It is widely acknowledged that we need to continue to decarbonise the power and heat that we use as we contribute to the global shift towards efficiency improvements, the use of sustainable energy sources - and the use of low/zero carbon technologies in these sectors.
This need for greater efficiency extends to the power plants we use to generate power and heat, as well as the heat sources themselves. The amount of cooling required for any given size power plant is determined by its thermal efficiency, and it is nothing to do with the heat sources themselves (such as coal, natural gas, solar thermal or nuclear). The conventional steam Rankine cycle (a thermodynamic cycle where heat is converted into mechanical energy and is then typically transformed into electricity) used in power plants emits two-thirds of the energy into the atmosphere, released via cooling towers. The use of either water or air for cooling depends on the availability and affordability of water – and while dry cooling enables water savings, it has a negative impact on how efficient the plant is.
To address the challenge of increasing efficiency in steam power plants while also saving water, the Heat and Energy Storage sub-team within the Centre for Thermal Energy Systems and Materials (CTEM) at Â鶹´«Ã½AV is researching Cold Thermal Energy Storage (CTES). One potential solution comes in the development of an air-rock thermocline – a technology which takes advantage of the drop in air temperature during the night to improve the overall performance of steam plants, by storing steam turbine exhaust load in a rock-bed. In this approach, cold thermal energy is captured and stored during the night within rocks in a packed bed, and then is later ‘released’ in front of a dry cooling system to cool down the condenser, enhancing the system’s efficiency and reducing water consumption in thermal plants.
This use of CTES formed part on an EU H2020-funded project, working to improve the efficiency of solar thermal plants in areas with high solar radiation and to reduce water consumption. The aim of this work was to increase the Technology Readiness Level (TRL) – a scale originally used by NASA which assesses technologies and their readiness for onsite deployment - from 1/2 to 4/5.
Transforming research into application
Such EU projects have supported several MSc group and individual projects within Â鶹´«Ã½AV’s Energy and Power MSc programme. Supervised by Dr Kumar Patchigolla, a group of students at Cranfield have designed and built a 150 kWth prototype test rig at Cranfield (as part of the EU WASCOP project) implementing the use of air-rock thermocline technology for the improvement of dry cooling in power plants. (Figure 1) This is supporting the construction of a second 1000m3 CTES at the La Africana CSP power plant in Spain – a thermal power plant with a design capacity of 50 Megawatt electric (MWe). This second prototype aims to demonstrate the performance improvements that can be achieved in evaporative cooling systems.
As part of these EU projects, one of the group members – Jon Gillard – has now completed his PhD and is undertaking Research Fellow activities to progress these technologies forward. The research includes thermodynamic modelling of the CTES and integration with a whole plant model. This whole plant model can also be used in conjunction with other innovative CTES technologies, such as water-rock thermocline, latent heat storage and hybrid cooling, to enable investigations into the maximisation of their utility in terms of increased power generation efficiency while reducing water consumption. The development of this whole plant model is now being used as teaching material for the Applied Thermal Energy Systems module.
Future practise
Going forward, the group is now exploring options for the integration of these schemes/designs into Cranfield’s own heat networks. Furthermore, they are working on absorption cooling that utilises the exhaust heat from the steam plants to provide space cooling and water (through desalination), as well as electricity to benefit Official Development Assistance communities.
Read more about activities and research being carried out by the Centre for Thermal Energy Systems and Materials (CTEM) at Â鶹´«Ã½AV.
Fig 1: 150 kWth air-rock thermocline system at Cranfield consists of ~40 tonnes of decorative rock