Answer by Dale Blundell (Atkins)
CHP is usually sized on heat demand, as utilising all or most of the heat that the CHP creates is central to determining the economics of the CHP installation. A feasibility study will optimise the plant size by assessing the economics of a range of CHP sizes. Overall economic performance falls when all the heat cannot be used, and so a careful balance must be achieved between the drop in performance and the increased savings from maximising self-generation.
Boilers are normally used to meet winter peak loads, and to provide back-up supply when the CHP is out of service for maintenance. As CHP units are typically connected to the electrical distribution network, power demand in excess of the CHP output is readily met using imported power from the grid. Unless there is a requirement to run the site in isolation from the grid, for example at times of grid failure, there is therefore no need to size the plant for the very highest peak demands.
Even where there are times when the heat generated is more than required by the load, it can still be worth running the CHP at high load in order to benefit from the lower cost of self-generated electricity. The excess heat from the CHP at such times may be rejected to atmosphere via cooling towers or heat rejection radiators. At some sites the heat can be captured in a thermal store, allowing the timing of electrical production and heat consumption to be de-coupled. Alternatively it can provide cooling by use of an absorption chiller.
A good example of an absorption chilling application would be a commercial building where in peak summer the demand for heat dips below the thermal capacity of the installed CHP for a few hours per day. Rather than rejecting the heat to atmosphere, an absorption chiller could then generate chilled water that could offset the energy used by the existing electrically driven chillers used for air cooling.
When sizing the CHP unit, it is important that all other no-cost and low-cost energy efficiency measures have been taken into account to minimise or eliminate wasted energy consumption. This will help to avoid installing oversized plant, thereby maximising the return on investment.
Future changes in energy requirements should also be considered, especially the possibility of reductions in heat and/or power demands. As a rule of thumb, industrial and commercial applications that have a simultaneous demand for heat and power for more than 5,000 hours per year will be worth investigating in detail. In practice, CHP must be sized using daily demand profiles/data in order to accurately determine the actual quantities of heat and power that can be supplied to the application.
Where the thermal demand is variable, thermal storage can be employed to smooth the demand profiles, improving the economics of the CHP scheme.
Oversizing a CHP scheme can incur unnecessarily high costs – both operating and capital/installed costs, wasted energy in the form of rejected heat, lower returns for any exported power and lower part load efficiency.
Under-sizing a CHP scheme will mean a reduction in the benefits compared to a system that is appropriately sized.