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Energy Consumption and Power Demand

During cold freeze-ups, power demand and consumption are the highest.

The starting point for mapping out the heating costs and fuel options is the annual energy consumption. The easiest way to do this is to take the number of last year’s energy consumption and convert it into kilowatt-hours (kWh), or, in a bigger-scale case, into megawatt-hours (MWh).

Different units of measurement are used of different fuels, which makes comparing them challenging. Using kilowatt-hours makes the comparing easier. More information about conversion factors here.

The estimated power demand of a heating target defines the size of the heating system. A standard house uses averagely 20–30 watts/cubic metre. Correct sizing and estimation of power demand are the foundation of a well-functioning heating system. They save you from unnecessary investments and secure the sound functioning of the heating system.

Power Dissipation and Other Things to Consider

When estimating power demand and deciding on the size of the heating boiler, possible power dissipation and other factors causing energy loss, now and in the future, must be considered. Is it prospective that, for example, the space to be heated is someday extended? Or that a cold-kept hall building will be heated in the future?

Preparing for possible future projects in time is advisable, for it lessens futile workload and is above all cost-effective. Power demand should rather be calculated higher than lower the estimated, also because of the fuels’ quality variations. But do not go over the top. Too big a heating plant does not function in the optimal way and is not economical.

In addition to basic heating, producing hot service water requires averagely 1–2 kWh energy. If a bio heating plant is situated in a separate building, heat is transmitted through an underground pipe. Power dissipation in the pipe is approximately 15–30 watts/metre. Power dissipation in a 25 metre long pipe can be calculated to be approx. 1 kilowatt when rounded up. In practise this means that a 25 metre underground pipe looses for example five loose cubic metres of wood chips every year.

Peak Power

Peak power is the power demand of a heating target when consumption is the highest, for example during winter freeze-ups. In calculating the peak demand, annual oil consumption can be used as a starting point.

Divide the annual oil consumption with 250, and you will get the peak demand in kilowatts. Peak demand can also be calculated using building volume.

Example target

An oil heated detached house has used 6 000 litres of fuel oil during the heating season. In kilowatt-hours this is 60 000 kWh. The house has 300 cubic metres and the room height is 2,6 metres.

  • If the fuel oil was replaced with wood chips, the need would be approximately 75 loose cubic metres (m³ᶩ ͮ), because the energy content of wood chips is, depending on the quality, 700–900 kWh/m³ᶩ ͮ.
  • If the fuel oil was replaced with pellets, the need in the same situation would be 12 500 kg. One cubic metre of pellet has approx. 3 000 kWh, and pellet weighs approx. 625 kg/ m³.
  • The peak demand for an oil heated house can be calculated with the formula above: the annual oil consumption is 6 000/250=24. Thus the peak demand is 24 kW.
  • Peak demand can be calculated using the building volume as well. The building has 780 cubic metres. Because heating up a house takes approx. 20–30 w/m³, this target would need 16–24 kW during peak demand.

In the example, the size of the burner and boiler could be 40 kW. When using biofuels, the combustion equipment should be sized somewhat bigger taking into account power dissipation, fuel’s quality variation, and hot service water needs.


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