Energy Storage in Datacenters: What, Where, and How much

Energy storage- in the form of UPS units- in a datacenter has been primarily used to fail-over to diesel generators upon power outages. There has been recent interest in using these Energy Storage Devices (ESDs) for demand-response (DR) to either shift peak demand away from high tariff periods, or to shave demand allowing aggressive under-provisioning of the power infrastructure. All such prior work has only con-sidered a single/specific type of ESD (typically re-chargeable lead-acid batteries), and has only employed them at a sin-gle level of the power delivery network. Continuing tech-nological advances have provided us a plethora of competi-tive ESD options ranging from ultra-capacitors, to different kinds of batteries, flywheels and even compressed air-based storage. These ESDs offer very different trade-offs between their power and energy costs, densities, lifetimes, and energy efficiency, among other factors, suggesting that employing hybrid combinations of these may allow more effective DR than with a single technology. Furthermore, ESDs can be placed at different, and possibly multiple, levels of the power delivery hierarchy with different associated trade-offs. To our knowledge, no prior work has studied the extensive de-sign space involving multiple ESD technology provisioning and placement options. This paper intends to fill this criti-cal void, by presenting a theoretical framework for capturing important characteristics of different ESD technologies, the trade-offs of placing them at different levels of the power hi-erarchy, and quantifying the resulting cost-benefit trade-offs as a function of workload properties