Renewable energy sources have acquired a central role in the energy model of countries committed to reducing greenhouse gas emissions. Nevertheless, the intermittent character of some of these energy sources, such as solar or wind, puts on the table a challenge that must be overcome.

This challenge is caused by the need to have storage systems capable of accumulate excess energy that is produced at times of maximum generation to be used when energy production declines. Whether or not they coexist with other sources of backup energy, such as nuclear, renewables need to ally themselves with cheap and competitive storage technologies.

Fortunately, there are several solutions that can go hand in hand with renewable energies to act as that complement capable of store surplus energy whenever necessary. The manufacture of hydrogen, hydroelectric pumping or energy storage using compressed air are some of these technologies, but we cannot ignore one of the most obvious options: batteries.

There is no doubt that they fit into this usage scenario (Tesla’s Megapack batteries prove it), but they need to increase their competitiveness to establish itself as the preferred option in the development of large energy storage infrastructures.

This is the context in which a team of researchers from the Solar Energy Institute of the Polytechnic University of Madrid has designed a new battery technology that, on paper, is more economical and efficient than the lithium-ion solutions that are widely used today. And it is because the chemical element they use to contain large amounts of energy is abundant and relatively cheap.

Silicon is the great ally of thermophotovoltaic batteries (and not lithium)

The cheapest energy storage strategy we know of requires storing it in the form of heat. In the article published in the scientific journal Joule, researchers from the Solar Energy Institute propose using the surplus energy produced by wind and solar installations at times of maximum generation to melt some metals that are capable of retaining heat for a long time . According to them, the ideal candidates for their low price and optimal physicochemical properties are silicon and silicon alloys.

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The authors of this study claim that a liter of molten silicon is capable of storing more than 1 kWh of energy

These metals can be melted by subjecting them to a temperature of more than 1000 ºC, so that the energy that is necessary to invest in this process remains confined in the material itself in the form of latent heat. According to these researchers, one liter of molten silicon is capable of storing more than 1 kWhand, furthermore, that thermal energy is maintained for a period of time that can range from 10 hours to several days assuming a very moderate rate of dissipation.

From here the energy stored as latent heat in molten silicon elements can be recovered in two ways: in the form of heat or as electricity. The authors of this study claim that 50% of energy demand of the world’s population is accounted for as heat, and recovering it from molten silicon does not require carrying out any type of transformation, so the losses that occur when one type of energy is transformed into another do not occur.

Thermophotovoltaic batteries

Source: Solar Energy Institute of the Polytechnic University of Madrid

It is also possible to recover the energy stored in the form of latent heat as electricity, although in this scenario, as is logical, it is necessary to face the transformation of thermal energy into electrical energy. These researchers propose to address it by taking advantage of a very interesting property of silicon: its ability to emit a lot of light when subjected to a very high temperature.

According to the technicians of the Institute of Solar Energy, the efficiency of the thermophotovoltaic cells that they propose ranges between 30 and 40%.

To carry out this transformation it is necessary to use photovoltaic cells like those used in solar panels with which we are all familiar. However, the thermophotovoltaic generators described by these scientists in their study they are more efficient than conventional solar panels. Much more efficient.

And it is that, according to these researchers, they are capable of producing up to 100 times more electrical energy per unit area. This simply means that one square meter of thermophotovoltaic panel is capable of producing 100 times more electricity than a conventional photovoltaic panel of the same size. On paper, it’s not bad at all.

The doctor in Physics and Professor of Electronics at the Complutense University of Madrid Ignacio Mártil explained to us in the conversation we had with him at the beginning of 2021 that the most sophisticated photovoltaic solar panels currently available have an efficiency of approximately 24%. And, according to technicians from the Solar Energy Institute, the efficiency of the thermophotovoltaic cells that they propose ranges between 30 and 40%.

In addition, in the illustration that we publish above these lines we can see that the storage of energy in the form of latent heat in silicon elements or silicon alloys has a lower cost than the €4 per kWha much lower figure than that associated with the lithium-ion batteries we currently use.

This cost will increase when introducing the price of the container and the necessary thermal insulation material to minimize the dissipation of thermal energy, but even so, there is no doubt that this technology is very promising.

At the moment, these researchers have perfected a thermophotovoltaic battery prototype with a capacity of less than 1 kWh, so much remains to be done to scale this technology so that much larger storage capacities are feasible, and also to make it profitable. In any case, it is worth keeping a close eye on him.

Pictures | ThisIsEngineering | Pixabay | Solar Energy Institute (UPM)

More information | Joule


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