Micro CHP

Introduction

CHP is a process in which both space/water heating and electricity are produced at the same time. Traditional CHP units have been operating at an industrial / community scale since the 1970s. Now since 2000, with rising energy prices, micro CHP designed for individual buildings has become economically viable. Effectively the micro CHP unit replaces the conventional gas central heating boiler providing heat and hot water as usual, but additionally providing some of the building's electricity needs. The European Cogeneration Directive defines micro-CHP as all units with an electrical capacity of less than 50 kW.

Advantages and disadvantages

+ CHP provides security of electricity supply
+ Local generation of electricity is usually cheaper than buying off-grid
+ Micro CHP can use a wide range of fuels
+ Micro CHP has the potential to provide carbon savings
+ Excess electricity can be sold to the grid (‘net metering’)
+ Carbon savings of 15 – 20% can be achieved in small commercial applications
+ Carbon savings of 5 – 10% can be achieved in older, larger houses with high and consistent heat demands.
+ Domestic micro-CHP systems typically generate electricity at times which correspond with peaks in demand.
+ Micro CHP technology and its implementation is rapidly improving. Small improvements in electricity efficiency to current domestic units are likely to dramatically improve carbon saving potential

- Micro CHP is a relatively unproven technology
- Carbon savings in most smaller, newer domestic properties are insignificant
- Micro CHP is inefficient for short run cycles.
- There is a high incidence of unreliability in currently installed units
- On average, 50% of electricity generated in domestic applications is surplus. There is a current shortfall of available export reward tariffs for excess electricity directed to the grid.
- Lack of technical knowledge can limit the practical application of micro – CHP in commercial environments
- As grid carbon intensity reduces (from an increase in renewables), so the benefits from micro-CHP fall accordingly.
- Current life expectancy of micro-CHP units is reported to be relatively low
- High installation costs
- Some CHP units are heavy – requiring solid flooring
- Current payback period is in excess of 20 years, but will reduce along with increased production

Technologies

Internal Combustion Engine (ICE)

IC engine micro CHPs are currently the more common prime movers. ICs are more often used in small commercial premises and in large residential developments such as care homes. The internal combustion engine is a tried and tested technology, but engines in practice produce relatively high emissions as well as noise.

Stirling engine

Current developments in micro CHP technology are being concentrated on the Stirling engine. These engines are external combustion engines, which allow continuous, controlled combustion resulting in very low pollutant emissions and high combustion efficiency. In comparison with IC engines, Stirling engines have relatively long service intervals as well as lower running costs as well as being quieter – which makes them more suitable for domestic applications.

Fuel cell

Still in an early stage of development, and years away from the market, fuel cells have the potential to produce heat and power very efficiently with low emissions and noise levels. Fuel cells convert hydrogen and oxygen (the fuel) directly into electricity without any mechanical drive or generator. Heat is produced as a bi-product of the electrochemical process, with water as a waste product.

Fuels

Natural gas is the most common form of fuel for micro CHPs – though research is proceeding with a view to develop engines which can operate using more sustainable fuels such as biomass, woodgas and solar thermal.

Points to consider when specifying CHP

• Applicability - the target markets are for small commercial buildings and existing houses.
• Understand what the heating and power loading will be
• Ensure that maintainance is included in life-cycle costing
• Check the track record of CHP systems considered
• Care must be taken to optimise performance when integrating with an existing central heating system
• Occupants must be informed of how to obtain maximum performance from the CHP system

Movies

Further information

• The Carbon Trust (www.carbontrust.co.uk)
• The Combined Heat & Power Association (www.chpa.co.uk)