ABSTRACT
A solar water heater is a combination of a solar collector array, an energy transfer system and a thermal storage system. Active solar water heaters use a pump to circulate the heat-transfer fluid through the collector while passive or thermosyphon systems use thermally driven natural circulation of the working fluid. The amount of hot water a solar water heater produces depends on the type and size of the system, the amount of sun available at the site, the seasonal hot water demand pattern and the installation of the system. Solar water heaters are also characterized as open loop (also called ‘direct’) or closed loop (also called ‘indirect’). An open loop system circulates potable water through the collector. A closed loop system uses an antifreeze heat-transfer fluid loop (typically containing polypropylene glycol) to transfer heat from the collector to the potable water in the storage tank. Some systems also use a load side heat exchanger between the potable water stream and the hot water in the tank. Passive or thermosyphon systems rely on the natural circulation of water between the collector and the tank or heat exchanger in the tank. As passive systems do not rely on pumps and controllers they are more reliable and have a longer life than pumped systems. Passive systems do not require an electrical supply to operate and naturally modulate the circulation flow rate in phase with the radiation level. Passive systems can also be built with inherent freeze resistance so they can be used in areas that have extended freezing temperatures.
