Micro-CHP or m-CHP, units can be powered by a number of technologies to produce electricity.  These include; petrol, diesel and natural gas internal combustion engines, Stirling engines and of course fuel cells.

Internal Combustion Engines

Micro-CHP units that use existing internal combustion engine technology do offer some advantages in that they exhibit a low capital cost and can be rapidly cycled to follow the electrical load demand (load following).  The disadvantages of such units are that they have many moving parts, can be noisy and produce a high level of emissions.  One of the main disadvantages is that they have a relatively low electrical efficiency, which means a significant amount of heat is generated.

External Combustion (Stirling) Engines

A Stirling engine is a type of external combustion engine that uses a piston & crankshaft, gas within a sealed enclosure and a heat source.  When combined with a rotary generator Stirling powered m-CHP units can be quite efficient.  Stirling engines also require some time to warm up before they can produce useful power, and also the engine cannot change its level of power output quickly.  One of the key disadvantages is that heat and electricity is produced in disproportional amounts, typically in a heat to power ratio of 5:1 to 8:1.  This means that for the average home, more heat is produced than needed which may mean the unit may not be operating at is peak efficiency.  Therefore, it may be unlikely for a Stirling powered m-CHP unit to operate 24 hours per day.

Fuel Cells

Fuel cells offer a significant benefit in that there are virtually no moving parts, negligible emissions and very little noise.

While there are a number of different types of fuel cells (see Fuel Cell Fundamentals) the two types of fuel cells most suitable for m-CHP in the home with are PEMFC and SOFC.  PEMFC are very flexible in that they are suited for a number of applications; cars, buses, back up generators to name a few.  It can be argued that PEMFC are not the best technology for use in domestic m-CHP.  PEMFC require a pure source of hydrogen, which requires an external natural gas to hydrogen reformer and gas cleaning equipment.  PEMFC are quite responsive to changes in power demand and are best suited for load following installations, however their electrical efficiencies of only 30 to 35% put PEMFC powered m-CHP units just above the electrical efficiency of a coal fired electricity station - without the level of emissions.

SOFC powered m-CHP units are ideally suited for continuous operation, providing stable base load electricity 24 hours a day.  SOFC systems also have very high electrical efficiencies, which means less waste heat and more electricity for a given amount of fuel.  Other benefits of SOFC powered m-CHP units include; operation on natural gas utilising existing infrastructure, the ability to operate on other fuel types such as diesel, LPG, ethanol and other hydrocarbon fuels.  The level of emissions from SOFC m-CHP units are extremely low with no Nitrous Oxide or Sulphur Dioxide produced and around 60% less Carbon Dioxide than combustion based technologies such as a coal fired power station.