Heat and Power Division
The recovery and re-use of low-grade waste heat and effective utilisation of renewable resources for heat and power provision are essential components of a sustainable energy future. In the UK, for example, natural gas consumption for heating purposes is double that of electricity, while in the domestic sector the demand for heat is currently somewhere between 3 and 5 times that of electricity, even though the energy debate focuses mainly on the latter. The availability of most alternative energy sources (e.g. solar, wind) exhibits appreciable fluctuations (caused by day-night cycles, weather, etc.), while the consumption also fluctuates considerably, according to users’ needs. The resulting mismatch between energy supply and demand gives rise to a need for reliable and efficient energy storage systems to smooth out the imbalance. A disparity between supply and demand is also an important factor that arises when considering the opportunities for low-grade waste heat utilisation, which is abundantly available in the industrial and commercial sectors, and also in the domestic environment, especially when considering ‘over the fence’ solutions which offer an improved scenario, both from an investment and an emission reduction perspective, over the self-contained re-use and/or conversion of heat within individual plants.
The research activities conducted by the Heat and Power Division are:
Waste Heat Recovery
The heat and power division team is focused on technologies for thermal energy conversion and utilization; in particular, the low grade heat from renewable energy sources (solar and geothermal) and the waste heat from industrial processes. Many industrial processes produce immense amounts of thermal energy, albeit at low temperatures for which the Carnot efficiency that limits conversion to power is inherently low. The conversion of low grade heat into electricity can be achieved by means of Organic Rankine Cycle (ORC) systems, which are capable of converting this low grade heat more efficiently compared to conventional steam Rankine cycle systems.
This research aims to investigate how to make ORC systems capable of converting this low grade heat more efficiently. Previous research indicates that the main components which destroy exergy in ORC systems are the evaporator (heat exchanger) and expander. Therefore, the main focus of this research is to improve the exergy efficiency of these two components.
Work on expanders focuses on the use of volumetric machines, such as the reciprocating expander, as alternatives for conventional turbines in small-scale applications. In the evaporator, exergetic losses occur as a result of large temperature differences between the heat source and the cycle. The use of zeotropic working fluid mixtures can provide a better thermal match, thereby improving thermodynamic efficiency and power output from the ORC. Therefore, investigation of novel working fluid mixtures and additives which will further improve heat exchanger performance is essential.
Alternative heat engines for the conversion of low-grade heat to useful work are also being researched. The heat and power division is involved in the European Up-THERM project, which is aiming to provide an affordable alternative to conventional heat engine prime-movers for combined heat and power (CHP) applications (for more information see the Up-THERM project summary).
The CEP group is also involved in the research activities of the UNIHEAT project under Theme 4 - Thermodynamic power generation cycles for improved energy efficiency. UNIHEAT aims to increase the energy efficiency of several processes in oil refineries in addition to upstream and downstream operations (for more information see the UNIHEAT project website).
The group is also leading the “Energy-Use Minimisation via High Performance Heat-Power-Cooling Conversion and Integration: A Holistic Molecules to Technologies to Systems Approach” project (iHPC). The project is a four-year multidisciplinary project funded by the Engineering and Physical Sciences Research Council (EPSRC) aimed at minimising primary-energy use in UK industry with next-generation technological solutions for waste heat recovery and conversion to power or cooling.
Solar/Low Grade Heat PV and Thermal
The IEA projects that solar energy has the potential to cover one-third of the world’s energy consumption by 2060, while solar-thermal technologies in particular can make a significant contribution to primary fuel and emissions reduction targets, and energy security. A key challenge is in enabling feasibility in regions of low irradiance and at small scales of application where land area is limited. Solar thermal collectors are a mature technology for the conversion of the solar irradiance into thermal energy for industrial, commercial or domestic application. The solar irradiance can be converted directly into electricity by means of solar cells. Low-temperature thermodynamic cycles for thermal energy conversion, such as organic Rankine cycle (ORC) technology, offer the possibility of using simple and affordable non-concentrating solar collectors in small systems.
The Heat and Power Division is interested in whole-system modelling of the solar ORC as part of a versatile, co-generation (heating and power) or tri-generation (cooling, heating and power) system. Such a system has potential for domestic applications, where they can be integrated with conventional solar hot-water systems at low cost. Visit the solar ORC page.
Photovoltaic (PV) cells for the direct conversion of solar radiation to electricity also produce a significant amount of low-grade waste heat. The solar to electricity conversion efficiency of PV cells can be improved by cooling down the cells with a cooling medium. The low grade heat obtained from such a hybrid PV/thermal collector (PV/T) can be used in domestic heating applications or employed for cooling via an absorption refrigeration cycle. Visit the solar cooling page.
The desire to utilise energy efficiently has led recently to an increased interest in thermal energy storage (TES) systems. TES can play a vital role in future energy systems, by enabling a reduction in the discordance (in time or rate) between the production or availability of energy and its consumption. By storing thermal energy locally, for later use when this is required (i.e. for space heating, or hot water), the peak energy load can be reduced and a lower-cost, off-peak energy input utilised instead.
The integration of latent heat storage solutions into modern heating and cooling systems has the potential to enhance the overall system performance compared to standard hot-water systems (radiators and tanks) due to an augmentation of the stored heat by the latent heat of a suitable material.
Aiming to characterise the dynamic behaviour and performance of an active thermal storage system for domestic applications, CEP has undertaken an investigation involving computational predictions complemented by experimental measurements in order to capture the time-varying interplay between fluid flow and heat transfer in a system based on the use of a hydrated salt Phase Change Material (PCM).
Computational modelling makes use of CFD toolbox OpenFOAM®. The CEP laboratory is also equipped with facilities for the experimental study of TES systems.
The CEP Laboratory has also been active in thermo-economic analyses of electricity storage systems. More specifically, technologies of interest include the liquid-air and pumped-thermal electricity storage systems. The two technologies are considered at medium to large scales with the ability to utilise low carbon low cost electricity during off-peak periods for charging. The electricity is then supplied through the discharging cycle when it is of high demand.
Thermodynamic models of these technologies are used for their technical performance analysis and the identification of influential parameters. Preliminary economic assessments are undertaken based on economic models which also account for the thermodynamic characteristics associated with each technology for its charging, storage and discharging elements. Economic evaluations consider different cost components such as initial capital expenditure, energy costs, and operational and maintenance costs. Also, financial indicators such as power capital costs (i.e. capital cost divided by power capacity), energy capital costs (i.e. capital cost divided by the energy capacity), levelised cost of storage, and required sell-to-buy electricity prices are studied and compared for the different systems.
The economic competitiveness of newly proposed technologies can be challenging due to the lack of cost data or case studies. The combination of thermodynamic and economic analyses through a thermo-economic framework can give estimates of the technical and economic competitiveness of these systems under different scenarios as well as in comparison to other electricity storage technologies.
High-Temperature High-Efficiency Hybrid PV-Thermal Solar Systems
In a PV-T generator, PV cells convert sunlight to electricity, and thermal energy is removed from the cells via a cooling contacting fluid, for uses such as hot water provision or space heating or cooling, with the added benefit of actively cooling the PV cells and increasing their electrical efficiency. Such systems are capable of reaching 70% overall efficiency: less than 20% electrical efficiency and over 50% thermal efficiency. In most present applications, the electrical output of a hybrid PV-T system is the main priority. Hence, the contacting fluid is kept at low temperature to maximise electrical efficiency. However, this imposes a limit on its posterior use for heating or cooling purposes, and precludes the use of selective surfaces for thermal capture, compromising thermal efficiency. Hence, a design conflict arises between the electrical and thermal performance of hybrid PV-T systems.
The CEP laboratory is currently investigating for the development of High-Temperature High-Efficiency Hybrid PV-Thermal Solar Systems. Custom PV-T HIT solar cells capable of full infra-red absorption for thermal generation and high efficiency at high operation temperatures are ideal for PV-T applications. Research at the Heat and Power Division is centered in the modelling, design, fabrication and testing of the proposed PV-collector as part of a wider solar-energy-based heat, power and cooling (solar-based tri-generation) system. Collector-unit models will be incorporated into ‘system’ models of a complete energy infrastructure capable of the combined provision of heat-power-cooling, which will then be used to design PV-T collectors with optimal flow rates and geometries in chosen cases (specifically, maximising the thermal output, electrical output, and cooling capacity with suitable sub-systems technologies, as well as minimising emissions, and maximising flexibility/demand tracking). For more information, visit 'project website'.