DC power distribution systems make more efficient use of PV power when powering electronics that run on DC by eliminating transformer Losses.
LBNL has investigated DC direct use in residential buildings. They modeled a house with loads that could be served by a DC power supply such as computers, consumer electronics, and home entertainment equipment; LED lighting, and use of ECPM or brushless DC motors in heat pumps used for space heating and cooling, hot water heat pumps, and/or furnace ventilation fans. They found that residential loads have poor coincidence with the output of a PV solar system as they tend to peak in the late afternoon. Direct DC would account for an offset of only 5% of the residential loads, increasing to 13.5% given the provision of 10 kWh of storage. In the storage scenario, the roundtrip efficiency of the battery storage (81%) more than offset any savings from AC to DC conversion efficiency reductions that have been estimated at 3% to 5% in several studies (Vossos, 2013).Commercial building studies indicate that about 13% of their electrical energy consumption is due to distributing and converting energy into the proper form for powering equipment. Studies also found that AC to DC conversion at the building level coupled with a DC distribution system results in increased energy use. This is due to DC wiring losses (wiring costs would increase by an "order of magnitude" if conductors large enough to limit these losses were to be installed). The creation of DC distribution "islands" has been proposed to concentrate loads and minimize wiring runs while eliminating the need for repetitive power conversion steps. Finally, researchers estimate that DC distribution could save 8% of a PV-powered zero net energy (ZNE) building's electrical energy use as the availability of DC power allows DC-to-AC-back to-DC conversion losses to be eliminated. Costs increase if both AC and DC distribution systems are required (Porter, 2014).
Status:
Baseline Description: Conventional AC Distribution System
This is a building strategy. There is no manufacturer that provides DC building distribution systems. This is also not a retrofit technology as it might best be applied to new zero net energy construction.
"Typical" Savings: 8% Energy Savings Reliability: 1 - Concept not validated
Studies indicate that energy consumption of a ZNE commercial building can be reduced by about 8% given direct use of PV-system provided DC power. (Note that energy use is low in a ZNE building due to building design and selection of appliances, so this is 8% of a small value and essentially means that the PV system size can be slightly reduced). Energy use would increase if an AC to DC converter was to be installed with only DC energy distributed throughout the building.
Simple payback, new construction (years): N/A
Simple payback, retrofit (years): N/A
Cost Effectiveness is calculated using baseline energy use, best estimate of typical energy savings, and first cost. It does not account for factors such as impacts on O&M costs (which could be significant if product life is greatly extended) or savings of non-electric fuels such as natural gas. Actual overall cost effectiveness could be significantly different based on these other factors.
Vagelis Vossos, 01/09/2013. Energy Savings from Direct-DC in U.S. Residential Buildings Energy and Buildings , 68
Suzanne Porter, 01/01/2014. Reviving the War of Currents: Opportunities to Save Energy with DC Distribution in Commercial Buildings 2014 ACEEE Summer Study on Energy Efficiency in Buildings
Technical Advisory Group: 2014 Commercial Building TAG (#9) TAG Ranking: 9 out of 21 Strategies (2014 Commercial TAG technologies ranked separately) Average TAG Rating: 3.13 out of 5 TAG Ranking Date: 03/17/2014 TAG Rating Commentary: Also for VFD, lighting systems and all digital devices This is particularly promising in buildings with onsite energy generation and/or energy storage in the DC realm. If the entire power infrastructure in the building is AC-based, the benefits of this are smaller. Are savings offset by increasing efficiency of power supplies? Limited to new construction as costs too high for retrofit?