Scientists from the Institute of Solar Energy at the Polytechnic University of Madrid and the Centre for Energy, Environmental and Technological Research in Almeria have proposed a new concept for storing solar energy: a compact thermophotovoltaic battery that operates at temperatures above 1,100°C. They combined three functions in one module: sunlight absorption, heat accumulation in molten material and the subsequent transformation of this heat into electricity. This format makes it possible to move away from large and complex solar stations with molten salts and generate electricity that does not depend on the time of day.
Unlike lithium-ion batteries in which energy is stored in chemical bonds, the proposed system stores energy in the latent heat of melting of the phase material. The solution is based on Fe-Si-B, which melts at 1,157°C and can accumulate more than 1,000 kW·h of heat per cubic meter. This is 6–10 times more than the widely available nitrate salts, which makes it possible to significantly reduce storage size. To charge the system, sunlight is amplified by about 900 times with heliostats and fed into a cavity absorber, which heats the container with the alloy until it melts. During discharging, the hot walls of the container start radiating in the infrared range, and the thermophotovoltaic elements capture this radiation and convert it into electricity. These elements from low-band semiconductors work effectively at temperatures of 1,100–1,200°C and provide an integral efficiency of over 40%.
The controllability of the unit is ensured by the ability of its key parts (the container with the melt and the block of TPV elements) to move relative to one another. This makes it possible to choose the optimal position for charging, discharging or storing heat. As a result, four modes are implemented: simultaneous charging and generation during the day, pure charging, night charging and long-term storage. In the latter mode, losses are negligible: calculations show that the system can retain heat for 18–19 hours with a container height of about 0.48 m, and for more than 24 hours with a container height of up to 0.72 m.
To check if the concept is functional, the researchers created a detailed digital model that included the calculation of solar optics, three-dimensional thermal dynamics and simulation of the TPV cells. Overall, more than 70 simulations were conducted, during which the geometry of the container, the profile of the solar flow and the parameters of the semiconductors were analyzed. The researchers found that the optimal configuration involves a thermal accumulator with a height of 0.48–0.72 m and InGaAs elements with a band gap of about 0.74 eV. This configuration provided the best balance between charging time, discharge duration and losses.
The cycle efficiency of the full sun-heat-electricity cycle exceeded 20%, reaching 23–25% in optimal conditions. The scientists managed to keep lateral heat losses at a level of about 7%. The biggest obstacles continue to include radiation losses through the open aperture and the uneven distribution of the solar flux, due to which the upper zones of the absorber get overheated and transfer part of the energy to the atmosphere.
The modular principle makes this system flexible in application: it can be used as a basis for high-temperature solar power plants or as local energy accumulators for industry and remote areas. At the next stage, the researchers plan to create an experimental prototype with a height of 0.24 m, which will allow them to confirm the results of the simulation and bring the technology closer to real-life implementation.
