Turning a source of heat – such as concentrated sunlight – into useful power (say, electrical power) is not an easy proposition. There’s a dizzying array of options – thermal engines using different thermodynamic cycles, photovoltaic arrays, thermoelectrics and thermionic conversion. The last was used extensively in early space power generators using small reactors or radioisotope heat sources, but left behind by thermoelectrics and Stirling cycle free-piston systems in more recent work. Now a new approach to “thermionic” conversion, focussing on electrons (thus thermoelectronic), has shown promising behaviour in experiments and out-standing performance in theory.
Thermionics previously had efficiency limitations due to “space current” – build-ups of electrons mutually repelling each other and choking the flow of current – so the new system uses external electric or magnetic fields to get the electrons going in the right direction. The system promises a high fraction of the Carnot Limit can be converted directly into electrical power. The Carnot Limit is a measure of how much useful work can be extracted from a thermal cycle – if the heat source temperature is Tin and the heat-sink temperature is Tout, then the Carnot Limit is:
CL = (Tin-Tout)/Tin
…say the source is 2000 K and the sink is 500 K, then the Carnot Limit is (1500/2000) = 0.75. In practice realistic thermal engines achieve a fraction of the Limit and thermionics & thermoelectrics achieve a low fraction. Efficiencies of 5-10% are typical. The new thermoelectronic approach promises efficiencies in the high 40-50% range, achieving the latter by acting as a “topping cycle” to a lower temperature steam system. For example a coal furnace burns at ~1500 C (1773 K), but a steam turbine runs at 700 C (973 K) and outputs at 200 C (473 K). Thus there’s significant loss due to the mismatch between furnace and steam power-cycle. A thermoelectronic converter covering the 1773-973 K range will add significantly to the overall power extracted by the power-plant pushing its efficiency above 50%. In this case a 45% efficient coal plant can be pushed to 54%, thus increasing the power output for no additional fuel costs and NO MOVING PARTS.
Switching to solar-power applications, imagine a thermoelectronic converter at the centre of a concentrator system which focuses sunlight to 500 times its normal intensity (temp ~1900 K.) By using a Photon Enhanced Thermionic Emission (a cousin of the Photoelectric effect) the system can convert raw sunlight to electrical power at over 40% efficiency. While maintaining a hard vacuum around the emitter-collector system is difficult here on Earth (but easy enough given the right engineering) imagine such a system in space! Hard vacuum everywhere! Even the densest squall of the Solar-Wind is a harder vacuum than a Thermoelectronic system needs here on Earth. Concentrators have to remain pointed at the Sun, but this isn’t excessively onerous engineering either.
One problem is the trick of efficiently losing excess heat to maintain the temperature differential that drives the system, but even this isn’t intractable engineering in space. Given the right “heat-pipes” the whole system can be built without moving parts, eliminating the main failure-point for mechanical thermal-cycle converter systems that have been proposed in the past.