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DC proponents look to home and data centres for energy efficiency

Image credit: Tom Chance

Beaten by AC in the early days of electrical distribution, DC’s time is coming.

What do you do when you find a financial crash has left your shiny new data centre empty? In Intel’s case, it provided the opportunity to find out whether a move from AC to DC distribution might save on its electricity costs. When bundles of sub-prime loans imploded and took the economy with them in 2008, the world’s largest chipmaker found it no longer had an immediate use for a new data centre built to support manufacturing at its plant west of Portland, Oregon.

Working with HP and Emerson Network Power, Intel reworked the 5MW electrical distribution network at the data centre to handle 380V of DC instead of the regular AC. The project followed on from earlier experiments at the Lawrence Berkeley National Laboratory (LBNL), 1,000km south of the Intel fab, which found that a shift to DC could improve energy efficiency by close to 30 per cent.

One of the reasons for the improvement was the elimination of the need to invert power provided through an uninterruptible power supply (UPS) system, many of which convert from AC to DC and back to AC. This reduces the time it takes to switch to battery power if the mains supply fails. Not having to invert to AC provided large savings. Since those studies in the mid-2000s, however, UPS designers have addressed some of the problems caused by double conversion by shifting to techniques that divert most of the energy around the double-conversion circuitry.

There is a further conversion from AC to DC inside the server, but this often feeds into multiple DC stages in any case from 12V to 48V on the backplane, down to less than 1V by the time the power reaches the processors and memories. Removing the rectifier and power-factor correction circuitry from the front-end AC power supply in the server can save several per cent. Designers can take advantage of greater flexibility in the DC input, potentially with some loss of efficiency. The advantage is that the UPS does not have to provide fully regulated 380V DC. As it runs during an outage the weaker regulation will help to eke more out of the battery as it drains.

The Intel project found that the core improvement from a shift from the 415V AC used in the US to 380V DC was up to 8 per cent. A study by Duke Energy suggested this could be pushed to 10 per cent. Proponents of 380V DC claim there are further benefits for data-centre owners. DC could help them pack more servers into the same space.

Intel found its DC data centre’s footprint was a third smaller than its AC predecessors. This could cut cable cost as the elimination of the skin effect meant the company could use thinner copper wiring than for AC distribution.

ABB claimed a 1MW 380V DC network it built for Swiss IT company Green in 2012 had 15 per cent lower capital costs than an equivalent AC system as well as being 10 per cent more efficient. But against this, at least in the early days of DC data centres, owners may have to balance increased cost of the actual servers.

The switch will not be easy. As most computer hardware is designed for an AC infrastructure, moving to DC will add cost because the specialised front-ends will not have the same economies of scale. Bigger data-centre operators will have an advantage in the short term because they have already moved to custom server and blade designs that are now making it to market through the hardware plans disseminated by the Open Compute Project (OCP). But there is a significant cost overhead for many to replace standard 240V AC equipment with bespoke versions that can take a DC input.

Although data-centre and light-industrial users continue to grapple with the problem of introducing DC into their buildings, the home may offer an easier path for realising the efficiency gains of moving away from AC.

Going local

As noted in Nick Carr’s book on Google, ‘The Big Switch’, Thomas Edison’s view of how electricity would be used was way ahead of his time. He saw electricity generation as being a local phenomenon, not something that would be performed at a distance in monumental power stations. A shift to local micro-generation through renewables, is beginning to realise some of Edison’s original vision, though not in the way he envisaged.

Despite the issues posed by the need to synchronise AC systems, the electricity distribution network looks likely to be the last stage to see DC introduced, because of the amount of infrastructure that is designed around AC. It is theoretically possible to reuse much of the cabling in distribution networks but so much else would need to be changed. There is much greater potential within housing or industrial estates.

One option at the estate level is to split the grid. Maintain access to the AC distribution network but introduce DC ‘microgrids’ that can operate at least for some of the time independently of the main supply grid. Although developed primarily for apartment blocks and estates as well as offices, retail parks and factories, the microgrid can potentially scale down to the individual house.

One component will be common to all microgrids - a battery. Moixa Technology’s Maslow is one example of how home microgrids could develop around the ability to store electrical energy.

Moixa’s choice of name derives from an acronym for ‘meter-attached storage low voltage’ but also refers to Abraham Maslow’s model of human needs. CTO Chris Wright says: “In the home, around electricity there is a hierarchy of needs. For a third of the installations we have done, there is a DC lighting circuit that stays on in the case of a power cut and some low-voltage things such as phone chargers. If there is a power cut, there are things you would want to stay on. If you can’t do your washing for an hour or so that’s OK. But sitting in the dark: that’s irritating.”

The battery in Maslow captures energy from a solar panel on the roof providing the residents with electricity even if the mains goes out. And if they are using DC lighting, the lights will stay on. Wright says social landlords find the system attractive, particularly those dealing with vulnerable people.

“They have a duty of care. If they can do something where the lights don’t go out if they have a power cut that’s a big advantage,” Wright says.

Avoid the feed-in

The nature of the feed-in tariff scheme in the UK, and its withdrawal, helps justify the move to a storage system like Maslow, Wright says. Initially, because feed-in rebates were calculated on generation and not export, it made sense for homeowners to use the electricity their solar installations generated. “Now that the feed-in tariff has more or less gone away, what you have left that has value is the electricity being generated,” Wright says. However, without storage users could not access all that energy. “Having storage connected enables much higher levels of self-consumption. And that has a lot of value.”

So far, more than 350 Maslows have been installed, with about a third supplying DC power and the majority feeding into conventional AC ring mains. The DC power reuses the ring mains normally set aside for lighting to avoid having to rewire the house.

The DC ‘nanogrid’ supported by a system like Maslow is not enough for the home. Cookers and other high-power appliances will still need an AC supply. As most homes have multiple mains supply rings, those allocated to lighting and sockets can be redeployed to DC, with a couple set aside for high-voltage appliances. To avoid rewiring the house, Moixa has focused on retrofitting existing cables to suit DC, which increases the potential for confusion between AC and DC systems. Clear labelling is vital to avoid the situation where electricians try to use the wrong tests but Wright says a training course on working with the DC system only needs half a day.

The DC voltage varies from 20 to 30W depending on the charge status of the battery. “We are looking at 48V but there aren’t very many LEDs that run at 48V DC,” Wright says, but adds that the losses of the lower voltage through existing wiring are neutral compared to the situation for incandescents. “LEDs are running at a tenth of the voltage but a tenth of the wattage of incandescent. So you end with much the same losses as when you started.“

In the Maslow DC installations, power to consumer gadgets is provided through smart USB sockets that convert the 30V supply down to the 12V expected by most portable devices today. Capable of delivering 100W at 20V, the latest version of USB, Type-C, will help push the amount of power that can be supplied through the cable higher.

“The adoption curve, we think, will be driven by the increasing roll-out of Type C USB. It can supply proper power. Already, Wimpey Homes are putting normal USBs in homes, so it’s not an unusual idea. It’s something that people will see a lot in the future,” Wright points out.

Future homes may not have many three-pin sockets and that may be accelerated by the trend to use high-capacity batteries in appliances such as vacuum cleaners that may be charged using USB Type-C cables.

Back in the data centre, companies are looking at ways to make the most of DC by bringing uninterruptible power supplies (UPS) and the servers together. Designs being promoted through OCP include one from Microsoft that puts the UPS inside each server instead of having centralised units. The Local Energy Storage (LES) places a lithium-ion battery backup alongside the power converter that feeds into the rack’s main power supply. Microsoft claims the distributed UPS design is 80 per cent cheaper than a centralised installation.

DC storage

Wider use of battery storage has the potential to change the way data-centre and light-industrial users think about UPS and electricity distribution. Not having to invert the DC to AC should simplify the installation of large-scale distributed storage technologies. However, Louis Scheffer, who heads Eaton EMEA’s distributed energy management operation, says storage and DC have yet to go hand-in-hand. Most users who are looking to shift from UPS to strategic storage that caches energy from local renewables are sticking with AC.

“We see a lot of discussion of DC and some people are doing bespoke installations but so far it’s not a large business. When we talk to people about their storage requirements, DC has not come up yet,” Scheffer says.

Storage may not drive the adoption of DC alone, but it represents a further source of infrastructural cost reductions that ultimately see a migration away from AC. As organisations become more aware of their energy consumption, more will weigh up including DC in their designs. *

Power networking

Thanks to the cabling that permeates most office buildings, offices do not have to wait for a whole new set of wiring to distribute low-voltage DC. When commercial building users started to turn to Ethernet to support not just computers and printers but security printers and ID-badge readers they wanted a way to deliver power to the them without running additional mains cables.

Spare pairs of copper strands in the eight-strand cables provided the ability to deliver low-voltage DC over Ethernet. Initially, the IEEE standard for Power over Ethernet (PoE) focused on wires that were ignored by lower-speed network adapters, using them to carry 50V DC.

Later versions have extended power delivery to even the data-carrying pairs. Transformers make it possible to combine the data signal with the DC voltage. Because the copper strands in Cat5 and similar cables are so thin, multiple pairs are needed to increase the total current capacity and cut losses from resistance. This is not just important for efficiency but to avoid bundles of cables laying in ceilings and walls from heating up dangerously.

The 24AWG conductors in Cat5 have traditionally been deemed safe to carry up to 360mA at 50V, with a high margin high enough to allow for differences in resistance of various cables. If half of the conductors are used for delivering power, the cables can carry up to 36W. But the voltage drop from a maximum length cable run means that the device at the far end would, in principle, receive just under 32W.

A move to Cat6, which is already in common use to support 100Mbit/s-plus communications, reduces resistance and will make it possible to push maximum power further.

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