variables, which are necessary to simulate the plant performance with reasonable accuracy. There are two types of Organic Rankine Cyle (ORC) units used in the plant; brine ORC and bottoming ORC. In this presentation dynamic model of a brine ORC is presented. This unit uses pentane as the motive fluid and it is powered by the separated brine from the geothermal fluid. It is found that the inlet brine properties including brine mass flow rate, and the ambient air temperature are the two most important parameters influencing the plant performance. Specifically, the plant performance is highly dependent on ambient air temperature as the ORC uses air-cooled condenser. On the other hand, the inlet brine property changes less significantly with respect to time compared to the ambient air temperature. Simulation has been carried out for 1000 hours operation where, inlet brine properties and ambient air temperature are fed as inputs to the computer model. The simulated plant performance has been compared with the actual plant performance data.