Design and Build of a 1 Kilowatt Organic Rankine Cycle Power Generator

Organic Rankine Cycle (ORC) systems are capable of utilising low-enthalpy geothermal sources. The aim of the Above Ground Geothermal and Allied Technologies (AGGAT) research programme is the development of ORC systems within New Zealand. An experimental scale ORC system, known as ORC-B, was built at the University of Canterbury to assist with the research and development of the system design and component selection process. The unit is a 1 kW ORC consisting of four key components: evaporator, expander, condenser and pump. Selection of the working fluid was found to be a constraining factor in the design. A refrigerant mix known as HFC-M1 was selected due to its desirable performance, high safety and ease of availability in New Zealand. The heat source for the system is the exhaust of a Capstone gas turbine. Heat is transferred to the unit’s plate-type evaporator by a thermal oil extraction loop. Cooling is supplied to the plate-type condenser from the available water supply. A scroll expander is used to extract work from the system and the evaporator is fed by a high-pressure plunger pump. The instrumentation selected allows measurement of the performance of the system as well as PID control. The system has been designed and constructed and is now ready for commissioning and testing. 1.1 ORC Background ORC systems are typically used for four major applications: waste heat recovery, geothermal power plants, biomass combustion plants and solar thermal plants. The primary difference between an ORC and a traditional steam Rankine cycle is the use of an organic working fluid, such as ammonia, pentane or a halocarbon. The application of organic working fluids allows the extraction of energy from a low temperature resource at a higher efficiency than conventional steam cycle technology (Macián, Serrano, Dolz, & Sánchez, 2013). Despite ORC systems and steam Rankine cycles being conceptually very similar, the use of an organic working fluid requires a higher level of caution to avoid leakage or contamination as the consequences are more severe. There are several ORC binary cycle plants currently installed in New Zealand utilizing geothermal resources. All these systems so far have been designed by overseas companies because the necessary expertise was not available in New Zealand. 1.2 University of Canterbury Background The University of Canterbury recently began aiding HERA and AGGAT in developing the research necessary for designing ORC systems. Together the three groups aim to develop the expertise required to design and fabricate ORC systems within New Zealand, making the technology more affordable and accessible. The University built a small scale ORC test-bed in 2012 named ORC-A. This unit was used to prove the feasibility of the system concept and to identify design requirements necessary for further development. This system successfully produced 400 W of electricity and identified many important requirements for ORC design and control. Figure 1 Initial ORC-A system for testing the system concept The University and AGGAT are now developing a second test bed to further improve upon the knowledge gained from the first unit. The ORC-B will be used to test different working fluids, turbines and control systems to gain greater capability for developing ORC systems. 2. ORC DESIGN PROCESS The AGGAT guideline, explaining the design process for an ORC plant, is shown in Figure 2. This design process is primarily for use with full scale plants as it ensures the plant will be economically successful.