Science has long warned that abandoning fossil fuels is essential to curb the climate crisis and protect the planet. As electricity costs soar, interest in renewable options like solar and wind is higher than ever.
Yet a major hurdle remains: no widely available technology can store and generate power on demand at a low cost. A team from Spain appears to have moved the needle with a photovoltaic battery concept offering substantial long‑term storage and low cost, with the potential to supply heat and electricity when needed.
The discovery comes from researchers at the Solar Energy Institute of the Polytechnic University of Madrid (IES-UPM). An article titled Hidden Heat Thermophotovoltaic Batteries, published in Joule, describes a system that uses surplus power from intermittent renewables such as solar and wind to melt inexpensive metals like silicon or ferrosilicon alloys at temperatures above 1000ºC.
Silicon alloys can store large amounts of energy during fusion processes, a form of energy known as latent heat.
For example, one liter of silicon can store more than one kilowatt‑hour of energy as latent heat, comparable to the energy stored in a liter of hydrogen pressurized at 500 bar. Unlike hydrogen, silicon can be stored at atmospheric pressure, which could make the system cheaper and safer.
Miniature photovoltaic plants
The system, already patented by UPM researchers, combines thermionic and thermophotovoltaic effects, enabling direct conversion of heat to electricity.
Unlike traditional thermal machines, this approach does not require physical contact with the heat source, because it relies on direct electron emission (thermionic effect) and photon emission (thermophotovoltaic effect).
Inside view of the latent heat thermophotovoltaic battery developed under the AMADEUS project and located at IES-UPM.
One key aspect is how stored heat is transformed into electricity. When silicon melts above 1000ºC, it emits energy that can be converted back into electricity by photovoltaic cells.
The thermophotovoltaic generators act like tiny solar plants, capable of producing far more power per square meter than conventional solar panels. If one square meter of solar panel yields about 200 W, a thermophotovoltaic panel can reach around 20 kW, with higher efficiency as well.
Because thermophotovoltaic cells operate with efficiencies ranging from 30% to 40% depending on the heat source temperature, they can outperform typical photovoltaic panels, which usually hover between 15% and 20% efficiency. This combination lowers moving parts and avoids complex heat exchangers, making the system compact, quiet, and economical.
A hundred times cheaper than lithium batteries
Researchers say latent heat thermophotovoltaic batteries can store large amounts of excess renewable electricity. “Most of this electricity is produced when there is little demand, so it will be cheap,” notes Alejandro Datas, a leading researcher at IES-UPM.
Storing energy in a simple, inexpensive system makes a lot of sense, according to the team, because it provides a practical way to keep energy available when demand dips. Silicon and ferrosilicon alloys can store energy for less than 4 euros per kWh, a figure cited as 100 times cheaper than current stationary lithium‑ion batteries.
In terms of total cost, the package includes the container and insulation, which will add to expenses. Still, the study indicates large installations could reach around 10 euros per kWh, especially when capacities exceed 10 MWh, since insulation costs would be a small fraction of the total.
It is not necessary to convert all stored heat back into electricity. If the system remains affordable, recovering 30% to 40% of the stored energy as electricity could make these units preferable to more expensive technologies, similar to lithium‑ion batteries in practice.
First prototype ready
Heat that cannot be converted to electricity can be redirected to buildings, factories, or cities, helping to cut natural gas use. Heat accounts for more than half of global energy demand and about 40% of CO2 emissions. Storing wind or photovoltaic energy in thermophotovoltaic latent heat cells could yield substantial savings and meet a portion of heating needs with renewables.
The researchers believe this kind of system could be a cornerstone in reducing dependence on fossil fuels, not only in electricity but also in thermal applications. The prototype described was developed within the laboratory‑scale European AMADEUS project and is now housed at IES‑UPM, with results published in the referenced study.
Silicon ranks as the earth’s second most abundant element after oxygen. This project marks more than a decade of work at IES‑UPM. However, further investment is required to bring the technology to market. The current lab model stores less than 1 kWh, while practical deployments will need storage in the multi‑MWh range to be financially viable.
The next challenge is scaling the technology and testing feasibility at larger scales. IES‑UPM researchers are assembling a team to pursue this goal. A formal reference report is available in the Journal of Energy Storage and forthcoming publications from the AMADEUS collaboration.