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Sizing CHP for tomorrow's demands

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Ian Hopkins, Director at ENER-G takes us through what you should consider when sizing your combined heat and power plant so that it runs just as well in half an hour, as it does in ten years time. Key questions include whether the site is heat led or electricity led, what the demand profile looks like hour to hour, week to week and month to month, and if the site's load will stay the same over time. 


CHP won't deliver for every development and detailed analysis and modelling is required to assess feasibility and then to ensure that it is sized and specified accurately to provide maximum efficiency - both in the short and long term, where energy demand patterns might change.

In conventional power and heating applications, plant capacity is usually dictated by maximum demand, resulting in the system operating predominantly at part load.  To gain the efficiency and economic viability of CHP plants, high utilisation is required; hence it is essential to understand the minimum energy demands during the running period, as well as the maximum demands.  

Sizing a future-proof CHP unit requires accurate measurements today and a measure of how the balance between heat and power baseloads might shift.  It is important to ask careful questions about what the CHP system will need to do in both the next half hour and in the next ten years.  

The process should start  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 system 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

Many CHP installations are oversized because the energy demand profile has not been assessed properly. To get the full benefits of CHP, the unit needs to run all day every day, and all the power and heat produced has to be fully utilised.

Ideally, the demand information would 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. As such, it is important to  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 your electricity supplier. Heat usage profiles are more difficult to assess, so you may need to utilise some temporary metering. 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.

Once you have established the demand profiles you can calculate the electricity and building's heat baseloads.

The useful output from a CHP gas engine is typically about 40% electricity, 45% heat. From the current and projected demands of the building and the business, you need 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, future considerations need to include an assessment of how the heat-to-power ratio of the building’s demand might change over time and to incorporate these scenarios into your planning..

Download the ENER-G guide: CHP project planning: How to determine site heat and power demands


Ian Hopkins is a Director of ENER-G Combined Power Ltd.  He is a technical sales and marketing professional and business leader with more than 15 years’ experience in delivering energy efficiency projects and strategy in Europe and the United States.

Further information: www.energ.co.uk/chp

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Guest Monday, 18 February 2019