This document discusses solar collectors, which absorb solar radiation and convert it to heat. There are two main types: flat-plate collectors, which are inexpensive but less efficient; and evacuated tube collectors, which are more expensive but more efficient. Flat-plate collectors consist of an absorber plate, glass cover, insulation and a heat removal system. They are used for water heating and pool heating. Evacuated tube collectors contain heat pipes in glass tubes under vacuum, achieving higher efficiency through smaller temperature differences. They support space heating and have applications in cooking, drying and power generation.