ANALYSIS OF ZEOTROPIC MIXTURE IN A GEOTHERMAL ORGANIC RANKINE CYCLE POWER PLANT WITH AN AIR-COOLED CONDENSER H. C. Jung1 and Susan Krumdieck2 1,2Department of Mechanical Engineering, University of Canterbury, Private Bag 4800, Christchurch 8041, New Zealand [email protected] [email protected] Keywords: Working fluid, Zeotropic mixture, Air-cooled The working fluid plays a vital role in the energy conversion Condenser, Geothermal, Organic Rankine cycle. efficiency of ORC machine. Numerous studies have been focused on the proper selection of pure fluids, including ABSTRACT refrigerants such as R-134°, R-123 and R-245fa and volatile The purpose of this research is to investigate a zeotropic hydrocarbons such as butane and pentane (Saleh et. al., working fluid mixture in terms of its performance in an 2007, Wang et. al., 2011). General selection criteria for a organic Rankine cycle (ORC) and the heat transfer potential fluid were proposed by Chen et. al. (2010). characteristics in an air-cooled condenser (ACC). The motivation of this study is that it is well known that the use The suggestion of using a working fluid mixture in vapour of a mixture improves the efficiency of an ORC system. compression systems was first made in 1888 by Pictet However, the behaviour of the mixture in the condensers is (Radermacher 1989). The benefits of using organic fluid not well understood. A standard ORC unit utilising hot mixtures as the working fluid in ORC systems were spring water to generate electricity is considered for the investigated by Angelino and Paliano (1998). By using a analysis. A mixture of two fluids, isobutane and pentane, is mathematical model they demonstrated that optimal used as the working fluid. Numerical models of the ORC selection of fluid composition can improve the ORC power plant and ACC are developed and simulations are performance, based on the use of mixtures of siloxanes and conducted with the pure fluids and their mixtures with four hydrocarbons. Recently, more research has been conducted different compositions. The optimum operating conditions to evaluate the performance of ORC systems with mixtures of the system are estimated for each fluid, and they are then as the working fluid (Borsukiewicz-Gozdur and Nowak, applied to a condenser, designed to use pentane, to calculate 2005, Wang et. al., 2010, Heberle et. al., 2012). Despite and compare the heat transfer parameters. The mixtures these efforts, the effects of mixture on the ACC performance display a significant increase in the system performance still remain to be studied and compared with the compared to the pure fluids. A highest exergetic efficiency performance of pure fluids to obtain an optimised design of of 24.2% is achieved for the system using a mixture of 80% the system and components. isobutane/20% pentane. However, with the exception of the mixtures having the isobutane composition between In the present study, a standard ORC unit utilising hot spring approximately 35% and 60%, all of the mixtures require an water (this heat source is available from Waikite Valley in additional surface area of the condenser compared to that for Rotorua) to generate electricity is used as a case study to pure pentane due to their relatively low heat transfer investigate a zeotropic working fluid mixture in terms of its characteristics. Therefore, the design of the ORC system and effect on the ORC system performance and heat transfer ACC involves a compromise between the system efficiency characteristics in the ACC. A mixture of two fluids, and condenser size when mixtures are employed as the isobutane and pentane, is used as the working fluid. The working fluid. performance parameters to be measured include the net power output and Second Law efficiency for the ORC unit 1. INTRODUCTION and the heat duty, convective and condensation heat transfer coefficients, surface area, and log mean temperature The rising scarcity of non-renewable energy resources and difference for the ACC. Computational models of the ORC the environmental issues with their use means that demand system and ACC component are developed to estimate the for renewable energy is increasing. Geothermal energy is performance. one of the clean, sustainable energy resources and the amount of geothermal energy is virtually endless in the 2. NUMERICAL METHOD earth. The temperatures of geothermal fluids range from 50 The thermodynamic analysis of the power cycle for a to 350 C and most of resources consist of the low to standard ORC system is first presented. The Second Law moderate temperature (below 150C) liquids. It is possible efficiency is then described for the exergy analysis of the to extract and convert the abundant brine into electrical ORC power plant. Lastly, correlations to determine the power via the organic Rankine cycle (ORC), and the convective and condensation heat transfer coefficients for installation of ORC power plants has been rapidly growing the ACC are introduced. over the past two decades all around the world. 2.1 ORC model To promote the continuing growth of the use of ORC plants in the regions where a supply of cooling water is not A schematic flow diagram of the standard ORC power plant available, it is necessary to employ air-cooled condensers is shown in Figure 1. The thermodynamic processes (ACC) to provide a means of condensing the working fluid. undergone by a working fluid mixture are illustrated on a The additional advantage associated with the use of ACC temperature-entropy diagram in Figure 2. In the evaporator, depends on the fact that the dry condensing method is not a geothermal fluid increases the temperature of the working subject to government regulations and environmental fluid in a liquid form until it reaches a slightly superheated concerns relevant to treatment and disposal of water. state. The vapour expands through the turbine, generating electricity in the generator. The vapour from the turbine 35th New Zealand Geothermal Workshop: 2013 Proceedings 17 – 20 November 2013 Rotorua, New Zealand passes through the ACC that turns it back into liquid. The powers and the plant efficiency are respectively expressed working fluid is conveyed to the pump where it is fed into by: the evaporator to complete the cycle. The numbers designate the state points of the working fluid. Wcycle mgm wf (h1 h2 ) Wp (3) The system is analysed on the assumption that it is at a steady state and the geothermal liquid behaves as pure Wnet Wcycle Wf (4) water. The state at the exit of the turbine (state 2) is specified by defining the isentropic turbine efficiency as: Wnet plant,I (5) Qin h1 h2 t (1) h1 h2s The power consumption of the fan in the ACC is determined by assuming it is 20% of the cycle power. Since the (see the nomenclature list for definition of the symbols in emphasis of the present study is on the analysis of mixtures, this and other equations). In order to find the process the power needed for the geothermal fluid supply pump is conditions at the pump outlet (state 6), its isentropic not considered in the net work of the plant. efficiency is given by: 2.2 Exergy efficiency h h The exergy efficiency based on the Second Law of 6s 5 (2) p thermodynamics is introduced on the basis of the work done h6 h5 by Dipippo (2005). In this case of geothermal power plant, The property data are evaluated on the assumption of exergy means the maximum work that can theoretically be constant pressure across the heat exchangers and piping achieved from the geothermal source at the reservoir system because pressure drops are less significant sources of conditions relative to its surroundings. The specific exergy irreversibility in a well-designed system compared with the can be written as: irreversibility occurring in the turbine and pump components. It is also assumed that heat losses from the e hres h0 T0(sres s0 ) (6) system components to the surroundings are negligible. where hres and sres are the enthalpy and entropy of the 1 geothermal fluid at the reservoir state, and T0, h0 and s0 are the properties evaluated at the dead state (i.e., when the fluid Turbine- is in equilibrium with the surroundings). When the fluid is a generator liquid at the dead state, it is appropriate to take the enthalpy Geothermal and entropy values for a saturated liquid at the wet-bulb fluid temperature. Shell & tube 2 Air-cooled condenser evaporator The maximum work output that can be obtained from the geothermal water is therefore expressed as: 6 Pump 5 Wmax m wf e (7) Air The Second Law efficiency of the plant is defined as: Figure 1: Schematic of a standard ORC power plant. Wnet plant,II (8) Wmax 2.3 ACC model The ACC is taken into account with circular finned-tubes staggered horizontally. In the ACC, while the working fluid flows inside the tubes, heat is transferred from the working 8 1 fluid to cooling air in the following three consecutive Evaporator Expander modes: vapour to air in Process 2-3, vapour/liquid mixture Temperature 7 to air in Process 3-4, and liquid to air in Process 4-5. The 2 actual heat rejection in each process is calculated by: 6 3 Pump Condenser 4 Q Uo AFTLM (9) 5 Entropy Here the overall convective heat transfer coefficient is based on the air-side surface area and is given by: Figure 2: Temperature-entropy diagram for a standard ORC power plant using a working fluid mixture. 1 U o 1 Aair " Aair " 1 The flow rate of working fluid is computed by equating the R wf R A R air h A A w air h thermal energy supplied from the geothermal fluid with the wf wf wf o air (10) energy acquired by the working fluid.