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Buffer tank

The thermal buffer tank decouples generation and demand. It is not an energy source but a time buffer: it absorbs heat when a generator can run favourably and releases it when it is needed. In doing so it determines runtimes, cycling and – through the standby losses – the utilisation ratio of the whole plant.

Capacity

The usable capacity follows from volume and usable temperature spread. One cubic metre of water holds about 1.16 kilowatt hours per kelvin of spread; a 1,000-litre tank with 20 kelvin spread therefore holds about 23 kilowatt hours. That is little compared with the daily demand of a building and much compared with the cycle time of a generator – precisely its purpose. What matters is therefore not the volume alone but the spread: raising the return temperature reduces usable capacity.

Stratification and losses

A tank only works with clean stratification: hot at the top, cold at the bottom. High volume flows, unfavourable feed-in points or a boiler working into the tank destroy it – the tank is then mixed and loses its effect. Equally important are the standby losses, which occur continuously regardless of whether heat is drawn. In the annual balance this shows up as heat production being systematically slightly higher than heat demand: the difference is exactly these losses. For a tank losing about two kilowatt hours per day they add up to several hundred kilowatt hours over the year.

Management through switching thresholds

The tank is managed through a hysteresis: the generator starts when the state of charge falls below the switch-on threshold and runs until the switch-off threshold. Both thresholds are adjustable in EPOS-Plan as a percentage of capacity and directly determine cycling and runtime. A narrow band leads to frequent cycling, a wide one to long runtimes and higher losses. The effect is particularly clear in summer operation of a heat pump: as long as the tank covers hot water demand the machine stays off – visible in the result chart as cycles of charging and standstill.

Assignment to generators

A tank is assigned to one or more generators, so the configuration reflects the hydraulic connection. Whether heat pump and peak-load boiler charge the same tank, whether the buffer is reserved for the CHP unit alone or whether it sits in a bypass covering only peak load has considerable consequences for runtimes and stratification. The common variants are described with principle diagrams under Fundamentals. For CHP units a rule of thumb is 60 to 100 litres per kilowatt of thermal output, and the buffer must not be charged by the boiler. For heat pumps the volume is governed less by output than by requirements: utility lock-out periods, hot water peaks, defrost support and the wish to store PV surplus thermally.

Tank as a heat source

In EPOS-Plan a buffer tank can also act as the heat source – for instance in the booster concept, where the second heat pump uses the preheated tank as its source, or for a tank fed from waste heat or solar thermal. The tank content then limits the extractable heat; the adjustable regeneration rate models how quickly it is recharged.

Sizing in practice

Calculate several volumes and watch four figures: operating hours and cycling of the base-load generator, the course of the storage charge level, the minimum peak boiler output required, and the annual balance of production and demand whose difference shows the storage losses. A tank that is too small cycles, one that is too large costs investment and losses without gaining runtime – the sensible range is usually clearly visible in the chart.