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Why size really does matter for cogeneration

Why size really does matter for cogeneration

Discover the key points that engineers need to consider in order to correctly size cogeneration units for optimum energy savings.

For engineers involved in specifying cogeneration, it is essential that the cogeneration unit is correctly sized – in fact, it is crucial to a project’s viability. An under or oversized unit will neither achieve the principal energy and cost savings, nor a satisfactory payback period.

An undersized cogeneration unit may operate at 100% output, but potential cost and CO2 savings are missed because of the shortfall in energy, meaning:

  • Additional electricity is imported from the grid.
  • Boilers are required for any shortfall in heat.
  • Payback periods are increased.

Engineers need to be aware that an oversized cogeneration unit may provide the full building load, but similarly will not deliver the potential savings:

  • The cogeneration unit will operate below its full output rating (perhaps causing lower efficiency) unless export of generated energy is an option.
  • Potential savings from funded and incentives that support cogeneration.
  • Payback periods will be longer.

To size a cogeneration unit correctly for any site, engineers should consider certain key points:

1. Heat and electrical power profiles

The efficiency of existing systems needs to be established – checking if any further improvement measures can be identified. Reliable hourly data for electrical and heat demands is required to accurately determine the site’s energy profiles.

2. Electrical and thermal load tracking

Fluctuations in electrical and thermal energy demands may be accommodated by setting the cogeneration unit to track or match the energy profiles by varying or modulating its output.

3. Agreeing baseline energy costs

The cogeneration unit should be sized to operate at its optimum baseline electrical and thermal output. Agreement will be needed if it is necessary to:

  • Provide any shortfall in electricity from the grid and heat from on-site boilers.
  • Operate the cogeneration unit slightly above the thermal baseline to produce higher electrical output for increased financial savings.

4. To dump or not to dump?

Where the cogeneration unit operates above the thermal baseline, it may be necessary to ‘dump’ or reject any excess heat energy to keep the system operational. ‘Dumping’ is commonly achieved by using a suitably sized dry-air cooler/radiator.

5. Future spark spread implications/sensitivities

Spark spread is the difference between the purchase price of the cogeneration fuel and the sell price of the electric power output and significantly affects the overall benefits of cogeneration. Fuel costs and can have a profound effect on spark spread.

6. Applying utility inflation

To ensure the financial viability of cogeneration projects, engineers need to carry out an analysis of the associated fuel and electricity tariff levels and calculate the effect of any potential inflationary rises.

7. Rules of thumb for savings calculations

  • Cogeneration units need to operate with a high and constant heat demand for at least 4,500 hours per year.
  • Cogeneration applications ideally require a spark spread of around 3 or more.
  • Cogeneration needs to operate well, meaning an electrical efficiency above 20%.


  • Use accurate energy profiles to size and match cogeneration units.
  • Select cogeneration units to meet energy baselines and use heat dumping if beneficial.
  • Always assess utility costs and associated inflationary rises.