Advanced Thermomagnetic Cooling for Ultrahigh Power Density Electrical Machines

据估计，电机占全球发电量的99%以上，占所有电能利用率的50%以上。随着我们向更可持续的碳中和经济迈进，它们的作用将更加明显。以英国汽车工业为例，它是欧洲经济增长最快的行业，使用了英国30%以上的一次能源。2017年，英国汽车产量约为200万辆。通过用更节能的产品更新报废产品，如电动和混合动力电动汽车（EV和HEV），这种强劲的增长将提高能源使用效率，并帮助实现英国政府的二氧化碳减排目标——到2020年，二氧化碳排放量在1990年的水平上减少34%，到2050年减少80%。运输电气化的趋势将导致对动力系统（机器和驱动器）研究的巨大需求。为了保持竞争力，电机制造商努力提高电机的功率密度和效率。然而，机械工业是一个相对成熟的行业，如果没有新材料或激进的冷却技术，进一步提高机器效率和功率密度的余地很小。这对于机器端绕组来说尤其如此，因为端绕组通常具有最高的温度，因此对机器的可实现效率、功率密度和寿命影响最大。提出了喷雾冷却、淹水冷却或半淹水冷却定子的方法。这两种方法都非常有效，因为冷却液与端绕组直接接触。然而，由于喷嘴的腐蚀和侵蚀，喷雾冷却的可靠性和稳健性存在问题。此外，喷雾冷却和淹水定子通常都需要配备机械泵，过滤器等的闭路液体（油或去离子水）供应，这增加了资本和运营成本，同时也导致有效机器功率密度的降低。为了克服传统冷却技术面临的挑战，本项目旨在开发一种新型的机器端绕组热磁液冷。热磁冷却介质以铁磁流体为基础，铁磁流体是一种不导电、对温度敏感的流体，主要由铁磁纳米级颗粒（如铁、钴、镍等）在液体载体（如合成油、碳氢化合物等）中组成。当这种液体在外磁场下经历温度变化时，流体表现为智能流体，即在较低温度区域（远离热源）比在较高温度区域具有更高的磁化强度。因此，产生净磁驱动力，自驱动流体流向受热区域（温度较高的热源）。由于这一特殊功能，热磁液体冷却将是自我调节，无泵和维护免费，因此非常具有成本效益。在本项目中，通过采用多物理场优化方法，将电磁和热磁域结合到具有铁磁流体冷却的机器的单个框架中，该项目旨在实现与带有转子安装风扇的强制风冷机器相比，温度降低bbb30℃。这是重要的，因为机器温度的降低，特别是绕组温度的降低，不仅增加了机器的寿命，例如，绕组温度增加10℃将使绕组绝缘寿命减半（对轴承寿命的影响类似），而且由于减少了功率损耗，还增加了机器的效率。

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.