This is currently by far the largest application for solar thermal in Europe. These systems are normally designed to cover 100% of the hot water demand in summertime and 50-80% of the total annual hot water demand.

There are two main types of solar DHW systems, those designed as thermo-siphons or those designed with forced circulation.

Thermo-siphon circulation (also known as natural flow)

Thermo-siphon systems are most common in the frost-free climates in the south of Europe but developments have now made the systems suitable for all of Europe including frost areas.

Thermo-siphon systems use gravity to circulate the heat transfer medium (e.g. water) from the collector to the tank, as hot fluids are lighter than cold. The medium is heated in the collector, raises to the top of the tank and cools down on its way back to the collector as the heat is transferred to the DHW. A thermo-siphon system is thus rather simple as it works without pump and controller but it requires on the other hand that the tank be placed above or beside the collector, which in most cases means on the roof. A thermo-siphon system can be operated as pre-heater or cover the whole DHW demand if it is equipped with a supplementary heater (often an integrated electric heater). A well- designed thermo-siphon system shows high and reliable performance.

DHW thermo-siphon systems for a one-family house have typically a 2-5 m2 collector and a 100-200 litres tank.

Thermo-siphon system

Diagram of a thermo-siphon system

Forced circulation

Forced circulation systems are most common in Central and Northern Europe. The forced circulation makes it possible to separate the tank and the collector as the heat transfer medium (fluid) is pumped between the tank and the collector. This means for example that the tank can be placed inside the building or even in the cellar, which often makes it easier to integrate the solar system with a heating system (see combisystems below). The aesthetics of avoiding a tank on the roof is another advantage.

A forced circulation system is more flexible but also more complex than a thermo-siphon system, because it needs a pump and a controller. A forced circulation system can be operated as pre-heater or cover the whole DHW demand if it is equipped with a supplementary heater (e.g. an integrated electric heater or gas burner). A well-designed forced circulation system shows the same high and reliable performance as a thermo-siphon system.

DHW forced circulation systems for a one-family house have typically a 3-6 m2 collector and a 150-400 litres tank.

Forced circulation system

Diagram of a forced circulation system

1 Collector - 2 Tank - 3 Heat exchanger - 4 Control unit

5 Expansion Tank - 6 Back-up heater - 7 Consumer

Larger DHW systems for multi-family houses, apartment blocks, hotels, office buildings etc. are also common throughout Europe. The collector surface of such systems varies from ten to several hundred square meters. Larger DHW systems are generally designed for a low solar fraction (low solar coverage of the hot water demand) and operate on a lower temperature resulting in a high system performance (thermal output/m2 of solar collector). Most large DHW systems are designed with forced circulation but multiple thermo-siphon systems are also used when the conditions are suitable.