Solar Thermionic‐Thermoelectric Generator (ST²G): Concept, Materials Engineering, and Prototype Demonstration

Abstract

Concentrating solar technologies represent economically viable alternatives to distributed PV since the optics production cost per unit surface is far lower than that of PV cells. The reduction of the active conversion area allows for expensive multijunction cells with efficiency as high as 45% to be used in concentrating PV (CPV). [1] However, CPV is struggling commercially because high-density arrangements are prevented by issues in excess heat removal. On the other hand, concentrating solar power (CSP) technologies, [2,3] based on transfer of high-temperature thermal energy through a fluid toward thermomechanical engines, are promising for large plants characterized by thermal-to-electric efficiency around 35%, with the important capabilities of energy cogeneration and storage.The solar thermionic-thermoelectric generator (ST 2 G) explored here takes the advantages of CSP and CPV to provide an effective alternative concept for future solar technologies by combining the hightemperature operations, typical of CSP, to the compactness, scalability, reliability, and long operating lifetime of the solid-state converters, typically used in CPV systems. By combining a thermionic energy converter (TEC), operating at high but practical temperatures (<1100 °C) that can be achieved with point-focus solar concentrators and the development of low-work-function materials, with a thermoelectric energy generator (TEG) thermally connected in series, the ST 2 G technology can meet requirements of economic cost-effectiveness guaranteed by high-temperature solid-state converters. [4] The thermionic-thermoelectric solid-state technology, characterized by solar-to-electric conversion efficiency feasibly >40%, is comprehensively proposed and discussed for conversion of concentrating solar power. For the first time, the related solar generator prototype is designed and fabricated by developing advanced materials functionalized for the specific application, such as thermally resistant hafnium carbide-based radiation absorbers, surface-textured at the nanoscale to obtain a solar absorptance >90%, and chemical vapor deposition diamond films, acting as lowwork-function (2.06 eV) thermionic emitters. Commercial thermoelectric generators and encapsulation vacuum components complete the prototype. The conversion efficiency is here evaluated under outdoor concentrated sunlight, demonstrating thermionic stage output power of 130 mW at 756 °C, combined to the maximum thermoelectric output power of 290 mW. The related solar-to-electric conversion efficiency is found to be 0.4%, but, once the net thermal flux fed to the conversion stages is considered, a thermal-toelectric efficiency of 6% is revealed. Factors affecting the performance of the present prototype are analyzed and discussed, as well as a strategy to rapidly overcome limitations, in order to prepare an efficient and highly competitive solid-state conversion alternative for future concentrating solar plants.

Energy Applications of Magnetocaloric Materials

“…Over the last 20 years, in addition to thermoelectricity, much research on solid‐state thermal energy harvesting ( Figure 2 ) has also focused on thermionics and thermophotovoltaics . Researchers have also investigated new frontiers in spin‐caloritronics, spin‐Seebeck, and anomalous Nernst effects .…”

“…In addition to the conversion of mechanical energy to electrical energy through piezoelectric, [75,76] magnetoelectric, [77] magnetostrictive, [78] triboelectric, [79,80] and electrostrictive [81] effects, various technologies have been developed for the solid-state harvesting of thermal energy, [64,77,82,83] including from low-grade thermal sources. [82,84] Over the last 20 years, in addition to thermoelectricity, much research on solid-state thermal energy harvesting (Figure 2) has also focused on thermionics [85][86][87] and thermophotovoltaics. [88][89][90] Researchers have also investigated new frontiers in spin-caloritronics, [91,92] spin-Seebeck, [93,94] and anomalous Nernst effects.…”

Work function and negative electron affinity of ultrathin barium fluoride films

“…3 Generally, in order to improve the thermionic device performance, the materials with a low work function ϕ must be selected for both cathode and anode. 4,5 For a TIPV device, the thermionic anode must be developed taking into consideration three specific characteristics: (1) ϕ lower than that of the thermionic cathode; (2) capability to collect the electrons thermally emitted by thermionic emitter; (3) optical transparency for the blackbody radiation from the emitter. A good strategy for this development is represented by the deposition of a functional coating on the TPV surface.…”

“…The anode has to guarantee an efficient collection of electrons and the optical transparency to the infrared (IR) radiation emitted by the cathode and absorbed by the TPV cell (made of GaAs or InGaAs) that could convert the absorbed radiation into electricity 3 . Generally, in order to improve the thermionic device performance, the materials with a low work function φ must be selected for both cathode and anode 4,5 . For a TIPV device, the thermionic anode must be developed taking into consideration three specific characteristics: (1) φ lower than that of the thermionic cathode; (2) capability to collect the electrons thermally emitted by thermionic emitter; (3) optical transparency for the blackbody radiation from the emitter.…”

Thermionic enhanced heat transfer in electronic devices based on 3D Dirac materials

“…Thermionic generators show promising applications in harvesting thermal energy 22,23 . In addition, they have been used to convert solar energy to electricity 24 . The investigation of thermionic cooling can be dated back to 1994, when G. D. Mahan described thermionic refrigeration 25 .…”