Excellent description of options for California buildings https://www.synapse-energy.com/sites/default/files/Decarbonization-Heating-CA-Buildings-17-092-1.pdf

Heat pumps for space heating  descriptions start on page 10  (page numbers at bottom of each page).

Cold climate heat pumps are described on page 16

Market Barriers and Opportunities begins on page 17

 

Most interesting: CO2 refrigerant Heat Pump Water Heaters are discussed page 23

CO2 heatpump manual is at https://www.sandenwaterheater.com/sanden/assets/File/Sanden_sanc02_technical-info_10-2017_4.pdf