This section gives guidance on specifying combined heat and power (CHP) systems for space heating, hot water and chilled water (via absorption chillers) in new and existing buildings to meet relevant energy efficiency requirements in the Building Regulations. Guidance on the design of community heating systems can be found in Section 6 of the Domestic Building Services Compliance Guide.
CHP units are normally used in conjunction with boilers. The majority of the annual heat demand is usually provided by the CHP plant, while the boilers are used to meet peak demand and in periods when the CHP unit is not operating (for example at night or when undergoing maintenance).
CHP units may on a relatively small scale supply single buildings, or on a larger scale supply a number of buildings through a community heating system. The most common fuel is natural gas, which can be used in spark-ignition gas engines, micro-turbines, or gas turbines in open cycle or combined cycle.
6.2 Scope of guidance
The guidance in this section covers CHP systems with a total power capacity less than 5 MWe used in commercial applications. The CHP units may or may not supply community heating.
Guidance on community heating systems with micro-CHP (having a total power capacity less than 5 kWe) and other heat generators is available in the Domestic Building Services Compliance Guide.
6.3 Key terms
Combined heat and power (CHP) means the simultaneous generation of heat and power in a single process. The power output is usually electricity, but may include mechanical power. Heat outputs can include steam, hot water or hot air for process heating, space heating or absorption cooling.
Combined Heat and Power Quality Assurance (CHPQA) is a scheme28 under which registration and certification of CHP systems is carried out according to defined quality criteria.
CHPQA quality index is an indicator of the energy efficiency and environmental performance of a CHP scheme relative to generation of the same amounts of heat and power by alternative means.
Power efficiency is the total annual power output divided by the total annual fuel input of a CHP unit.
6.4 CHP in new and existing buildings
CHP plant in new and existing buildings should have:
a. a minimum CHPQA quality index (QI) of 105 and power efficiency greater than 20%, both under annual operation
b. a control system that, as a minimum, ensures that the CHP unit operates as the lead heat generator
c. metering to measure hours run, electricity generated and fuel supplied to the CHP unit.
The CHP plant should be sized to supply not less than 45% of the annual total heating demand (i.e. space heating, domestic hot water heating and process heating) unless there are overriding practical or economic constraints.
Calculating the carbon dioxide emissions from a CHP heating system
CHP may be used as a main or supplementary heat source in community heating systems. To calculate the carbon dioxide emission rate for a new building for the purposes of showing compliance with the Building Regulations, the following data will need to be entered into an accredited NCM tool such as SBEM:
a. the proportion (P %) of the annual heat demand (H MWh) to be supplied by the CHP plant (HP). This is needed as the CHP unit is normally sized below the peak heat demand of the building and will also be out of service for maintenance purposes
b. the overall efficiency ratio of the CHP plant (E) as defined by Equation 5 and taking account of part- load operation and all heat rejection predicted by an operating model:
E(annual useful heat suppliedannual electricity generated net of parasitic electricity use)/annual energy of the fuel supplied
(in gross calorific value terms) Equation 5
c. the heat to power ratio of the CHP plant (R), calculated for the annual operation according to Equation 6:
Rannual useful heat supplied/annual electricity generated net of parasitic electricity use
The carbon dioxide emitted in kg/year for the heat supplied by a gas-fired CHP plant is then:
where 216 and 519 are the assumed carbon dioxide emission factors in g/kWh for
mains gas and grid-displaced electricity respectively.
The carbon dioxide emitted for the balance of heat supplied by the boilers is then calculated by the NCM tool as for a boiler only system.
6.5 Supplementary information