We have a fully funded PhD position available focusing on air to air heat pumps in the UK. The details are here https://www.findaphd.com/phds/project/school-of-engineering-phd-scholarships-air-to-air-heat-pumps-for-equitable-demand-flexibility-and-year-round-thermal-comfort-in-uk-homes/?p198898
Please apply if interested. Please let me know if you have any questions.