Fundamentals › Boilers and peak load

Almost every supply concept needs a generator that provides the last few per cent of the annual heat at high capacity and low investment cost. Boilers, immersion heaters or a district heating connection take on this role. They run a few hundred hours a year, but they secure supply on the coldest days and allow the heat pump or CHP unit to be sized considerably smaller and therefore more economically.

Role in the generator set

EPOS-Plan assigns a dispatch order to all generators. The peak load generator stands at the end of this chain and is only switched on when base load generators and storage can no longer cover demand. The simulation shows which share of the annual heat falls to it. Typical values are five to fifteen per cent for a bivalently sized heat pump and twenty to forty per cent for a CHP unit optimised for power-to-heat ratio.

Types of boiler

Gas and oil condensing boilers

Condensing boilers reach high utilisation rates provided the return temperature stays below the dew point, roughly 55 °C for natural gas and 47 °C for oil. In systems with high return temperatures the condensing advantage largely disappears, which is represented in the calculation through the utilisation factor. The modulation range determines how well the boiler handles part load without cycling.

Biomass boilers

Pellet and wood chip boilers are better suited to base load than to peak load because of their thermal inertia and unfavourable part load behaviour. They require an adequately sized buffer store to limit the number of starts. Fuel logistics, storage space and ash disposal enter the economic analysis as operating costs.

Electric peak load

Immersion heaters and electric boilers are inexpensive to install and can be controlled instantly. Their performance factor of one makes them expensive in continuous operation, but for a few hours a year or for the use of photovoltaic surpluses they are often the most economical solution. EPOS-Plan takes into account whether the electricity is drawn from the grid or from on-site generation.

District heating as peak load

An existing district heating connection can cover the peak load provided the capacity charge does not consume the entire saving. The contractually agreed connected load is decisive: it follows the actual maximum power drawn and can be reduced by storage and a smoothed operating regime.

Sizing

The peak load generator is sized on the difference between the building heat load and the capacity available from the remaining generators at the bivalence point. In a redundant design it covers the full heat load and additionally provides backup. The annual load duration curve in EPOS-Plan shows directly how many hours require which capacity, so that redundancy and oversizing can be told apart cleanly.

Cycling and minimum run time

An oversized boiler cycles during the transition seasons, which reduces efficiency and service life. A hydraulically well integrated buffer store, a sufficient modulation range or a cascade of two smaller units provide a remedy. The simulation reports the number of starts; more than around 6,000 starts per year is a clear indication of unfavourable sizing.

Practical notes

In refurbishment projects an existing boiler is often the cheapest source of peak load because no investment is required. It should however be assessed with regard to remaining service life, flue gas route and future fuel availability. For the economic analysis it should also be noted that fossil peak load is increasingly burdened by carbon pricing and by regulatory requirements for the share of renewable heat.