The biggest downside of renewable energy is that we can’t control when the sun will shine or winds will blow. That factor alone ensure that renewable energy cannot supply all of our energy needs at the moment. Of course, there are other types of renewable generation that are more predictable, tidal energy and hydropower for example, but we don’t yet have the technology to exploit tidal power to its maximum potential, and hydropower needs specific geographical features, and not enough of them exist to supply our energy. It’s not just on an hour by hour, or day by day basis that renewables need assistance, it is also season by season. We use the majority of our energy to heat our homes in winter, the time of year when we see least of the sun. In the summer, when lots of countries need air conditioning, there is less wind. One solution for both of these problems is storage. There’s been a lot of work done on storage for the shorter term, but until now, storage over longer periods hasn’t really been spoken about too much.
Using molten salts is one technique that has been investigated to store excess renewable energy, but that was fir shorter term use. Now, scientists have developed a method of “freezing” the energy for use months later. The new “freeze-thaw battery” was designed by scientists at the Department of Energy’s Pacific Northwest National Laboratory. They have developed a prototype that is about the size of a hockey puck.
Initially, the battery is heated to 180oC turning the molten salt electrolyte to liquid and allowing ions to flow through, creating chemical energy. The battery is then cooled to room temperature, turning the electrolyte into a solid, freezing the energy-carrying ions in place and locking in the energy. To release the energy just requires the battery to be heated again. The freeze-thaw concept dodges the battery idle self-discharge problem and retained 92 percent of its capacity over 12 weeks. The team used abundant and easy-to-source materials - the anode and cathode are solid plates of aluminum and nickel, respectively. Sulphur was also added to the electrolyte to enhance the battery’s energy capacity.
The battery is able to use a fiberglass separator between the anode and the cathode because of the battery’s stable chemistry. Most higher-temperature molten-salt batteries use a ceramic separator, which can be more expensive to make and is susceptible to breakage during the freeze-thaw cycle.
The battery’s energy is stored at a materials cost of about $23 per kilowatt hour, which was measured before a recent jump in the cost of nickel. The team is now exploring the use of iron, to try bring the materials cost down to around $6 per kilowatt hour, roughly 15 times less than the materials cost of today’s lithium-ion batteries. The battery’s theoretical energy density is 260 watt-hours per kilogram—higher than today’s lead-acid and flow batteries.
The work by was published online March 23 in Cell Reports Physical Science. Other authors of the paper include PNNL researchers Evgueni Polikarpov, Nathan Canfield, Mark Engelhard, J. Mark Weller and David Reed, and former PNNL scientist Xiaowen Zhan. Battelle, which operates PNNL, has filed for a patent on the technology.