Products: Low-voltage DC power for houses

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- CommentAuthorwookey
- CommentTimeDec 12th 2010
I saw something interesting at HEAT10 conference last week, which I think is worth passing on. A system to supply all-purpose DC power in the home.

It's not launched yet so the website is not very useful, but I saw their pre-production kit and talked to their chief engineer and boss-man and was impressed.

This is potentially disruptive technology. They have solved the perennial problem of a low-voltage DC home network (what voltage to use) by supplying 'all of them'. DC power is distributed at whatever voltage your battery or supply is (e.g. ~24V or ~12V) then each socket converts to the end-device voltage according to the cable you plugged in (using current and volatge-setting resistors in the connector), so you'll get 5V for USB devices, 19V for your laptop, 9V for the phone etc. Very neat.

Their focus is using a house battery (e.g 0.5-1kWh LiFePO4) and small off-grid PV system (that you can just buy at the DIY store and install in a couple of hours) but the DC distribution tech is actually independent of that aspect.

I don't know if it'll be cheap enough to be attractive, but it is pretty neat. Should be ready for launch 'very soon', which I suspect means a small number of months in practice.
http://www.moixaenergy.com/page.asp?pageid=25
- CommentAuthorDamonHD
- CommentTimeDec 13th 2010
What about power loses / cable sizes at the lower voltage?

And dealing with the 5:1 ratio in UK insolation year-round?

(My off-grid battery is ~0.5kWh and I can tell you it's bumping along on empty even when I doubled the panel area more-or-less for this winter.)

In other words, though what you've described sounds very good, I think it's only a tiny piece of the puzzle!

Rgds

Damon
- CommentAuthorevan
- CommentTimeDec 13th 2010 edited
Bah, my idea :)

http://www.greenbuildingforum.co.uk/forum114/comments.php?DiscussionID=5578&page=1
- CommentAuthorSteamyTea
- CommentTimeDec 13th 2010
Evan
That's another fortune you have lost
- CommentAuthorevan
- CommentTimeDec 13th 2010
Tell me about it...
- CommentAuthorSteamyTea
- CommentTimeDec 13th 2010
I have a special bank account full of cash from bright ideas, it started with nothing in it and most of it is left.
- CommentAuthordavidfreeborough
- CommentTimeDec 14th 2010 edited
Posted By: wookeyDC power is distributed at whatever voltage your battery or supply is (e.g. ~24V or ~12V) then each socket converts to the end-device voltage according to the cable you plugged in (using current and volatge-setting resistors in the connector)
What's the advantage of this over converting 240V AC to whatever DC voltage is required by the device?

Haven't they just integrated a multi-voltage DC-DC converter into the socket? Is this any more efficient than using the AC-DC converter shipped with the device?

Are any efficiency gains enough to compensate for the additional losses due to battery leakage & low voltage distribution? Or is it intended for use with non-grid connected photovoltaics?

David
- CommentAuthorDamonHD
- CommentTimeDec 14th 2010
Staying on LV DC does have an advantage with an off-grid PV/wind system.

Rgds

Damon
- CommentAuthorSteamyTea
- CommentTimeDec 14th 2010
May be worth mentioning that what we think as low voltage and the electrical industry thinks of as low voltage are not the same.

Low Voltage is between 50 and 1000Vrms for AC and 120 to 1500V for DC.

Extra Low voltage, which is between 0 and 50Vrms is what we are on about here and this comes in 3 flavours SELV, PELV and FELV. Stick with SELV (separated Extra Low Voltage) if you can.
- CommentAuthorrhamdu
- CommentTimeDec 14th 2010
Technical language is indeed sometimes a bit pedantic.

For example what we think of as tea is <100C and what the steam industry thinks of as steam is >100C. So technically there is no such thing as steamy tea.
- CommentAuthorCWatters
- CommentTimeDec 14th 2010
Understanding LV vs ELV is important when you read Part P of the Building Regs.

Installing your own ELV DC light (eg 12V) can be notifiable whereas installing the LV equivalent (240V) isn't :-)
- CommentAuthorSteamyTea
- CommentTimeDec 15th 2010
Posted By: rhamduno such thing as steamy tea.
- CommentAuthorrhamdu
- CommentTimeDec 15th 2010
Posted By: CWattersUnderstanding LV vs ELV is important when you read Part P of the Building Regs.

Sure. Those are the definitions in that context, and they are clearly stated there. Also in BS7671.

But everyday English isn't ruled by building regs or British standards. When a client calls an electrician because 'my low voltage lights have failed', the electrician should turn up with replacement 12V bulbs and transformers.

As a former professional editor (dedicated to wiping out jargon), retraining as an electrician (required to learn and use jargon) I try to see this issue from both sides.
- CommentAuthorDamonHD
- CommentTimeDec 15th 2010
Indeed. Jargon is not usually about elitism but rather for conveying facts accurately and succinctly amongst experts.

(Apparently I'm not an expert given that ST introduced me to the SELV terminology for the first time, which is what my off-grid system is, basically, except possibly where the mains-powered ADSL modem hits my off-grid server over Ethernet, and explains why my 'leccy Part-P friends suggested *not* grounding it anywhere.)

Rgds

Damon
- CommentAuthorwookey
- CommentTimeDec 15th 2010
Rather than write a long answer here I suggest you go read the corresponding thread over on the Navitron forum:
http://www.navitron.org.uk/forum/index.php/topic,12372.0.html

But just quickly: David, yes these DC/DC converters are much more efficient than the AC->DC converters that came with your various devices. Typically 90% vs 60%, although there are exceptions. Clearly if you _also_ can save the inverter losses from DC up to AC in the first place then it really starts to make sense, but few of us are off-grid.

Damon - on a household scale, with low-power devices, cable losses can be very small. See navitron thread for a load of calcs :-)
- CommentAuthorDamonHD
- CommentTimeDec 15th 2010
Sure, I run a little off-grid system myself, but even for my very low power system and with hugely over-rated cables I estimate cable losses of well over 1%: how tempting will it be to 'save' on 'expensive' cabling.

PeopleWhoShouldKnow[citation needed] often run their systems at 48V to stay within low-voltage regs but minimise cabling costs/losses.

Rgds

Damon
- CommentAuthorSteamyTea
- CommentTimeDec 15th 2010
Lets stop speculating

R=p l A^-1
Where R= Resistivity in Ohms
p (rho)= ohms per metre
l= length in metres
A=area of cable in m^2

p for copper is typically 1.68Ã—10^âˆ’8

Can work it all out now
- CommentAuthorDamonHD
- CommentTimeDec 15th 2010 edited
At 1/20th the voltage (12V vs 240V) you'll need 20 times the metal to get the same proportional losses.

It rules out big appliances entirely: don't try to run your 12V 2kW washing machine (ours uses that when heating water or at the peak of its spin) on the other side of your house from the batteries, for example, unless you have a huge busbar running between them. Oh gosh, no there probably aren't any 12V 2kW washing machines, you're right. B^>

I observe fractions-of-a-volt drop in my cabling, even for low-power applications, which is insignificant at 240V but non-trivial at 12V. Indeed I use 13A mains wiring (eg good for loads of 3kW) for loads of a few W at 12V and even then the losses are noticable even if not huge.

And handling peak battery charging currents with >200Wp of off-grid solar on the far side of my house from the batteries is non-trivial too, eg in winter when there is least time for charging and least energy to be wasted. The fattest wiring I've ever handled.

Rgds

Damon
- CommentAuthorbrig001
- CommentTimeDec 15th 2010
Or you could just look the volt drop per metre up in the regs (or here http://www.tlc-direct.co.uk/Figures/Tab4.7.htm ) for the current you are using and multiply the volt drop per meter by the length in metres and by the current to get "wasted" power. This is of course heat, which isn't necessarily waste.

On a brighter note, I'm glad you are back SteamyTea - I heard a rumour that you were no more!
- CommentAuthorDamonHD
- CommentTimeDec 15th 2010
ST can't exist in the same way that bumblebees cannot fly! %-P

Rgds

Damon
- CommentAuthorfostertom
- CommentTimeDec 15th 2010
and interseasonal solar heat storage into uninsulated ground cannot work, and MF insulation, and homeopathy, and dowsing, and and ....

When anecdotal and exemplar evidence persistently contradict 'scientific method', expect 'science' to produce a 'paradigm shift' and then claim it was right all along (it was just the 'old' scientists who were wrong).

Bumblebees flying ... trout launching themselves up waterfalls from standing start ... Schauberger's SS-sponsored 'Trout Turbine' anti-gravity machine finally took off and destroyed itself on the last day of WW2 (luckily for us) ... 3 days later well-informed US scientific snatch squad grabbed the lot (in deepest Czech-land), knowledge suppressed ever since (except for use in e.g. Stealth planes which in theory shouldn't fly esp at their actual performance).
- CommentAuthorPaul in Montreal
- CommentTimeDec 15th 2010 edited
Posted By: fostertomexcept for use in e.g. Stealth planes which in theory shouldn't fly esp at their actual performance
Hmm, I know a few people in the aerospace world so I don't think this is true. Stealth planes can only fly with active control of their flight surfaces - there is no stable "default" setting that allows the plane to, for example, fail safe. But they can easily fly with the performance they have with computer control of all the surfaces. There's no magic going on there, just well understood fluid mechanics.

Posted By: fostertominterseasonal solar heat storage into uninsulated ground cannot work,
Drakes Landing Solar Community has insulated storage and dry ground conditions to prevent ground water from removing the stored heat. Uninsulated storage may work, but not as well as insulated storage.

As for bumblebees not being able to fly, that's mainly folklore. Check out the Straight Dope on that myth: http://www.straightdope.com/columns/read/1076/is-it-aerodynamically-impossible-for-bumblebees-to-fly

(since linking and quoting doesn't work at the same time, here's an excerpt from the above):

"According to an account at www.iop.org/Physics/News/0012i.1, the story was initially circulated in German technical universities in the 1930s. Supposedly during dinner a biologist asked an aerodynamics expert about insect flight. The aerodynamicist did a few calculations and found that, according to the accepted theory of the day, bumblebees didn't generate enough lift to fly. The biologist, delighted to have a chance to show up those arrogant SOBs in the hard sciences, promptly spread the story far and wide.

Once he sobered up, however, the aerodynamicist surely realized what the problem was--a faulty analogy between bees and conventional fixed-wing aircraft. Bees' wings are small relative to their bodies. If an airplane were built the same way, it'd never get off the ground. But bees aren't like airplanes, they're like helicopters. Their wings work on the same principle as helicopter blades--to be precise, "reverse-pitch semirotary helicopter blades," to quote one authority. A moving airfoil, whether it's a helicopter blade or a bee wing, generates a lot more lift than a stationary one.

Paul in Montreal.
- CommentAuthorBrianR
- CommentTimeDec 15th 2010
Glad to see the idea of a house based DC power supply I raised here 21Dec08 is getting more interest.

I believe less copper will be required because of the use of stored power in so many devices (electric drills take little average current). Mains based switchers are likely to stay more expensive because making high voltage switchers is expensive - they have to withstand kV spikes. They are more bulky because of the input smoothing capacitor.

Modern lighting requires 3W against older 60W bulbs and older mains circuits wasted copper because of the long runs to light switches. A newer DC installation could provide remote switches at the socket e.g. by radio, timer, or IR control. Background charging of devices could defer to more urgent consumption requests.

The newer DC sockets can be far smaller and their better safety could mean they will be allowed into bathrooms and other places where mains sockets are banned.

I would agree that washing machines and electric cookers etc should not be included.
- CommentAuthorfostertom
- CommentTimeDec 15th 2010
Thanks Paul - you're usually impeccably right, and so am I, somehow (i.e. right, not impeccable!)

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