Large-Scale Energy Storage Needed
There is a growing need for battery systems that are able to hold a lot of power for several hours or days. This is because solar and wind power are intermittent, fluctuating throughout the day and from day to day.
Another factor is the energy demand of industrial processes, such as metallurgy. These factories need power 24/7, usually to generate a lot of heat in the 1,000-1500C range.
Using lithium-ion batteries like for EVs is unlikely to be a solution that works at scale. They are simply too expensive, not durable enough, and consume too much raw materials.
This is why many different alternative battery chemistries have been considered for utility-scale energy storage. We look at most of these options in our article “The Future Of Energy Storage – Utility-Scale Batteries Tech“, including lithium-iron-phosphate, sodium-ion, redox-flow, iron-air, molten metal, nickel-hydrogen, sodium-sulfur, etc.
All of these batteries store energy in the form of electricity one way or another, usually through the oxidation and reduction of a metal.
The issue is that even very common metals like iron or aluminum would still require a lot of mining. So what if we store energy in another form?
Storing Heat
There is a surprisingly large number of options for storing energy without using electricity as a means of storage. We looked at them in further detail in “Non-Chemical Alternatives To Batteries For The Energy Transition“. These options include:
- Compressed air.
- Gravity batteries (rocks, pumped hydro).
- Heat batteries.
- Flywheels.
- Thermal solar.
Each of these options, battery and non-batteries, have advantages and disadvantages. The ideal storage energy solution would fit a few characteristics:
- No need for metal and rare materials.
- Long-duration energy storage.
- Low cost.
- Easily scalable without limitation on available sites or resources.
- Can be used as electricity with minimal losses.
For the first 4/5 items, heat storage would fit the bill. Sand batteries, like those from the Finnish company Polar Night Energy, can store excess energy from the summer into the winter, a must for powering with renewables the heating systems in the frigid and sunless winters of the North. And it uses only low-grade sand and a bit of metal for the frame and piping.
Source: Polar Night Energy
A similar heat battery is envisioned by Rondo Energy, with bricks that can be heated to as much as 1500C, using the heat for industrial processes like cement or steel production.
These heat batteries work well when the desired product is heated in the first place, whether it is district heating for apartments or industrial heat.
Source: Rondo Energy
However, heat batteries were thought to be a poor match for providing energy to the grid, as the efficiency of converting heat back to electricity is usually in the low 20-30% range. This would mean that as much as 4/5ths of the energy “stored” is actually lost. Or, to put it another way, the stored energy is suddenly x5 more expensive than when it was produced.
Even with solar energy now on par with fossil fuel energy, it is still far from being five times cheaper. However, this issue might be only temporary, with innovation in a technology called thermophotovoltaics.
Thermophotovoltaic Cells – Photovoltaics, But With Heat
Thermophotovoltaics is the idea of creating power in a similar way to traditional photovoltaics but with heat rather than sunlight —or, more precisely, infrared light emitted by heated materials.
In a traditional photovoltaic panel, any light frequency below a certain threshold is not energetic enough to create a current and is wasted. This is the case for all infrared light, as infrared light electromagnetic waves are too low on energy.
This is not a new technology in itself, but it has always suffered from too low an efficiency level to be adopted widely. This was true until the recent achievement by researchers at the University Of Michigan, which released their results in a scientific publication titled “High-efficiency air-bridge thermophotovoltaic cells”.
They claim to have achieved a record 44% efficiency for temperatures below 1,500°C, which is much higher than previous results, which only reached 37%.
And the researchers think they have a way to reach 50% “in the not-too-distant future”.
Source: Cell
How Do Thermophotovoltaic Cells Work?
First, the storage material, which can be stone, sand, carbon, bricks, ceramics, etc., must be heated, either with excess electricity from renewables or direct solar heat.
At 1435°C, about 20-30% of the infrared photons it will emit have enough energy to generate electricity in the researcher team’s thermophotovoltaic cells.
Source: Design Boom
The other infrared frequencies were not possible to convert into electricity with the semiconductor used in the thermophotovoltaics panel.
So, how did they get to 44% from there? The ingenious “trick” was to add a thin layer of air to the thermophotovoltaic cell just beyond the semiconductor and a gold reflector beyond the air gap.
This kept the photons with the right level of energy trapped and ready to produce electricity. The others would be sent back toward the heated material, re-heating it. They then could have another chance to be sent toward the thermophotovoltaic cell with the right light frequency.
Essentially, it allows the cell to recycle what it did not catch the first time, boosting the overall conversion efficiency.
Future Heat Batteries
An efficiency of 44% or even 50% might not look that impressive when compared to pumped hydro (70-85%) or lithium-ion batteries (85-95%).
Source: EESI
But this would be ignoring that mass deployment of utility-scale energy storage has as much to do with economics as with technology.
Batteries have a lifespan measured in years before needing recycling. There is also simply not enough lithium and even less copper, nickel, or cobalt to deploy them at that scale, especially when most of this metal production is already required for EVs.
In contrast, heat batteries can last decades with little to no maintenance needed. They require almost no material beyond rock, sand, or clay. They can be built anywhere and at any size, even truly gigantic, with almost no effect on the ecosystems, contrary to hydroelectric.
Lastly, heat is one of the most efficient methods when the energy needs to be stored for weeks and months, with virtually all types of batteries “leaking” power over time at a much higher rate than a well isolated heat storage system.
“It’s a form of battery, but one that’s very passive. You don’t have to mine lithium as you do with electrochemical cells, which means you don’t have to compete with the electric vehicle market.
Unlike pumped water for hydroelectric energy storage, you can put it anywhere and don’t need a water source nearby,”
Stephen Forrest, University Professor of Electrical Engineering at U-M
So, overall, a heat battery can be the perfect energy storage solution to smoothen out renewable intermittency. And even store away excess energy of the windy or sunny months into the winter for temperate climates, or into the rainy season for tropical climates.
Heat Storage / Thermophotovolatic Companies
1. II-VI Marlow / Coherent
II-VI Marlow is a branch of II-VI Inc., an industry leader in the (currently rather small) field of production of thermophotovoltaic cells. In 2022, II-VI Inc. acquired laser manufacturer Coherent Inc. and subsequently changed its company name.
The company is an expert in advanced materials used in lasers, optics, and photonics, such as indium phosphide, epitaxial wafer, and gallium arsenide. It grew largely thanks to multiple acquisitions over the last decade.
Source: Coherent
The company”s thermophotovoltaic activity is only a small part of its overall revenue, with optic fiber, laser, and others making up the bulk of its revenues. This might, however, be good news for investors, as it also means it has the capital to deploy new technology quickly at an industrial scale if innovation opens new markets.
Source: Coherent
For example, the massive market of utility-scale storage for excess renewable energy…
As Michigan University has “applied for patent protection (…) and is seeking partners to bring the technology to market“, it would not be surprising if a company like Coherent ended up being the one to bring the new thermophotovoltaic cells to the market. Especially considering its preexisting expertise in the industrial production of thermophotovoltaics and similar advanced materials.
2. Sumitomo Electric Industries (SMTOY)
The thermophotovoltaic cells used by the Michigan University researchers were made with InGaAsP (indium gallium arsenide phosphide)”.
“So we investigate whether translating the air-bridge architecture from ternary to quaternary group III–V absorbers (InGaAsP lattice matched to InP substrates) can enhance the efficiency within the target range of emitter temperatures.”
If this technology becomes a standard format for utility-scale energy storage, we will need InGaAsP, and a lot of it.
So it would make sense in this scenario to bet on the company that has stayed for 30 years as the leader in the production of InGaAsP, the Japanese conglomerate Sumitomo.
For more than 30 years, the Semiconductor Division of Sumitomo Electric has maintained its leadership position as the world’s largest manufacturer of gallium arsenide (GaAs) and indium phosphide (InP).
The name Sumitomo has become synonymous with quality in III-V materials. Sumitomo Electric achieved this reputation by providing its global customers with superior-quality substrates that result in higher yields and devices with consistent electrical characteristics.
Sumitomo Compound Semiconductor – About Us
Sumitomo Electric Industries, and its compound semiconductors department, is the branch of the larger Sumitomo conglomerate, one of the largest worldwide sōgō shōsha (general trading companies).
Besides InGaAsP, the main of Sumitomo Electric Industries’ activity is producing:
- Telecom equipment (optical fiber, 5G)
- Automotive wiring and high-power cables
- Electronics (flexible printed circuits, data wires, filtration membranes)
- High-quality materials (carbide, cutting tools, prestressing steel strand).
Source: Sumitomo
The expertise in mass producing high-quality material, the leadership in InGaAsP production, and the business connections that come with being part of a sōgō shōsha / general trading company should ensure that Sumitomo Electric Industries will be one of the main beneficiaries of a mass adoption of thermophotovoltaics.
The growth should also be good for telecom (digitalization, AI) and EVs’ afferent products (cables, wiring), which are both booming sectors.