This study provides an overview of new trends in the development of cooling systems for electric motors. It includes a summary of academic research and patents for cooling systems implemented by leading motor manufacturers at TRL9. New trends in the cooling management of air and liquid cooling systems are discussed and analyzed with a focus on temperature distribution and its influence on the power-to-dimension ratio of electric motors. The prevailing cooling method for synchronous and asynchronous motors is air cooling using external fins, air circulation ducts, air gaps, and fan impellers to enhance efficiency and reliability. Internal cooling with rotor and stator ducts, along with optimized air duct geometry, shows potential to increase the power-to-dimension ratio and reduce motor size. Liquid cooling systems offer a power-to-dimension ratio of up to 25 kW/kg, achieved through redesigned cooling ducts, stator heat exchangers, and cooling tubes. However, liquid cooling systems are complex, requiring maintenance and high ingress protection ratings. They are advantageous for providing high power-to-dimension ratios in vehicles and aircraft. Discussions on using different refrigerants to improve efficient motor cooling are underway, with ozone-friendly natural refrigerants like CO₂ considered to be promising alternatives to low-pressure refrigerants with high global warming potential.

During the last decade, many industrial and medical applications have shown a requirement for low-temperature-cooling usage (from −40 to −80 °C), which cannot be efficiently obtained via the conventional refrigeration systems usually employed for medium-temperature applications (from 0 to −40 °C). A proper ultra-low-temperature (ULT) refrigeration system design is essential to achieve the desired output. The performance can be maximised via the suitable selection of the configuration and refrigerant for a specific temperature range. This work contributes a detailed overview of the different systems and refrigerants used in ultra-low-temperature applications. Different systems, such as single-stage vapour compression, multi-stage, cascade, auto-cascade, and air refrigeration cycles, are presented and discussed. An energy analysis is then carried out for these systems identifying the optimal system design and refrigerant selection to achieve the highest performance. This paper aims to provide the reader with a comprehensive background through an exhaustive review of refrigeration systems suitable for ultra-low-temperature applications. The effectiveness of these systems is proven numerically, mainly based on the temperature level and purpose of the application.

Heat pumps integrated with renewables enable carbon-neutral heating/cooling, requiring a multi-scale strategy, interdisciplinary research, and cross-sector collaboration.