Forschungsgruppe-NET - Hochschule Offenburg

Solar Heat

Potable water heating

A characteristic feature of large-scale solar thermal system for potable water heating is the division into collector circuit, buffer circuit and potable water circuit, all of which are separated by a heat exchanger.

Systems with potable water preheating require an additional reservoir (preheating tank) that is fed by the circulation pump with solar energy from the buffer storage. Health and safety regulations require the preheating tank to be equipped with a legionella control unit to ensure that the water is heated up to 60°C at least once a day. The system with the preheating tank has proven to be the most reliable, especially since the amount of water used is not fed via the heat exchanger directly. To keep the two circuits separate, the useable water is provided by a separate pump which feeds the water to the heat exchanger on demand. Unfortunately, the systems’ efficiency decreases with high reverse-flow temperatures in the buffer storage; they are often due to the high temperatures in the preheating tank.

Continuous flow systems do not fed the potable water into a preheating tank. Instead, the water runs through a heat exchanger, where it can absorb the heat from the buffer storage circuit. The flow rates of the buffer storage and discharge circuit ideally correspond to the actual amount of water used, which causes considerable fluctuation. The main advantage of continuous flow systems, as compared to the preheating tank system, lies in the low reverse-flow temperatures in the buffer storage, which makes the system more efficient. Furthermore, it is no longer necessary to provide an extra potable water tank, a fourth circulation pump or the legionella control device that is a requirement for preheating systems. Continuous flow systems are thus preferable to those with preheating tanks.