Solar-energy conversion usually takes one of two forms: the 'quantum' approach, which uses the large per-photon energy of solar radiation to excite electrons, as in photovoltaic cells, or the 'thermal' approach, which uses concentrated sunlight as a thermal-energy source to indirectly produce electricity using a heat engine. Here we present a new concept for solar electricity generation, photon-enhanced thermionic emission, which combines quantum and thermal mechanisms into a single physical process. The device is based on thermionic emission of photoexcited electrons from a semiconductor cathode at high temperature. Temperature-dependent photoemission-yield measurements from GaN show strong evidence for photon-enhanced thermionic emission, and calculated efficiencies for idealized devices can exceed the theoretical limits of single-junction photovoltaic cells. The proposed solar converter would operate at temperatures exceeding 200 degrees C, enabling its waste heat to be used to power a secondary thermal engine, boosting theoretical combined conversion efficiencies above 50%.

http://absimage.aps.org/image/MAR10/MWS_MAR10-2009-006451.pdf

Photon Enhanced Thermionic Emission for Solar Concentrator Systems

Thermionic Converters and Low-Temperature Plasma

Abstract Challenging space mission scenarios include those in low altitude orbits, where the atmosphere creates significant drag to the S/C and forces their orbit to an early decay. For drag compensation, propulsion systems are needed, requiring propellant to be carried on-board. An atmosphere-breathing electric propulsion system (ABEP) ingests the residual atmosphere particles through an intake and uses them as propellant for an electric thruster. Theoretically applicable to any planet with atmosphere, the system might allow to orbit for unlimited time without carrying propellant. A new range of altitudes for continuous operation would become accessible, enabling new scientific missions while reducing costs. Preliminary studies have shown that the collectible propellant flow for an ion thruster (in LEO) might not be enough, and that electrode erosion due to aggressive gases, such as atomic oxygen, will limit the thruster lifetime. In this paper an inductive plasma thruster (IPT) is considered for the ABEP system. The starting point is a small scale inductively heated plasma generator IPG6-S. These devices are electrodeless and have already shown high electric-to-thermal coupling efficiencies using O 2 and CO 2 . The system analysis is integrated with IPG6-S tests to assess mean mass-specific energies of the plasma plume and estimate exhaust velocities.

System analysis and test-bed for an atmosphere-breathing electric propulsion system using an inductive plasma thruster

Thermionic energy conversion (TEC) using nano materials is an emerging field of research. Graphene can stand high temperature as high as 4600 K in vacuum. Its work function can be engineered from its high value (for monolayer/bilayer) 4.6 eV to 3. 7 eV, 3.8 eV, 3.5 eV and 3.4 eV, through the use of K2CO3, Li2CO3, Rb2CO3, Cs2CO3 as shown by Kwon et al43 and to as low as 0.7 eV as shown by Yuan et al. Such remarkable property along with good electrical conductivity, high dielectric constant makes engineered graphene an ideal candidate to be used as both emitter and collector in a thermionic energy converter. Since large size parabolic trough solar concentrator is now available, there is a great possibility of harnessing solar energy efficiently and cost effectively, if complex functional relation of efficiency of solar energy conversion with solar insolation, area of solar concentrator, work functions (We, Wc) & temperatures, emissivity of emitter and collector, space charge control etc. can be understood clearly. Using modified Richardson-Dushman equation this work focuses on theoretical studies of these aspects, excluding the space charge control. What is necessary for high efficiency is good combination of (We, Wc), with a difference > 0.5 eV, proper collector temperature and high solar energy input. Such solar energy conversion would reduce the dependence on silicon solar panel and has great potential for future applications.

Theoretical studies of thermionic conversion of solar energy with graphene as emitter & collector

Even today, thermal-to-electrical energy conversion is mainly performed by thermomechanical engines activated by hot fluids, such as Stirling engines or turbines. Although such converters are characterized by a high efficiency (roughly >20% at temperatures >400°C), they have several limitations in terms of bulky size, reduced scalability (efficiency depends on size), and frequent maintenance of moving mechanical parts. Conversely, solid-state converters are preferable due to their small volume, up- and down-scalability (efficiency does not depend on size), and long lifetime, because of their operational static nature. Among them, thermionic and thermoelectric solid-state converters are receiving increasing attention from both scientific and industrial communities. In recent years, their applicability has extended from the long-standing sectors (nuclear and aerospace) to novel sectors (concentrated solar energy, energy recovery from energy storage, and metallurgical processes), boosting research and development activities on the related materials and concepts. In this chapter the main historical development and recent results, the physical principles and limitations, and the production strategies and innovative concepts will be described and discussed.

Thermionic and thermoelectric energy conversion

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 such thermoelectronic generators will require further substantial efforts, we conclude that a highly efficient transformation of heat to electric power may well be achieved.

Thermoelectronic Conversion of Solar Energy and Heat into Electric Power

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Thermoelectronic Conversion of Solar Energy and Heat into Electric Power