DC electricity networks

The Energy Saving Trust estimated in 2007 that by 2020, 45% of the electricity consumption in a household would be “entertainment, computers and gadgets and LED lighting” [1] – all DC-powered (Direct Current).

Greg Reed, director of the Power & Energy Initiative at the University of Pittsburgh is quoted in Technology Review saying that “Within the next 20 years we could definitely see as much as 50 percent of our total loads be made up of DC consumption... It’s accelerating even more than we’d expected.” [2]

Solar panels produce DC power and have to be coupled with an inverter to feed into the mains, costing easily 10-15% of the generated power in the process. [3, 4] So why not use the DC directly in devices that use DC anyway?

Some server farms have now started investing in DC distribution systems. Siemens is currently running an EU-supported research project called “DC Components and Grid” (DCC+G) [5]

Adding to the trend is the increasing prevalence of electric cars, most of which currently have transformers to charge on AC (Alternating Current) but can most quickly and efficiently be charged with DC electricity.

External AC to DC adapters (otherwise known as 'power supply units' or PSUs) have become a lot more efficient and smaller with the advent of switched-mode operation (they switch the mains power on and off thousands of times per second rather than using big coils to change voltages at set ratios). Since 2005 there is an internationally agreed efficiency rating scheme for external power supplies [6] resulting in Roman numerals shown inconspicuously on the PSU, with older (but already switching) units typically rated III or IV, but today’s units most often carrying a V. Ratings of VI or higher are likely to be reserved for future use.

Lighting has gone from tungsten GLS bulbs (General Lighting Service) that ran on AC (though technically they operate as well on DC) via Halogens based on the same principle, to Compact Fluorescent Lamps (CFLs) and Light-Emitting Diodes (LEDs). LEDs run on DC and require a transformer when connected to AC power. CFLs are a bit more complicated – they use a ballast (the plastic bottom part the CFL) that converts mains AC into DC and then back into much higher frequency AC (in the kHz range). The BRE study on DC distribution from 2002 found that savings of around 4% could be achieved by skipping the first part of that. [3]

High power home appliances (such as freezers, washing machines, dishwashers, kettles, toasters, blenders etc.) still use AC without transformation. But more powerful and efficient DC motors are being developed, and DC fridge-freezers already exist.

In 2002, BRE was quite sceptical as to the potential of using DC power from solar panels directly. But the fully DC-powered energetically self-sufficient home may warrant a fresh look. Moshe Kinn wrote an MPhil Thesis at the University of Manchester in 2011 on the topic, which may be a good starting point. [7]

A big concern is the possible length of cable with higher transmission losses at lower voltages. But if it makes sense for a server farm, it could well make sense for a home.

[edit] Related articles on Designing Buildings Wiki

[edit] External references (all web resources last accessed on 2014-02-21)

[1] The ampere strikes back: How consumer electronics are taking over the world, Energy Saving Trust 2007
[2] www.technologyreview.com/news/427504/edisons-revenge-the-rise-of-dc-power/
[3] BRE for DTI 2002: The use of direct current output from PV systems in buildings:
webarchive.nationalarchives.gov.uk/+/http://www.berr.gov.uk/files/file17277.pdf
[4] www.solar-facts.com/inverters/inverter-efficiency.php
[5] siemens
[6] International agreement document from 2005 showing draft implementation timelines, published by the Power Sources Manufacturers Association (PSMA):
www.psma.com/ul_files/forums/energy/July2005EPSInternationalEfficiencyMarkingProtocol_rev2.pdf The latest available version of the US Environmental Protection Agency’s ENERGY STAR implementation:
[7] www.dcisthefuture.org/papers