Evacuated tubes consist of two glass tubes fused at the top and bottom. The space between the tubes is evacuated to form a vacuum, which provides excellent insulation against heat loss and helps to retain up to 97% of thermal energy.
A copper pipe (called a heat pipe) containing a small amount of purified water runs through the centre of the inner tube. The one-way coating on this tube absorbs the sun’s thermal energy and heats the water in the heat pipe. The water evaporates and rises to the manifold at the top of the pipe, which is connected to a slow-flow circulation pump that then sends the heated water to a storage tank. Water is heated during daylight hours but can be used at night or the next day because the tank is insulated.
Evacuated tubes can extract heat from the air on an overcast day and don’t need direct sunlight. This provides an advantage over solar panels, which do not perform as well in the early morning or late afternoon. Evacuated tubes are designed to operate in warm or cold weather.