Low grade waste heat recovery in an intercooled recuperated closed cycle gas turbine

Abstract

The context of the work is the study of a 180 bar nitrogen recuperated and intercooled turbo-compressor of 300 MWe. The net efficiency of this closed cycle gas turbine is assessed between 37 and 37.5%, but which could be increased by considering waste heat recovery techniques. Among waste heat recovery technologies which are currently used in conventional gas turbines we find organic Rankine cycles (ORC). This kind of machine is similar to a conventional steam cycle energy conversion system, but uses an organic fluid instead of water. A main challenge is the low level of temperature of waste (90°C). Preliminary studies are carried out on the design of ORC using three different fluids: traditional and mature R245fa technology, innovative R125 cycle and high pressure low temperature transcritical CO2 machine. Different architectures are compared and advantages of using transcritical cycles have been underlined. From a thermodynamic point of view the best solution would be to use R125 cycle at a supercritical pressure at precooler secondary side and at a subcritical pressure at intercooler secondary side. A supplementary power of more than 15 MWe could be achieved. The resulting combined cycle could reach a net efficiency of 39%.