It should be noted that, although a number of practical devices have been demonstrated using the technologies described below, they may not have been exploited for residential micro CHP, but rather in more niche applications.

For further discussion of the relative merits of these technologies see section on PAPERS.

Thermo-ionic generators produce electricity by heating one surface and cooling, (removing heat) from another; there is a vacuum between the two surfaces.  The hot surface (cathode) emits electrons which are collected on the cold surface (anode).  The resulting flow of electrons produces a direct current (DC).

Normally these devices require very high temperatures and achieve relatively low electrical efficiencies.  However, as with other micro CHP technologies, efficiency is not necessarily the only, or even the critical factor influencing viability.  Cost, simplicity and life expectancy are also key factors.

Further information on thermo-ionic devices:

Borealis Technical Limited

Eneco

Thermo-ionic Conversion and Related Basic Physics

Odysseus Mission

Planck's Linear Oscillator Concept Of Matter Can Explain Thermionic Direct Conversion Of Heat To Electricity

Thermoelectric generators convert heat directly into electrical energy, using a phenomenon called the "Seebeck" effect.

Typical efficiencies are around 5–8%. Early Seebeck-based devices used bimetallic junctions and were bulky, whereas more recent devices use semiconductor p-n junctions. These are solid-state devices and thus have no moving parts.

Although not currently available as a micro CHP application, this technology is supplied for power generation purposes by TELGEN..

The image above shows a prototype device intended to recover energy from the waste heat in a vehicle exhaust.

Thermo-photovoltaic (TPV) electric power generators comprise a gas burner pre-heated using exhaust heat, with the main flame heating a radiant emitter.  The emitter is surrounded by photovoltaic cells which are particularly sensitive to infra-red (heat) radiation.

TPV electric generators are currently used in military and outdoor recreational contexts, for example recreational vehicles (RVs), and have been proposed as a quiet, low emission power source for electric vehicles.

Current development is focussed on improving the efficiency of TPV.  One particularly promising technology is the DRAX burner which will heat the emitter to a much higher temperature, emitting more of the near infra-red and visible radiation that the photovoltaic cells require. 

On the assumption of an achievable radiator temperature of 1300ºC from a conventional burner, 1500ºC from an air pre-heated burner, and 1700ºC from a DRAX burner, the fraction of energy available for conversion from both Gallium Antimonide and Silicon photovoltaic cells is significantly increased using the high temperature DRAX burner:

Gallium Antimonide

w Ordinary Bunsen burner 21%

w Air pre-heat burner 27%

w DRAX burner 32%

Silicon

w Ordinary burner 2.9%

w Air pre-heat burner 5.4%

w DRAX burner 8.4%

The gas turbine is a well established technology for power generation with typical efficiencies in Combined Cycle mode of up to 60%. 

However, as the size of turbines is reduced it becomes increasingly difficult to achieve high conversion efficiencies, and micro-turbines (such as those from CAPSTONE) with power outputs of 30kWe and 65kWe achieve only 25% and 29% electrical efficiencies respectively.

In 2013, MTT is trialling the ENERTWIN, a 3kWe (14.4kWt) micro CHP package, with an electrical efficiency of 16%; products are expected to become available in mid-2014.

Although this efficiency is below that achievable from ICE products, it is believed that the use of a single moving part should provide additional benefits in service life, reliability and cost.