In all the devices you mentioned, the COP can be improved by: 1) reducing the heat losses from the system and 2) Improve the thermal conductivity of the heat transfer media employed in the system.

Hope this helps answer your question.

Professor Yehia Khalil

Yale University

USA

In addition to what Professor Khalil mentioned you need to use an efficient  well oriented solar collector which can maximize the attracted solar energy as much as possible.

Thank you respected professors Dr. Kamal and Dr. Khalil for your help and support.