New Technique Gives Old Batteries a Second Life

As we make the switch to renewable energy, batteries have a huge part to play in the speed of the transition, and also its success. At the moment, scientists across the globe are trying to find new battery chemistries that will store excess energy until it is needed by the grid. But there are also other ways to make the storage we have operate more efficiently, either during storage, or when converting the energy back to AC to feed back into the grid.

At the moment, a normal storage system would consist of a high-voltage battery pack and a central inverter. The battery pack is made from several permanently interconnected modules, and often can feature dangerous voltages of up to 800 V. The central inverter is responsible for converting the stored voltage into an AC voltage using pulse width modulation to switch the high voltage of the power storage unit on and off. The resulting voltage is then smoothed by a grid filter so that it complies with the standards and requirements of the electricity grid. This method works well, but it can be inefficient with relatively high conversion losses.

Technical University of Munich spin-out, STABL Energy has been working on a different way to convert the voltage to AC. The company’s "modular multilevel inverter" approach is already widely used in high-voltage DC transmission. STABL has taken the technique and optimised it for stationary battery storage systems. Instead of using static module connections, the company connects them dynamically giving power storage that consists of safe battery modules with voltages of less than 60 V. To generate the voltage, the individual battery modules are connected in series one after the other. The step voltage generated by this method resembles the desired mains voltage much more closely than traditional inverters, allowing the use of a smaller grid filter and minimising losses. The technique makes the central inverter unnecessary.

The company’s modular approach simplifies the electricity storage system design and enables a low-cost sustainable energy supply that reduces energy losses by up to 70 %. It also save up to 40 % of the system’s operating costs, while having nearly the same initial outlay. Another large advantage of the STABL system is that it can use second life batteries with different residual capacities to operate. Usually battery storage systems use battery modules of the same series, with the same internal resistances and health state.

The company’s first pilot systems have been put into operation in Germany and Switzerland using MOSFETs from Infineon and retired passenger car batteries. The storage units have passed CE and grid connection standards for Germany and are connected to the German grid, a first unit in Switzerland has also been connected. The Swiss installation uses 27 discarded KIA batteries with a nominal capacity of 98 kWh to serve a residential neighbourhood as temporary storage for solar energy. The batteries were discarded as they no longer had the capacity to be used in vehicles.

The basic electronic components used are 100V OptiMOS 5 MOSFETs from Infineon. STABL Energy chose Infineon because of the low on-resistance, as well as high thermal performance and product reliability.