Buffer (heating technology)
In a heating system, a buffer is understood to be a heat storage tank that is filled with water . It is used to compensate for differences between the generated and consumed heat output in order to be able to decouple heat generation from consumption in terms of time. This improves the operating behavior, efficiency and durability of many heat generators.
The disadvantage is the continuous loss of heat, which results from the temperature difference between the contents of the storage tank and the environment. The loss can be reduced through thermal insulation (avoiding thermal bridges ).
A special case of the heat buffer is the so-called boiler , in which the heat generation is directly integrated. The most common forms are electric boilers or gas boilers.
The counter-concept to heating with a heat buffer is a flow heater . With this, the heat is produced “ just in time ”, that is, at the same time as consumption. This mode of operation is only economically feasible if the heat source can be switched and regulated very quickly and has a sufficiently high degree of efficiency even under partial load (e.g. electric heating or gas burner ).
Central heating, in particular with solid fuel
The buffer is often used in central heating systems with solid fuel firing, which achieve their optimum efficiency under full load and whose heating output is difficult to control. If the device is switched on and off frequently, losses in efficiency and a reduction in service life due to the formation of condensation can be expected.
Log firing systems, for example, heat both the heating circuit and the buffer at the same time. When all of the wood in the boiler has been burned, the heating system is fed from the buffer. Also pellet show the best efficiency at full load. However, since their performance is designed in such a way that sufficient energy is provided even at very low outside temperatures and high hot water requirements, the actual power requirement is significantly lower than the design performance at almost all times of the year. Intermittent operation is therefore necessary in the transitional periods (autumn, spring) : The buffer then stores the energy during the operating phase of the heating and then releases it as required. The clocking of the heating circuit pump (switch-on time / switch-off time), which inevitably occurs in the transition period when there is less heat demand, would result in the combustion control of the wood-burning stove preventing clean and, in particular, efficient combustion of the fuel without the use of a buffer tank. The use of a buffer storage means that the solid fuel stove / wood stove always works under ideal conditions and operating temperature. Excess heat is fed into the storage tank and accessed from there via the heating circuit pump if required.
Two water circuits are required for buffer storage operation. One also speaks of circulations. Circuit one connects the solid fuel boiler or wood stove with water register to the buffer tank via a loading pump. A so-called temperature difference control controls the charge pump. This control system compares the temperature in the solid fuel boiler and the temperature of the buffer tank. When the temperature of the solid fuel boiler is higher by a value that can usually be set between 2 and 20 degrees, the control switches on the charging pump and the storage tank is heated up.
The required heat is pumped into the radiator circuit by the second heating circuit pump, usually controlled by a room temperature controller with a thermostat.
A third circuit is required if a separate hot water tank is also available. This is fed with hot water via a control thermostat and a separate charging pump if required. A so-called priority circuit is used for the electrical control. The heating circuit pump is switched off for the duration of the charging process, i.e. while the charging pump for the hot water is in operation. In this way, sufficient warm water is generated even when high heating output is required at cold outside temperatures.
As a rule, in central heating systems that are operated with solid fuel, with a buffer storage volume of 60–70 liters per installed kilowatt of heating output, there is a sensible compromise between storage capacity and heat losses.
For single-family houses of the usual size, (well- insulated ) buffer tanks with 800–2000 l are sufficient. In order to save space in the house for the storage tank, it can also be built from an underground storage tank .
Greenhouses often have relatively large heat stores between 100 and 150 m³. During the day, when the sun heats up the greenhouse, the storage tank is heated, which then releases the heat inside the greenhouse at night. During the day, the CO 2 from combustion can be used for plant growth. The required boiler output is reduced by the buffer, since in extreme weather conditions (cold winter nights) the buffers and boilers provide heat.
These steel tanks are mostly designed as stratified storage systems. This means that cold water is sucked in at the bottom of the buffer and the heated water is then introduced into the upper area. So that the water layers do not mix, the flow and return are designed as pipes with many small holes over the entire length of the tank.
A buffer storage tank is not absolutely necessary for heat pump heating , but it improves the annual coefficient of performance . In particular, buffer storage in heat pumps play a role in the question of the load balancing potential of heat pumps in the power grid in the context of the energy transition .
A buffer for all-day use of solar energy is common . The heat from solar radiation is absorbed during the day by means of appropriate collectors and temporarily stored so that it is available around the clock for heating the building.
In low-energy houses , generously dimensioned buffer storage tanks should be able to store the energy gained over several days in order to be able to bridge periods of bad weather in the transition period without having to resort to fossil energy sources.
If the solar circuit only reaches a low temperature level when the sky is overcast, then with a stratified charge storage tank it is possible to store the only slightly heated liquid if there is still warmer liquid in the upper area of the storage tank.
Seasonal storage facilities have such a large storage volume that the thermal energy stored in summer is still available in winter.
Buffer storage tanks with integrated drinking water heating are also called combination storage tanks (tank-in-tank solution). If the drinking water is heated according to the flow principle, it is also referred to as a fresh water station .
- Joachim Berner: The tank filling makes it all. In: Solare Wärme: Das Solarthermie-Jahrbuch 2019 , publisher: Solar Promotion GmbH, from February 27, 2019, pp. 40–43