Combined heat and power (CHP), or cogeneration, is the production of electricity and useful heat in a single process, generally at the site where it is needed. The heat derived from electricity generation is not wasted but is used for central heating or hot water. Alternatively, waste heat from a boiler is used to drive an engine to generate electricity.
This greatly improves the overall efficiency, which is typically around 80-90%. This compares wiith an average efficiency of only 40% for central electricity generation in the UK, where much of the energy is lost as heat and during distribution along power lines.
CHP schemes are well suited to the needs of community buildings such as hospitals, hotels, and leisure centres, where there is demand for both heat and electricity throughout the year. However, smaller-scale mini or micro CHP systems are available for small businesses and households.
What is micro CHP?
A micro CHP unit is defined by the European Cogeneration Directive as any CHP unit with an electrical capacity of less than 50 kWe (kilowatts electrical). However, the term usually applies to units with less than 5 kWe. Such a unit replaces the central heating boiler, and supplies some of the household's electricity, with the potential to feed surplus electricity back into the grid.
The domestic systems currently available in the UK use one of three technologies: a Stirling engine, an organic Rankine cycle, or a fuel cell. These generally require connection to both the gas and electricity grids, although some have the potential to run on different fuels, such as liquefied petroleum gas (LPG), with the potential to work off-grid. Any surplus electricity generated can be exported to the electricity grid, and qualifies for a Feed-in Tariff. Externally the units look similar in size and shape to any conventional gas boiler.
These are units based on combustion of gas to produce primarily heat, which can then be used for hot water and space heating, and also to generate electricity by means of either a Stirling engine (i.e. a 'heat engine') or an organic Rankine cycle. The output of heat is automatically adjusted to suit demand in the building while maximising the amount of electricity generated, which can be used on site within the building, or exported to the grid.
Some units consist essentially of a burner, heat exchanger and Stirling engine; electricity is generated by the magnetic piston inside the Stirling engine, which is driven up and down between electrical coils.
Boilers incorporating the organic Rankine cycle produce electricity via a mini-turbine-like device, which is driven by an organic fluid in a closed circuit. When heated the liquid vaporises and expands thus propelling the turbine, which powers a generator. The vapour than passes through a heat exchanger, where it condenses back to a liquid.
Inside a micro-CHP unit. Image courtesy of GreenSpec
At the heart of these units is a fuel cell, which uses the gas to produce primarily electricity, 24/7, with an efficiency typically of around 60%. Heat is a by product, recovered to heat hot water. These are not generally suitable for single dwellings, but more appropriate for apartment blocks, small businesses, community buildings and the like. One has been installed at the Madeley Centre near Newcastle under Lyme (see Fuel cell helps power Madeley Centre).
Fuel cells are devices that convert hydrogen and oxygen into water, producing electricity and heat in the process. If the fuel is hydrogen, there are zero direct carbon emissions. In the absence of ready supplies of hydrogen, current designs use natural gas or LPG, which entail some emissions.
In some situations, larger capacity, or 'mini-CHP' units might be more suitable, depending on the type of dwelling and occupancy. Commonly, these use an internal combustion engine powered by natural gas or liquid petroleum gas (LPG) primarily to generate electricity, typically 5 kW or more, while producing less usable heat than a boiler-based system.
Points to consider
- Micro and mini CHP can cut carbon emissions, albeit by relatively small amounts, typically 5-15%
- Electricity is generated at times of peak demand
- Installation costs are high
- CHP units based on combustion engines can be noisy and produce exhaust fumes
- From April 2010, micro-CHP under 2 kW capacity is eligible for a feed-in tariff for the electricity generated (for the first 30,000 units installed), currently to the tune of 13.95 pence per kilowatt hour. See article on Feed-in Tariffs for more details.