Thermoelectronic Energy Conversion

Thermoelectronic Energy Conversion – Highly-Efficient Conversion of Heat and Solar Radiation to Electric Power

 

Figure 1: Photograph of a prototype generator. The glowing orange disk (left) shows the back of the resistively heated emitter; the yellowish disk edge on the right shows the reflection of the glowing emitter on the collector surface [6].
Figure 1: Photograph of a prototype generator. The glowing orange disk (left) shows the back of the resistively heated emitter; the yellowish disk edge on the right shows the reflection of the glowing emitter on the collector surface [6].

Electric power may, in principle, be generated in a highly efficient manner from heat by means of thermoelectric power generation with vacuum as thermoelectric material. As the conversion efficiency of this process tends to be degraded by electron space charges, the efficiencies of such generators have amounted to only a fraction of those fundamentally possible. Utilizing a novel electrode scheme, we have found a way to solve this problem [6]. (J. Renewable Sustainable Energy "Open Access" or cond-mat / article)



Conversion of heat into electric power as done, for example, by concentrating-solar power plants or fossil-fuel power stations, faces the problem that the highest acceptable input temperatures of the converters are usually significantly lower than the temperatures generated by focused solar energy or by combustion. Coal, from which 40% of the world‘s electricity is currently generated [1], is burned in power stations at ~1500°C, whereas, due to technical limitations, the steam turbines to which the combustion heat is delivered are operated at ≲700°C, to give but one example. The temperature gap causes losses because the maximum conversion efficiency, called the Carnot efficiency, decreases with the ratio of the output and input temperatures of the converter.



Thermionic generators [2] can operate with extremely high inlet temperatures. They could therefore close this gap and generate electric power with extraordinary efficiency. They can be considered thermoelectric devices that use vacuum as their thermoelectric material [3]. This allows the parasitic heat conduction from the hot to the cold electrode to be radically decreased. The system efficiency further benefits from the high output temperature at which the generators can be operated: the output heat can be used to drive mechanical engines that operate in this low temperature range near the Carnot limit, boosting the total system efficiency [4].

Figure 2: Scanning electron microscope image of a 200-μm-thick laser-cut tungsten foil with a honeycomb pattern used as gate electrodes in the thermoelectronic generators investigated.
Figure 2: Scanning electron microscope image of a 200-μm-thick laser-cut tungsten foil with a honeycomb pattern used as gate electrodes in the thermoelectronic generators investigated.

The conversion process is straightforward: electrons are evaporated from a heated emitter electrode into vacuum, then the electrons drift to the surface of a cooler collector electrode, where they condense. If the process is used for solar energy harvesting, the quantum nature of light can be exploited for greater efficiency gains by using photon-enhanced thermionic emission (PETE) [5]. As a result of the electron flow, the chemical potentials of the emitter and collector differ in terms of voltage and an output current can be sourced through a load resistor. However, as the output current of the thermionic process is reduced to a few percent by electron space charges, the efficiencies of thermionic generators have only achieved a fraction of what is fundamentally possible. Turning this elegant operation into commercial devices has not been possible to date.



With S. Meir of our former group at Augsburg University and T.H. Geballe at Stanford University, we have developed a new method to solve the space-charge problem using electric and magnetic fields (condmat / article) [6]. To remove the static space charges, a positively charged gate electrode is inserted into the emitter–collector space to create a potential trough. In a virtually lossless process this trough accelerates the electrons away from the emitter surface and decelerates them as they approach the collector. A nominally homogeneous magnetic field H applied along the electron trajectories prevents loss of the electrons to a gate current by directing them through holes in the gate on helical paths circling straight axes. This process turns the static space-charge cloud, which previously blocked the electron emission, into a useful output current.

Model calculations, verified experimentally in a prototype system (Figure 1), show that now there is no fundamental hurdle left preventing the practical, highly efficient heat-to-electric-power conversion. By using this process the efficiency of state-of-the-art coal combustion plants may be increased by a considerable fraction, yielding a corresponding reduction of CO2 emissions. We also envision the possible use of thermoelectronic generators as solar energy converters converting solar radiation to electricity with unprecedented efficiency, or for converting fuel-combustion heat in cars.

 

Figure. 3: Heat-to-electric-power conversion efficiencies calculated as a function of the gate voltage. The diagram shows the efficiencies of stand-alone thermoelectronic generators working at a series of emitter temperatures and of systems comprising a thermoelectronic generator as topping cycle (<em>d</em><sub>ec</sub> = 30μm). In the combined-cycle systems, the thermoelectronic generators operate between <em>T</em><sub>e</sub> and <em>T</em><sub>s</sub> = 600°C. The image also lists the efficiencies of hypothetical thermoelectric generators with figures of merit of ZT = 2 and 10 at temperatures between <em>T</em><sub>i</sub><sub>n</sub> and 200°C [6].
Figure. 3: Heat-to-electric-power conversion efficiencies calculated as a function of the gate voltage. The diagram shows the efficiencies of stand-alone thermoelectronic generators working at a series of emitter temperatures and of systems comprising a thermoelectronic generator as topping cycle (dec = 30μm). In the combined-cycle systems, the thermoelectronic generators operate between Te and Ts = 600°C. The image also lists the efficiencies of hypothetical thermoelectric generators with figures of merit of ZT = 2 and 10 at temperatures between Tin and 200°C [6].

[1] Key World Energy Statistics 2012, Technical Report, (International Energy Agency, 2012).
[2] W. Schlichter, Annalen der Physik 47, 573 (1915).
[3] A. F. Ioffe, Semiconductor Thermoelements and Thermoelectric Cooling (Infosearch Ltd. London, London, 1957).
[4] B. Y. Moyzhes and T. H. Geballe, Journal of Physics D: Applied Physics 38, 782 (2005).
[5] J. W. Schwede et al., Nature Materials 9, 762 (2010).
[6] J. Renewable Sustainable Energy 5, 043127 (2013) "Open Access" or Cond-mat / arxiv.org/abs/1301.3505

 
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