DC Voltage point of diminished returns 12,24,48,96 and so on.

System · February 2008

Hey guys, first off i wanted to thank all of you who post tirelessly day after day answering all the questions us newbies ask over and over again, never-the-less i did search for my answer although i was unsuccessful.

I do know from searching however that 12,24,48 and so on reduce the wire size needed to get the power to its destination but what im curious about is this:

at what point does that voltage because too high in the sense of products available to run off that voltage (IE: Lighting, Water Pumps, Heating Units, AC Inverter for the occasional ac item) and also being able to issue a charge to the battery banks in say a 96 volt configuration.

I was also wondering if anyone had a link favorited about the drop offs of converting DC to AC as i'd love to have like a spread sheet or something that reflected how much of my "power network" would best be suited as keeping it DC instead of going to AC.

Thanks all for your consideration. I do have another question about efficient home heating so if you have some expertise in that category feel free to go on over to the Energy Use & Conservation forum and flex your muscles

Solar Guppy · February 2008

Re: DC Voltage point of diminished returns 12,24,48,96 and so on.

48Vdc is the end of the line for generally available products.

The reason is mostly regulatory ...

mike95490 · February 2008

Re: DC Voltage point of diminished returns 12,24,48,96 and so on.

If you are going Grid-Tie, you can get inverters that accept over 500 VDC, reduces your wire gauge to reasonable values

• Output power: 5.0 / 4.5 kW (240 / 208 Vac)
• CEC efficiency: 95.5% / 95% (240 / 208 Vac)
• Certified MPPT range: 240-550 Vdc (600 VOC)
• Tested MPPT range: 235-550 Vdc

Solar Guppy · February 2008

Re: DC Voltage point of diminished returns 12,24,48,96 and so on.

Mike is writing about the Xantrex GT inveters, they do not power any type of offgrid loads but are designed to sell back power to a low impeadance grid.

The OP mentioned "batterys" and loads that would be a a stand-a-lone inverter, so I think 48V stands in regards to OP's question

mike95490 · February 2008

Re: DC Voltage point of diminished returns 12,24,48,96 and so on.

SecretsKept wrote: »

I was also wondering if anyone had a link favorited about the drop offs of converting DC to AC as i'd love to have like a spread sheet or something that reflected how much of my "power network" would best be suited as keeping it DC instead of going to AC.

Visit this thread:
http://forum.solar-electric.com/showthread.php?t=2224

where it was just recently discussed.

Wire capacity @ 12V vs Appliances you can buy @ 12V, and more.

H2SO4_guy · February 2008

Re: DC Voltage point of diminished returns 12,24,48,96 and so on.

Exeltech has 108 volt DC pure sine wave inverters. I sure want one and they are modular! You can start with 1,000 watts and add 1,000 modules as you go! What a cool idea. I sure want one, BUT the challenge is charging the batteries. What do you do, charge them in sections?

Skip

System2 · February 2008

Re: DC Voltage point of diminished returns 12,24,48,96 and so on.

Mike,

I read that post and it only refers specifically to 12vdc vs 120vac and does not address availability for products DC above 12v, my question more specifically address this:

Solar Guppy best answered my question when stating that 48vdc is virtually the end of the line for products, my final question is this this:

Do you think its cost effective to specifically have products engineered for lets say 96vdc for stuff like the heavier appliances that chew power or is it more worth it to just get a heavy duty inverter and buy AC appliances instead.

If anyone has any values they would be willing to share that would be super, the kind of info we can start compiling is about inverter efficiency if anyone knows of any....

for example i have a APC 3000RMXL3U which has a wattage output of AC 2400 Watts, i'll perhaps order a watt-meter and see what it pulls from the batteries under full load or perhaps i can find that info on their documentation, that might tell us the efficiency of those units and whether its cost effective to go with AC for stuff like that.

I mean i know its inevitable that i'll be using some degree of AC power in my home but i'd like to minimize that to consumer products like ipods, and laptop charger etc as a pose to the more permanent stuff that can be DC like lighting, water pumps, possibly refridge, washer/dryer.

I know i posted alot, i hope it all makes sense i kinda jumped around in thought there a bit. Let me know what you think, and thanks for indulging me =P

mike95490 · February 2008

Re: DC Voltage point of diminished returns 12,24,48,96 and so on.

SecretsKept wrote: »

Do you think its cost effective to specifically have products engineered for lets say 96vdc for stuff like the heavier appliances that chew power or is it more worth it to just get a heavy duty inverter and buy AC appliances instead.

The only products above 12V are inverters and chargers, no washing machines, or ceiling fans. I've seen some 50hz stuff, commercial, in Europe, up to 96V, but only inverters and chargers, no appliances.

If this will go into a home, with homeowners insurance, you need to look at appliances and equipment with UL lab stickers. Otherwise, you generally void your ins. policy.

There may be some 24v gear out there, for the cross-country truckers market (coffee pot, TV) but most of everything is for the 12V RV market, or regular 120VAC.

In a couple of years, this may change, as automakers are about to change their base voltage from 12V to 24 or 36VDC, to reduce the amount of copper and its' losses in cars.

n3qik · February 2008

Re: DC Voltage point of diminished returns 12,24,48,96 and so on.

I would stay at 12/24 volts, maybe go 48 volt, Why?? well it will go bad at some point in time, then one will have to replace it. If its an odd voltage you may have to wait weeks to get a replacement compared to overnight for something that is mass produced.

mike90045 wrote: »

In a couple of years, this may change, as automakers are about to change their base voltage from 12V to 24 or 36VDC, to reduce the amount of copper and its' losses in cars.

It was set to goto 48VDC, but it has seen some setbacks due to arching of contacts for switches/relays. They could not make the components at the price level the auto companies wanted to pay.

System2 · February 2008

Re: DC Voltage point of diminished returns 12,24,48,96 and so on.

well, one thing to note is the basic construction of certain products in your home, a refridgerator is for lack of a better basic description, an air conditioner consisting of a phase change design. That is, that it has a expansion coil, a compression tank, and a compressor not to mention other electronic components which have DC counterparts as well. The toughest thing to power in a fridge is the compresser which for again, lack of a better description is much like a motor with a piston, a motor simply enough can run on DC, thus it would be easy to have a DC fridge made and well within the UL specifications of safety.

A washer has 2 main components, a motor and a water pump, again both DC components not to mention relays, and electronic components all of which have DC counterparts.

A dryer is a shade different but again still the same, another motor and also a heating coil, now the coil that runs off DC often has larger power constraints so with that said i dont know that it would be economical to power a dryer at all except on days when its too cold to dry them on a line, thats is to say however that i dont have a better solution, which i do =P

a water pump from your well, basicly a motor, dc powerable and well you get the idea.

now i dont conceide that what i've stated is fact, its just my limited understanding of these components, if someone has a better description that disproves my theory slap it in, i love learning new things =P

BB. · February 2008

Re: DC Voltage point of diminished returns 12,24,48,96 and so on.

The issue with DC motors are that a true electro-mechanical DC motor needs brushes and commutator to produce the needed magnetic fields to produce rotation. These are high wear components and generally will not last very long and will produce ozone.

The typical "DC" motor we see on DC fridges/fans/etc. are actually three phase motors with electronic commutators. Very efficient and reliable. No brushes or wear components--other than the bearings.

However, if you are going to use a lot of AC motors anyway, looking at a central inverter and using common (and cheap) 120/240 VAC 50/60Hz mass produced motors/appliances is usually the most cost effective solution.

Today, there is little advantage to use specialized DC equipment over energy star 120 VAC appliances. Look at the off-grid / RV DC refrigerators vs a "cheap" Energy Star unit. The use almost the same amount of power and the 120 VAC fridge is about 1/3 (or less) the price. Use the money saved to add panels/batteries/inverters to your system.

-Bill

System2 · February 2008

Re: DC Voltage point of diminished returns 12,24,48,96 and so on.

Thanks BB. that was the kind of input i was looking for.

Now curious, anyone happen to know common efficiencies of inverters and just how much power is lost in conversion? i know the two systems (ac dc) are hard to compare but i was just curious if i made my entire home AC if it would be a waste of money in the sense i'd need a larger array to compensate for that loss or if it would still be advantagous to light it with DC and use DC where its still a readily available product i'd be buying to minimize that loss as much as i can.

Ultimately this is the last house i'll ever build / live in, im not planning on expanding it except to meet my needs should they grow (which they wont) so my precision planning is crucial now so im not making costly revisions in the build or worse post-build phases.

So that brings me back to one last reiteration, do 96v systems exist or is 48v about the highest you see? i know someone mentioned about 108v inverters which would be great i wouldnt mind going a bit higher but the problem is charging and management. You could possibly solve this issue with mechanical / magnetic relays that would systematicaly disconnect as many batteries as needed from a given configuration and re-allign them to the configuration needed for charging but that seems to be inefficient and really over-kill just to accomodate what?......a 108v system which the effort for may be more money than its worth. Love to hear your thoughts.

thanks guys you are all so very intelligent, informative and kind!

Solar Guppy · February 2008

Re: DC Voltage point of diminished returns 12,24,48,96 and so on.

Check out http://www.consumerenergycenter.org/erprebate/inverter_tests/summaries/

above link is a list links for test reports of all CEC listed inverters. The newest generation of inverters have made large gains in efficiencys and now are in the 90-95% range for efficiencys.

For whole house offgrid, 120vac with a newest generation split-phase inverter is the clear choice. As an example check out the Xantrex XW-6048

http://www.consumerenergycenter.org/erprebate/inverter_tests/summaries/Xantrex%20XW6048.pdf

BB. · February 2008

Re: DC Voltage point of diminished returns 12,24,48,96 and so on.

One other reason, I forgot to add earlier, for inverter 120/240 VAC vs raw 12, 24, 48, 192 VDC battery systems is that batteries are virtually "unregulated" voltage sources whereas your 120/240 VAC system is well regulated and all of the 120/240 VAC devices are designed to run within the well defined range. When you you look at a 12 VDC battery system, the load terminal voltage of even a well designed distribution system, the input voltage will be between ~10 and 15.5+ VDC... Turns out that very few "consumer" DC devices operate well over that range of input voltage. Towards the 10 volt end, either the device "browns out" or can over current and burn out. At the high side of 15.5 volts, many devices simply fail.

The other question, I am sure that you can find specialized equipment that runs at other (and higher) voltages to help you save DC bus wiring costs... And you could even build/program your own electronics to control battery voltage/charging curves.

However, I would tend to stay away from these types of solutions. I am an engineer and in my "youth" I would have really enjoyed rigging together home made solutions to a solar power plant. However the issues of trying to maintain such a system over the next 2-4 decades (after that, I won't be around to care), becomes a big pain in the rear.

Yes, you can save wiring/copper costs with higher distribution voltages, but 10 years from now trying to find parts/replacements while your family is complaining that there is now power (or you are complaining that you have to run the generator 18 hours a day waiting for a 3 day express shipment--or weeks/months just to find a part) because the kids need to use their computers, the home is cold/hot because you are conserving power while the charger is down, etc., and you end up having to re-wire because your favorite 192 volt used computer center UPS that you purchased from EBay after the 2009 dot-com bust is no longer available...

Yes, you can save some energy by going to a higher battery voltage (and save wiring costs), but for a well designed system you are only talking about saving a few percentage points (typical wiring losses are in the 2-5% range).

Your better bet is to conserve on the "load" side of your usage. From a "standard" home type situation, you can probably save 25-50+% of your energy usage with conservation, insulation, new Energy Star Appliances, moving "loads" from electrical to alternative sources of energy (natural gas, solar thermal, wood stove, thermal air heat exchanger, etc.). Way more than you will ever save by increasing your DC battery bus voltage.

And once you have reduced your overall power need, your DC/AC needs will be a lot less, and the need for a "high voltage" DC system reduced too...

-Bill

System2 · February 2008

Re: DC Voltage point of diminished returns 12,24,48,96 and so on.

BB, Solar Guppy,

Just the input i was looking for, thanks for your informative and very revealing points. Needless to say i was just thinking that there may have been some hidden advantages and now that i know just how efficient the newer inverters are i can say that i'll be going with an AC home rather then a hybrid.

On a side note i'd always planned to minimize my energy consumption with propper efficient energy star appliances, good practices etc, the challenge however to meet my power needs even above that as i have very large computer power consumption, although advances in computer power consumption are making leaps and bounds especially in the data center enviroment like i employ here from my home =/. i just dont see giving up my business that i love so much.

Thanks again guys, this has been a rather good thread.

niel · February 2008

Re: DC Voltage point of diminished returns 12,24,48,96 and so on.

you may not want to discount using some items direct from battery if possible for although they've improved their efficencies on inverters there is still a conversion loss that is appreciable.

DC Voltage point of diminished returns 12,24,48,96 and so on.

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