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Non-Chemical Alternatives To Batteries For The Energy Transition

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Are Batteries The Right Idea?

The preferred approach to remove fossil fuels from industrialized economies has been electrification. The goal is to replace ICE (Internal Combustion Engine) vehicles with EVs (Electric Vehicles) and produce heat with heat pumps instead of combustion. It is something we investigated in detail in our article “The Future of Mobility – Battery Tech”.

The problem is that for this change to be actually ‘green', the electricity needs to come from renewable or sustainable sources – which, depending on who you ask, may include nuclear.

In both cases, the changes in electricity demand during the day and season mean that you would need to build a lot of spare capacity that would stay idle most of the time. This is especially true for renewables, as solar underperforms in cold & cloudy winters, and wind can stop blowing for weeks at a time.

So there is a dire need for long-term energy storage. One solution can be utility-scale batteries, leveraging new chemistries that offer lower costs or other advantages. It is something we investigated in detail in our article “The Future Of Energy Storage – Utility-Scale Batteries Tech“. Another option is to produce some form of liquid fuel that can be stored away until we need it, either liquid hydrogen or liquid ammonia for example, something we detailed in our articles “The Other Hydrogen Fuel – Top 5 Green Ammonia Stocks” & “Algal Biofuel: The Next Energy Revolution?

But what if the proper way to store energy for days, weeks, months, or even years was not batteries? In this article, we will look at all the non-chemical options for energy storage.

Compressed Air

The Pros of Compressed Air

Compressing air requires a lot of energy. This same energy can be partially collected back when decompressing the stored air.

A key advantage of this technique is that it uses only mechanical parts like pipes, pumps, and tanks, themselves requiring only simple materials like steel, instead of rare materials like rare earth, cobalt, lithium, etc. It could even rely on a depleted natural gas reservoir, providing an absolutely massive “tank” at a very low cost. And of course, air is a fully free and available supply anywhere.

Another advantage is that compressing air is a well-understood technology, with a very well-established supply chain, that could be ready for scaling up quickly if required. It is also a system that can store energy for a very long time with little to no losses.

The Cons of Compressed Air

A key problem in making compressed air efficient is a physical phenomenon that occurs when you compress any gas. Heat is generated during the compression. This heat generally has to be dissipated to avoid damage to the compression system, and translate into lost energy. The air also cools when decompressed, often needing warming systems.

30-40% of the energy can be lost due to this. It can even be worse for the less efficient designs. So far the best efficiency at scale has been achieved in China at 30% losses, at the 400 MWh Zhangjiakou plant designed by the Chinese Academy of Sciences.

So while compressed air could be an acceptable option for low-cost long-term storage between seasons, the unavoidable 30%+ losses of energy are a drag on the system economics. Only if the heat produced during compression could be recycled in another process could these systems become truly efficient.

Gravity Batteries

In physics, any object going up is considered as acquiring a higher level of “potential energy”. This energy manifests as downward movement as soon as that object is allowed to go down. From this fact stemmed the idea of using gravity to store energy.

Pumped Hydro

This is the most commonly used form of utility-scale energy storage today, making for 90% of global grid energy storage capacity in 2020, according to the Department of Energy Global Energy Storage Database.

Hydropower generation uses the energy potential in water from rainfall, stores it behind dams, and releases it to produce power. Pumped hydro uses electricity to pump back the water up the hydropower station.

The storage dam can also be built only for the purpose of pump storage, and require just a hilly landscape compared to large hydropower dam requiring often mountainous terrain. Alternatively, an underground reservoir can also be used.

Source: Energy.gov

The energy losses are generally in the range of 20-30%. Storage can last multiple months with limited losses, at least in regions that are not too hot. The capital requirement can be quite large, but the pumped hydro installations are also extremely long-lasting.

An intriguing idea only discussed but never tested at scale, is to use electrolysis to turn the water at a lower altitude into hydrogen. This hydrogen, lighter than air, is then piped up “energy-free” upward. It is then reformed into water (returning part of the energy consumed by the electrolysis). That water is then producing power again on its way down through a hydropower turbine.

While potentially with a much better efficiency overall than pumped hydro, the size of electolyzers required could make the system still quite expensive.

Concrete/Rock Energy Storage

Water is not the only thing that can be stored up to accumulate energy with gravity. Rocks, concrete blocks, and other ultra-heavy objects can be used as well.

In this case, the technology required is even simpler than with pumped hydro, as this mostly relies on cranes and alternators for storing the energy and producing it back. Cranes and rocks are not really rare materials either.

The great advantage of these systems is their efficiency. Most of the lost energy in pump hydro comes from turbulence in the water, reducing the efficiency of the pump. In contrast, cranes and cables can provide losses as low as 15% (a quarter less losses than with pumped hydro).

Currently, the companies Gravitricity and Energy Vault (NYSE – NRGV) have been pushing for these concepts.

Gravitricity recycles abandoned mine shafts to access already-made and easy-to-handle wells of several hundred meters of depth to store energy with its GraviStore system.

Energy Vault instead is aiming for dedicated constructions, with blocks going up and down depending on the energy demand.

With hundreds of thousands of mine sites all over the world, this could have a much larger potential than many realize and benefit from the fact that the infrastructure already exists. The company is looking at a partnership with Swedish industrial giant ABB to finalize its technology development, as well as access the massive pool of ABB’s mining customers.

Source: ABB

The limitation of “solid energy storage” with gravity is that the energy stored on just a few tens or hundreds of meters is not that large. One option is to simply have enough of it, like hundreds of thousands of mine shafts like Gravitricity, or a massive dedicated “warehouses” like Energy Vault.

Another option is to have a lot longer depth. For that, deep-sea beds near the shore are an option. A floating platform could stand above such an area, and lower or raise a weight over several thousands of meters. This concept is called Deep Ocean Gravitational Energy Storage (DOGES). One company working on this concept is the French startup Sink Float Solutions.

Sink Float Solutions claims a potential for up to 80% efficiency and very cheap costs.

“Currently, offshore cranes with the largest load capacity can lift masses of 4000 tons, which, for a vertical speed of 20 km/h, would correspond to a power of 200 MW. Several winches on the same site can be used in parallel.”

The actual economics of these systems will highly depend on their durability, with the corrosion from seawater of the mechanical parts the main concern.

Sand/Heat Batteries

A lot of electricity is used to produce heat, either for industrial use or to keep houses and apartments warm in winter. Instead of storing electricity like a chemical battery, a sand battery looks to store heat directly.

The idea is to build a  large silo (like the ones used for grain), fill it up with sand, and add isolation to it. The sand is then warmed up to 400C. Because sand has rather low thermal conductivity AND high thermal mass, it can store a LOT of energy. It is also very resistant, not suffering any damage even at such high temperatures.

This concept relies on very simple technology, with standard pipes, air pumps, electric resistance, steel, and low-grade sand that cannot be used for applications like concrete.

The leader of this technology is the Finnish company Polar Night Energy. This is a concept that works especially well in Nordic countries with pre-existing “district heating”, where hot water for radiators is produced in a centralized location for hundreds or thousands of flats. This is notably the case in Scandinavia, Germany, and most of Eastern Europe and ex-USSR countries.

Because the energy losses are minimal, the energy can be accumulated during the long sunny months of summer (which can last 20-24 hours/day in Finland). The excess photovoltaic production can this way be stored to keep the country warm in winter and reduce the seasonal spike in power demand induced by the need for heating.

We could imagine further iterations of the concept could be used to warm greenhouses, or rely on thermal solar systems.

A similar concept of a “heat battery” is also being developed by Rondo Energy. These bricks can be heated to as much as 1500C. Because of the higher temperature, this application could be usable not only for district heating, but also for industrial applications like cement production, metallurgy, chemicals, or mining.

Source: Rondo Energy

Meanwhile, the company Stiesdal is also working on a heat battery using crushed rocks. The grid storage capacity is expected to range from anywhere between 10 hours and 10 days.

Lastly, the giant cement manufacturer Holcim is also developing a solution relying on cement, which stores heat when dry and releases it when hydrated. Because the cycle can be repeated infinitely, it should come at a low cost over a long enough period of time.

This is designed to store any surplus heat in an industrial setup. Maybe it could be used as a complement to a compressed station, reducing the loss from the air heating up when compressed.

With 300kW of energy per cubic meter of density, it could store a lot of power with a few thousand tons of relatively cheap and abundant cement. Only revealed in 2022, the concept is currently in development.

Flywheels

Another way to store energy is to keep it in the form of mechanical movement. Flywheels are such devices that store energy by rotating a mechanical wheel in a vacuum on a magnetic bearing, rotating at speeds as high as 20,000 to 50,000 rotations per minute. The system can store or give back energy by accelerating or slowing the flywheel.

The main advantage of these systems is that they are very quick to react in mere milliseconds. So not only they can store energy, but they can also help stabilize the frequency of the electric grid, something that is harder to do without massive centralized power plants like coal, gas, or nuclear power plants.

Companies like Siemens Energy have conducted pilot projects using flywheels, like in South Australia.

Other startups are also working on this concept like Stornetic, with a system expected to last at least 20 years and a 100,000 charge/discharge cycle.

Source: Stornetic

Another company is S4 Energy, with 60 MWh operational or under construction, and a pipeline of 1,050 MWh. The reactivity of the flywheel can also help make some operations more efficient when there are surges in power demand, like the cranes at the port of Rotterdam, where a flywheel is used to collect back the energy when a load is lowered. The company's flywheel, KINEXT, is especially heavy (5 tons) and moves relatively slowly (“only” 1,800 rotations per minute), making it very long-lasting,  with low operating cost, and easy to maintain. The flywheel has only 8% losses of its energy stored, which is slightly better than even lithium-ion batteries.

While very efficient and reactive, the total energy stored of a flywheel is lower than other solutions for their cost, so they are likely to stay confined to niche applications and for grid frequency stability.

Thermal Solar

Photovoltaic panels convert solar rays into power directly. In comparison, a solar thermal power plant collects the sun's heat and concentrates it to produce electricity.  Since heat is easier to store with some inertia, such solar stations can still produce power many hours after sunset, right when energy consumption peaks.

This idea, which lost popularity in the face of increasing yield of photovoltaic panels, might make a comeback now that the intermittency of renewables becomes a larger issue as they become a larger part of the power generation mix.

Source: Energy.gov

While promising, this concept has historically been plagued by high costs. Carrying the heat from the collector to the generator has proven difficult, with dangerous heat and waste hydrogen being produced. Another design using molten salts has suffered from leaks and reliability issues.

It is also problematic for wildlife, as birds can be regularly incinerated by the invisible and very concentrated sunrays.

In the future, less massive and therefore less ecologically disruptive designs might bring concentrated solar power back in the spotlight.