There is always a tendency for CHP to be oversized, especially when the selection is based on thermal output. This leads to reductions in annual running time, and considerable loss of financial saving for the client. In extreme cases it can lead to physical damage due to excessive shutdowns and failures.

How much heat and power do you need?

When sizing a CHP system the basic principle must be remembered – the combined production of heat and power. You can’t have one without the other and the size relationship between them must remain fixed under all load conditions.

There are many different drivers behind the use of CHP systems. These tend to influence the sizing decision, often wrongly.

Sizing CHP systems

Some software packages used to select plant tend to take a ‘top-down’ look at CHP size. These packages often decide the total collective energy requirement for the building, and seek to provide a certain percentage of this, from the CHP. This is not the same thing however as working out the base load and starting from a ‘bottom-up’ assessment.

We will go so far as to say, several proprietary commercial software modelling tools used often for this application, are fundamentally flawed. Practical experience has shown that schemes designed from this method, have often produced results post-installation that nowhere near meet the expectations of the original designer.

The root cause of this is usually over sizing, leading to excessive return of water temperature during low load conditions, resulting in premature shutdowns of the plant.

Key questions when sizing a CHP system

During the design process it’s important to ask the following questions:

  • Where will these energies be used?
  • What is the base load for electricity and thermal demand on the building, and do they occur concurrently?

Keep in mind that the heat output of the CHP is a direct consequence of it doing work and creating electricity – you can’t have one without the other.

This information will determine the building’s ability to accept the energy created by the CHP and enable its use.

It can be demonstrated that a smaller machine running for more hours a year can often provide a better energy output and carbon saving, than a larger machine running less frequently.

Other specification considerations

Issues of spatial arrangement, acoustic cost, and other install considerations are always benefited by designers leaning towards under sizing rather than over sizing.

There are two common sizing approaches. For a new building, energy estimates must be used, but for existing buildings actual energy consumption data is available and this can therefore be input into various cost studies to compare the effect of CHP with the current energy situation. This is much more reliable as it gives a realistic indication of the current base load of the building. In situations of doubt, or when critical financial targets must be met, there is no substitute for installing electric and heat metering equipment to obtain energy data measured at source.

Conservative is the principal. When in doubt – undersize.

To find out more about the correct design and specification of a CHP system read one of our recent CHP case studies.