A cool electrocaloric effect
Electrocaloric phenomenon is being exploited to develop a gas-free, CO2 efficient cooling solution for refrigeration and air conditioning systems. The project, led by the UK’s National Physical Laboratory (NPL), aims to develop a prototype electrocaloric cooler to replace vapour compression, magnetic and thermoelectric-based technologies.
Project leader Tatiana Correia explains, ‘Over the last two decades the development of novel refrigeration technologies has been a scientific priority. This is partly due to the 1997 Kyoto protocol to reduce greenhouse gas emissions. A large percentage of emissions is due to vapour compression refrigeration technology and air conditioning that use polluting refrigerants such as chlorofluorocarbons and hydrofluorocarbons.’
The operating principle of electrocaloric refrigeration is analogous to that of magnetic and gas compression cooling, but capitalises on the ability of some materials to change temperature when an electric field is applied or withdrawn under adiabatic conditions. The phenomenon has been known for more than 50 years. However, the effect previously observed was too small to be realisable in a cooler. ‘A large electrocaloric effect has already been found in thin films, but these had such a small thermal mass that it was not a viable technology for a real cooling application. Multilayer thick films to produce large electrocaloric cooling power might be the answer,’ she adds. ‘Electrocaloric cooling is environmentally friendly too. We predict it will have efficiencies of 60–70%, unlike thermoelectric cooling technologies that operate at a maximum of 13%. The process will also eliminate expensive magnets currently required in magnetocaloric technology.’
So far the largest electrocaloric effect has been found in ferroelectric materials such as barium titanate, lead zirconium titanate and lead-magnesium-nobium titanate. often used in sensors and capacitors.
‘The raw materials are cheap, easy to integrate and have high cooling power. And, assuming we just use ceramic-based materials, the fabrication process is widely used,’ adds Correia.
But developing electrocaloric materials has not been without its challenges. New materials need a large electrocaloric effect at room temperature and much work has been done to develop new compositions that do not contain lead.
Measurement of electrocaloric materials, traceability and accuracy has also been problematic. To help with this, NPL is working with other European measurement institutes on a new multimillion pound project funded by the European Metrology Research Programme, called Metrology of Electro-Thermal Coupling (METCO). The consortium will bring together leading research centres and industry to develop a more accurate measurement system.
Correia explains, ‘The direct measurement of the electrocaloric temperature change in thin films is a challenge as this is attached to a thicker substrate and the heat is quickly leaked away to the substrate, which makes the temperature reading very difficult’.
Further down the line, the electrocaloric coolers could be used for thermal management of power electronics in integrated circuits and hybrid and electric vehicles, ‘but commercialisation is a longterm goal,’ says Correia. ‘Many material and design aspects need to be addressed and we need an accurate datasheet of electrocaloric materials.’ The project will be completed in November 2016.
Did you know?
Cooling and heating of residential and commercial buildings represents 40% of the EU27’s used energy and contributes to 36% of global warming emissions
Wow, that’s compressive
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