But solar energy is a functional description, involving a variety of classification numbers, depending on which aspect you want to check, such as solar heat collection or garbage disposal.
Give a few examples:
B60L 8/00 electric traction with natural forces to provide electricity, such as solar energy and wind power.
B64g1/44 ... solar cells using radiation, such as deployable.
F24J2/00 solar thermal utilization, such as solar collectors
Or, you can directly use solar energy as a keyword to search in the patent name or abstract, but you will miss some by comparing the classification number.