Combined Solar Thermal Gas Turbine and Organic Rankine Cycle Application for Improved Cycle Efficiencies

Concentrating Solar Power (CSP) technologies are considered to provide a significant contribution for the electric power production in the future. Different kinds of CSP technologies are presently in operation or under development, e.g. parabolic troughs, central receivers, solar dish systems and Fresnel reflectors. In such applications, electricity is produced by thermal energy conversion cycles. For high MW-class CSP applications usually water/steam cycles (Rankine cycles) are used. Alternative technologies, especially for central receiver applications, are open and closed gas turbine cycles (Brayton cycles), where higher receiver fluid outlet temperatures can be applied. Therefore, there is the potential of higher cycle efficiencies and the advantage of reduced water consumption. The paper presents the results for design considerations to improve a gas turbine cycle of a 2 MWel class industrial gas turbine for solar-thermal application, where solar heat is fed in by a central receiver technology. The reference process is improved significantly by application of an intercooler between the two radial compressor stages and a recuperator, which recovers heat from the exhaust gases to the compressed air before the air is further pre-heated by the solar receiver. Hybrid operation of the gas turbine is considered. In order to further improve the overall cycle efficiency, the combined operation of the gas turbine and an Organic Rankine Cycle is investigated. The ORC can be coupled to the solar-thermal gas turbine cycle at the intercooler and after the recuperator. Therefore, waste heat from different cycle positions can be transferred to the ORC for additional production of electricity. The investigations have been performed by application of improved thermodynamic and process analysis tools, which consider real gas behavior of fluids and a huge number of organic fluids for application in ORCs. The results show that by choice of a suitable organic fluid the waste heat recovery can be further improved for the investigated gas turbine cycle. The major result of the study is that by combined operation of the solar thermal gas turbine and the ORC, the combined cycle efficiency is approximately 4%-points higher than in the solar-thermal gas turbine cycle.