Learn how to calculate the CO2 emissions savings of Combined Heat and Power (CHP) projects compared to traditional energy networks.
One of the primary benefits of on-site cogeneration is that by capturing the heat produced during the generation of electricity rather than wasting it, as is the case with conventional energy production, the efficiency of the process more than doubles. As such, the amount of fuel required to run a building is reduced, together with the associated CO2 emissions. In order to evaluate the financial and environmental benefits of specifying a cogeneration system, the fuel and CO2 emissions savings must be determined.
In principle, this can be assessed by first calculating the energy use and CO2 emissions that would be produced if the project relied wholly on conventional heating and grid electricity, then calculating the energy use and CO2 emissions for the project with cogeneration to identify a comparative saving.
As the benefits are always derived from a comparison with separate heat generation and grid electricity, it is essential that the assumptions for both the conventional and cogeneration cases are as accurate as possible, so the fuel and CO2 emissions benefits are not overstated.
The significance of CO2 emission factors
Key components of CO2 emissions calculations are the CO2 emission factors for the fuels, which represent the amount of CO2 produced for each kilowatt hour (kWh) of energy used during the operation of the building. The figure includes the direct CO2 emissions resulting from combustion of the fuel, and the indirect ‘up-stream’ emissions associated with supplying the fuel (typically extraction and transportation).
For fossil fuels, such as mains gas, the CO2 emission factors are relatively stable, given they are mainly composed of the CO2 direct emissions from combustion of the fuel. However, the situation is more complex for grid electricity, as the CO2 emission factor depends on the balance of energy sources used for generation. The factor will therefore change as the grid is progressively decarbonised.
The CO2 emission factors used by the National Calculation Methodologies (SAP for dwellings and SBEM or DSM software for other buildings) are three-year averages for compliance purposes. Fifteen-year projected CO2 emission factors (for example 2013-27) are also available, and may be used to assess the possible longer-term emissions reductions that cogeneration can deliver.
CO2 emissions compliance
CO2 emissions targets set by building regulations and standards address the use of energy for conditioning and lighting buildings. These targets are derived from the predicted CO2 emissions of a notional building, which is identical in shape to the proposed building, but has a default level of fabric performance and a default configuration of building services.
The appropriate National Calculation Methodology (NCM) is used to calculate the annual CO2 emission rate per square metre for both the notional building and the proposed building. The value for the efficiency of a cogeneration plant in the NCM calculation has to be derived from a detailed analysis of the proportion of overall heat demand it will meet.
The building will meet the compliance targets if the CO2 emission rate for the proposed building does not exceed that of the notional building. Provided the thermal performance of the building fabric is close to that of the notional building (and there is little reason for that not to be the case), the proposed building utilising cogeneration will have a significantly lower CO2 emission rate than the notional building.
Using cogeneration to achieve CO2 emission rates well below those required by building regulations will be particularly beneficial for projects being assessed for BREEAM, as low emission rates are essential for achieving BREEAM’s Excellent and Outstanding ratings.
- By capturing the heat produced during the process of generating electricity, cogeneration reduces the amount of fuel required to run a building and the CO2 emissions associated with it.
- In order to evaluate potential fuel and CO2 emissions savings from cogeneration, your calculations for both conventional and cogeneration outputs must be as accurate as possible.
- Knowing the CO2 emission factors of the fuels in question (including the direct emissions from fuel combustion and the ‘up-stream’ emissions from supplying the fuel) is essential to accurate CO2 emissions calculations.
- CO2 emissions targets are derived from the predicted emissions of a notional building.
- Using cogeneration to achieve CO2 emission rates well below those required by building regulations will be particularly beneficial for projects being assessed by BREEAM.
Reducing CO2 emissions is crucial throughout any Combined Heat and Power (CHP) project. Learn how to simultaneously manage CHP emissions and compliance without compromising your system’s efficacy here.
Topics: CHP / Cogeneration