Other Technologies

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There are numerous technologies with potential for micro CHP applications. Although there is an optimum thermal and electrical output to match the requirements of any given home, it is possible to supplement the thermal output to a greater or lesser extent and to use the resulting electrical output to whatever use is most relevant. For example, although the very low power outputs of thermo-ionic devices may not make a substantial contribution to the overall electricity consumption of the home, it may at least cover the parasitic losses (pumps and fans) of a central heating system and make it possible to continue heating even during a power cut.

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

Thermo-ionic

Thermo-photovoltaic

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 devices, efficiency is not necessarily the only 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

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 ('solar' cells) which are particularly sensitive to infra-red (heat) radiation.

TPV electric generators are 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. DRAX burners currently require access to pressurised air, with at least a few tens of millibar pressure. Further development could reduce the pressure requirement, allowing a low power fan to be used, which could then be powered by the TPV itself without losing significant output power. The DRAX burner might offer further advantages in compactness, in that its intensity of emission would be increased 50% over Air pre-heat. Working on the assumption of an achievable radiator temperature of 1300 C from a conventional burner, 1500 C from an air pre-heated burner, and 1700 C from a DRAX burner, a simple calculation gives some idea of the advantage which the DRAX burner would have. With the simplifying assumption of black body radiation, these are the figures for both Gallium Antimonide and Silicon photovoltaic cells:

Gallium Antimonide

Fraction of energy available for conversion

w Ordinary Bunsen burner 21%

w Air pre-heat burner 27%

w DRAX burner 32% (20% improved over Air pre-heat, 50% over Bunsen)

Silicon

w Ordinary burner 2.9%

w Air pre-heat burner 5.4%

w DRAX burner 8.4% (60% improved over Air pre-heat, 190% over Bunsen)

Clearly the DRAX burner would give spectacular increases in efficiency using silicon cells. Silicon has the advantage of very low cost, robustness and lower surface reflection losses which further boost its performance relative to the idealised GaSb result above.