An earlier article covered some of the more obscure renewable energy production technologies, but the energy transition to renewables also needs the help of efficient energy storage technologies, to help the intermittent generation techs such as solar and wind power meet their full potential. While they might not boast the same fame as Li-ion batteries or pumped hydro storage, some technologies could make a big impact on the world of renewables. Let’s cover some of the lesser-known ones:
A. Bricks.
Yes. The road to sustainability might be paved with bricks rather than batteries. Bricks with high heat capacity, volcanic rocks, sand, or gravel, all these can make for quite a good heat storage technology, they are sturdy, cheap, don’t degrade, and are widely available. The principle is pretty simple: you use renewable energy to power electric heating elements, which as with other thermal energy storages (TES) have a power-to-heat efficiency of almost 100% (where else would the energy go?), thus warming up bricks to high temperatures, anywhere between 100°C to 2000°C. These bricks should be in a very well insulated container to limit the tremendous potential heat losses, then the energy can be stored for long durations with minimal loss. Then, the stored heat is transferred to water to turn it into steam, which is then fed into a turbine, thus generating electricity. Other possibilities include heating the system using waste industrial heat, or providing the stored heat as hot air for heating or industrial purposes.
The first obvious remark is: “2000°C is really hot. My oven can’t go that hot, even my gas stove cannot. And that would burn most things.” And indeed, that much is true, which is why very specific materials are needed. Regarding the high-temperature heating elements, I would refer you to this article, but to sum it up, some materials of various kinds can be used to generate and withstand very high temperatures. Tungsten is one of the most notable examples, as it was used in old filament lamps and reached up to 3000°C during operation. However, if not in a vacuum or oxygen-free environment, it cannot go that hot without getting severely oxidized.
The second remark is about heat losses. Heat losses are a big deal in high-temperature thermal energy storage systems, and keeping them in check is all about good insulation. The hotter the storage medium gets, the faster it loses heat, thanks to the steep temperature difference with the surrounding air. For a system running at a more moderate 500–600°C, decent insulation can keep daily losses down to around 1–2%. But when you crank things up to around 2000°C, the heat starts escaping much faster—potentially 5–10% per day—because of the way radiation grows with the fourth power of temperature. Advanced insulation materials, like vacuum-insulated panels or aerogels, can cut these losses significantly, but they’re pricey, so it’s a balancing act between keeping costs reasonable and maximizing efficiency.
B. Molten salt thermal energy storage
Bricks might be the plainest form of TES, but they are not the sole one! Apart from technologies simply warming up a solid, there are additional technologies requiring a slightly more thought-through planning. Among them, molten salt thermal energy storage, or MSTES. This topic was briefly touched upon in the post on energy generation, when discussing concentrated solar power. The idea is somewhat similar, a heating element is plunged in a liquid material (molten salt, often NaOH which is safe and widely available), to heat it up from a few hundred degrees to the 5-800°C range. The hot(ter) fluid is sent to a hot tank, stored until needed to superheat water to steam at several hundred degrees. Cooled down, the fluid then rests in a “cold” tank, still very hot to stay well above its melting point, awaiting renewable power to heat it back up.
An important drawback of high-temperature TES is that, while they can store and supply heat at a very high efficiency, they usually rely on steam turbines when their purpose is to supply electricity. Steam turbines in this context usually have an efficiency below 50% and can drop as low as 25%. Therefore, even though all electricity going in can be successfully stored, most of it cannot be retrieved as electricity, severely hindering the relevance of such systems in some applications. As a comparison, large scale battery storage usually reaches an efficiency of 80 to 90%. This means that while high-temperature TES systems excel in storing and supplying heat, their low electricity-to-electricity efficiency limits their competitiveness for applications requiring direct electrical output.
C. Phase-change material TES
The aforementioned points work in a very simple way: You provide heat to a material, it heats up. You need energy, you cool it down to harness its stored heat. This is called sensible heat storage. However, a slightly more complex approach exists, using phase-change materials. When you heat water, it warms up to 100°C then starts boiling, with water itself staying at 100°C. This means extra energy doesn’t make the water go hotter, but makes it change to its gas phase. The reverse process can also happen, where, upon phase-change when cooling down, a material releases heat. This is the case with candles, as the wax releases some heat when solidifying. This allows storing more energy in the same amount of material.
This simple concept forms the backbone of a storage solution that’s efficient and reliable. PCM TES relies on physical reactions, not chemical ones, making it a usually more resilient solution. It also sidesteps the issue of rare earth metals and toxic chemicals, as PCM electricity storage systems for the most part use hydrates and molten salts such as sodium and magnesium sulphate hydrates, or a Calcium Chloride-Sodium Chloride salt mixture, which are all widely available and safe for human consumption. More information on the topic can be read in this article.
However, while PCM TES is great for storing heat, it still bumps into the same efficiency limits as other systems when trying to generate electricity.
D. Water thermal energy storage
Water thermal energy storage (WTES) can play a role in electricity generation, but it’s not its strongest suit. WTES is much better suited for heating applications, especially when paired with solar thermal systems. Solar collectors can heat water during sunny periods, storing it in well-insulated tanks or underground pit storage systems. These setups are common in district heating networks, where large-scale water storage can supply homes and businesses with heat for days or even months. Seasonal storage systems, such as solar-heated water pits, store excess summer heat for use during the colder months, making them an efficient option for balancing energy needs over time.
For industrial or residential applications, WTES systems excel in simplicity and reliability and just like other TES, they can not only pair with solar, but also with other renewables, capturing excess energy during peak production and feeding heating elements, then releasing it when needed. They’re also scalable, ranging from small residential systems to large district or industrial-scale solutions. Allowing buildings to keep the heaters off for a while longer, WTES help reduce their dependency on fossil fuels or their peak-period electricity use, and thus contribute significantly to reducing greenhouse gas emissions. They also lower operational costs for both heating and cooling by leveraging renewable energy and minimizing waste. All that using water.
E. Flywheel Energy Storage
Flywheel energy storage is a fascinating and somewhat underrated technology in the world of energy storage. The idea is straightforward: a motor uses surplus electricity to spin a heavy rotor at high speeds, storing energy as kinetic energy. When the energy is needed, the system reverses the process, using the rotor's momentum to drive the motor as a generator, converting the stored kinetic energy back into electricity. This closely resembles the idea behind electric vehicles regenerative braking, where the brakes help recharge the battery a little bit as they slow the car down. It’s a simple, mechanical approach with a lot of potential.
One of the standout features of flywheels is their quick response time. They can absorb and release energy almost instantly, making them a great fit for stabilizing power grids or supporting renewable energy sources like solar and wind, which can be unpredictable. They’re also incredibly durable—flywheels can go through tens of thousands of charge and discharge cycles with little to no degradation, unlike batteries that lose capacity over time. This makes them ideal for applications where reliability and longevity are critical.
Flywheels do have their challenges, though. One of the biggest is energy density—they can’t store as much energy as batteries or other storage systems of the same size. For example, a typical flywheel system is more suited to short-duration storage, like balancing a power grid for a few seconds to minutes, rather than storing large amounts of energy for hours or days. Efficiency is another factor: while modern flywheels can achieve round-trip efficiencies of up to 85-90%, some energy is always lost due to friction and air resistance, even with advanced designs like magnetic bearings and vacuum-sealed chambers.
Despite these limitations, flywheels are a practical and eco-friendly choice for certain roles. They’re low maintenance, don’t rely on rare materials, and can operate in a wide range of conditions. Whether it’s smoothing out fluctuations in renewable energy output or providing backup power in critical systems, flywheels are a reliable option for bridging the gaps in our energy systems.
F. Hydrogen storage
Ah. The trendy one. Hydrogen storage is one of those technologies that’s been gaining a lot of attention as we look for clean energy solutions. The basic idea is to produce hydrogen (usually through electrolysis, where water is split into hydrogen and oxygen using electricity) and store it for later use, often as a fuel or to generate electricity. Hydrogen is pretty versatile—it can be used in fuel cells to power vehicles, or in industries where it’s needed for things like making fertilizers or refining metals. The challenge lies in storing it efficiently, since hydrogen is the smallest and lightest element, making it tricky to keep under control.
There are a few ways to store hydrogen, and they each come with their pros and cons. One common method is storing it as a gas, pressurized in high-pressure tanks. This is what you see in most hydrogen-powered vehicles, like buses or trucks. While it works well in the short term, storing hydrogen at high pressure takes a lot of energy, and the tanks themselves need to be strong and heavy, which can limit their efficiency and portability. Another method is storing hydrogen as a liquid, which requires supercooling it to cryogenic temperatures (around -253°C). This makes it denser, so you can store more hydrogen in a given space, but the process of cooling and maintaining those temperatures is energy-intensive.
Solid-state hydrogen storage is another interesting option, where hydrogen is stored in solid materials like metal hydrides. The idea here is that the hydrogen reacts with the material to form a solid compound, which can then be safely stored and later "released" by heating it up. This method can be more compact and safer than storing hydrogen as a gas or liquid, but it’s still in development and doesn’t yet match the convenience of liquid or gas storage for larger-scale applications.
Once hydrogen is stored, it can be converted back into energy in a couple of ways. The most common method is through a fuel cell, where hydrogen reacts with oxygen to produce electricity, with water as the only byproduct. Hydrogen is one proton and one electron, and when this happens, think of it as the protons joining the oxygen to make water while the electrons go their own way, generating electricity. This makes hydrogen fuel cells a clean, efficient energy source, especially for vehicles and stationary power systems. Alternatively, hydrogen can be burned in a combustion engine, though this is less common for portable energy storage and is more often seen in industrial applications. The beauty of hydrogen is that it can be used to generate energy without emitting any harmful substance, making it an appealing option for a carbon-free future as long as its production is just as clean.
While hydrogen storage is still facing some hurdles—mainly around efficiency, cost, and scalability—it holds a lot of promise. As we continue to improve storage technologies, hydrogen could become a key player in the clean energy transition, offering a way to store renewable energy for long periods and help power everything from vehicles to entire industries.
G. Pneumatic Energy Storage
Pneumatic energy storage is a cool idea where energy is stored in the form of compressed air. The basic concept is pretty simple: you use electricity to compress air and store it in large, strong containers, often underground in caves or specially designed tanks. When you need the energy back, the compressed air is released, and as it expands, it drives a turbine or generator to produce electricity.
The nice thing about pneumatic storage is that it’s relatively simple and doesn’t require a lot of complicated technology. It’s also flexible, because the system can be built in most settings, as long as you have a place to store the compressed air. In some setups, excess energy from renewable sources like wind or solar can be used to power the compressors, which makes it a pretty efficient way to store energy from those intermittent sources. Plus, it’s a lot safer than some other storage methods since air doesn’t pose the same risks as chemicals or highly pressurized liquids.
However, like any energy storage tech, pneumatic storage has its challenges. The energy density, or how much energy you can store in a given space, isn’t as high as with other systems like batteries or pumped hydro. Also, when you release the compressed air, it tends to cool down rapidly, which can make it less efficient if you don’t have a way to recover that lost heat. That’s why some advanced pneumatic systems include heat exchangers to capture and reuse the heat during the compression and expansion process, improving efficiency.
Overall, pneumatic energy storage is an interesting option, especially for large-scale storage where space isn’t a huge concern. It’s not quite as compact or efficient as some other methods, but it’s still a solid contender in the mix of renewable energy solutions.
H. Adiabatic Compressed Air Energy Storage (A-CAES)
Adiabatic Compressed Air Energy Storage is a more advanced twist on the traditional pneumatic energy storage idea. Instead of just compressing air and storing it in a container, A-CAES focuses on doing all of this without losing energy to heat during compression or expansion. The term "adiabatic" refers to processes that occur without heat exchange with the surroundings. This means the system keeps all the energy from the compression phase instead of letting it go into heating the air. Neat, right?
Here’s how it works: In a traditional compressed air system, when you compress the air, it gets really hot. This heat is usually lost, which reduces efficiency. With A-CAES, the heat generated during compression is captured and stored in a thermal reservoir (like a tank of gravel, sand, or even a special salt). Then, when the air is released to generate power, it’s first passed through this stored heat, warming the air back up before it expands. This way, the air can expand and produce power more efficiently, since warmer air can do more work when it expands. The catch is that the setup is a bit more complex and requires more infrastructure to store the heat, so it’s not as simple as just having a big tank of compressed air.
Overall, A-CAES is a clever way to make compressed air energy storage more efficient. While it might not be as widely used yet as other methods, its ability to store energy more effectively by handling heat is a pretty exciting development for the future of energy storage.
I. Biological Energy Storage
Sounds like science-fiction, but it’s true! Biological energy storage is one of those cool, up-and-coming ideas that taps into nature’s own processes to store and release energy. Essentially, the concept revolves around using living organisms—like algae, bacteria, or plants—to capture and store energy, usually in the form of chemical bonds, and then later release that energy when needed. While it’s still pretty experimental, the idea is undeniably fascinating, blending biology with energy storage in a sustainable way.
One approach involves using bacteria. Some types of bacteria can consume waste products (like CO2 or organic waste) and produce energy in the form of electricity, often through microbial fuel cells (MFCs). These bacteria can "breathe" metals (such as iron or manganese) and transfer electrons to a surface, generating electrical current. It’s like harnessing a tiny bio-battery that continuously generates energy from waste materials. If scaled up, bacteria-powered storage systems could potentially provide a unique form of energy generation and storage, especially for off-grid or low-energy communities.
However, more commonly, plants themselves are definitely natural energy storage systems. Through photosynthesis, plants convert sunlight into sugars and other chemical compounds that they store within their cells. We burn wood, and we harvest the sugars for ethanol production (although this delves more into energy production than storage per se) but newer methods involve creating bio-hybrid systems that integrate plant-based energy storage into power grids.
Though it’s mostly still in the early stages, biological energy storage shows a lot of promise as a sustainable and potentially low-cost way to store energy. Imagine a future where bacteria could help power our cities, using the same processes that have been working for millions of years. Thanks, Mother Nature!
That's mostly it regarding the underdogs of energy storage! If you're interested in sustainable energy, Fremsyn can assist you both with the conversion of your company's fleet to greener vehicles and with assistance for all things related to biogas and biomethane!
Lesser-known renewable energy storage methods