The jury is still out on this one. A new report - World Flywheel UPS Markets - has just been published on the subject, looking at the global market and sales of flywheel energy storage systems within power quality applications as backup devices. Flywheel are great in principal but there are some barriers to overcome when used as part of a power continuity plan. The primary weaknesses are the higher capital cost and low runtime available. The first means that the devices are only really suitable for large applications of several hundred kVA or more - (with uninterruptible power supplies such as the Riello Master Plus UPS). Secondly, they have to compete as a substitute product against sealed lead acid batteries which can offer minutes and hours of runtime rather than just several milliseconds. Sure, flywheels do have a compact footprint and offer a ‘green’ technology but their benefits only really accrue over several years use - think battery replacement and disposal. So, my view is that if the flywheel industry finds some way of cost-effectively extending the runtime available and/or lowering the entry barrier costs, they may be onto a more competitive winner.