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CHP efficiency

When sizing plants, ask careful questions about what the CHP will need to do in both the next half hour and in the next ten years.

Sizing a CHP plant for a building starts with an audit of current and future demands of heat and power.

Site demand information will show how demand profiles peak and fall with:

•             Time of day       

•             Day of the week

•             Season of the year

When a CHP plant has been sized against the building’s normal patterns of consumption, it is always wise to compare the economics and environmental benefits with those of a larger and smaller plant. 

Theory says a well-designed CHP system will use all the heat and power produced, but a larger CHP plant generating surplus heat may show greater economy and environmental benefits in future. The chief considerations are:

•             Planned energy efficiency measures that would reduce the current demand for heat and/or power.

•             Planned changes to the business, the building or the occupancy that will increase or reduce energy demand.

For example, in a sports centre, is a swimming pool planned? In a hotel, will the fabric of the building be insulated, or will double-glazing be installed? For any business, will the staff numbers grow?

Don’t guess watt

Forecasting energy demand is not an exact science, but as little as possible should be left to pure estimation. Success with a CHP project depends on the sizing. CHP installations can be oversized because the energy demand profile has not been assessed properly. To get the full benefits of CHP, the unit needs to run as continuously as possible with all the power and heat produced being fully utilised.

The demand information should be based on heat and power consumption measured every hour for one year. Annual or monthly electricity and gas meter make no allowance for seasonal variations, particularly in heat. So get as close as possible to hourly or even half-hourly consumption figures.

Electricity usage profiles can be obtained by looking at half hour meter data from the electricity supplier. Heat usage profiles are more difficult to assess, so some temporary metering may need to be used. Monthly fuel bills will indicate some degree of seasonal variation. For weekly and daily profiles it is important to understand the operating pattern of the building and to add to that a short-term monitoring exercise or audit.

Loads to think about

Once the demand profiles are established, the electricity and heat baseloads for the building - the levels below which the site electrical and heat demand seldom fall – can be calculated. From the current and projected demands of the building and the business, it is important to establish whether the CHP sizing will be based on the electricity or heat demand.

A heat led sizing will meet the site’s heat demands. It may produce surplus electricity that can be exported or leave a need for top-up power. The economics of exporting power then becomes a priority issue. A power-led sizing could produce excess wasted heat, so it may be worth considering a smaller unit.

Once a CHP unit has been sized on a current heat to power ratio, considerations of the future need to include an assessment of how the heat-to-power ratio of the building’s demand might change over time.