Based on the blue energy principle, where mixing of salt and fresh water generates energy, ETE scientist Jan Willem van Egmond was able to build a working battery unit where electricity could be stored and retrieved using salt gradients.
‘This is our newest design of a concept to store energy’, says van Egmond. He is holding a solid, silvery looking piece of plexiglas with holes and milled canals. ‘We will test this prototype of our ‘blue battery’ during the coming months.’ Van Egmond is working on a sustainable technology for large scale electricity storage. Eventually, this could make wind and solar energy a lot more attractive, since it can store surplus energy generated by these renewable energy sources.
Sustainable large-scale energy storage is a top priority as the energy supply slowly changes from fossil fuels to more renewable energy based on biomass, wind or sun. In 2014, about 12.5% of the EU’s energy consumption originated from renewable resources. The EU aims to increase this share to 20% by 2020. But energy generated from wind or sun can be rather unpredictable. Therefore, one of the issues that needs to be resolved is electrical energy storage to cover periods with low energy generation due to the absence of sun or wind. ‘A widely applicable safe and sustainable technology to store energy peaks to compensate for low energy output is still lacking’, van Egmond states. ‘To generate substantial amounts of renewable electricity and become more independent from fossil fuels, big-scale energy storage is fundamental.’
Several systems to store energy are currently in use, for example the well-known lithium-ion batteries used in many consumer products. Large scale electricity storage using lithium-ion batteries is not a good alternative though: these batteries are expensive and relatively unsafe. In addition, they contain small amounts of rare materials, making recycling expensive. For this reason, about 99% of all large-scale energy storage in the world is installed in elevated water reservoirs. During peak hours, water is pumped to higher elevation using excess electricity. When energy production is insufficient, electricity is recovered by water flowing down through a turbine. However, for this system sufficient elevation is required, making its application relatively limited. ‘We actually need a sustainable energy storing system that is environmentally safe, cost-effective and applicable in flat terrain’, says van Egmond. ‘I think our concept of ‘blue battery’ does the job!’
Fresh and salt water are separated by these membranes, creating such salinity gradients. At times when surplus energy is available, electrical energy is used to migrate salt ions against the salinity gradient from a low to high salinity (fig. 1) and the battery is charged. At times when additional electricity is required, the process is reversed by mixing fresh and salt water, generating electricity and discharging the battery. So, reservoirs of fresh and salt water potentially contain energy and may serve as environmentally friendly energy storage. ‘This same principle is used since the 1960’s to make fresh drinking water from sea water’, explains van Egmond. ‘We modified an old idea into a modern way to store large amounts of energy.’
As always, the whole project started with a good idea, followed by designing and testing different battery models. After trial and error, van Egmond managed to design a working concept. Now he is strongly focused on improving the process and increasing the efficiency of the system. Better membranes, different salt solutions in different concentrations and a range of temperatures were tested. Step by step, this resulted in a more efficient battery. ‘I expect big improvements in efficiency and storage capacity in the very near future’, the scientist says. ‘The ultimate goal would be to install blue batteries near residential areas. The size of an Olympic swimming pool is sufficient to sustain about 200 houses.’ According to van Egmond the water-based blue battery will be a very competitive system for energy storage due to its low costs, safety and environmentally friendly design, based on water.
This research is part of the blue energy theme, supported by A.Hak, Alliander, AquaBattery, Fujifilm, Landustrie/Desah, REDstack, W&F Technologies.
Key publication: Van Egmond et al. 2017. The concentration gradient flow battery as electricity storage system: Technology potential and energy dissipation. The Journal of Power Sources 325: 129-139.