Thermal Radiation as Electricity Generation’s Nemesis

Producing electricity from high-temperature heat is central to our energy system. Thermodynamic efficiency increases as the temperature difference across a heat engine increases, major traditional sources of energy inherently produce high temperatures, and waste heat recovery holds great promise. In the future, concentrated solar and thermal energy storage systems may also supply heat at high temperatures on a large scale.<br/><br/>The established technologies of converting high temperatures to electricity are based on turbines. These are large, complex, and expensive devices. To reach high conversion efficiencies at low cost and in flexible form factors, static heat engines are desirable. A prime example is a thermophotovoltaic cell, in which thermal radiation is harnessed by a photovoltaic device.<br/><br/>The most direct approach to generating electricity from heat is to use the hot electrons themselves as the working fluid. Indeed, thermionic energy conversion is a deceptively simple concept: heat an electrode to the point of evaporating electrons (historically known as thermionic emission), collect these electrons on a cold electrode, and circulate them back to the hot electrode through the electrical load. This elegant idea has been known since early 20^th century but, despite significant progress over the decades, its performance has remained limited and it has not become a practical technology. To understand why, one must look past the apparent simplicity of the thermionic converter.<br/><br/>As in other heat engines, the fundamental challenge in a thermionic converter is that of enabling the flow of electric charge while restricting the flow of heat. In this device, the Coulomb repulsion of the emitted electrons hinders further electron emission and transport. The simplest way to mitigate this space charge effect is to place the cold collector close to the hot emitter—at a distance of a few micrometers or less. At such small emitter-collector gaps, however, near-field radiative coupling causes major energy loss.<br/><br/>A thermionic converter is in fact a highly complex device. The emitter is strongly out of equilibrium—a hot soup boiling with many excitations and ripe with nonlinear effects. In addition to the thermal emission of electrons and far-field and near-field thermal radiation, thermal conduction and thermoelectric and other solid-state transport phenomena within the emitter, collector, and leads need to be accounted for. Add nanomaterials and nanostructures to the mix and you also have new physics. To analyze a thermionic converter realistically and accelerate technology development, all this must be treated simultaneously. Combing thermionics with complementary technologies such as thermoelectrics and photovoltaics will make the problem even richer. In the first part of this talk, we will discuss a multiphysics approach to modeling thermionic converters and some of the unexpected insights it provides in the context of example devices.<br/><br/>To engineer a thermionic converter to favor electron transfer over radiative coupling, we need to understand both phenomena to a greater degree than traditionally considered in these devices; fine-grained knowledge of the electron and photon energy exchange channels is required. In the second part of the talk, we will argue for the need for quantitatively meaningful spectroscopic approaches in this context, report on recent experimental developments on thermal electron and photon spectroscopy, and discuss directions to further advance this area.