Industrial waste heat recovery using supercritical carbon dioxide cycles (SCOTWOHR)

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.

This project will deliver an improved understanding of the performance of small-scale supercritical (sCO2) systems for waste heat recovery from energy intensive industries leading to higher system efficiency and potentially reduced costs through optimised design and improved operational aspects

Firstly, impact is expected in the two areas of the economy and knowledge transfer. A defined deliverable from this research program are recommendations on the most appropriate system configurations that can be implemented within different applications. This will give energy-intensive industries such as glass, iron and steel, cement, and oil and gas the opportunity to significantly reduce their carbon footprint.
Furthermore, there is a concerted international effort to improve the understanding of sCO2 systems through research and development as well as demonstration projects and this research will provide the potential for the UK to lead in the development and manufacture of these systems. Furthermore, results can be used for the validation of existing simulation tools to improve understanding of system behaviour. This will ultimately lead to better component design; either measured through higher efficiency at the design point, or higher efficiency over a wider range of operating conditions as well as optimal cycle configurations.
The widespread implementation of sCO2 technology could impact society in many ways. Not only will it help the UK government to meet its targets for renewable energy production and reductions in greenhouse emissions, thus positively impacting our climate, but it will also help reduce reliance on imported fossil fuels. This will improve the UK's future energy security. Furthermore, sCO2 technology could be used for efficient power generation replacing in the future steam power plants in nuclear reactors as well as concentrated solar power systems.
Through the proposed research programme the installation of a new test facility at City will strengthen the position of the research team as an international leader in waste heat recovery systems research. Additionally, through the improvements to the existing test facility at Brunel, it is expected that this project will maintain, and further enhance Brunel's international reputation as a leading research centre for sCO2 and heat exchanger technology. This will help in attracting leading students and staff to both universities who will benefit from the extensive knowledge of sCO2 systems, which will ultimately help in the development of leading sCO2 research engineers in the future.

The transfer of knowledge to industry will be obtained through regular progress meetings with beneficiaries and obtaining feedback to ensure that this research remains highly relevant to the requirements of the industry. A workshop will be arranged to disseminate key findings to relevant industries and researchers, whilst also providing a springboard from which to direct further research and development activities. Both City and Brunel regularly organise conferences and industry days and will use those to disseminate the developed knowledge.

The specialist knowledge developed as a result of this research will be actively used during taught courses at the two universities. This will ensure the development of highly skilled graduates trained in the design and analysis of sCO2 cycles, compressors, expanders and heat exchangers. This will be obtained by bringing relevant material into lectures, in addition to using the background of this project to define industrially relevant undergraduate and MSc research projects.
Wider dissemination will be achieved through Journal and conference publications in addition to a project website to provide updates, reports and links suitable for the lay audience. Furthermore, digital media will be utilised to both inform the public about research activities and upcoming events, and to explain the various aspects of technology to the public.