A solar water heater is a device that generates hot water using solar heat energy. A solar collector that is attached to a water storage tank and positioned on the top of a building makes up a standard solar water heater. Depending on the system, unheated water can either be heated directly by being circulated from the tank through the collector or by a high-capacity heat-exchange fluid that has been warmed in the collector and transmits its heat through tubes in the water in the tank. “Active” solar hot water systems employ energy to circulate the heat-exchange fluid and power mechanical pumps and controllers, whereas passive solar hot water systems transmit heat from the solar collector to the unheated water without the use of mechanical means.
Mechanical pumps and differential controllers are used in active solar hot water systems to control and guide the flow of the heat-transfer fluid or water from the solar collector to the tank. When the water in the tank cools below the collector’s temperature, the controllers detect the temperature differential between it and the water in the tank and activate the pump. Some pumps use line power (mains electricity), while others use power produced by solar photovoltaic panels. While some solar-powered systems simply circulate the fluid when the Sun is out and store the heated water in well-insulated tanks for space heating at night, others rely on mains energy as a backup for nights and cloudy days. Since the flow of water in active solar hot water systems is not solely dependent on gravity, the water storage tanks can be placed inside the roof area or anywhere else that will reduce heat loss to the cold air. Because of this, these tanks can be integrated with hot water cylinders in household space heating systems, and the solar hot water system can be utilised to pre-heat water in the cylinder in the winter for space heating.
The most effective passive systems, which rely on gravity rather than power, are found in warm climes without nighttime or winters freezing. Some passive systems employ a thermosiphon setup that relies on convective and gravitational heat transfers. Gravity pulls cold water from a height down to the solar collector, where it runs through the collector and heats up before rising by convection to return to the storage tank. The integrated collector storage system is another kind of passive system. In this system, the collector serves as the tank’s top and heats the water there.
The effectiveness of the collector and the overall system design largely determine the output of a solar hot water system. An efficient solar hot water system must be designed with the collector and storage tank sized appropriately for the amount of hot water that will be used. Because the performance of a single component (the solar absorber) determines the efficiency of individual collectors, whereas the performance of an entire system depends on a variety of factors, the efficiency of both is rated separately (water and ambient temperatures, system configurations, insulation, water volume, the type of collector, heat-exchange mechanism efficiencies, the location and local weather at the installation, the amount of sunlight received by the collector, etc.). All solar water heaters must also include temperature restriction and overheating mitigation features, especially if they are deployed in warm locations.