Micro Combined Heat and Power (micro-CHP) is a technology that provides an opportunity to use an energy source (normally gas) to effectively produce heat and electricity. Unlike larger CHP systems (mini and full CHP) the application of micro-CHP is typically limited to one property and does not include any need for 'district' heating.

Micro-CHP is not classed as a renewable technology (unless fuelled by renewable biofuels) but properly applied it can reduce carbon emissions and total energy costs. CHP is sometimes referred to as 'cogeneration'.

It will not replace the need for a connection to the mains electrical supply grid but will provide a source of electrical power that can be used directly on the premises or exported back to the grid. The term micro-CHP normally indicates that the output of the system is less than 5kWe (5 kW electrical) combined with a heat output suitable to replace a domestic or small commercial boiler. (This differs slightly from the EC definition of 'micro-cogeneration' as being under 50kWe).

Micro-CHP products have been available for some years and the technology is evolving to provide systems that can be included within a domestic or small commercial application that require similar installation and maintenance to that of a gas-fired boiler.

Micro CHP types

There are number of different technologies that can be used for micro-CHP, including stirling engines, rankine engines, internal combustion engines, and fuel cells. Apart from fuel cell technology the systems typically produce 4 to 8 kWth (thermal) heat for each kWe of electricity. If further heat is required (beyond that produced as a result of electrical generation) a supplementary heat exchanger (within the unit) is normally used to supply heat directly to the water for the heating system.

Stirling engines

This is the most prevalent technology in today's domestic micro-CHP market and the only technology specifically recognised by the MCS.

Unlike the internal combustion engine a stirling engine does not have any combustion process within its reciprocating engine. A sealed cylinder containing a gas (such as helium) is heated at one end by the burning gas and then cooled at the other by the return heating water so that it expands and subsequently contracts and is arranged to actuate a piston to drive an electrical generator. The specific configuration of the engine will depend on the manufacturer but the majority of the moving parts are within a sealed, and so clean environment. The most common application is of the free piston Stirling engine.

The driving heat could be provided by a number of sources including recovered heat and heat from solar sources however in UK micro-CHP applications the energy typically will be produced by burning natural gas.

Since the system relies on heating and cooling process it will take several minutes before electricity is produced whilst it heats up.

Stirling engine micro-CHP typically will convert under 10% of the energy to electricity and the remainder goes to heat. The overall system is likely to be in excess of 90% efficient.

Internal combustion engines

Using well established technology an internal combustion (IC) engine is powered by the expansion of hot gases within the engine. These can be fuelled by petrol, diesel, biofuels, LPG or natural gas and linked in with a generator to produce electricity. Heat is then taken from the engine’s cooling water and exhaust manifold. They can have a higher electrical efficiency than a Stirling engine but are larger and noisier and not commonly applied to the UK domestic market. The practical size range is from less than 1kWe to 5 MWe or more but typically in the UK the size (and noise) have meant that they are more commonly used in larger applications located in plant rooms. Well designed systems can convert 25% of the input energy to electricity.

There is a strong European (and Japanese) market for IC systems that are designed for use both inside and outside domestic or small commercial buildings.

Rankine engine

These are based on the same concept that drive many large scale power stations that use water in a closed system. Just like the Stirling engine these are driven by heat (normally provided by burning natural gas) that works on a closed system containing an 'organic' fluid that requires lower temperatures and pressures than a water based system (using for example silicone oil, or a refrigerant). The fluid is heated to create a gas that is then used to drive a 'turbine' that generates electricity - this video shows the technique for a larger system. There are very few of these in the marketplace - see the manufacturer's site genlec.com for more details.

The Rankine engine has been used to develop systems driven by lower temperature sources potentially for use with waste heat or solar energy.

Fuel cells

A chemical reaction drives a fuel cell where the hydrogen in the natural gas reacts with the oxygen from the air to generate electricity and create heat. Micro-CHP would most likely use solid oxide fuel cells (SOFCs) that consist of a solid electrolyte sandwiched between anode and cathode layers. When natural gas is passed over the anode, it breaks down to release hydrogen. When this combines with oxygen from the cathode, electricity, water and heat are produced - with as much as 80% of the output being electrical.

Current technology means that the cost of fuel cells (both in purchase and operation) is still too high to make them commercially viable for micro-CHP but they potentially offer a noiseless, low emission system with high efficiencies.

A UK company plans to begin wholesale supply of domestic units in 2014 and there is substantial background on fuel-cell applications at UK Hydrogen and Fuel Cell Association.

Micro-CHP units are increasingly designed to take up a similar space to a domestic or small commercial boiler but their current cost (2012) are several times that of a similar output heating only boiler. Their simple external appearance may be deceptive as they will also have more complex controls and mechanisms than a typical condensing boiler.

A recent review including these technologies as applied in Europe can be seen here.

Heat and power from micro-CHP

Micro-CHP consumes the energy source (usually natural gas) at point of use to generate heat and electricity. Grid supplied electricity is likely to be less efficient in converting energy consumed at the power station to electricity supplied to the premises, (probably less than 50% efficient). However CHP can only claim high efficiencies if the heat (produced as a 'by product' of electrical generation) is being usefully employed at all times. This can make it difficult to justify CHP where there are low heating demands such as in new, or small domestic premises. Similarly it would not be suitable where there are other renewable heat systems such as solar thermal heating. Due to the laws of thermodynamics the heat efficiency of a micro-CHP will be less that for a heat-only boiler.

And so currently micro-CHP are designed as 'heat led' systems (they operate when there is a demand for heating water) - if fuel cells become more available then 'power led' systems (probably linked in with some thermal storage) may become more feasible.

Micro-CHP application

The micro-CHP unit is increasingly designed as a replacement for a domestic or small commercial boiler and has similar requirements for connections to a gas supply and the heating system. There will be additional electrical connection requirements and eventually smart metering will be applied to provide information on the exported electricity. There is a ratio of 1kWh(electricity):6kWh(heat) over the year in the 'average' UK home and so the output from an ideal micro-CHP system would match this.

A trial was undertaken by the Carbon Trust (reported on in 2011) of 87 micro-CHP installations in typical UK households and small commercial applications. It found that the larger domestic systems (that used greater than 15,000kWh heating) and small commercial premises were most likely to benefit from savings in carbon. On average small domestic installations benefit (in carbon terms) by a few percent compared to using a condensing boiler and grid electricity.

Micro-CHP functions best with extended operating periods and minimised cycling on and off. For short running cycles (about 1 hour or less) the electricity consumed by the system can reduce, or even eliminate, the benefits. By linking a thermal store into the systems cycling can be reduced and heat can be reserved for use later in the day. The application of low temperature heating systems (such as underfloor heating) can readily utilise heat from thermal stores and can provide lower return water temperatures - this improves the efficiency of Stirling engine micro-CHP.

Under the MCS it is the contractors responsibility to evidence that the micro-CHP is of an appropriate size and design to properly satisfy the system needs of the building.

Planning and Regulation

Requirements for micro-CHP are included in the building regulations and in England this is explained in the DCLG Domestic Building Services Compliance Guide 2010 Edition and also the Non-domestic Edition.

Unlike regular boilers, the performance of micro-CHP systems are not given in terms of a seasonal efficiency (ie SEDBUK) but by the Heating Plant Emission Rate (HPER) for an application in a particular property. The HPER gives an indication of carbon emissions related to the needs of a specific property and is given in kg CO? per kWh. Since the current systems are heat led the HPER is given for each kWh of heat generated (for total heating and hot water), but the carbon value includes that for both heat as well as power generation.

Generally the SAP assessment is used to show compliance in domestic applications. To comply the SAP rating must be acceptable and the system with a micro-CHP should have an improved total rating compared with that of a standard boiler. In existing buildings that are using micro-CHP fuelled by something other than natural gas the HPER must be calculated to be equivalent to a gas boiler of SEDBUK efficiency of 86%. MIS 3007 provides detail of the neccesary calculations to prove compliance for both new and existing buildings.

Under the Microgeneration Certification Scheme (MCS) the Microgeneration Installation Standard: MIS 3007 provides approved guidance on the installation requirements for micro-CHP installations - this is particularly important if government funding is being sought for a project but does, in any case, provide a useful source of information.

For non-domestic buildings the regulations set a minimum Combined Heat and Power Quality Assurance (CHPQA) Quality Index and a minimum electrical power efficiency measured across yearly operation. The CHPQA is based on the ability of a system to be able to produce heat and electrical energy more effectively than using the same fuel in a power station.

Where the micro-CHP operate with other boilers the micro-CHP must be the lead heat generator and normally sized to supply at least 45% of the annual heat requirement. Non-domestic installations must include metering to measure hours run, electricity generated and fuel supplied. The micro-CHP will be included in the overall assessment of the building performance when undertaking the National Calculation Methodology (most likely using SBEM) analysis. 

Government Funding

Gas powered micro-CHP installations are recognised as a small scale low carbon technology by the UK Government. Systems under 2kWe can attract Feed in Tariff payments (FITs) (available for up to the first 30,000 UK installations) for a duration of 10 years - note that there had been fewer than 500 Ofgem registered installations by the end of 2012. Systems must be installed under the auspices of the MCS to attract funding and there may be a requirement in future for a dwelling to have an EPC of at least band D to be eligible for FITs.

FITs comprise of 2 elements -

• Generation tariff – a payment for each unit (kWh) of electricity generated. Currently (April 2013) this is 12.89p/kWh. The payment is regardless of whether the generated electricity is used in the building or exported back to the grid. To measure the generation there must be an Ofgem approved total generation meter (typically less than £50).
• Export tariff – electricity that is not used on site can be exported back to the grid and a payment is paid as an additional payment (4.6p/kWh as at April 2013) in addition to the generation tariff.

References to buy or borrow

CIBSE AM12 'Small Scale CHP' provides a comprehensive reference in the design and application of non-domestic micro-CHP

BSRIA BG 2/2007 'CHP for existing buildings: Guidance on design and installation' does not vcover domestic micro-CHP but does look at how larger CHP can be usefully integrated into systems.

B&ES TR37 - 'Installation of CHP' has little specifically on micro-CHP but covers the principles and underlying engine technology as well as much information on good practice when installing and operating larger scale CHP equipment

Web Sites and freely downloadable resources

The Carbon Trust has a whole area developed to the emerging micro-CHP technology that includes downloads of informative resources and also Good Practice Guide 388 - 'Selecting, installing and operating CHP in buildings - a guide for building services engineers'. They also have a particularly useful guide on FITs for small businesses

Electrical Safety Council's Connecting a microgeneration system to a domestic or similar electrical installation (in parallel with the mains supply) provides detailed and well written guidance on the considerations required when connecting a micro-chp system.

Energy Saving Trust has a page with up to date guidance and links to appropriate legislation and government web resources

Department of Energy & Climate Change - Micro CHP - A great place to catch up with all the government funding and guidance

Combined Heat & Power Association- A well established trade organisation with some resources mainly looking at larger applications of CHP but still has relevant material that covers the underlying principles and application.

Standards and Regulations

The Microgeneration Installation Standard: MIS 3007 (available at no cost) provides the requirements for installation of micro-CHP to comply with the MCS scheme and provides a reference to other required standards.

The DCLG Domestic Building Services Compliance Guide 2010 Edition and the Non-domestic Edition includes requirements for the performance of micro-CHP systems