From Germany, Australia, Israel, Norway to Poland ... The speakers at the first international conference dedicated to a new kind of battery — “the storage of energy in its thermal form at very high temperatures” — held last November in Madrid, came from 11 different countries. It was proof that the subject is of interest to many people.
The participants discussed the challenges of transforming electricity into heat, storing that heat and then converting that energy into electricity. But why dedicate so much time to something that seems like a fool’s game? “You lose 50% of the electricity when you convert it to heat and 50% of heat when you convert it to electricity!” MIT Professor Asegun Henry told Les Echos. “Without a doubt, this system will cost 10 to 100 times less than lithium-ion batteries.''
Storing electricity at lower prices has become a priority for making the shift to renewable energies. Since the sun sets and the wind stops blowing, how can variations in production be overcome? “The distribution of electricity only tolerates very small margins of error,” recalls Jürgen Weiss, economist and associate director of the American firm The Brattle Group (a consulting firm for businesses and public administrations). “We must therefore find innovative systems for storing energy over periods of a few days, weeks, months …”
The needs are immense: the capacities of electricity storage must multiply by 13 from 2019 to 2024, according to the Scottish energy and raw materials consultancy Wood Mackenzie. It is a market estimated rise over five years to 64 billion euros.
For the moment, most of this growth comes from the establishment of “farms” where dozens of huge lithium-ion batteries are lined-up. The largest such farm in the world occupies 1 hectare in South Australia. “But these batteries are expensive, have to be replaced every five or ten years, are complicated to recycle and can only provide electricity for a few hours,” says Andrew Maxson, manager of the New Solutions program at Epri (Electric Power Research Institute), an American research institute.
One of the most seriously studied solutions consists of using the electricity produced by a photovoltaic power plant or wind turbines to heat a material, then inverse the operation and transform the heat to electricity. Another alternative: store the materials at very high temperatures, over 1,000°.
The first option allows the use of technologies which have already been mastered. “Up to 600°, without needing, for example, special steels,” insists Adrienne Little, technical manager of the “heat exchanger” at Malta, a company based in Cambridge, Massachusetts. From the X laboratories of Alphabet, the parent company of Google, Malta wants to use electricity to, on the one hand, heat a material and, on the other, to cool a liquid. The difference in temperature between the two would then make it possible to produce electricity using a heat engine.
The second option of heating at a higher temperature allows more energy to be stored for longer, but involves developing new technologies. What material would be used to store heat? “We opted for carbon heated to 1,000°,” explains Justin Briggs, co-founder of Antora energy, a California startup supported by Stanford, Caltech (California Institute of Technology), Shell and the United States department of energy, “using a solid simplifies the whole process.” "We are currently testing a mixture of boron, sodium and iron," explains Alejandro Datas, researcher at the Institute of Solar Energy (Polytechnic University of Madrid) and responsible for the Amadeus project.
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Funded by the European Union, with a budget of 3.3 million euros for the period 2017-2019, Amadeus aims for a temperature of 2,000°. It brings together researchers from seven countries. "With our Polish colleagues, we are trying to develop the right mix of 'heated material' and 'container'," said Merete Tangstad, professor in the department of materials science and engineering at the Norwegian University of Science and Technology at Trondheim.
Last step: develop the best technology to convert heat into electricity. The Australian firm 1414 Degrees (the temperature at which silicon melts) uses the heat accumulated in silicon to produce very hot air which powers a turbine. "We can then supply the electricity or heat, to the electricity grid or to industries," says Kevin Moriarty, president of 1414 Degrees.
At very high temperatures, one can also use TPV (thermophotovoltaic) systems. "Any object at a non-zero temperature emits infrared electromagnetic radiation which can then be converted into electrical energy by a photovoltaic cell," explains Rodolphe Vaillon, research director at CNRS and specialist in near-field TPV devices (distances of less than 1 micrometer between the "radiator” and the photovoltaic converter).
With Alejandro Datas, he had the idea of mixing TPV and thermionic devices, where a current is produced by the transfer of electrons from a hot cathode to a cold anode. "A conversion efficiency of 30% can theoretically be achieved, while a macroscopic near-field TPV cell by itself would not exceed 10%," calculates Rodolphe Vaillon.
Meanwhile in the United States, Asegun Henry is exploring another avenue at his MIT labs: equipping photovoltaic cells with mirrors that will reflect part of the heat and therefore extend the life of the material. Like all his colleagues, Henry hopes to be able to develop the first pilot in the next two to five years.
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