Heat Storage and Domestic Hot Water

Solution advantages

  • Variety of options to meet domestic hot water demand in all contexts
  • Allowing combinations and storage from renewable energies
  • Efficiently sorting domestic hot water and supporting space heating

Share of domestic hot water energy consumption

An estimated 10-20% of the overall energy consumption for heating purposes within EU is used for domestic hot water (DHW).

Differentiation of domestic hot water systems

The main task of all domestic hot water systems is to provide the user with the desired comfort, which means delivering a minimum requested amount of hot water with a minimum temperature.

Appliances for providing a more or less constant supply of domestic hot water are commonly known as water heaters or DHW systems. The choice of applicable systems depends on the detailed requirements and the situation on-site. Basically it is possible to differentiate the systems into two general principles: on-demand water heater and storage water heater.

On-demand water heater

On-demand systems heat water instantly as it flows through the appliance (also known e.g. as tank-less water heater). The temperature difference between cold (inlet) and hot (tap) water and the required flow rate (e. g. for hand washing, showering or bath) define the necessary heat power.
Small tank-less water heaters could be located right where the water is used and are only linked with this one tap. Larger tank-less water heaters are also common in centralised systems such as single flats up to one or two family houses. Common energy sources are gas or electricity. Appliances which are capable of supplying both space-heating and on demand DHW are known as "combi" boilers.

Storage Systems

Direct heated storage tanks are a combination of a vessel with e. g. a gas or oil burner, electric resistance heater or an air source heat pump.
Where hot water space heating appliances are used,  drinking hot water cylinders are usually heated indirectly by primary water from the appliance or by an electric immersion heater (often as back-up to the boiler).

Indirect heated storage tanks (unvented cylinders) incorporate one (or more) internal heat-exchanger which could be connected with any appropriate heat source like heat pumps, gas, oil or biomass boilers, solar panels, district heating or combinations of those. 

Specific storage technology is the “stratified storage tank” (often use in solar DHW supported systems). This technology is specifically designed to force the effect of difference in mass between cold and hot water to prepare a specific “hot” area on upper part of the tank. This “hot” or “process” area provide the user with domestic hot water, while the lower “cold” area contains the heat exchanger (or directly) fed by the input of the solar panel. Between cold and hot area is a second heat exchanger which could – if necessary – heat up the solar heated water to the desired temperature of the domestic hot water.

Electric water heater
Electric water heater
Gas instantaneous water heater
Gas instantaneous water heater
Cross section domestic hot water tank
Cross section domestic hot water tank
Cross section stratified storage tank
Cross section stratified storage tank