Science has long urged a shift away from fossil fuels toward renewable energy to tackle the climate crisis. With electricity prices at historic highs, attention on solar, wind, and other renewables has never been sharper.
Yet a key obstacle remains: there is no economical system to store and deploy this energy on demand. A team of researchers in Spain appears to have found a path forward with a photovoltaic battery system that promises large storage capacity at low cost over extended periods, while also providing heat and power when needed.
The discovery comes from researchers at the Institute of Solar Energy of the Polytechnic University of Madrid (IES-UPM). The work, described in the article Hidden Heat Thermophotovoltaic Batteries and published in the journal Joule, envisions using surplus renewable generation from intermittent sources such as solar and wind to heat cheap metals like silicon or ferrosilicon alloys to temperatures above 1,000ºC, enabling energy storage via latent heat.
Silicon alloys can store substantial amounts of energy during fusion processes. This form of energy is referred to as latent heat.
For instance, a liter of silicon can store more than one kilowatt-hour of energy as latent heat, a quantity comparable to the energy held by one liter of hydrogen compressed 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 the UPM researchers, combines thermionic and photovoltaic effects, enabling direct conversion of heat into electricity.
Unlike traditional thermal machines, this approach does not require physical contact with the heat source, relying instead on the direct emission of electrons (thermionic effect) and photons (thermophotovoltaic effect).
Inside view of the latent heat thermophotovoltaic battery developed in the AMADEUS project and located at the IES-UPM.
One central feature is the method by which stored heat is turned into electricity. Silicon becomes highly active when melted above 1,000ºC, allowing the radiated heat to be converted back into electrical energy through photovoltaic cells.
The thermophotovoltaic generators behave like compact photovoltaic units that can produce far more power than conventional solar plants. In simple terms: if one square meter of a standard solar panel yields about 200 W, a square meter of thermophotovoltaic panel could deliver around 20 kW. Not only is the output higher, but efficiency improves as well.
Because thermophotovoltaic cell efficiency typically ranges from 30% to 40% depending on the heat source temperature, compared with 15% to 20% for typical solar PV modules, the potential gains are meaningful.
Using thermophotovoltaic generators instead of traditional heat engines (such as Stirling, Brayton, or Rankine cycles) avoids moving parts, fluids, or complex heat exchangers. This could keep the system compact, quiet, and cost-effective.
A hundred times cheaper than lithium batteries
Research indicates latent heat thermophotovoltaic batteries can store large amounts of excess renewable electricity. “Much of this electricity will be produced when demand is low, making it inexpensive to store,” notes Alejandro Datas, a researcher and project leader at IES-UPM.
“Storing energy in a cheap system makes sense, because it would be inefficient to trap such inexpensive electricity in an expensive container,” Datas continues. In particular, silicon and ferrosilicon alloys can store energy at less than 4 euros per kilowatt-hour, roughly 100 times cheaper than current stationary lithium-ion batteries.
The study acknowledges that total costs rise when considering containment and insulation, but if the system scales, costs can drop toward about 10 euros per kWh for large capacities, typically above 10 MWh, with insulation representing a small fraction of total cost.
It is not necessary to recover all stored heat as electricity. If the system is inexpensive enough, even 30% to 40% energy recovery as electricity could beat other more expensive options. The researchers compare this to lithium-ion batteries in terms of application logic.
First prototype ready
When heat cannot be converted to electricity, it can be delivered directly to buildings, factories, or cities, reducing natural gas consumption. Heat accounts for more than half of global energy demand and about 40% of global CO2 emissions. Storing wind or PV energy in thermophotovoltaic latent heat cells could deliver substantial cost savings while meeting significant heat needs from renewable sources.
Developing such a system could lessen dependence on fossil fuels, not only in electricity but also in the thermal sector, Datas concludes. The initial prototype, developed within the European AMADEUS project framework, is now available at IES-UPM, with early experimental results published in the study.
Silicon is the second most abundant element in Earth’s crust after oxygen. This achievement reflects more than a decade of research at IES-UPM. The current laboratory prototype stores less than 1 kWh, but energy storage capacities above 10 MWh will be required for the technology to become profitable.
The next challenge is to scale the technology and test feasibility at larger scales. IES-UPM researchers are assembling a team to make this possible.
Reference report: https://www.sciencedirect.com/science/article/abs/pii/S2542435122000423?via%3Dihub