Solar water heaters are made up of solar collectors, and some kind of water storage for example a solar geyser.
Close-coupled systems are where the water storage unit and collector, are coupled together normally on a roof, whereas a split unit would have the collector on the roof and the water storage unit mounted within the roof void.
Direct circulation systems circulate household water through the collectors and into the water storage tank/geyser. They work well in climates with no frost and freezing conditions.
Indirect circulation systems circulate a non-freezing heat transfer fluid through the collectors and a heat exchanger where the solar heat is transferred from the heat transfer fluid to the domestic water in the water storage tank/geyser.
Active solar hot water systems utilize pumps to circulate fluids throughout the system. Some form of external power is required for operation.
Passive solar hot water systems circulate either domestic water or a heat transfer fluid through the system without the use of a pump, this is called thermosyphoning. Passive systems require no external energy to operate and being simple, tend to be very reliable with little or no maintenance needed.
Open-loop solar hot water systems are direct systems because the water travels through the solar collector.
Closed-loop solar hot water systems have the solar fluid remain within a single circuit at all times. All closed-loop solar hot water systems are classified as indirect systems because the sun heats some type of solar fluid and that heat is transferred to the domestic water through a heat exchanger. Closed-loop systems can use flat plate or evacuated tube. Flat plate collectors are by far the most common.
Two types of solar collectors are used for residential applications, flat plate and evacuated tube.
The water storage/geyser like a normal electric geyser has a heating element, although the element is down rated to a 2Kw or 3Kw depending whether the solar geyser is a 200-litre or 300-litre unit.
To ensure that there will always be sufficient hot water for the household, these modern solar units are extremely well insulated and incorporate their own
built-in thermostatically controlled electric element as a standby to keep the water temperature at an optimum level of 55 degrees C. This means that any conventional geyser can be eliminated. To cope with large demands, more than one solar hot water system can be installed in series to give a big enough supply.