ETC use liquid–vapour phase change materials to
transfer heat at high efficiency. These collectors feature a
heat pipe (a highly efficient thermal conductor) placed
inside a vacuum-sealed tube. The pipe, which is a sealed
copper pipe, is then attached to a black copper fin that fills
the tube (absorber plate). Protruding from the top of each
tube is a metal tip attached to the sealed pipe (condenser).
The heat pipe contains a small amount of fluid
(e.g. methanol) that undergoes an evaporating-condensing
cycle. In this cycle, solar heat evaporates the liquid, and the
vapour travels to the heat sink region where it condenses and
releases its latent heat. The condensed fluid return back to
the solar collector and the process is repeated. When these
tubes are mounted, the metal tips up, into a heat exchanger
(manifold) as shown in Fig. 5. Water, or glycol, flows
through the manifold and picks up the heat from the tubes.
The heated liquid circulates through another heat exchanger
and gives off its heat to a process or to water that is stored in
a solar storage tank.