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Proceedings World Geothermal Congress 2010 Bali, Indonesia, 25-29 April 2010 Dynamic Modelling and Simulation of an Organic Rankine Cycle Unit of a Geothermal Power Plant M. Imroz Sohel1*, Susan Krumdieck1, Mathieu Sellier1, Larry J. Brackney2 1 Department of Mechanical Engineering, University of Canterbury, Christchurch, New Zealand. 2 Department of Electrical and Computer Engineering, University of Canterbury, Christchurch, New Zealand. * Corresponding author. Email: mso32@student.canterbury.ac.nz; mohammed.imroz.sohel@gmail.com Tel: +64 3 3642987 ext 7243; Fax: +64 3 3642078; Keywords: Geothermal power, dynamic modelling, binary cycle, ORC. ABSTRACT This article presents a dynamic model of a 5.4 MW binary cycle unit of a geothermal power plant. The plant is an Organic Rankine Cycle (ORC) where pentane is used 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 the ambient air temperature as the ORC uses air-cooled condenser. Moreover, the inlet brine property changes less significantly with respect to time compared to the ambient air temperature. Simulation has been carried out where, brine inlet 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. It has been found that the developed model is very competent in predicting plant performance. The average percentage error of the theoretical and the observed power outputs remain within 5%. 1. INTRODUCTION Geothermal plants have a large number of parts involving an enormous number of flow recirculations and splitting, and connections of several subsystems. The overall plant behaviour, both static and dynamic, cannot be simply inferred from that of its components, rather it is essentially determined by their interaction (Casella 1999). Although a wide range of literature is available on the steady state operation of a geothermal power plant, dynamic models are not very common. Dynamic models are very useful in two areas, firstly, in controlling and monitoring the system for transient conditions, particularly when the load demand or the quality or flow of geothermal fluid suddenly changes (Wei et al. 2008). It is necessary to keep the proportion of liquid and vapour phases in the condenser and evaporator within acceptable ranges to avoid stalling or temperature shocks. Secondly, it is very convenient for the plant operators to know what the power output could be under given ambient conditions, plant parameters and geothermal fluid flow rate. The plant operators need to inform the dispatcher beforehand how much power they can produce as the price of the electricity depends on it. Normally, there is a huge penalty for the plant owners, if they produce more than promised to avoid grid overload (Pritchard 2002). A dynamic model can ease this problem by predicting power output in advance with high accuracy. This paper presents a dynamic model of a 5.4 MW binary cycle unit of a geothermal power plant. The brine inlet properties and the ambient air temperature are used as the inputs to the model. The developed dynamic model meant for predicting the plant performance depending on weather condition and brine inlet properties. Therefore, transient phenomenons i.e. starting up or shutting down processes are not discussed here. These are done manually in the plant. The following section introduces the plant. Modelling of each of the component then follows. Results and discussions are presented in the last section of the paper. 2. THE ORC UNIT Figure 1 shows the process flow diagram of the ORC unit. It is powered by brine from the geothermal fluid. The brine, which is at about 205°C, passes through the vaporizer- separator providing heat for the ORC unit. Pentane taking heat from the brine changes its phase from liquid to vapor in the vaporizer then superheated in the separator. It is then passes through the turbine producing work. Then it passes through a recuperator before being cooled by an air-cooled condenser. The recuperator recovers some of the heat from the pentane vapor coming from the turbine reducing the heat load in the condenser. The pentane liquid after the cycle- pump passes through the recuperator, where it is heated to an elevated temperature. Then the pentane liquid enters the vaporizer completing the cycle. Figure 2 presents the T-s plane presentation of the cycle. 2.1 Modeling of the Components Following sections present modeling of each of the components. The components are modeled separately and then they are connected to form the model of the geothermal power unit. The model developed for this study were solved using commercial software packages Matlab/Simulink® and gPROMS® (MathWorks 2008; PSE 2008). 1PDF Image | Dynamic Modelling and Simulation of an Organic Rankine Cycle Unit of a Geothermal Power Plant
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