Startup Mixergy has caught a significant break this week, as Centrica has announced it will be using the firm’s platform to control residential hot water tanks as part of a Firm Frequency Response (FFR) project in the UK. This is another wrinkle in the tale of using water tanks as battery capacity, but one that might finally have the proper legs to take off. The water tanks are being aggregated as part of a larger Virtual Power Plant (VPP), which is the term used to describe a collection of assets that are treated like a grid-scale power generation appliance. In this case, it’s actually a bunch of hot water tanks in homes, which can fire up their heating elements as…