Should we dual-purpose energy storage in datacenters for power backup and demand response?

Prior work has shown the benefits of Energy Storage Devices (ESDs), such as batteries, to smoothen/flatten power draws in Datacenters, for reducing demand during peak tariffs (for op-ex savings) and under-provisioning the power infrastructure (for cap-ex savings). Until now, all prior studies for such smoothening, referred to as Demand Response, have considered re-purposing existing UPS unit batteries for demand response. It is not clear if such dual usage - handling power outages and demand response - is the most effective option since the needs (energy and/or power), mandates (best effort vs. hard stipulations), costs, availability and health degradation considerations could be very different. In this paper, we study the design space of choices for provisioning ESDs for these dual purposes - separate ESDs for each purpose, common pool of ESDs for both purposes, and soft-reservations in this pool with possible re-purposing dynamically based on demand. Our evaluations show that: (i) provisioning lead-acid batteries for a peak “power” load needed to handle power outages already comes with sufficient energy capacity that is more than adequate to automatically supply the energy needs for demand response; (ii) this makes it economically attractive to use the same UPS batteries, originally intended for Power Outages, for Demand Response as well, despite any consequent health degradation (due to repeated discharges); (iii) the ability to handle the needs during a power outage is not compromised despite the dual-purposing of these UPS batteries; and (iv) the non-orthogonality of the power and energy capacities of these batteries (i.e. provisioning for the high power needs during an outage automatically comes with a lot of energy capacity) suggests the possibility of having different Energy Storage Technologies for the two purposes and we show that a heterogeneous/hybrid option using Ultra-capacitors or Flywheels for Power Backup and batteries for Demand Response is a more cost-effective option.