At first glance, it doesn't seem that complicated: a large water cylinder in your basement stores or buffers heat. When there is excess heat, the cylinder is heated up. It transfers this heat as and when it is needed. Generally, cylinders are used for the intermediate storage of excess heat, as is the case with wood heating systems and solar thermal systems, neither of which generate heat constantly or at the same rate. They also bridge power-OFF periods for heat pumps and improve efficiency, e.g. when burning solid fuels (such as wood).
As heat should be stored in a cylinder for the longest possible time, its thermal insulation plays an important role. Good thermal insulation can be recognised, for example, by a certificate such as the ErP label or the fact that the outer surface of the insulation is approximately at room temperature whilst the inside of the cylinder is at 90 °C. If the insulation is poor, it is not only the cylinder that loses heat, but its connections also incur substantial losses. Consequently, you should ensure excellent thermal insulation not just for the cylinder itself but also for the connecting pipework. State of the art.
Generally, almost all commonly used cylinders store energy in the form of hot water. Amongst these, the buffer cylinder represents the simplest method of retaining heat for your heating system over long periods. A large, well insulated water cylinder stores heat from solar, oil, gas, pellet or heat pump heating systems, and transfers this energy to the heating system when required.
The most efficient buffer cylinder technology is the stratification cylinder. As its name suggests, this type of cylinder stratifies water at different temperatures, as occurs in lakes: the coldest layer is at the bottom and the warmest at the top. One benefit of this is that even after a short heat-up time, sufficient energy for DHW at the required temperature is available in the upper section of the cylinder. Furthermore, strong turbulation is avoided when the cylinder is being reheated, retaining the temperature stratification inside the cylinder. This means you will always have hot water available quickly whilst saving energy as well. In addition, the lower temperatures in the bottom sections of the cylinder increase the efficiency of the solar thermal system as well as the efficiency of condensing boilers.
The primary reason for this is that renewables may be free and in theory infinitely available, but sadly the sun does not always shine. For such circumstances, you should have a well insulated and highly efficient cylinder that can retain heat just like a battery for use at a later date.
Combining a solar thermal system, heat pump and cylinder is very environmentally responsible. In a building with excellent thermal insulation (such as the KfW 40 house or a passive house), such a solution would enable you to operate without fossil fuels all year round, provided you also use green electricity. You would definitely need electricity to operate the heat pump in winter on days when the solar thermal system provides insufficient energy.
In such cases, the solar thermal system would generally be used to provide DHW. In the meantime, the heat pump would heat up your heating water buffer cylinder. Depending on the system, excess solar energy can also be used to back up the heating system. Either way, with this kind of heating system you would have a highly efficient and sufficiently sized cylinder that can store the excess heat in good weather and transfer it as required.
Many factors influence whether a cylinder is a wise purchase for you. For example, the size of your apartment or house, the type and number of heat generators, the number of occupants, peak times for DHW demand, hygiene requirements, the inclusion of renewables, etc.
One further tip: always check whether the cylinder you have in mind is optimised for your heat generator. For example, solar thermal systems and heat pumps require cylinders with one or more internal indirect coils. One indirect coil for a heat pump should be designed for low temperature levels to allow the pump to operate efficiently. Should you require DHW hygiene to be of a particularly high standard, perhaps because several children or older people share your household, or because your property remains unoccupied for lengthy periods, you might want to consider a freshwater module or a DHW system with a legionella protection function (which heats the water to above 60 °C).