Flat Solar Collectors are currently the most common. Covered Flat Solar Collectors are essentially  composed of a glass covering, a collector panel, and thermal insulation encased in a metal or plastic box.
ANIMATION >

Vacuum tube collectors are proposed to reduce the loss of heat to the exterior. The heat captured by each element (each vacuum tube) is transferred to the panel, generally made of copper, inside each tube. Thus, the heat transferring liquid in the tube is heated, and, because of the vacuum inside the tube, the loss of heat to the exterior is minimized. ANIMATION >