Via Nextbigfuture.com: Arxiv - Thermionics (electronics for converting heat to electricity) 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.
See also: Power from Heat… High-Efficiency Thermoelectronic Converters
...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.
Highly-Efficient Thermoelectronic Conversion of Solar Energy and Heat into Electric Power
Abstract: Electric power may, in principle, be generated in a highly efficient manner
from heat created by focused solar irradiation, chemical combustion, or nuclear
decay by means of thermionic energy conversion. As the conversion efficiency of
the thermionic process tends to be degraded by electron space charges, the
efficiencies of thermionic generators have amounted to only a fraction of those
fundamentally possible. We show that this space-charge problem can be resolved
by shaping the electric potential distribution of the converter such that the
static electron space-charge clouds are transformed into an output current.
Although the technical development of practical generators will require further
substantial efforts, we conclude that a highly efficient transformation of heat
to electric power may well be achieved.