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Summary

Distribute DC Power from PV to Electronics

Commercial Building Distribution Systems: DC vs. AC

DC power distribution systems make more efficient use of PV power when powering electronics that run on DC by eliminating transformer Losses.

Synopsis:

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).

Energy Savings: 8%
Energy Savings Rating: Concept not validated  What's this?
LevelStatusDescription
1Concept not validatedClaims of energy savings may not be credible due to lack of documentation or validation by unbiased experts.
2Concept validated:An unbiased expert has validated efficiency concepts through technical review and calculations based on engineering principles.
3Limited assessmentAn unbiased expert has measured technology characteristics and factors of energy use through one or more tests in typical applications with a clear baseline.
4Extensive assessmentAdditional testing in relevant applications and environments has increased knowledge of performance across a broad range of products, applications, and system conditions.
5Comprehensive analysisResults of lab and field tests have been used to develop methods for reliable prediction of performance across the range of intended applications.
6Approved measureProtocols for technology application are established and approved.

Status:

Details

Distribute DC Power from PV to Electronics

Commercial Building Distribution Systems: DC vs. AC

DC power distribution systems make more efficient use of PV power when powering electronics that run on DC by eliminating transformer Losses.
Item ID: 553
Sector: Residential, Commercial
Energy System: Power Systems--Building Power Distribution
Technical Advisory Group: 2014 Commercial Building TAG (#9)
Average TAG Rating: 3.13 out of 5
TAG Ranking Date: 03/17/2014
TAG Rating Commentary:
  1. Also for VFD, lighting systems and all digital devices
  2. 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.
  3. Are savings offset by increasing efficiency of power supplies? Limited to new construction as costs too high for retrofit?

Synopsis:

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).

Baseline Example:

Baseline Description: Conventional AC Distribution System

Comments:

Manufacturer's Energy Savings Claims:

Comments:

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. 

Best Estimate of Energy Savings:

"Typical" Savings: 8%
Energy Savings Reliability: 1 - Concept not validated

Comments:

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.

Energy Use of Emerging Technology:
Currently no data available.
Technical Potential:
Currently no data available.
First Cost: Currently no data available.

Cost Effectiveness:

Simple payback, new construction (years): N/A

Simple payback, retrofit (years): N/A

What's this?

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.

Reference and Citations:

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

Rank & Scores

Distribute DC Power from PV to Electronics

2014 Commercial Building TAG (#9)


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:

  1. Also for VFD, lighting systems and all digital devices
  2. 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.
  3. Are savings offset by increasing efficiency of power supplies? Limited to new construction as costs too high for retrofit?


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