For years, science has warned that phasing out fossil fuels is essential to curb the climate crisis and safeguard the planet. With electricity prices reaching historic highs, interest in renewable energy sources like solar and wind has surged even more.
Yet a practical hurdle remains: there is no economical, scalable system that can store and deliver renewable energy on demand. Researchers from Spain claim to have addressed this gap with a photovoltaic battery concept that promises large storage capacity at low cost over long periods, potentially supplying both heat and electricity when needed.
The findings come from the Institute of Solar Energy at the Polytechnic University of Madrid (IES-UPM). The work is described in an article titled Hidden Heat Thermophotovoltaic Batteries, published in the journal Joule. The proposed system uses surplus energy from intermittent renewables such as solar and wind to melt inexpensive metals, including silicon or ferrosilicon alloys, at temperatures above 1,000ºC.
Silicon alloys store substantial energy through fusion-like processes, a form of latent heat.
For example, one liter of silicon can hold more than one kilowatt-hour of energy in latent heat, a quantity comparable to the energy contained in one 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 IES-UPM researchers, combines thermionic and photovoltaic effects, enabling direct conversion of heat into electricity.
Unlike traditional thermal machines, this approach does not require mechanical contact with heat sources, as it relies on direct electron emission (thermionic) and photon emission (thermophotovoltaic).
One key aspect is how stored heat is transformed into electricity. Silicon emits light when melted above 1,000ºC, allowing the radiated heat to be converted back into electricity using photovoltaic cells.
The thermophotovoltaic generators act like compact solar plants, capable of delivering far more power than conventional solar panels. In simple terms: if one square meter of a solar panel yields about 200 W, a square meter of thermophotovoltaic technology could produce around 20 kW, with higher efficiency as well.
Thermophotovoltaic cells achieve efficiencies between 30% and 40% depending on the heat source temperature, compared with typical solar PV efficiencies of 15% to 20%. This higher efficiency, along with fewer moving parts, makes the system compact, quiet, and potentially economical.
Replacing traditional heat engines such as Stirling, Brayton, or Rankine cycles, the approach avoids circulating fluids and complex heat exchangers, further simplifying the design and reducing maintenance needs.
A claim of being markedly cheaper than lithium batteries
Researchers indicate latent heat thermophotovoltaic batteries can store large amounts of excess renewable electricity. Energy storage is especially valuable when demand is low, enabling wholesale sale at lower prices. This aligns with the idea that storing energy as heat can be a cost-effective strategy for balancing grids, according to the IES-UPM team.
In particular, silicon and ferrosilicon alloys can store energy at a cost under 4 euros per kWh, a claim that suggests a potential order-of-magnitude advantage over current stationary lithium-ion batteries.
It is acknowledged that total costs rise when including containment and insulation; however, preliminary analyses indicate large installations could reach around 10 euros per kWh, especially for configurations exceeding 10 MWh, where insulation costs become a smaller fraction of total expenses.
Converting only part of the stored heat back into electricity is feasible. If the system is priced to be sufficiently cheap, recovering 30% to 40% of its energy as electricity could still outperform more expensive technologies, according to the research team. This concept is likened to the behavior of lithium-ion storage in terms of energy return.
First prototype ready
A portion of the heat that cannot be converted to electricity can be supplied directly to buildings, factories, or cities, helping to reduce natural gas use.
Heat accounts for more than half of global energy demand and a significant share of CO2 emissions. Storing wind or solar energy in thermophotovoltaic latent heat cells could cut costs while meeting substantial heat needs with renewables.
Thus, developing this system could contribute to lowering reliance on fossil fuels not only in electricity generation but also in the thermal sector, according to researchers. The current prototype was built within a European laboratory-scale project called AMADEUS and is available for study at IES-UPM, with experimental results published in the cited article.
This marks more than a decade of work at IES-UPM. While the technology holds promise, it requires further investment before reaching commercial viability. The present laboratory prototype stores less than 1 kWh, but the goal is to scale storage to well over 10 MWh to achieve profitability.
Future work aims to test scalability and feasibility at larger scales. To support this, IES-UPM researchers are assembling a team to make scale-up possible. The study references support from peer-reviewed sources and reports on the topic.
Reference report: https://www.sciencedirect.com/science/article/abs/pii/S2542435122000423?via%3Dihub