Storing energy with water and table salt

Being able to store large amounts of energy becomes more important as we move further into the energy transition. Lithium-ion technology, while efficient, is also costly. Moreover, the raw materials for such batteries are becoming scarcer and mining them can lead to environmental damage. With AQUABATTERY, there is a cheaper and environmentally friendly alternative to store electricity: using table salt and water. 

Where exactly the idea of storing energy with water and table salt came from is hard to say. The fact is that Emil Goosen, COO at AQUABATTERY, has been fascinated by the concept since his student days. Long ago, Goosen, along with a fellow student (now a colleague), participated in the "Energy Battle," a student competition in Groningen. "We were invited there by Fujifilm to look at new applications for its recently developed membrane. During that competition, we worked on an idea to make a battery based on water and table salt, by storing energy in fresh and salt water. In doing so, we used that then-new Fujifilm membrane. At the same time, a project was underway on the Afsluitdijk to generate energy using fresh and salt water. This also produced drinking water from salt seawater, using the same membrane. You could see the first process as a discharging process and the second as a charging process. That's how the idea came to us to combine these two processes in a flow battery."

Operation

Anno 2024, AQUABATTERY is countless lab setups and pilot plants away and the young company is the only party in the world that has managed to get this technology past the research phase. "It may not seem like rocket science, but it is still quite a challenge to get the whole process operational," Goosen knows.

The technology behind AQUABATTERY works as follows: there are three vessels, all filled with a solution of water and table salt. That solution passes through a stack, which, after adding electricity and using a number of membranes, separates the solution into an acid and a base (HCl and NaOH, dissolved in water). Both the acid and base end up in their own vessels. In more detail, the equation is as follows: H2O plus NaCl become HCl (acid) and NaOH (base) with the help of electricity. In the reverse reaction, where the acid and base pass through the stack again, electricity is released and the concentrations of salt and fresh water are restored to baseline. 

The membranes used are similar to those used in the production of drinking water, as well as those used in an electrolyzer stack. The membranes are formed by polymers. The electrodes are currently coated with titanium, but an alternative is now being worked on. "We are constantly looking at optimization possibilities in terms of the components and materials we use," Goosen clarified.

Local storage

AQUABATTERY is now in the phase of somewhat larger pilots. Goosen: "We are now making flow batteries on the scale of shipping containers. In a year or two we will probably have outgrown that." Currently, for example, work is underway on a pilot installation at a Deltares solar farm in Delft. Deltares is keen to explore the possibilities of local storage of electricity. "The installation we are going to build has about 10 hours of storage capacity," he said. 

Levelized cost of storage

The AQUABATTERY has a number of indisputable advantages over Li-ion batteries: no costly and environmentally polluting minerals are used and the battery does not degrade during its lifetime. That lithium is many times more expensive than water with table salt is obvious. "The most expensive part of our flow battery is the stack. The expected lifetime of a stack is twenty years," Goosen knows. "How much cheaper our solution is compared to Li-ion actually depends mainly on the storage life of energy. It makes quite a difference whether you store energy for four, eight or a hundred hours. What matters then is the levelized cost of storage. Energy storage solutions are compared based on that value. That value is determined by the efficiency of your solution, lifetime, as well as other CapEx factors."

Understanding levels

Both process safety and process efficiency require an understanding of the levels in the three different vessels. Goosen: "To be clear, the three vessels never become empty during the process. The water from the tank containing the salt solution simply runs back to the vessel after passing through the stack. Only the ions are transported to the base and acid vessel. So the levels in the vessels also do not vary very much at different stages of the process, but you still need to keep an eye on them. During the process, osmosis could occur, which of course affects the levels. Water can also expand and contract and you will notice this in the levels as well. You may also have leakage somewhere. In short: insight into the levels is really necessary."

Equally important, the data from the level meter is used to determine the state of charge (SOC) of the battery. Goosen: "The concentration of volumes multiplied by the levels in the vessels determine the SOC. That provides a reliable indicator of charge." 

Type of measurement

The speed of measurement is not the most important criterion when selecting a measurement method. Measuring once or twice a minute is more than sufficient for this application, Goosen knows. "During the pilot phase, we reviewed a lot of solutions. We started with carbon electrodes suspended in water. Then we tested different types of ultrasonic sensors, where condensation proved to prevent reliable readouts. We also experimented with a float measurement, which we used as an underfill and overfill device. That worked in itself, but it doesn't provide real real-time data, so you can't control a process on that."

After some time, the AQUABATTERY team decides to go with radar level measurement. "This type of measurement best fits our application. We then compared different sensors from different suppliers, and then we came to VEGA quite quickly. They supply good sensors at competitive prices. Based on our specifications, we were recommended the VEGAPULS C11 from the BASIC series."

Accurate and simple

For VEGA, the application at AQUABATTERY is relatively simple, says Jeroen Pellicaan, Account Manager at VEGA. "We are dealing with tanks without agitators, where no foaming occurs. Based on these conditions and the customer's other specifications, a BASIC radar level sensor is a smart choice. You then have a very accurate and reliable level measurement for a relatively small investment, which is also extremely easy to use."

One of the advantages of non-contact radar level sensors is that the sensor itself does not come into contact with the medium. Pellicaan: "With this type of solution, you could otherwise still have to deal with crystallization on the lens or contamination in other places. Our sensor is suspended above the liquid and that makes for a maintenance-free solution."

Fast in operation

The sensor was installed by an AQUABATTERY employee himself. "It works very intuitively indeed," Goosen knows. "It's a matter of grabbing the manual and then the sensor is up and running very quickly."

At AQUABATTERY, the data from the level meter is sent through the 4-20 mA output to a PLC and then to the in-house battery management system (BMS). "In this we have programmed an underfill and overfill alarm, so that we never have any surprises." 

Scalable

Goosen is betting on large-scale applications with AQUABATTERY, at solar farms, wind farms and in places where energy is generated locally for "behind the meter" storage. "Our smallest unit will probably soon be the 100 kW for business applications. An application as a home battery for households is too small for us."

AQUABATTERY's units are highly scalable. "It's really a matter of adding more barrels to the application and you can move on again. It's also possible to use larger barrels." 

The big advantage of being able to store energy locally and use it again at a later time is that you don't have to use the overloaded power grid. Nevertheless, Goosen is also realistic: "You're not going to make it with battery storage alone, but as part of the big picture, local storage is a very good idea." 

AQUABATTERY's units are expected to be commercially available starting in 2026.