The benefits of CHP when compared to importing electricity and using boilers to generate heat include;

  • Savings on total energy costs for the user
  • Improved efficiency of fuel use
  • Reduced emissions
  • Independence & security of power supply
  • Exemption from the Climate Change Levy for energy costs.


The UK Government is committed to increasing the country's CHP capacity and it forms a key element of it's strategy to reduce the country greenhouse gas emissions. It is widely predicted that further legislation and incentives will be introduced in the near future to encourage the use of CHP.
 

Turbec T100 in greenhouse applicationThe key to the success of a CHP system is getting the sizing right. Many CHP installations are oversized because the facilities energy demand profile has not been assessed properly.

To get the full benefits of CHP, the unit needs to operate to its full potential, and all the power and heat produced has to be fully utilised. Hence, in assessing whether CHP is appropriate for a building or development, it is essential that the electricity and heat demand profiles are established. It is common for annual electricity and gas meter data to be used to calculate 'average' power and heat demand. This type of calculation makes no allowance for seasonal variations in demand, particularly heat.

Electricity usage profiles can be obtained by looking at half hour meter data, which can be requested from your electricity supplier. Heat usage profiles are more difficult to assess. Monthly gas bill readings can be used to assess daily and hourly usage. Clearly there is a degree of inaccuracy in using monthly data to derive hourly figures, but it's much better then starting with annual data!

Once demand profiles have been established, it is then possible to calculate the respective electricity and heat 'baseloads' for the building; the baseload is the minimum hourly usage. You then size your CHP unit based upon the appropriate baseload. In many instances, if the assessment process is done properly, the size of the appropriate CHP unit will be much smaller than anticipated. In some cases, it will not be viable.

The output from a CHP unit is typically approx 40% electricity, 60% heat, so you need to establish whether the CHP sizing is electricity or heat led. For example, if you had an electrical baseload of 100kW, a unit of this size would provide circa 150kW of heat. Hence, if your heat demand baseload is 150kW or above, the CHP size would be electrically led. However, if the heat baseload is below 150kW, it would heat led (i.e. you would choose unit based upon the thermal demand).

Note that it is not normal to run CHP during the night time 'Economy 7' period. This is because, over this period, electricity is cheaper to buy, and it is not economically attractive to produce your own power. Hence, CHP is usually assessed and operated for 17 hours/day.
 

The economic viability of CHP is, first and foremost, dependent upon the unit size being appropriate for the facility. It is essential that all of the electricity and all of the heat from the CHP package is fully utilised for the vast majority of the time.

The cost benefits achieved by CHP unit are a function of;

  • Electricity savings; power produced that would otherwise have been imported from the grid.
  • Heat savings; heat generated that would otherwise have been using on site gas fired boilers


Set against the benefits, the costs that need to be taken into account in assessing the economic viability of CHP comprise;

  • Installation costs
  • Costs of fuel input required
  • Maintenance costs


The costs of CHP, in terms of £/kW installed, reduce as the size of the unit increases. The same applies to the cost of maintenance. In addition, the larger the unit, generally the better the efficiency.

Although clearly every potential CHP scheme needs to be assessed individually, simple payback periods for an appropriately sized CHP installation are typically 3 to 5 years.