Increased pressure on reducing the carbon footprint from energy intensive industry such as glas, iron and steel, cement and oil and gas, with substantial waste heat streams is leading to the need to develop efficient and cost-effective waste heat recovery technologies. With waste heat stream at temperatures typically below 500 deg C, and low flow rates that mean commercially available steam power generation systems are unsuitable, attention is focused on other waste heat recovery technologies. Thus, significant research efforts have focused on the next generation of thermal-power systems, operating with novel working fluids such as organic fluids and supercritical carbon dioxide (sCO2). The ORC, which uses an organic working fluid, has been proven for conversion of heat between approximately 100 and 350 deg C into electricity, and commercial systems are available. However, ORC systems remain associated with high investment costs, whilst organic fluids are often flammable, unstable at high operating temperatures, and associated with a detrimental environmental impact. Alternatively, CO2 is an extremely promising candidate with benefits including low cost, is non-flammable and has a lower environmental impact than organic fluids. It facilitates compact components owing to high fluid densities, and high cycle efficiencies can be obtained at moderate heat-source temperatures. Despite its significant potential, sCO2 systems for waste heat recovery applications have not been commercialised yet, due to significant technical challenges that need to be overcome. This includes the development of suitable heat exchangers and turbomachinery, as well as the identification of optimal systems that adequately address the trade-off between performance and complexity

The focus of this proposal is to conduct original research to improve the fundamental understanding of the performance sCO2 cycles and the design aspects of the key components, namely compressors, expanders and heat exchangers. Computational and experimental methods will be used to investigate the performance and design characteristics across a wide range of operating conditions. These studies must account for the complexities of using sCO2 that exhibit complex fluid behaviour not observed in conventional fluids such as air and steam, in addition to considering the high-speed flows, and two-phase conditions close to the critical point at the compressor inlet, and the corrosive nature of sCO2 with low level of humidity to the heat exchanger materials. Ultimately, the results from these studies will improve the existing scientific understanding, and will facilitate the development of new performance prediction methods for the cycle and components. Understanding these aspects will not only lead to improved performance prediction, but could also lead to improved component design in the future. Within this project the new prediction methods will be used to investigate and compare the performance of different cycle architectures and component designs. The results from these comparisons will enable the identification of the optimal systems that can operate across a wide range of heat input and load conditions, and therefore best facilitate improvements to sCO2 systems.

The primary outcomes of this research will be improved fundamental understanding of the performance of sCO2 cycles and component designs and validated performance models for compressors and expanders. Furthermore, recommendations will be made on the most appropriate system configurations that offer improvements to operational aspects, thus enabling the future commercialisation of small-scale sCO2 technology for waste heat recovery. Therefore this project has the potential to stimulate investment and create new jobs within the low carbon energy market, whilst positively contributing to the UK's existing research portfolio in waste heat recovery from energy intensive industry.