Investors recently chipped in $15 million to fund battery startup EOS Energy Storage, a company that says its batteries could eventually compete with natural-gas power plants to provide power during times of peak demand.
Cheap energy storage is becoming increasingly important as greater numbers of wind turbines and solar panels are added to the grid. If renewable energy is to replace the fossil fuels that dominate power supplies and serve to backup wind turbines and solar panels, very large-scale, inexpensive batteries like the ones EOS is developing will be needed (see “Wind Turbines, Battery Included, Can Keep Power Supplies Stable,” “Battery Could Provide a Cheap Way to Store Solar Power,” and “A Solution to Solar Power Intermittency”).
EOS is trying to commercialize a type of battery that’s based on inexpensive materials: water, zinc, and air (see “Startup Promises a Revolutionary Grid Battery” and “Years in the Making, Promising Rechargeable Metal-Air Batteries Head to Market”). Such batteries—in which zinc reacts with oxygen in air to generate electricity—have been around for a long time, but it’s been difficult to make them rechargeable. Electrodes deteriorate, for example, and the batteries are inherently inefficient because of the difference in voltage levels when charging and discharging them—they waste nearly half the energy it takes to charge them.
EOS has addressed these issues in a couple of ways. It uses a slightly acidic water-based electrolyte that helps prevent deformations of the zinc electrode that can damage the battery.
The company is also supplementing the zinc-oxygen reaction with reactions between zinc and a mixture of up to six other materials (it won’t identify the type of compounds). The other reactions help reduce the difference between charge and discharge voltages, improving the efficiency from 60 percent to almost 75 percent. The mixture of reactions makes the battery more difficult to operate, but George Adamson, vice president of R&D, says that today’s battery management software is up to the task.
The decision to make use of these extra reactions was the result of a bit of serendipity. Impurities were causing unwanted side reactions in the original zinc-air prototypes. But then the researchers noticed the beneficial impact on voltage. “Once we realized that,” Adamson says, “we started searching on purpose for multiple combinations of reactions.”
EOS has built a two-kilowatt prototype. Eventually, its batteries will be packaged inside a shipping container to make one-megawatt batteries than can store six megawatt-hours of electricity, enough to power a typical U.S. home for six months. It plans to build a pilot manufacturing plant by the end of the year or early next year, and to start making full-size one-megawatt batteries by the end of 2014.
EOS wants to produce batteries that cost as little as $160 per kilowatt-hour and last for 30 years. Current batteries that cheap would fail after only a couple of years of service. The U.S. Department of Energy has set a goal of batteries at $100 per kilowatt-hour that can be recharged 5,000 times with 80 percent efficiency, saying that at that point batteries could be widely adopted for grid storage. EOS says its batteries can last 10,000 charges, which could make up for the higher upfront cost and lower efficiency of its batteries.
But the company hasn’t reached its goals yet. It says it’s “well within” $300 per kilowatt-hour. EOS has completely charged and discharged the most recent iteration of its battery cells over 1,000 times, and the batteries have so far retained 90 percent of their capacity. Typically, batteries are designed to retain 80 percent of their capacity at the end of their life, so the current rate of capacity loss is too fast for a 10,000-cycle battery.
But, Adamson says, much of the capacity loss is from electrolyte levels falling too low. In one experiment, topping off the batteries restored capacity from 80 percent to 96 percent of the original capacity. Manufactured batteries will come with a mechanism for automatically topping off the electrolyte, which could improve the durability of the system.
EOS says it’s teaming up with seven utility companies to test the battery and design it to the performance specifications they need—it will announce the partners in the next couple of weeks.