As we are trying to limit our dependency on fossil fuels and solar panels and wind turbines are increasingly saturating our landscape, we find ourselves in front of yet another, potentially bigger, challenge: not the production of green, low-carbon energy, but its storage over both short and long periods of time. These are some of the most promising energy storage solutions to date.
For a long time, our attention has been focused on the production end of the renewable energy challenge. As a result, we have installed thousands of solar panels and built numerous on- and off-shore wind turbines and parks. And yet, even if we succeed in installing enough of these green energy solutions to – theoretically – power our lives and businesses, we will not necessarily be able to decarbonise the system. In fact, since wind and solar energy are only available intermittently and production can’t be fully controlled, we will remain dependent on a more traditional energy system to guarantee continuity and will face potentially large disturbances on the grid during peak production times to boot.
Unless we can find an efficient, affordable and preferably scalable solution to store our locally produced green energy, that is. Plenty of researchers, start-ups and more established companies are hence busy developing, testing and finetuning such energy storage solutions, often using the same basic principles and technologies as their starting point and expanding on their efficiency and durability.
Lithium-ion batteries are the most commonly used solution for green energy storage today, but the technology is tainted with a myriad of issues. For one, there’s the lithium supply chain. Lithium supplies are becoming increasingly hard to access, availability is dropping and prices are rising.
And then there are the technological obstacles: the degradation of lithium-ion cells, the quick drop in efficiency, the impossibility to completely discharge and recharge, safety issues due to overheating, a relatively short lifespan, and so on. While lithium-ion batteries may remain a useful solution for small scale and short term energy storage, we will need to look at alternatives to move forward.
Redox Flow Batteries
Like lithium-ion batteries, redox flow batteries store energy in an electrochemical form. Instead of storing the energy within the electrode of its own structure, however, a redox flow battery consists of two tanks filled with highly conductive electrolyte fluids – one on the positive side, one on the negative – separated by an electrochemical cell. The negolyte and posolyte solutions are separated by a membrane through which electrons are exchanged to charge or discharge the battery.
One advantage of redox flow batteries is that power and storage capacity can be scaled and adjusted separately: the amount of electrolyte fluid and hence the size of the tanks determines the storage capacity; the electrochemical cell the electrical power. In addition, redox flow batteries can store energy for a larger amount of time, can be fully charged and discharged over 20.000 times without loss of capacity and have a longer lifespan than lithium-ion batteries.
A potential bottleneck, however, is the material used as the active ingredient. Most redox flow batteries use vanadium, which isn’t necessarily cheaply available, although researchers and companies are experimenting with other sources and materials to reduce costs and increase efficiency.
When it comes to finding ingenious solutions, there’s no better source of inspiration than nature. So why not use one of nature’s most basic physical principles: gravity? By pumping large amounts of water upward to a high reservoir at times of energy surplus and releasing that same water to flow downwards when the energy demand outweighs the immediate production, pumped hydro systems have provided an efficient energy storage solution for many decades already and can easily be connected to a green energy supply.
While these systems can provide an enormous storage capacity at a relatively low cost, there are some challenges too. First and foremost, pumped hydro systems can’t just be built anywhere. One needs to take into account geographical features including elevation, but also the vicinity of natural river ecosystems, making sure not to disturb them. In addition, these are projects catered to the largest scale, which inevitably means that construction is a long-term and high-cost endeavour.
20.000 MWh Storage Capacity in Swiss Water Battery
In 2022, the Nant de Drance pumped storage power plant in Valais, Switzerland went into operation. The project took all of 14 years to build and includes two water reservoirs, 17 kilometers of underground tunnels and six turbines powered by water cascading down a steel pipe. By raising the water level of the upper reservoir, Vieux-Emosson, its capacity was doubled to a total of 25 million m3, thus resulting in a storage capacity of 20.000 MWh. Nant de Drance has a rated power of 900 MW and can switch from pumping at full power to running the turbine at full power within five minutes. With the addition of Nant de Drance, the installed capacity of pumped hydro storage in Switzerland increased by 35%, to a total of 3.462 MW.
© Nant de Drance | Sebastien Moret
Energy Storage Tower
Taking its cues from the pumped hydro systems, but looking to create a more scaleable solution applicable to a wide range of geographical situations, the Swiss company Energy Vault has developed a gravity-based storage solution using purpose-built stacked blocks.
The system uses a proprietary mechanical process of lifting and lowering 30-ton composite blocks to store and dispatch electrical energy and can theoretically be built anywhere. It is said to provide storage capability from two to eighteen hours with a lifetime round trip efficiency of over 80% and a technical lifetime of over 35 years. While essentially a grid-scale solution, its modular Over the past couple of years, Energy Vault has started construction on a number of these energy storage towers in the United States, Middle East, Europe, China and Australia, with a first commercial-scale deployment in 2020.
[Picture : © Energy Vault]
The thermal storage of surplus electrical energy is a well-known, often low-tech and low-cost solution. While there are plenty of ways to go about thermal energy storage, the most known and straightforward form involves the use of surplus energy to heat water, which can later be used for district of central heating or as a hot water source in its own respect. While this type of thermal storage works relatively well on a small scale and a short term, it is not the most efficient long-term solution.
As an alternative to more traditional forms of thermal storage, the Finnish, research-based company Polar Night Energy has developed a sand battery. Consisting of a thickly insulated silo filled with 100 tonnes of low-grade builders’ sand, two district heating pipes and a fan, the battery uses excess electricity from wind turbines and solar panels to heat up the air inside the sand, allowing the temperature inside to rise to over 600°C. When necessary, the battery discharges the hot air through the heat-exchange pipes, thus warming water in the local district heating network.
The biggest advantage of the sand battery is its storage capacity. Not only does the battery, when full, store 8 MWh of thermal energy, but the sand is also very effective at retaining the heat over long periods of time and thus storing the energy for months on end. The only way to efficiently discharge the energy, however, is in the form of heat, limiting the value of this technology to those applications requiring heat rather than electricity.
© Polar Night Energy
Power to Gas to Power
Long-term Hydrogen Storage
There are plenty of discussions on the role of hydrogen in the energy transition, most of them focused on its value as a fuel. Yet, hydrogen can also be considered as a medium for energy storage, namely by converting (green) electricity into hydrogen by electrolysis, storing that hydrogen en eventually re-electrifying it. Overall, the round trip efficiency of this process is lower than that of many other technologies – electrolysis usually has a conversion efficiency of 65 to 70%; re-electrification in fuel cells or gas power plants doesn’t surpass an efficiency of 50 to 60% – but the appeal is mostly in its high storage capacity and the ability to store energy for much longer periods of time, potentially even a season.
While small amounts of hydrogen can be stored in pressurised vessels, such a particular energy storage application calls for a long-term and large-scale storage solution. Underground salt dome caverns are the most likely candidate here, allowing for large amounts of hydrogen to be stored at 200 bar or 2.900 psi, corresponding to a potential 100 GWh of stored electricity. However, there are still certain dangers and complexities to this type of storage, including the need for a constant pressurisation. Hence, researchers are constantly looking for other and potentially safer hydrogen storage solutions, for instance by using a cryogenic flux capacitor to store as many fluid molecules as possible in the smallest, lightest-weight volume possible, and to supply those molecules rapidly on demand for end-use applications.