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
Borealis Technical Limited
Thermo-ionic Conversion and Related Basic Physics
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
The image above shows a
prototype device intended to recover energy from the waste heat in a
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:
w Ordinary Bunsen burner 21%
w Air pre-heat burner 27%
w DRAX burner 32%
Ordinary burner 2.9%
Air pre-heat burner 5.4%
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.
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.