The primary energy factor and carbon intensity of waste energy

The energy performance of appliances, buildings and groups of buildings is commonly assessed in terms of equivalent carbon emissions or primary energy, usually following procedures defined by regulations. The most appropriate treatment of energy (usually heat) that is rejected as waste from one process, but which may be a useful input to another process, is not obvious and has proved to be problematic. This note describes and illustrates how the situation can be handled by adapting a procedure that is already in use for somewhat similar purposes. The direct users of the approach are likely to be developers and system designers, but since the procedures are often set by regulation, the most important potential users are those responsible for the regulations The principal features of the procedure are that • The use of waste energy reduces the primary energy (or carbon emissions) attributed to the “donor” process: by allocating some of it to the eventual user: an equitable principle. • The total primary energy (or carbon emissions) of the output streams equals the input to the transformation process: a fundamental requirement • The PEF values of outputs are never negative: required for physical plausibility • If there is only one useful output, all the primary energy is assigned to it: for consistency with the fundamental assumptions of the policy metric • The PEF assigned to the recipient user is lower than that of the alternative supply (if its PEF is zero, so is the that of the waste heat) The principal challenge is the need to define a “counterfactual” alternative source of energy for the recipient’s energy needs: options for this this discussed in the note Practical application : The question of the most appropriate convention for the assignment of primary energy and carbon emissions to the application of energy flows that have traditionally considered to be waste has gained importance in recent years, especially in the context of energy distribution systems that transfer energy between buildings. It seems reasonable that the “donor” of the waste energy should be incentivised by a reduction of the primary energy consumption or carbon emissions that is attributed to them (with the reduction balanced by a compensating attribution to the user of the “waste” energy). This is not reflected by the currently widespread convention that waste energy can always be considered to have a carbon intensity and primary energy factor (PEF) of zero. This Technical Note sets out a procedure to achieve this. It is suggested that its application would result in a more equitable and consistent attribution of primary energy and carbon emissions and therefore more robust design of systems that reuse “waste” energy.