Theories, advances, challenges, and innovations in photon-enhanced thermionic emission technology for concentrating solar systems

Photon-enhanced thermionic emission (PETE) technology holds the potential to become an eminent power generation technology when coupled with concentrated solar power systems. The theoretical conversion efficiency of a PETE converter can surpass 50 % when combined with a thermodynamic cycle. This review summarizes the research process of PETE converters, including prototype experiments, detailed physical models, and optimization of key components. The research topics of electrode materials and space charge effect are of long-term significance. Some studies have demonstrated that PETE remains confronted with certain challenges. The corresponding innovative methods and solving strategies are analyzed in this work. To remove obstacles for the development of PETE converters, we have identified two promising routes. The first route is coupling a novel anode-incident PETE configuration to the solar absorber of the thermodynamic cycle. The second route is employing a photovoltaic anode to realize the efficient cascade utilization of solar photons. In both routes, the Cs vapor charged in the electrode gap can be excited by solar photons, thus overcoming the space charge effect. This review elucidates the synergistic potential between PETE converters and high-efficiency thermodynamic cycles, facilitating the utilization of clean energy.