October-November Special Focus: Energy Efficiency, Buildings and the Electric Grid
An increasing fraction of the most efficient appliances on the market operate on direct current (DC) internally, making it possible to use DC from renewable energy systems directly and avoid the losses inherent in converting power to alternating current (AC) and back, as is current practice. Products are also emerging on the commercial market that take advantage of that possibility.
Lawrence Berkeley National Laboratory researchers Vagelis Vossos, Karina Garbesi, and Hongxia Shen investigated the potential savings of direct-DC power distribution in net-metered residences with on-site photovoltaics (PV) by modeling the net power draw of the 'direct-DC house' with respect to today's typical net-metered AC-house configuration. Both houses were assumed to have identical DC-internal loads based on an analysis of 32 electricity end-uses, all of which were found to be DC-compatible.
Model comparisons were run for 14 representative cities across the United States, using hourly, simulated PV-system output and residential loads. The modeling tested the effects of climate, load shifting, and battery storage, as well as considered partial load conditions. A sensitivity analysis determined how future changes in power system component efficiencies might affect potential energy savings.
Results showed that net-metered PV residences without storage could save 5 percent of their total electricity load by using DC-internal appliances, and those with battery storage could save 14 percent of their total load. The residence without battery storage would achieve only a modest savings because the time of peak PV production (midday) does not coincide with the peak residential load (late afternoon/evening). However, residential PV systems incorporating battery storage could achieve much higher savings because the system can both save and use the generated power in DC form.
The project also found that direct-DC energy savings are sensitive to power system and appliance conversion efficiencies, but that they are not significantly influenced by climate.
V. Vossos, K. Garbesi, H. Shen. "Energy Savings from Direct-DC in U.S. Residential Buildings." Energy and Buildings. 68 (2014),pp. 223-231.