The next big thing in the energy sector: Photovoltaic generated DC electricity

Nov 05, 2013 by Rajendra Sing
Clemson graduate student Githin Alapatt tests a silicon sample using the Copper Oxide Deposition System. Credit: Clemson University

(Phys.org) —Energy consumption continues to grow. The costs of generation and transmission of energy must come down for the increased consumption to be sustainable. Energy must be generated without depleting resources, without causing pollution, and without incurring waste. Transmission of energy too must be efficient. These ideal goals, when realized, would enrich lives, regardless of economic distinction.

A viable solution is the onsite generation of electricity using the photovoltaic (PV) method of converting solar energy directly into electrical energy. The PV method uses semiconductor devices called . With constant reduction of the cost, this method is the most promising direct current (DC) power source for rich and poor globally. Due to the availability of all over the world, PV generation is not hostage to the usual geo-political constraints. Thus, it can power an revolution just like the information revolution powered by the internet continues to shape our world today.

Rajendra Singh, D. Houser Banks Professor in the Holcombe Department of Electrical and Computer Engineering and PhD student Githin F. Alapatt at Clemson University, along with and Akhlesh Lakhtakia, Charles Godfrey Binder (Endowed) Professor in Engineering Science and Mechanics at the Pennsylvania State University, recently examined the most promising types of solar cells to power every home. On October 23, 2013 they published a paper entitled "Making Solar Cells a Reality in Every Home: Opportunities and Challenges for Photovoltaic Device Design" in IEEE Journal of Electron Devices (Volume 1, number 6, June 2013 Issue).

Prof. Singh says that, as he had predicted in 1980, "the vast majority (~90 %) of installed more than 100 Giga Watt PV systems employ traditional photovoltaic panels made of silicon." Silicon is the second most abundant element in Earth's crust. Due to lack of functional reliability and the concomitant lack of economic bankability, PV systems employing solar concentrators have not made a significant impact worldwide. The use of thin films of semiconductors such as cadmium telluride, amorphous silicon and copper indium gallium arsenide is still to make a major commercial impact. PV modules comprising organic and dye-sensitized solar cells shall not play a role in bulk power generation, without fundamental breakthroughs in material synthesis and performance.

For increasing the efficiency of single-junction solar cells beyond the current limitations, several new approaches have been proposed by scientists. These approaches are based on concepts such as multiple exciton generation, carrier multiplication, intermediate band gap and hot-carrier extraction etc. However, after applying manufacturing criteria to ensure commercial viability, the Clemson and Penn State researchers became pessimistic about the efficacy of these approaches for bulk power generation.

The researchers have proposed a new multi-terminal multi-junction architecture for inexpensive PV electricity generation. Efficiency will exceed the currently feasible 25%. The proposed architecture is based on the use of currently commercial silicon solar cells and thin-film solar cells made of materials (such as copper oxide) that are abundant in Earth's crust. Management of the flux of solar photons through the solar cells is expected to boost efficiency, but the additional manufacturing costs to be incurred thereby remain unknown, according to the researchers.

Prof. Singh says that "the creation of local DC power grids can save power being lost in the transmission and unnecessary conversion from DC to alternating current (AC) and then back to DC." Most electronic appliances and electric loads operate on DC and by transmitting and converting AC power to DC about 30% of the total power generated is lost. Today, PV electricity generation and distribution on a DC microgrid is the best way to power villages without access to electricity. It is also the best option to replace aging and transmission infrastructure in USA and other developed countries.

Explore further: PV production grows despite a crisis-driven decline in investment

More information: IEEE Journal of Electron Devices Society (Volume 1, number 6, June 2013 Issue): Making Solar Cells a Reality in Every Home: Opportunities and Challenges for Photovoltaic Device Design, ieeexplore.ieee.org/xpl/articleDetails.jsp?reload=true&arnumber=6589128

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MR166
1.4 / 5 (18) Nov 05, 2013
More green BS spouted by "Green Science" which is based on consumer ignorance. Even the new florescent light standard requires AC and some point or the other. Also a "Grid" requires high voltages in order to keep the power losses to a manageable level without using distribution cables the diameter of a telephone pole. Perhaps they are suggesting that each home should be wired for a 20KV DC service. Yeah, right.

Don't get me wrong High Voltage DC (250 KV +++)transmission systems are in use today but before distribution they are converted to low voltage AC for home use. Also I really doubt that a bank of solar cells operate at much more than 1KV DC before they are converted to AC. Otherwise a failure of one bank of cells would bring down the entire site if it were to be wired in series to produce say 100KV DC.
VendicarE
1.2 / 5 (6) Nov 05, 2013
"Even the new florescent light standard requires AC" - 166Tard

That is because florescent lighting uses high voltage to ionize the gas inside the tubes.

This is not the case for LED lighting, which is better suited for DC operation.

DC wiring for in home lighting should be driven by building codes and industry lighting and retail regulation.
Bob_Wallace
1.6 / 5 (13) Nov 05, 2013
Local DC power grids, such as running a server farm on DC is feasible.

Going to large scale DC grids is unlikely and unnecessary. We don't loose much electricity converting from DC to AC (about 2%). AC is so established that creating a parallel grid to take the place of the AC grid and switching over everything we own to DC doesn't make sense.
Egleton
1 / 5 (11) Nov 05, 2013
"More Green Bullshit?" Nice exposure. No-one asked you to drop you pants.
Shootist
1.3 / 5 (14) Nov 05, 2013
Want fission. Must have fission. Fission is the only solution until (if) fusion is developed, or large scale solar power plants are orbited. iow, LONG TIME.

North America alone can start with 100 1000 megawatt fission plants. Someone go tell the luddite greens to shove it up their algore.
MR166
1.5 / 5 (15) Nov 05, 2013
VD you are really an idiot. Every appliance that has a motor or a transformer needs to be replaced in a DC system. Virtually every device in a home would need to be replaced in a DC system. Plus that, limiting distribution voltage to a reasonable 240V would require huge wires on the grid.

"Going to large scale DC grids is unlikely and unnecessary. We don't loose much electricity converting from DC to AC (about 2%). AC is so established that creating a parallel grid to take the place of the AC grid and switching over everything we own to DC doesn't make sense."

Read the article BOB!!!!!!

It claims that 30% of the power is lost in the conversion. As I said "Green BS" one way or the other.
baudrunner
1.6 / 5 (13) Nov 05, 2013
Did somebody dig this article out of a pile dated 1960's? What's the matter with you people at PhysOrg?

Rajendra Sing ought to take a few classes in electricity, if she thinks this is the next big thing. Photovoltaics (ie. solar cells) already generate DC energy. It has to be converted to AC for consumer product use because DC is just too dangerous to play with. One touch of an exposed domestic DC house current contact will kill you. I've taken taken 120 volts at seven amps of AC current for seconds and survived (don't ask). The way we already do this is we transmit AC after conversion from the DC source and "rectify" it to DC in the product for consumer use. That ain't gonna change, trust me.
Eikka
1.7 / 5 (11) Nov 05, 2013
"DC microgrids" operating on low residential level voltages only carry a mile or so, and have tremendous losses and costs with heavy gauge wires required to carry the power. DC microgrids operating on higher voltages require up and downconverters at both ends, which are functionally DC-AC-DC converters. It is better you split the system at the AC step and trasmit AC to the consumer to avoid one extra conversion in the transmission system.

The main fallacy of the article is the suggestion that you can do away with wall warts and separate power supplies when using DC instead of AC, because you can't charge your cellphone on 240 Volts DC, and you can't run your washing machine or electric stove off of 5 volts DC, and wiring your entire home for both voltages, or any intermediate voltage you might need, like 22 volts for your laptop, is expensive and silly.

In fact, your biggest loads like fluorescent tubes, stoves, heaters, fans, motors, run directly on AC without conversion.
MR166
1.2 / 5 (13) Nov 05, 2013
Baudrunner actually AC is more dangerous than DC because DC will cause your muscles to jerk and move from the source whereas AC will cause them to freeze and stay connected to the voltage. I also have received my share of shocks.
Eikka
1.3 / 5 (11) Nov 05, 2013
I've taken taken 120 volts at seven amps of AC current for seconds and survived (don't ask).


No you haven't. Your body does not conduct electricity well enough to pass 7 amps. You would have to be made out of metal. In reality, the most you've probably passed is 100 milliamps.

And AC is the more dangerous current because it changes polarity all the time, and that pulsing is actually what incapacitates your nerves and stops your heart.
italba
5 / 5 (3) Nov 05, 2013
The article is about "micro grids" in villages without electricity, not about developed countries electrical needs. If you have to start from zero, and you need only the base devices, a DC grid can be a viable solution. We have motorhome and boat devices already working with DC current, energy from solar, wind or micro hydro sources can be easily stored in cheap lead batteries. You'll need ticher wiring, but this technology is for distributed energy generation, not centralized ones. You only need to be connected with your close neighborhood, to share the power for peak needs.
Eikka
1.4 / 5 (11) Nov 05, 2013
This is not the case for LED lighting, which is better suited for DC operation.


When you change 240 VAC to DC you lose about 1.2 volts in the bridge rectifier which represents a whopping 0.5% loss in efficiency.

A bigger loss happens in the switching mode regulator that turns that DC back into AC and then transforms it down to a lower voltage, and turns it again into DC to drive a constant current through the LEDs, because you can't run LEDs directly off the line voltage - you need some sort of regulator to limit the current and eliminate any fluctuations and spikes.

italba
5 / 5 (2) Nov 05, 2013
...you can't run LEDs directly off the line voltage - you need some sort of regulator to limit the current and eliminate any fluctuations and spikes...
Currently sold led lamps for homes already have rectifier and regulators built inside, and led lamps for cars use DC current directly, so you can use them both ways.
Eikka
1.7 / 5 (12) Nov 05, 2013
Most of you didn't read at all the article or haven't understood it. They are talking about "micro grids" in villages without electricity, not about developed countries electrical needs.


Umm.. are you sure YOU read the article?

It says:
It is also the best option to replace aging power generation and transmission infrastructure in USA and other developed countries.


You only need to be connected with your close neighborhood, to share the power for peak needs.


Have you ever tried to run a 12 volt cable pair over a hundred meters?

With a copper cable as thick as your finger, you can barely transmit 10 amps without serious efficiency losses. 120 Watts - that's just about enough to run a fridge and some lights at 7% transmission losses.

If you try to run something that needs power, like a hairdryer or a toaster oven that takes something like 400 Watts, you're going to lose 2/3 of your power beacuse the line drops to about 4 Volts and 100 Amps.
Eikka
1.9 / 5 (13) Nov 05, 2013
Currently sold led lamps for homes already have rectifier and regulators built inside, and led lamps for cars use DC current directly, so you can use them both ways.


LEDs can not run directly from a DC voltage source. LED is a diode, and diodes start conducting very large amounts of current once the voltage exceeds the treshold where they light up - so much that they will be almost instantly destroyed.

They need current limiting regulators. The LED lamps in cars simply have them built-in.

The current limiting regulators can be linear regulators, like a simple resistor or a feedback controlled transistor - which waste a lot of power - or they can be switching regulators which chop DC into AC and then smooth it back to DC at a precise voltage where the LED passes just the right amount of current.

Bob_Wallace
1 / 5 (10) Nov 05, 2013
VD you are really an idiot. Every appliance that has a motor or a transformer needs to be replaced in a DC system. Virtually every device in a home would need to be replaced in a DC system. Plus that, limiting distribution voltage to a reasonable 240V would require huge wires on the grid.

"Going to large scale DC grids is unlikely and unnecessary. We don't loose much electricity converting from DC to AC (about 2%). AC is so established that creating a parallel grid to take the place of the AC grid and switching over everything we own to DC doesn't make sense."

Read the article BOB!!!!!!

It claims that 30% of the power is lost in the conversion. As I said "Green BS" one way or the other.


BOB!!!!! read the article. And the article is wrong. 30% loss is from the days of mechanical inverters. Even square wave inverters were much more efficient than what the article claims.

Perhaps you should gather some facts prior to calling BS.
cantdrive85
1.4 / 5 (11) Nov 05, 2013
If the tech becomes cheap enough and available in such forms as roofing tiles or building cladding/siding that could be distributed across entire neighborhoods, industrial parks, or cities you could very definitely supplement the grid. What portion of our power needs could be satisfied with DC power supply? 20%, 40%, 60%? I don't think it's about replacing the grid, but finding ways to supplement the system with the least impact.
El_Nose
not rated yet Nov 06, 2013
A truly great public works project would be the US federal government dedicating itself to creating a superconducting eletrical energy transmission backbone. linking all major areas and then spreading it out to at minimum to have a connection in every county with over 80k people.

built underground a corridor spanning the US carrying power... selling it to Canada and Mexico
Mike_Massen
1 / 5 (9) Nov 06, 2013
MR166 mumbled with
Baudrunner actually AC is more dangerous than DC because DC will cause your muscles to jerk and move from the source whereas AC will cause them to freeze and stay connected to the voltage.
No !
DC is far more dangerous because thats the (non-reversing) current that locks the muscles, the occasional jerk away from a high voltage DC is a fortunate chance reaction, often the muscles lockup. DC also polarises pathways, try standing on dry rubber in rubber shoes for more than a couple of minutes whilst lifting the cap off a 300V DC battery bank to check level or hydrometer etc and wondering why a tingle can be progressively felt up to the elbow - even through all that insulation... !

AC, with the voltage reversing allows some biochemical respite provided any local heating effect isnt excessive and that respite allows one sometimes a release...

david_king
1 / 5 (11) Nov 06, 2013
A localized DC grid would be handy when the AC grid goes down. Not having to burn candles for light and having the ability to keep gas heating appliances running safely might be indispensable in the wintertime. It's the sort of infrastructure that could develop very quickly in the event of a natural disaster provided the local Harbor Freight could power up it's cash registers.
italba
not rated yet Nov 06, 2013
It is also the best option to replace aging power generation and transmission infrastructure in USA and other developed countries.
In very fringe areas, why not?
Have you ever tried to run a 12 volt cable pair over a hundred meters?
Why 12 V? 24 V is too much? And, I wrote, this technology is for distribute power generation. Why do you have to run a 100 meters cable when you have your own generator and battery? Connection with your close neighbors will be useful, for instance, when you have to use a washing machine (with external hot water source, obviously).
If you try to run something that needs power, like a hairdryer or a toaster oven that takes something like 400 Watts, you're going to lose 2/3 of your power because the line drops to about 4 Volts and 100 Amps.
Sure, how can they live all those poor people in rural Africa or Asia villages without an hairdryer or an electrical toaster?
javjav
1 / 5 (1) Nov 06, 2013
This is a non sense. The cost of the copper wires (which can only rise) for making a parallel DC grid would be much higher than the cost of the Silicon PV material ( which can only go down) that is needed to compensate from DC/AC conversion, so a parallel grid will never worth it.

Even with that, DC power still makes sense, but only for direct connections from local PV panels to DC devices of similar voltage specs, plus a DC battery or a super capacitor as needed. For example a DC Air conditioned directly connected to PV panels in the roof, or a lamppost with a local DC battery & DC Lighting connected to its own PV panel. (The last example not only saves the cost of the parallel grid, but also the cost of the AC grid itself).
Eikka
1.4 / 5 (10) Nov 06, 2013
Why 12 V? 24 V is too much? And, I wrote, this technology is for distribute power generation.


Because 12 volt RV batteries were mentioned.

Why do you have to run a 100 meters cable when you have your own generator and battery?


To have an electrical connection with your close neighbor. Even within the house you have tens of meters of wiring.

Connection with your close neighbors will be useful, for instance, when you have to use a washing machine (with external hot water source, obviously).


A washing machine on spin cycle draws so much power that it would be impossible to run in a 12-24 volt microgrid without losing tremendous amounts of energy to transmission loss.
MR166
1.3 / 5 (13) Nov 06, 2013
"Perhaps you should gather some facts prior to calling BS. "

Well Bob, if converter efficiencies are as high as you claim and I have no reason to dispute your figures, doesn't that make the whole article BS since the whole reason for the new grid is poor converter efficiency?

As I said in my response to you one way or another this is a BS article.
Eikka
1 / 5 (9) Nov 06, 2013
No !
DC is far more dangerous because thats the (non-reversing) current that locks the muscles


Nope. You're wrong. Nerves don't work in a continuous fashion, they work by pulsing so that when your muscles are pulling, the nerves that control them are sending an AC signal. That's why the AC current, not DC, causes a locking cramp.
Eikka
1.4 / 5 (9) Nov 06, 2013
Sure, how can they live all those poor people in rural Africa or Asia villages without an hairdryer or an electrical toaster?


Literally poorly.

The point is, that low voltage microgrids experience high voltage losses along the transmission cables unless you put in ridiculously heavy gauge wires.

To get 100 Watts through to the other house without too much loss, you need heavy copper welding cable that costs $3 a foot! If a household cannot afford a toaster oven, they can't afford the cables either.

MR166
1 / 5 (14) Nov 06, 2013
Actually Bob we are both on the same side of the argument here and my "read the article" statement was stupid.
MR166
1 / 5 (14) Nov 06, 2013
Half of these articles arise from the "Publish or Die" environment of academia. I don't care how brilliant you are, when you are forced to say something when you have nothing new to add, you will say something stupid.
italba
not rated yet Nov 06, 2013
@Eikka:
Because 12 volt RV batteries were mentioned.
ONLY YOU mentioned 12 V!
To have an electrical connection with your close neighbor ... tens of meters of wiring.

You are thinking in a monodimensional way! Try to figure a village where at least an house out of five has a solar generator and a backup battery, and imagine that every house is connected with its neighbors in a conductive cable mesh. You can easily understand that each house can draw current from multiple sources in a few tenth meters radius, and there is no need that a single cable must carry all the load.
...they can't afford the cables either...
With 24V you'll need a cable size about twice (4x cross section) of a 110 V grid. And there are aluminium cables too.
A washing machine on spin cycle draws so much power that it would be impossible to run in a 12-24 volt microgrid...
There are low current (and low cost) washing machines. Look for camping washing machines.
Bob_Wallace
1 / 5 (10) Nov 06, 2013
Peak efficiency for grid tie inverters is generally over 95%. The best inverters have a 98% efficiency rating. Commercial solar plant inverters have 98% or better efficiency ratings. Going from AC to HVDC and back down to AC again loses about 0.75% at each of the two steps.

We're simply not wasting much electricity moving from DC to AC.

The power bricks we use for our gadgets use advanced switching designs with efficiencies in the 80–90% range. And we're pulling small amounts of power when we use them. Not much electricity is being wasted going from AC to DC. 20% of 40 watts is small power.

Setting up a separate DC grid in order to avoid rather small conversion losses doesn't make sense to me. I'd need to see some numbers, a grid takes a lot of hardware and labor. Might make sense to just install some extra solar panels and a few more wind turbines. Sometimes it's cheaper to over produce than eliminate inefficiency.
baudrunner
1 / 5 (10) Nov 07, 2013
Eikka: It was actually around three amps AC at a very low frequency, and my other hand was grounded back into the circuit because I was touching the chassis. This was old equipment that used vacuum tube triodes and such, not modern day electronics. I maintain that DC transmission over the grid is indeed impractical because it is far too dangerous. Ask the experts.

Russia's electricity grid used to deliver DC straight into the homes of its people in the old days. Did you know that?. They have long since switched to AC. We will never, ever, see DC power coming into our houses. You can bet the farm on that.
VendicarE
1 / 5 (2) Nov 07, 2013
"VD you are really an idiot. Every appliance that has a motor or a transformer needs to be replaced in a DC system." - 166TARD

Only a moron like yourself would presume that it is a one or the other situation.

It truly is unfathomable to your peanut sized brain that houses will soon be wired for both AC and DC power, and that home lighting, clocks, computers and other electronic equipment are best served by DC.

God you are such a fucking moron.
VendicarE
1 / 5 (1) Nov 07, 2013
"DC will cause your muscles to jerk and move from the source whereas AC will cause them to freeze and stay connected to the voltage." - 166Tard

You can't keep anything straight in that little pea brain of yours can you.

You have AC and DC confused.
VendicarE
1 / 5 (1) Nov 07, 2013
"Russia's electricity grid used to deliver DC straight into the homes of its people in the old days. Did you know that?" - baudrunner

Did you know that DC transmission in the U.S. was used in at least one portion of NYC until 2007?

http://boingboing...nyc.html
baudrunner
1 / 5 (10) Nov 08, 2013
I don't know what the fuss is. We have a lot of people posting statements who have absolutely no knowledge of electrical theory. What say you, VendicarE? And no, I did not know that.

Ac is converted to DC in the rectifier stage of the power supply in every electronic device made. Apart from addressing the danger issue, AC is just more efficient because power dissipation losses are drastically reduced compared with DC. And DC can cause fires.

Another reason to use AC, albeit a minor one. In Southwestern Ontario, the timing of the 60 cycle current coming from their wall outlets is corrected by the atomic clock in Ottawa to unparallelled precision. This means that you can design a simple digital electronic clock that uses the 60 Herz timing pulses to maintain the correct time on the display. I actually did that. I designed and built a cheap digital clock that keeps time more accurately than a Bulova watch.
VendicarE
not rated yet Nov 08, 2013
Thank God for gubderment standards.

In any case, getting back to DC power... It will soon be the preferred method of powering home LED lighting as it is more efficient than AC for that use, and the bulbs can be greatly reduced in complexity, and cost and at the same time be made more reliable.

DC is also best for powering compute devices, home monitoring, cameras etc.

High power devices, like ovens, water heaters, motors etc, are better served with AC.
Bob_Wallace
1 / 5 (10) Nov 08, 2013
Yep. We'll soon build a brand new DC grid and double wire our houses.

That's for sure. Right after we tame unicorns.

(Time to turn this thread off....)
VendicarE
not rated yet Nov 09, 2013
Most houses in North America are already quadruple wired BobbieTard.

One set of wires carrying 110 AC.
Another set carrying 220AC
Another set carrying telephone
Another set carrying cable.

Many Modern homes contain another set, for a household networking system.

Still others have another set for a home alarm system.

Like a child, you are just looking for excuses not to act.

Rutzs
1 / 5 (12) Nov 09, 2013
Perhaps they are suggesting that each home should be wired for a 20KV DC service. Yeah, right.


We are a long way away from converting our homes to DC, but research is already underway to determine what the challenges would be for such a feat.

Some benefits to using DC distribution & transmission systems would be: a) removing losses from inverters for any homes using renewable energy systems... b) removing the issues of harmonics due to increasing electronics and energy efficient appliances... c) long distance transmission, etc etc...

Of course current appliances and equipment within the home would need to be retrofitted for DC use, and yes we are a long way away from it, but it doesn't mean we can't look at the problems now...

Just because you think AC is the only way, doesn't mean you are right. Don't be a biggot.

If we make a breakthrough in solar panel efficiency(much greater than the existing 10-15%), you will definitely see this article coming back to haunt you.
Eikka
1.2 / 5 (11) Nov 09, 2013
One set of wires carrying 110 AC.
Another set carrying 220AC


That's actually a single set of wires carrying two phases and a ground. You get 110 Volts from phase to ground and 220 volts from phase to phase.

Another set carrying telephone
Another set carrying cable.


These are not very heavy gauge wires.

ONLY YOU mentioned 12 V!


Motorhome systems are commonly 12 volts.

imagine that every house is connected with its neighbors in a conductive cable mesh.


I can imagine it would be relatively expensive to install unless your houses are practically wall-to-wall in a single cluster. Villages aren't generally densely built. Buildings where you'd most need it, like on a farm, are usually very remote.

With 24V you'll need a cable size about twice (4x cross section) of a 110 V grid.

And that is exactly why it's expensive. Metals are.

Look for camping washing machines.


Those things are a joke. Barely better than washing by hand. And they're 12 V
Eikka
1.4 / 5 (11) Nov 09, 2013
We have a lot of people posting statements who have absolutely no knowledge of electrical theory.


Indeed.

AC is just more efficient because power dissipation losses are drastically reduced compared with DC. And DC can cause fires.


There is no practical difference in power loss for the same voltage of AC/DC over relatively short distances. Over very long distances, AC loses due to capacitive leakage and radiation losses.

There is no universal mechanism by which DC power would cause fires any more than AC would.

Eikka: It was actually around three amps AC at a very low frequency, and my other hand was grounded back into the circuit because I was touching the chassis.


Your body cannot pass even three amps. It simply does not have low enough resistance. To pass three amps at 120 volts you need a resistance of less than 40 Ohms, whereas the resistance of the human body from hand to hand, even if the skin is broken and wet, is no less than 500 Ohms.
Eikka
1.4 / 5 (11) Nov 09, 2013
Just because you think AC is the only way, doesn't mean you are right.


The most likely solution is to use both, but not in the way the article suggests.

Every time DC voltage is shifted up or down, it has to be inverted into AC. If you have more than one intermediate voltage level between distribution and end user, it makes no sense to transform the power to DC at every step because it would only require another conversion to AC again.

Therefore if our long distance transmission grids operate on HVDC, the local grids that connect to it would work on AC to avoid unnecessary conversions. The medium-voltage local distribution networks would also be AC to easily step down to the various voltages used. Only the very end of the network would be DC, where you don't need to convert to any more lower voltages, but there you don't get very large DC networks because the low voltages won't carry far.

Although the problem still remains that all your DC devices need different DC voltages.
Eikka
1.4 / 5 (10) Nov 09, 2013
Local transmission grids actually operate at 2-4 kV and over. The low voltage 240-110 Volt grids are literally just the last couple hundred meters from the local transformer to a handful of nearby houses, and there you get a problem:

AC-AC transformers work both ways. AC-DC converters only work one way. If your low voltage network where your solar panels connect to is AC, then the solar power you push in gets automatically pushed up the network. If it's DC, you need a separate converter to shift the voltage up to distribution level.

If you don't convert, then your solar panels will only be connected to your immediate neighbors, who are likely to have solar panels as well, so they don't need your power. What you really want is to transmit the power to someone else who doesn't, who is probably hundreds of miles away, and that won't happen with a local low-voltage DC grid.
VendicarE
5 / 5 (1) Nov 09, 2013
"We are a long way away from converting our homes to DC" - Rutzs

Why in the world would you wan't to do such a silly thing?

AC is very useful. So is DC. Both have their applications. The application of DC is primarily in local generation for battery storage and retrieval for lighting, actuators, and electronics.
VendicarE
1 / 5 (1) Nov 09, 2013
"That's actually a single set of wires carrying two phases and a ground. You get 110 Volts from phase to ground and 220 volts from phase to phase. " - Eikka

One phase = 2 wires (3 for modern installs)

0 - Ground
1 - Return
2 - Supply 110

For 220 you add another wire

3 - Supply -110

22- requires additional wiring.in north America - although some codes allow the return to be dropped for 220 V devices.

Modern homes of course require 15 amp wiring, 20 amp wiring for kitchen appliances, 30 amp wiring for electric clothes dryers and 50 amp wiring for electric stoves.

But NumNutz thinks that it is impossible to run some low voltage wire through the house to power DC devices.

He/They are looking for excuses to be mindless fools.

Eikka
1.2 / 5 (10) Nov 09, 2013
One phase = 2 wires (3 for modern installs)


That depends on how you define "phase".

http://en.wikiped...ic_power
david_king
1 / 5 (11) Nov 09, 2013
There's a perfectly serviceable DC transmission line from the Celilo, Oregon to the outskirts of LA, 850 miles away. It has 2 conductors just over 3" in diameter and has a capacity of 3.1 gigawatts.
http://en.wikiped...Intertie
FainAvis
1 / 5 (6) Nov 09, 2013
Here is a serious practical difference between AC and DC. (Australian supply voltages are 240v AC 50hz.)
Years ago I had a DC 240v 2KVA gasoline driven generator for house construction. I used a hand held power saw to cut rafters etc. Inevitably, sometimes I stalled the saw by making it work too hard. The generator locked the stator to the rotor so hard that the generator flipped upside down. If I bolted the generator to something really secure, I could fry the stalled saw armature in half a second. That is because universal (commutator) motors have very low resistance at stall, almost zero counter-emf. These saws carry a warning not to run at low voltage, else they will overheat.
The same model saw running off a 240v AC 2kva generator just buzzes annoyance for several seconds. The AC generator strains momentarily then recovers.
As well, switches burned out sooner using DC.
Eikka
1 / 5 (7) Nov 10, 2013
switches burned out sooner using DC.


That's because there is no zero-crossing in DC so it takes a longer time to extinguish the contact arc on pulling them apart. The solution to the problem is to have a small value capacitor in parallel with the switch.

italba
not rated yet Nov 10, 2013
@Eikka:
Motorhome systems are commonly 12 volts.
Wrong! You can have 24V models too, at will. On trucks and boats 24V is more common.
...Villages aren't generally densely built...
Do you figure villages something like American subdivisions?
Metals (cables) are (expensive).
That's why if you don't need to be connected with the nearest (maybe hundreds of miles away, in developing countries) electrical grid point you can save a lot of money!
...camping washing machines ... are a joke. Barely better than washing by hand.
Just think about washing all your family's clothes day by day and you surely change your mind. Those little machines works the same of bigger ones, you just have to fill them by hand with warm water for washing and cold water for rinse. The only difference, the clothes comes out very wet, but you can just hang them to dry.
ryggesogn2
1 / 5 (8) Nov 10, 2013
The most dangerous electrical power system is 28VDC typically found in military systems.
The voltage is sufficient to overcome skin resistance and not cause much discomfort and the 28VDC systems usually have high current limits providing sufficient current to stop a heart, which is in the milli amp range at the heart.
Tesla coils operating at very high frequency produce high voltage, low current and is safe for humans to contact as the low current conducts along the skin.
Eikka
1 / 5 (6) Nov 10, 2013
Just think about washing all your family's clothes day by day and you surely change your mind.


Camping washing machines only wash something like 5 pounds per filling, and all they really do is agitate the water a bit. The style of machine is called a pulsator; there's a rotating wobbly plate at the bottom that goes back and forth, and then spins for a bit, and then oscillates again until the timer runs out. You have to manually pour and drain the hot water, cold water, and then squeeze the clothes dry.

If you have any more clothes to wash, it's more efficient to put them in a large bucket and agitate it by hand. It's very much a waste of time to wash anything more than two shirts and some underwear with a tiny 12 Volt pulsator machine.

Do you figure villages something like American subdivisions?


I'm imagining villages as a handful of houses laid out something like this: http://www.regia....lage.gif
Eikka
1 / 5 (7) Nov 10, 2013
That's why if you don't need to be connected with the nearest (maybe hundreds of miles away, in developing countries) electrical grid point you can save a lot of money!


The problem is that just lighting up your garden shed with low voltage DC is really inefficient.

This is not against microgrids, but against the idea of very low voltage microgrids. The idea that you have multiple paths between source and drain doesn't really help much, and here's why, in the form of a famous physics problem:

http://www.mathpa...h668.htm

With an infinite grid of resistors, the path between two adjacent nodes is R/2. The fact that each node may have a battery doesn't mean anything, because the batteries will immediately start drawing current from other nodes as their voltage drops under load.

So even in the ideal case, you only save half the cable, but we started out with 4x the cable, so it's still twice as expensive to build with 24 V compared to a 110 Volt grid.
Eikka
1 / 5 (7) Nov 10, 2013
And that said, there's nothing stopping you from using a multipath 110 Volt DC grid, so it really still costs 4x the cable to build with 24 V instead of 110 V.

Although you have to add in the cost of wiring every house with every adjacent house instead of just passing a cable from one to the next, which also saves on cable by eliminating redundant paths.

And you have a big problem in safety with the multipath system because a short circuit at one node can be fed with current from many paths and the current along any one of them may be insufficient to trip the breakers.

Then, if we're talking about developing nations, there's the problem of parts availability. AC motors, as used in things like washing machines, don't need speciality electronics. Just iron and enameled copper wire. They can be fixed with very primitive tools.

DC motors either use brushed commutators, which wear out in use, or have special electronics to commutate which are not easily mended due to lack of spares.
GrayTech
1 / 5 (7) Nov 10, 2013
We are victims of false images fed to us, by a flawed paradigm. Economics is joke science. I don't argue that economics has no value. I do believe that it has whole facets of opportunities for huge improvements that would make it much more valuable and less dangerously prone to misuse. Some examples of what needs to be fixed.

1 = Industry that happens at the small scale, and cost benefits that happen in a personal = non commercial economy are routinely being almost completely discounted. The happily self sufficient farmer being almost no economic value to their nation is the classic example. They start to become an economic force when they go in debt for tractors, get an outside job to pay off some debt, end up in getting a divorce, and paying for rehab for their neglected children.

2=Depth and stability of economic base is not counted more important than growth. A Steady State economy should be a goal. Endless growth is a doomed path.

...so
A small tech for security? Why?
Eikka
1 / 5 (7) Nov 10, 2013
The most dangerous electrical power system is 28VDC typically found in military systems.
The voltage is sufficient to overcome skin resistance and not cause much discomfort and the 28VDC systems usually have high current limits providing sufficient current to stop a heart, which is in the milli amp range at the heart.


DC current doesn't easily stop one's heart because it doesn't interfere much with nerve signals. You need about 5 times the current from DC to be deadly, and even then it's because of tissue heating and electrolytic effects that lead to internal burns and blood poisoning.

28 Volts across the body with needles stuck under your skin would produce about 60 milliamps of current, which would be deadly, but isn't because direct current doesn't cause muscle cramps. The power dissapated along the path would be about 1.6 Watts which would become a problem in a minute or so, but would not cause any immediate concern other than to release the cable from your hand.
Eikka
1 / 5 (7) Nov 10, 2013
What is dangerous in DC systems is that there is often a prominent AC signal on top of the DC voltage that is coming back from equipment like commutating DC motors and other switching loads.

The commutation of a motor causes a voltage spike to travel back towards the battery, and if you are caught in between it will act the same as grabbing a live AC wire, because that's what it is.
Osiris1
1 / 5 (7) Nov 10, 2013
The old Edison argument. Low voltage DC is what the article wants, however, this requires a LOT of copper or Aluminum.... Simple, just look at Ohm's law and see. Wire size is driven by current, and a lot of power in low voltages will require stupendous wire sizes and weight......a total waste. Better to invert the DC to AC at higher voltages, just like Westinghouse said over a hundred years ago. By the way, all that wire would be lost the first time metal thieves looted it, negating ALL 'theories' of saving money.
cantdrive85
1.2 / 5 (9) Nov 10, 2013
These complaints and concern over the raw material aspect are really just an engineering problem.
http://www.cornel...koti.pdf

If we want to solve the problems of the future without lowering the standard of living we had better start thinking creatively.
italba
not rated yet Nov 10, 2013
@Eikka:
Camping washing machines only wash something like 5 pounds per filling ...You have to manually pour and drain the hot water, cold water, and then squeeze the clothes dry.

Wrong! See http://www.electr...3012.htm
... villages ... laid out something like this: http://www.regia....lage.gif

Your link doesn't work, you can find on Wikipedia some real world villages.
The problem is that just lighting up your garden shed with low voltage DC is really inefficient.
I was talking about villages, not farms! Small houses, 1 or 2 rooms, led lightening, power to recharge phone or laptop computer, small tv, small refrigerator and, maybe, a ceiling fan. Mean power needed 100W or less.
italba
not rated yet Nov 10, 2013
@Eikka:
...against the idea of very low voltage microgrids.
You can use any voltage you want, you have to consider the cost and efficiency of multiple voltage transformations. Solar panel needs a battery to be useful all day long, so you'll better use the battery voltage directly. You can easily find 12V or 24V DC electrical devices (and spare parts), so you'll better stay with those voltages. That's all! If you want to use 110V to connect your neighbours, you'll have to buy and maintain an up-converter for each solar panel and battery and a down-converter to recharge the batteries from the grid. And connect a generator to an AC grid is not so easy as to connect it to a same voltage DC grid. By the way, a short circuit can be as dangerous on a 110V AC grid as in a 24V DC one, there are fuses for that.
Urgelt
3 / 5 (2) Nov 11, 2013
Well, this is amusing. We're refighting the same old battle fought between Tesla and Edison.

There's a reason Tesla lost. DC distribution of power over more than a few meters is simply too clunky (and expensive) to bother with it.

What's next? A Phys.org article trumpeting the benefits of broadcast power? Antigravity? Going to tell us the best way to store ghosts in canisters?

Are the editors on vacation, or what?
Eikka
1 / 5 (7) Nov 11, 2013
Solar panel needs a battery to be useful all day long, so you'll better use the battery voltage directly.


And you can have arbitrarily high voltages in a battery. Simply stack them up. 110 Volts is no problem. Most 110/220 volt devices like computer power supplies will readily work with DC, or require only minor modifications to do so.

Wrong! See http://www.electr...3012.htm


And as you might see, that washing machine consumes 380 + 160 Watts to run the washer and spinner. And it works on 220 - 240V, 50/60 Hz.

That's the problem I was walking about in the first place. At 12 volts you can reasonably transmit only about 100 Watts, and at 24 Volts about 200W over anything but very short distances. Going from house to house, you get significant transmission losses and voltage losses at anything over that.
Eikka
1 / 5 (7) Nov 11, 2013
I was talking about villages, not farms! Small houses, 1 or 2 rooms, led lightening, power to recharge phone or laptop computer, small tv, small refrigerator and, maybe, a ceiling fan. Mean power needed 100W or less.


The mean power doesn't really count because the problem is the instantaneous power you need to pull through the cable. Suppose all your devices are on at the same time, which is not unplausible.

Phone charger 5 W
2x 20 Watt LED lamps (seriously, those 7-8 Watt bulbs are NOT 60 Watt equivalent)
Laptop computer 50 W
Small TV 50 W
Small fridge 50 W
Ceiling fan 24 W

Sum total 169 Watts.

Then you also forget the wifi router or DSL modem for the computer, satellite reciever for the television, the washing machine you wanted earlier...

A simple 12 volt coffeemaker will draw 120 Watts alone.
antialias_physorg
not rated yet Nov 11, 2013
There's a reason Tesla lost. DC distribution of power over more than a few meters is simply too clunky (and expensive) to bother with it.

It actually makes a lot of sense for large distances (that is why the connections between national grids in the EU are DC - and also why the connections accross the mediterranean to north African solar/thermal powerplants are planned to be DC. So if we're talking DC powergrid we're not talking about the last few meters but a complete changeover from (or parallel grid to) the existing AC one.
Eikka
1.5 / 5 (8) Nov 11, 2013
Your link doesn't work, you can find on Wikipedia some real world villages.


Very well.

When you're talking about developing countries, the villages you see are built something like this: http://goo.gl/maps/bdnXB

When you're talking of the developed world, the villages already have high voltage AC distribution to connect them with other nearby villages and within the different parts of the village, so your low voltage DC system would be cumbersome and redundant.
Eikka
1.5 / 5 (8) Nov 11, 2013
(that is why the connections between national grids in the EU are DC


Very few such connections actually exist, and nearly all of them are underseas cable because that permits the use of the water itself as the return path.

http://en.wikiped...rope.svg

Most of the connections within the Continental Synchronous Area are actually plain old AC.
antialias_physorg
5 / 5 (1) Nov 11, 2013
The longer the cable the more advantageous (as the losses in DC lines are less than those in AC lines). Once you have reached a length where converter losses are compensated it#s all good.

And when we're talking about a full conversion from an AC infrastructure to a DC infrastructure then the converter infrastructure changes to something a lot more efficient, too.
But given that that would mean replacing a lot of AC machines I don't see that happening any time soon.
(At least until energy prices or demand rise substantially - neither of which is likely as renewables aren't going to become more expensive and gains in efficiency seem to be nullifying increased demand for the most part).
TheGhostofOtto1923
1 / 5 (2) Nov 11, 2013
losses in dc lines are lower
Losses if what?
"[for DC] the voltage drop due to the resistance of the system conductors was so high that generating plants had to be located within 1.6 km or so of the load."
cantdrive85
1.5 / 5 (8) Nov 11, 2013
Well, this is amusing. We're refighting the same old battle fought between Tesla and Edison.

There's a reason Tesla lost. DC distribution of power over more than a few meters is simply too clunky (and expensive) to bother with it.

What's next? A Phys.org article trumpeting the benefits of broadcast power? Antigravity? Going to tell us the best way to store ghosts in canisters?

Are the editors on vacation, or what?

Tesla=AC, Edison=DC, you got it backwards, Tesla "won".
Eikka
1 / 5 (4) Nov 12, 2013
losses in dc lines are lower
Losses if what?
"[for DC] the voltage drop due to the resistance of the system conductors was so high that generating plants had to be located within 1.6 km or so of the load."


The Edison DC system had to generate straight domestic level voltage right at the powerplant because he had not figured out ways to transform voltages up and down. He had to account for the voltage loss in the system in such ways that a distribution point further away from the generating station would be fed with a higher voltage from the generator to compensate for the loss along the way, and then a constant load be kept to keep the voltage at that point from rising.

Edison used some tricks like rotary transformers to extend distribution range, but ultimately at a high cost. He couldn't get the voltage to stay up or stop varying with the load without huge losses of energy, whereas Westinghouse could wire in the Niagara Falls with little trouble.
Eikka
1 / 5 (3) Nov 12, 2013
The problem with AC power at first, and why Edison didn't start with it in the first place, was that nobody had figured out how to build an AC motor. Commutated permanent magnet DC motors and dynamos were known as science toys since forever, so Edison simply collected all the works, put it together and started selling it as one package.

VendicarE
not rated yet Nov 12, 2013
"There's a reason Tesla lost." - Urgeit

Tesla promoted AC. Edison promoted DC.

Tesla won. Edison was a fool and a fascist.

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