Thermophotovoltaics (TPVs) convert predominantly infrared wavelength light to electricity via the photovoltaic effect, and can enable approaches to energy storage^1,2and conversion^3–9that use higher temperature heat sources than the turbines that are ubiquitous in electricity production today. Since the first demonstration of 29% efficient TPVs (Fig. 1a) using an integrated back surface reflector and a tungsten emitter at 2,000 °C (ref.¹⁰), TPV fabrication and performance have improved^11,12. However, despite predictions that TPV efficiencies can exceed 50% (refs.^11,13,14), the demonstrated efficiencies are still only as high as 32%, albeit at much lower temperatures below 1,300 °C (refs.^13–15). Here we report the fabrication and measurement of TPV cells with efficiencies of more than 40% and experimentally demonstrate the efficiency of high-bandgap tandem TPV cells. The TPV cells are two-junction devices comprising III–V materials with bandgaps between 1.0 and 1.4 eV that are optimized for emitter temperatures of 1,900–2,400 °C. The cells exploit the concept of band-edge spectral filtering to obtain high efficiency, using highly reflective back surface reflectors to reject unusable sub-bandgap radiation back to the emitter. A 1.4/1.2 eV device reached a maximum efficiency of (41.1 ± 1)% operating at a power density of 2.39 W cm^–2and an emitter temperature of 2,400 °C. A 1.2/1.0 eV device reached a maximum efficiency of (39.3 ± 1)% operating at a power density of 1.8 W cm^–2and an emitter temperature of 2,127 °C. These cells can be integrated into a TPV system for thermal energy grid storage to enable dispatchable renewable energy. This creates a pathway for thermal energy grid storage to reach sufficiently high efficiency and sufficiently low cost to enable decarbonization of the electricity grid.

Systems of many nanoparticles or volume-discretized bodies exhibit collective radiative properties that could be used for enhanced, guided, or tunable thermal radiation. These are commonly treated as assemblies of point dipoles with interactions described by Maxwell's equations and thermal fluctuations correlated by the fluctuation-dissipation theorem. Here, we demonstrate the equivalence of different theories for these systems and provide a complete derivation of many-dipole thermal radiation, showing that the correct use of the fluctuation-dissipation theorem depends on the definitions of fluctuating and induced dipole moments. We formulate a method to calculate the diffusive radiative thermal conductivity of arbitrary collections of nanoparticles; this allows the comparison of thermal radiation to other heat-transfer modes and across different material systems. We calculate the radiative thermal conductivity of ordered and disordered arrays of SiC and $\mathrm{Si}{\mathrm{O}}_2$ nanoparticles and show that thermal radiation can significantly contribute to thermal transport in these systems, because packed nanoparticles have low phonon thermal conductivity. We demonstrate that the radiative heat transfer strongly depends on the materials and geometrical arrangement of the nanoparticle array, and we verify our calculations by comparison to the exact solution for a one-dimensional particle chain.

Recent theoretical works predict the ability of surface phonon polaritons to increase thermal conduction along nanoparticle chains, but measurement of this effect has proven elusive. Here we demonstrate a new approach to observe thermal conduction by surface phonon polaritons – packed beds of SiO₂ nanoparticles. When we modify the interstitial material with adsorbed water or a coating of ethylene glycol, we experimentally resolve thermal conductivities as high as 1.5 and 18 times the phonon value, respectively. Although existing models do not fully explain our results, we develop a new scaling relation that suggests thermal conduction is primarily dependent on the 3D density of states. An examination of classical heat transport mechanisms shows that these are unlikely sources for the observed thermal conductivities, suggesting that surface phonon polaritons play an observable role in SiO₂ nanoparticle bed heat transfer properties.