The incorporation of supercritical carbon dioxide power cycles in waste heat recovery

It is estimated that in the United States alone, enough energy is wasted in industrial applications to power between 43 and 110 million homes. Much of this energy is wasted through hot exhaust streams often in power generation and in the manufacture of steel. The recovery of portions of this energy is clearly necessary from either an economical or environmental standpoint, however the temperatures at which these exhaust streams run are often too low for conventional energy recovery methods. The incorporation of supercritical carbon dioxide (S-CO2) as a working fluid in power cycles aims to provide a platform on which this energy can be recovered. Literature identifies many techniques to improve S-CO2 cycle efficiencies, however few outline how to modify this for waste heat recovery applications, where energy output is the key performance indicator. Through the modification of several existing numerical tools, the simulation of several cycle configurations was possible. This allowed for the verification of several thermodynamic principles involved with optimising a S-CO2cycle for the application of waste heat recovery. Specifically, a context where a waste-heat source at 900 K flowing at 10 kg/s was simulated as this falls within typical waste heat ranges. This allowed for the comparison of several techniques for achieving higher performing cycles including optimising fluid states at key points in the cycle to minimize compression-work and maximize turbinework and the use of recuperation to reduce waste energy. In addition, the process of splitting the flow between different components to optimise recuperation and avoid its key limitation of its presence known as the pinch-point was investigated. This investigation shows the standard Brayton cycle to be capable of recovering 867 kW representing 13.69% of the aforementioned waste-heat source. This cycle was found to waste 5888 kW of power making it insufficient in terms of energy yield, but having a simplicity suitable for waste heat recovery. The recuperation cycle was found to yield 1248 kW representing 19.7% of the waste-heat source. This cycle only wasted 2713 kW of power showing the improvement over the standard Brayton cycle. Furthermore, the split recuperation cycle was found to generate 1360 kW of net-work recovering 21.5% of the source and only wasting 2419 kW. Also found was that as the complexity of cycles grow, often the required parts increase in size and quantity yielding an additional need for an economic analysis, and additionally a potential further study involving the recompression cycle and its suitability for waste heat recovery as well as a review of the numerical methods used