Combined Heat and Power output by DEFRA

The methodology

Emissions model

Combined Heat and Power plants produce both heat and electricity as outputs for consumption. The emissions produced in generating heat and electricity therefore constitute emissions attributable to the end-user. The emissions which are to be attributed to heat and electricity production respectively depend on (1) the emissions of the whole plant; and (2) the ratio in which heat and electricity are produced by the plant. However, the efficiency with which heat and electricity are produced from fossil fuel combustion is not the same, and it is therefore necessary to allocate disproportionate quantities of the plant emissions to electricity users and heat users respectively. This methodology assumes that heat is twice as efficient to produce as electricity, and weights the emissions attributable to heat and electricity production accordingly.

Emissions associated with both heat and electricity outputs are established on a per unit energy (e.g. kWh) basis. These values can then be used to calculate end-user-attributable emissions depending the quantity of heat and/or electricity consumed.

It should be noted that, in those cases where the entire (heat + electricity) output of a CHP plant is consumed by a single user, then the emissions attributable to that user are simply equal to the emissions associated with the plant - no apportioning of emissions into those associated with heat and electricity is necessary.

Model data

No data is provided to the emissions calculation - all values represent activity data to be specified by the user.

Related methodologies

In order to calculate heat-specific and electricity-specific emissions from CHP plants, it is necessary to establish the total plant emissions. The DEFRA source documentation refers users to their Annex 1 - Converting from fuel use to carbon dioxide equivalent emissions which contains data and a calculation methodology for the stationary combustion of fossil fuels. This methodology is available for energy-, mass- and volume-based fuel quantities. If the CHP plant uses biomass for energy production, this DEFRA methodology may be useful for calculating plant emissions.

Using this methodology

Choosing a specific activity type

There are no drill down choices necessary for this methodology.

Activity data required

This methodology requires that total plant emissions to be attributed to the heat and electricity generated in their appropriate proportions. It is therefore necessary that the total plant emissions, the quantity of heat produced and the quantity of electricity produced be explicitly specified by the user. These should be specified using the plantEmissions, heatProduced and electricityProduced profile item values respectively.

Also required is that at least one of the heat or electricity quantities consumed by the end-user is specified. These can be specified using the heatConsumed and electricityConsumed profile item values. Both type of consumption can be specified if appropriate.

Users can choose to specify their activity data in a comprehensive range of appropriate units.

Calculation and result

The methodology uses the proportion of heat and electricity produced - together with the efficiency weighting factor - to apportion the total plant emissions appropriately across heat and electricty outputs. These emissions are then converted to a per unit energy basis in order to be applied to the specific consumption of the user.

The emissions returned represent those associated with the consumption of heat and/or electricity specified. The following discrete amounts are returned:

• heatCO2e: CO₂e emissions associated with the heat consumption, if specified
• electricityCO2e: CO₂e emissions associated with the electricity consumption, if specified
• totalCO2e: CO₂e emissions associated with both heat and electricity consumption, as specified