A pump circulates the water from the tank up to the collector and back again. This system has a differential controller that senses temperature differences between water leaving the solar collector and the coldest water in the storage tank. In this way, the water always gains heat from the collector when the pump operates. A flush-type freeze protection valve installed near the collector provides freeze protection. Whenever temperatures approach freezing, the valve opens to let warm water flow through the collector.
Automatic recirculation is another means of freeze protection.
When the water in the collector reaches a temperature near freezing, the controller turns the pump on for a few minutes to warm the collector with water from the tank. In this way, water flows through the collector only when the sun is shining. The dc pump and PV panel must be suitably matched to ensure proper performance. The pump starts when there is sufficient solar radiation available to heat the solar collector. It shuts off later in the day when the available solar energy diminishes.