Advanced Thermomagnetic Cooling for Ultrahigh Power Density Electrical Machines

Electrical machines are estimated to contribute to more than 99% of global generation and more than 50% of all utilisation of electrical energy. Their role will be more pronounced as we move towards a more sustainable carbon neutral economy. Taking the UK automotive industry as an example, it is the fastest growing sector in the European economy, utilising more than 30% of our primary energy resources. UK automotive production is around 2 million vehicles in 2017. By renewing end of life products with more energy efficient ones, such as electric and hybrid electric vehicles (EV and HEV), this strong growth will increase the efficiency of energy use and help meet UK government targets in CO2 emission reduction - a 34% cut in 1990 CO2 emission levels by 2020 and 80% by 2050.

This trend of electrification in transport will lead to a huge demand in powertrain (machines and drives) research. To remain competitive, electrical machine manufacturers endeavour to increase power density and efficiency of electrical machines. However, the machine industry is a relatively mature sector and the margin for further improvement in machine efficacy and power density is slim without novel materials or radical cooling technologies. This is particularly the case for machine end-windings, which often have the highest temperature and hence have the biggest impact on machine achievable efficiency, power density and also life span. Methods such as spray cooling, flooded or semi-flooded stator are proposed for end-winding cooling. Both methods are very effective because the cooling fluid is in direct contact with the end-windings. However, due to corrosion and erosion of spray nozzles, the spray cooling suffers from reliability and robustness issues. Moreover, both spray cooling and flooded stator often require a closed circuit liquid (oil or deionised water) supply equipped with mechanical pumps, filters, etc. which adds to capital and operating costs while also leading to a reduction in effective machine power density.

In order to overcome the challenges facing the traditional cooling technologies, this project aims to develop a novel thermomagnetic liquid cooling for machine end-windings. The thermomagnetic cooling medium is based on ferro-fluid, which is an electrically nonconductive, temperature sensitive fluid mainly consisting of ferromagnetic nano-scale particles (such as iron, cobalt, nickel, etc.) in a liquid carrier (such as synthetic oils, hydrocarbons, etc.). When such liquid experiences a temperature variation under an external magnetic field, the fluid behaves as a smart fluid, i.e. it will have higher magnetisation in the lower temperature region (farther away from the heat source) than in the higher temperature region. As a result, a net magnetic driving force is produced to self-drive the fluid to flow towards the heated area (heat source with higher temperature). Due to this special feature, the thermomagnetic liquid cooling will be self-regulating, pumpless and maintenance free and hence very cost effective.

In this project, by adopting a multiphysics optimisation approach that combines electromagnetic and thermomagnetic domains into a single framework for machines with ferrofluid cooling, this project aims to achieve a temperature reduction of >30oC compared to a forced air cooled machine with rotor mounted fans. This is significant because the reduction in machine temperature, particularly the winding temperature, not only increases machine's life span, e.g. a 10oC increase in winding temperature will halve the winding insulation life (similar effect for bearings' life span), but also increases the machine's efficiency due to reduced power losses.

This project has important and far-reaching implications for both industry and academic research in novel electrical machine design with advanced cooling, particularly for high torque/power density applications where the ferrofluid cooling offers unique advantages. The impact of this project will be felt by the following beneficiaries:

1. Machine designers and material scientists. One of the outcomes of this project, i.e. the general framework of multiphysics (electromagnetic, thermal and thermomagnetic) optimisation of electrical machines, will enable machine designers to explore the ferrofluid cooling solutions for next generation electrical machine technologies. The workshop held towards the end of the project will provide a showcase for the framework and offer hands-on experience in machine design and thermal management (using thermomagnetic materials) to industrialists and academics. The aim of this work is to break down the barriers to thermomagnetic cooling adoption thereby encouraging industrial proof-of-concept research which draws on the outcomes of this project, accelerating progression through the technology readiness levels and pushing the market forwards.

2. The University of Sheffield. Through this project, the University of Sheffield will cement its position as a national leader in electrical machines and thermomagnetic materials, will be propelled to the forefront of electrical machine research worldwide and will strengthen its interdepartmental research links. The research staff involved (PDRAs, investigators, technical staff) will have unique interdisciplinary skills which will benefit future research at Sheffield and inform their teaching.

3. Industrial beneficiaries. The proposal has already attracted commitment from 4 partners representing a range of industries and research interests, and other partners will be recruited throughout this project. The potential for impact is demonstrated by the substantial financial commitment offered by each partner and their enthusiasm for the research articulated in their letters of support. To maximise the impact, in addition to the public workshop at the end of the project, interested industry partners will be invited to contribute to the direction of the research via the steering committee and by proposing application-specific demonstrators. As demonstrated by the letter of support, there has been considerable interest in this project. Several prospective partners have indicated the specific benefits to their businesses and have committed substantial resources to the realisation of our vision and ambition.

4. British economy and UK PLC. Britain has a world-leading track record in electrical machines and drives research and innovation and is positioning itself to be a world leader in some key market sectors such as renewables, automotive (electric and hybrid electric vehicles in particular) and aerospace. This project will help to safeguard and extend this leadership in technology to 2030 and beyond. It will provide a competitive lead to British engineering companies and strengthen the supply chain, benefiting the economy as a whole.

5. General public. To attract young people and under-represented groups into engineering and innovation, the project members will participate in public engagement activities with local schools doing outreach activities. They will also take part in public debate events, such as Café Scientifique, highlighting the importance of electric motors, explaining why the environment and cost have to be included in new thermal development to ensure longevity in the motor use. Also any ground-breaking results will be communicated to the general public via the departments' communication officer, who have experience of getting novel research results into general media (BBC, national newspapers).