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How to apply CHP to reduce a building's carbon footprint and operating costs

Using combined heat and power (CHP) to optimise the reduction of a building's carbon footprint and operating costs.

A combined heat and power (CHP) unit uses fuel (typically mains gas) to generate electricity, but utilises a substantial proportion of the heat created during the electricity generation process, making it more efficient overall. A typical CHP unit operating at 80% efficiency will provide 35 units of electricity and 45 units of heat for every 100 units of energy put in. Using a conventional gas boiler and grid electricity would require 145 units of input energy for the same output. That saving in primary energy through CHP usually carries through to reductions of CO₂ emissions, cost, or both.

The key to good performance

CHP achieves its CO₂ emissions and cost savings through its more efficient use of gas in electricity generation and the absence of transmission losses. Maintaining the electrical efficiency of the CHP is therefore central to its performance. As the efficiency of a CHP unit is highest when it is running continuously at full capacity, the system should be sized to ensure it can run at full capacity for as many hours as possible.

CHP sizing must therefore be carried out on the basis of a realistic assessment of heat and electricity demand, considering both annual and daily patterns. CHP will be most effective – both environmentally and financially – when the demands for heat and electricity largely coincide.

Assessing CO₂ emissions reductions

The CO₂ emissions reductions achieved by CHP can only be demonstrated by comparing the building’s emissions modelled with a CHP system against its emissions modelled with a conventional heating system. In order to remain compliant with the CO₂ emissions targets of Building Regulations and Building Standards, these calculations require the use of government-approved software which tests the design against a notional design with a default set of building specifications.

The algorithms used in the approved software, particularly the Simplified Building Energy Model (SBEM) are not sufficiently accurate to be used for sizing a CHP plant. It is possible to use this software to deduce a theoretical reduction in CO₂ emissions. If carried through however, this would result in a significantly oversized unit that would not perform effectively. It is therefore important that compliance testing is carried out on the basis of an accurately sized plant.

Managing conflicting benefits

On projects where financial savings are the main driver, it may be beneficial to run the CHP in order to maximise electricity generation and reject unwanted heat. However, designers must recognise that rejecting substantial amounts of heat will compromise the CO₂ emissions reduction that CHP is supposed to deliver.

Another strategy for managing the situations where electricity and heat demand are out of step makes use of thermal stores. The water in a thermal store (typically tens of thousands of litres) is heated when electricity demand is high, but heat demand is low. When heat demand is high, but electricity demand is low, the water is distributed to heating and domestic hot water systems. This allows the CHP to operate at full capacity and efficiency, maximising the utilisation of electricity and heat, and thereby reducing emissions.

But in every project, the key to reducing emissions and operating costs is sizing the CHP unit on the basis of an accurate assessment of heat and electricity demand.

Takeaways

  1. CHP is significantly more efficient than conventional gas supplies and grid electricity largely due to its usage of the heat created during the electricity generation process. The savings CHP makes on input energy usually carry over to reductions of CO₂ emissions and/or costs.
  2. As the efficiency of a CHP unit is highest when it is running continuously at full capacity, the system should be sized to ensure it can run at full capacity for as many hours as possible. CHP will be most effective – both environmentally and financially – when the demands for heat and electricity largely coincide.
  3. The algorithms of the government-approved software necessary to demonstrate the reduced carbon emissions that CHP enables are not accurate enough to be used for sizing a CHP plant. Therefore, compliance testing must be carried out on the basis of an accurately-sized plant.
  4. On projects that prioritise financial savings, it may be beneficial to run the CHP to maximise electricity generation and reject unwanted heat. But note that rejecting substantial amounts of heat will compromise the CO₂ emissions reduction that CHP is supposed to deliver.
  5. In every project, the key to reducing emissions and operating costs is sizing the CHP unit on the basis of an accurate assessment of heat and electricity demand.