Latent Heat Thermophotovoltaic Batteries

Latent heat thermophotovoltaic (LHTPV) batteries are a kind of power-to-heat-to-power storage (PHPS) system that combines very high melting point phase change material (PCM) with thermophotovoltaic (TPV) energy conversion. Electricity is employed to produce the solid-liquid phase transition in the PCM. Consequently, electrical energy is stored in the form of latent heat at very high temperatures (> 1000oC). When needed, stored energy is released as thermal radiation, and converted back to electricity on demand using TPV. Additionally, heat can be also delivered from the cooling of the TPV cells, or directly from the PCM to provide the heating demands. In this study we discuss on the techno-economics of LHTPV systems, focusing on parameters like the round-trip efficiency, the energy-to-power ratio (storage duration), the cost per energy and power capacities, and the levelized cost of storage, all of them depending on the specific selection of PCM and TPV generator. In a purely electrical system, the relatively low TPV conversion efficiency (< 50%) and the low cost of the PCMs (< 4 €/kWh) result in optimal system designs with small power-to-energy ratios, fitting long duration storage application. LHTPV systems delivering also heat can be used for short duration storage applications in fully electrified energy systems where there is heat demand. In all the cases, large-scale LHTPV systems are preferable to minimize the impact of thermal insulation in the total cost of the system. The use of lower melting point PCMs, like FeSiB, enable a significant reduction of the cost per energy capacity, favoring their use in smaller scale applications. On the contrary, higher melting point PCMs, like Si, enable a significant reduction of the cost per power, favoring their use in shorter duration applications. Preliminary experimental results using copper PCM and GaSb TPV cells are provided to illustrate the real operation of a LHTPV system.