Although not as effective compared to other solar thermal collectors, the flat-plate is the most commonly used collector in a number of countries. Made of an insulated weatherproof box, the flat-plate includes multiple translucent or transparent covers and a dark absorber plate. The pipes placed below the absorber plate have heat conducting fluids or water passing through them and as the liquid flows through the pipes, thermal energy is generated due to the heat from the sun.
Evacuated tubes also known as solar tubes are generally rows of glass tubes positioned parallel to each other. There are various types of tubes available such as glass-glass tubes, which comprises of a glass tube within another glass tube, the space between them evacuated of air to form vacuum. Glass-metal tubes on the other hand consist of a glass tube with a metal plate placed inside, which is connected with a flow pipe. Such tubes are considered efficient but they tend to lose the vacuum during the course of time.
Batch heaters are passive solar water heating systems. A batch heater is basically a huge tank full of water, insulated with glass and painted with black color. It absorbs the heat from solar rays and heats up the water inside the tank to generate heat energy. It is among the simplest and most practical of collectors that can be easily set-up in any location. Connected to the existing plumbing system, the heated water flows out of the tank and fresh water enters the tank from the bottom.