Solar Thermal systems collect or absorb solar energy as heat. They can be designed to generate high temperature heat (for electricity production and/or process heat), medium temperature heat (space/water heating and electricity generation), and low temperature heat (for water and space heating and cooling).
Solar Water Heater Basics
Solar water heating systems include storage tanks and solar collectors. Solar water heaters use the sun to heat either water or a heat-transfer fluid in the collector.
Most solar water heaters require a well-insulated storage tank. The tank can be a modified standard water heater, but it is usually larger and very well insulated. Solar storage tanks have an additional outlet and inlet connected to and from the collector.
In two-tank systems, the solar water heater preheats water before it enters the conventional water heater. In one-tank systems, the back-up heater is combined with the solar storage in one tank.
The hot fluid can be used to heat a space, such as in-floor heating system.
Both images are from Energy.gov.
Solar Swimming Pool Heaters
Most solar pool heating systems include a solar collector, filter, pump, and flow control valve.
Water in the pool is pumped through the filter and then through the solar collector(s), where it is heated and returned to the pool. In hot climates, the collector(s) can also be used to cool the pool during peak summer months by circulating the water through the collector(s) at night.