DC picogrids: A case for local energy storage for uninterrupted power to DC appliances

Abstract

An increasing number of appliances now operate on DC and providing uninterrupted power supply (UPS) to them through outages requires two conversions: first from an energy store, typically a DC battery, to AC mains and then from AC mains to the DC input required by the appliance. The energy storage and DC-to-AC inversion are usually centrally located and tied to existing AC distribution lines to amortize costs and battery capacity. In this paper, we argue that adding energy storage locally to each DC appliance and managing it intelligently can lead to higher efficiency and lower average cost. We term this topology a DC picogrid as it mimics a scaled down independent microgrid. Our contribution is the design and evaluation of a smart picogrid controller that a) identifies the power source and b) decides on battery charging or discharging based on the power source. As we expect DC picogrids to co-exist with AC UPSes, we must ensure that the DC picogrid does not draw power from the UPS's battery but charges from the macrogrid when available. To accomplish this, we exploit the fact that AC distribution from the macrogrid exhibits sufficiently distinct characteristics compared to an AC UPS or a diesel generator. Our picogrid controller uses a Hidden Markov Model for state estimation that uses temporally correlated fluctuations in line voltage and frequency for discrimination. We show through data from four settings that the controller can identify its supply source with over 90% accuracy, and that efficiency recovered from conversion losses could result in 30% reduction in energy consumption. © 2013 ACM.