Non‐conventional combined cycle for intermediate temperature systems

SUMMARY This work presents a non-conventional combined cycle comprising a topping supercritical Brayton cycle working with carbon dioxide (S-CO2) and a bottoming organic Rankine cycle (ORC). As shown by previous authors in the field of nuclear energy, carbon dioxide cycles are characterised by lower turbine inlet temperatures (around 1100 K) than conventional gas turbines, and they usually incorporate a highly efficient recuperative heat exchanger in order to increase system efficiency. Accordingly, temperatures of around 200 °C are achieved at the outlet of the recuperator, hence leaving a certain amount of heat at the disposal of a bottoming cycle. Presently, ORCs are the most appropriate commercially available technology for such low temperature heat recovery. In this paper, an analysis of ORCs with different organic fluids is presented. Initially, only pure substances are considered. Then, the performance of the same cycle working with mixtures of hydrocarbons is assessed finding a 7 percentage point improvement in global efficiency with respect to the stand-alone topping cycle when the bottoming system (heat recovery unit and ORC) is incorporated. Following this interesting result, the performance of a S-CO2-ORC, a conventional gas turbine and ORC and an ORC–ORC combined cycles are evaluated for a tower-type central receiver Concentrating Solar Plant. Isopentane (the optimum pure substance obtained in the analysis mentioned earlier) is used in all cases for the bottoming system, showing favourable results for the proposed combined cycle configuration. Additionally, a discussion about the most relevant techno-economic concerns is presented at the end of the work showing that complementary economic benefits must be expected because of the lower footprint of the heat transfer equipments when carbon dioxide is used. These positive features derive from the more interesting transport properties of carbon dioxide when it is used at very high pressure, what also contributes to reducing the size of turbomachinery by decreasing their volumetric flows. In summary, the work presents a very interesting proposal, both thermodynamically and economically, to be used in intermediate temperature systems even if this technology is not mature yet. Copyright © 2012 John Wiley & Sons, Ltd.