DC-AC Conversion; What a Waste

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kingpb

Senior Member
Location
SE USA as far as you can go
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Engineer, Registered
This topic is for general discussion regarding a major shift that needs to take place in the world with regards to how we think of renewable and alternative energy sources.

The laws of phsyics tells us that anytime we convert energy from one type to another, there are losses. Specifically, I refer to DC-AC conversion.

We have PV, wind, solar thermal, wave, landfill gas, and there are others. It seems the general theme with using these drills down to two problems that every project has to overcome:
1. How to deal with the peaks and valleys of production and use
2.
 

kingpb

Senior Member
Location
SE USA as far as you can go
Occupation
Engineer, Registered
This topic is for general discussion regarding a major shift that needs to take place in the world with regards to how we think of renewable and alternative energy sources.

The laws of physics tell us that anytime we convert energy from one type to another, there are losses. Specifically, I refer to DC-AC conversion.

We have PV, wind, solar thermal, wave, landfill gas, and there are others. It seems the general theme with using these drills down to two problems that every project has to overcome:

1. How to deal with the peaks and valleys of production and use
2. Getting the power from DC to AC

This country needs jobs, industry needs a boost, and the electrical infrastructure is old and in need of replacement. Why not start replacing it, or at least supplement it with something outside the norm.

The US should start planning for DC power use as a way to overcome the heavy losses imposed by the equipment itself when trying to go from DC-AC. Think of the savings and simplification of the systems. We spend so much time, effort, and money on trying to build a better mouse trap, instead we need to be thinking of how to do it better, and DC is viable.

Power generated during off peak times can easily be stored in batteries, to overcome the peaks and valleys. Batteries require no water, their is no air emission discharges. It seems we spend so much money on a project trying to make a chicken produce a duck, when what we need to do is simply re-think how to utilize the chicken more efficiently; i.e. cut out the middle man (dc-ac inverter). I think the War of Currents should rise again.

It would be nice to see a "green" developer, maybe even with a grant from the DOE, build a small community that is planned and based solely on DC systems.

I would like to hear all thoughts, concerns, and experiences.
 
Modern inverters do not have a lot of loss. We put a 60kW Solectria three phase inverter online yesterday. It runs at 95% efficiency. That is not much loss when you consider the voltage drop losses when trying to distribute DC power.
 

kingpb

Senior Member
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SE USA as far as you can go
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Engineer, Registered
95% is a reasonably good efficiency for in the box thinking. But translate that to 100MW of power, (BTW in DC there is no power factor) but nevertheless, 95% efficiency would mean 5MW of power wasted. What voltage would you be distibuting at, and what are the lengths. There is still going to be lines losses with AC.
 

ggunn

PE (Electrical), NABCEP certified
Location
Austin, TX, USA
Occupation
Electrical Engineer - Photovoltaic Systems
Power generated during off peak times can easily be stored in batteries, to overcome the peaks and valleys. Batteries require no water, their is no air emission discharges. It seems we spend so much money on a project trying to make a chicken produce a duck, when what we need to do is simply re-think how to utilize the chicken more efficiently; i.e. cut out the middle man (dc-ac inverter). I think the War of Currents should rise again.

It would be nice to see a "green" developer, maybe even with a grant from the DOE, build a small community that is planned and based solely on DC systems.

I would like to hear all thoughts, concerns, and experiences.
Easily stored in batteries? Are you kidding? Storage of large amounts of electrical energy in batteries is a formidable problem and one that hasn't been cracked yet. Lead acid is the most cost efficient battery technology we have come up with so far, and it's not great. It's lousy, in fact; the energy density for a lead acid battery is on the order of a few tens of kWh per ton.
 

kingpb

Senior Member
Location
SE USA as far as you can go
Occupation
Engineer, Registered
Easily stored in batteries? Are you kidding? Storage of large amounts of electrical energy in batteries is a formidable problem and one that hasn't been cracked yet. Lead acid is the most cost efficient battery technology we have come up with so far, and it's not great. It's lousy, in fact; the energy density for a lead acid battery is on the order of a few tens of kWh per ton.

That is not accurate on two points: lead acid is OLD technology. Lithium-ion has already been used successfully, and is the similar type as being used in electric vehicles.

Projects already completed range from 1MW up to 25MW.

http://www.pv-magazine.com/news/det...emonstration-project_100007784/#axzz20yoq5jwB
 

ggunn

PE (Electrical), NABCEP certified
Location
Austin, TX, USA
Occupation
Electrical Engineer - Photovoltaic Systems
That is not accurate on two points: lead acid is OLD technology. Lithium-ion has already been used successfully, and is the similar type as being used in electric vehicles.

Projects already completed range from 1MW up to 25MW.

http://www.pv-magazine.com/news/det...emonstration-project_100007784/#axzz20yoq5jwB

Lead acid is old but it is still, as I said, the most cost effective (Ah/$) way to store electrical energy; Li Ion is far more expensive. If size and weight were not an obstacle, electric cars would use lead acid batteries. The projects you mention are likewise too expensive to be widely implemented. Storage remains entrenched as the Achilles' heel of smart grid technology.

There has been much AC vs. DC debate over the years, but beyond the positive and negative attributes of each there is the overriding issue of how expensive it would be to convert the existing infrastructure from AC to DC, and that's not to mention the tremendous reluctance on the part of utility companies to embrace ANY changes to the status quo. It ain't gonna happen.
 
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Besoeker

Senior Member
Location
UK
This topic is for general discussion regarding a major shift that needs to take place in the world with regards to how we think of renewable and alternative energy sources.

The laws of physics tell us that anytime we convert energy from one type to another, there are losses. Specifically, I refer to DC-AC conversion.

We have PV, wind, solar thermal, wave, landfill gas, and there are others. It seems the general theme with using these drills down to two problems that every project has to overcome:

1. How to deal with the peaks and valleys of production and use
2. Getting the power from DC to AC

Yes, I agree. Storage is an issue that needs to be resolved for most renewable sources like PV, wind etc. A dark calm night can happen.
But there is one form of renewable that has the others beaten into a cocked hat. It's hydroelectric. The Itaipu dam (14,000MW capacity) is one such example. It works because of all that potential energy of the water stored behind the dam. Storage on a scale that makes all other efforts combined look like a waste of effort.

But not everywhere has the right geography to make such schemes feasible. So I guess we'll persist with other forms of renewables and try to work out how we can accommodate their on-availability output to meet our on-demand expectations and I've seen articles about the development of large scale sodium sulphur (NaS) batteries by a consortium of European and Japanese companies if I remember correctly.
I know HVDC transmission is used but normally for very large, long distances transmission systems. I don't think, at this stage, DC from batteries is suitable for local distribution for a number of reasons.

Firstly, the voltage varies significantly between charging voltage and nearly empty cells. It's generally a much greater range than we would expect most electrical equipment to deal with. So, you'd need a DC-DC converter to stabilise the voltage either at source or in whatever you're feeding.

Then you'd have to consider switchgear. Mostly it's designed around AC and has to be considerably de-rated on both current and voltage to operate on DC. So, either you'd have to replace it or operate at reduced ratings including the voltage. But to get the same power at reduced voltage you'd have to increase the current. Bigger conductors, bigger switches......major undertaking.

I'm saying it isn't impossible. But I think it would be starting with a clean sheet and discarding most of what's already installed where DC gets used.
 

jaggedben

Senior Member
Location
Northern California
Occupation
Solar and Energy Storage Installer
As long as the grid remains intact, DC utilizations are going to be pretty few and far between. In this country we are waaaaaaay far away from needing to store solar energy on the grid scale (although we could swiftly change this with the right policies). As long as the peak output of all solar remains below what the grid can re-distribute, there's simply no need, and grid-tied solar is the most cost-effective way type of solar. A grid tied inverter costs about the same as batteries and a charge controller, and you don't have to change your appliances.

Now if you are talking about houses or even small villages where there currently is no grid or where it is unreliable enough to consider severing ties, DC utilization makes more sense. But even if you are doing even a small micro-grid, AC coupling is an option, and the lower voltage drop in AC coupling will make up for the costs of inverters in many cases, even if you are starting from scratch. Meanwhile almost every electrical object that people want to use is built for AC power and it is time consuming and expensive to find DC alternatives, when even possible.

There are some countries where they are basically starting from scratch, and I think in those places things may evolve differently. (And there is one country, Germany, that in a few years will have to figure out storage or stop installing more solar.) But here it is going to remain dominated by AC for probably the rest of our lives, although as the world's energy crunch continues the grid may become less reliable and things may start to change. That's kind of the doomsday scenario anyway, that ordinary people won't be able to afford much electricity of any kind and that most resources will go toward the struggle to maintain what was previously built.
 
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tom baker

First Chief Moderator
Staff member
Look for a new article on DC power in the 2014 NEC. There are companies that have DC power systems for T bar ceilings that distribute power via the grid to luminaires. DC power makes a lot of sense when you consider all the AC to DC conversion for TVs, radios, cell phones etc.
 

Besoeker

Senior Member
Location
UK
Look for a new article on DC power in the 2014 NEC. There are companies that have DC power systems for T bar ceilings that distribute power via the grid to luminaires. DC power makes a lot of sense when you consider all the AC to DC conversion for TVs, radios, cell phones etc.
But, as I posted above, how would you stabilise the voltage without power electronics?
What voltage would you use?
How would you ensure that switchgear can accommodate that?
If you wanted to move just your house from AC to DC, what parts of the installation would you need to revise?
What appliances would you expect to work with a DC supply?

We do a fair bit of DC work in the industrial sector. Drives fed with a common DC link. That can be done and is done. But the source is invariably AC.
 

sgunsel

Senior Member
DC was a loser for power distribution when Edison tried it and it still is for all the same reasons. There may be a few niche applications for high voltage DC, but for everything else, AC is far more practicable. Every major energy consuming device in your house uses AC. Those gizmos that use DC often use multiple DC voltages, so voltage conversions will always be needed. Electronics today are continually decreasing their operating voltage, so it is not uncommon to find 12, 5, 3, 1.5 VDC in the same device. Any device that required more power, like a toaster or coffee maker, would need quite a few amps (83+) to develop 1 KW at 12 VDC; my air conditioner would need 830+ amps! The copper would be $$$$.

Battery storage is a big loser too and will continue to be so for the near future. Batteries are expensive, consume power while charging and discharging, and have severely limited lifetimes in any other than standby operation. Batteries are very effective for cell phones and other small electronic devices that consume very little power - they are definitely more portable than a long extension cord, but are only practical where there is no alternative. Next time you buy a battery, figure out the cost per kwh. It won't be anywhere near the $0.10 to $0.15 per kwh you pay the utility.
 

kingpb

Senior Member
Location
SE USA as far as you can go
Occupation
Engineer, Registered
But if your going to look soley at $/Kw then why are we switching from coal, which has the cheapest cost, to predominantly gas turbines. Which didn't we already live through the gas turbine market crash. Becasue of efficiency? Hardly, the difference is like 38% for the newest coal technology to 42% for gas turbines. What about the effect/toll on the environment for gas capturing. You can't tell me it's carbon footprint, with the water, chemicals, and contamination to the ground water supply make it overall any better. Once the eco people start regulating it because of damage to environment, then what? Prices for gas will sky rocket. It will be a disaster.

Switching to DC. Think of all the devices out there that use DC internally. Think of all the conversion devices that could eliminated, which for every conversion it has an efficiency, i.e. losses. Simpler systems can be utilized. At what point do we as a society quit looking for ways of throwing money around to improve the mousetrap, when in reality we should step back and say, why not build a better mousetrap. What makes more sense for long-term health of the US?

In this country, I believe we are at a crossroads. The utility infrastructure is deteriorating, and has been for many years. As plants, equipment, and systems are replaced with newer "efficient" models we are simply buying into another 40 years of the same old, same old. it's time to think outside the proverbial box, and look for ways to rebuild from the ground up. Essentially re-invent concepts not simply recycle old ones and put on a new bow.

You can put lipstick on a pig, but it's still a pig! (to be PC - no offense to the pigs out there.............)
 

ggunn

PE (Electrical), NABCEP certified
Location
Austin, TX, USA
Occupation
Electrical Engineer - Photovoltaic Systems
Switching to DC. Think of all the devices out there that use DC internally. Think of all the conversion devices that could eliminated, which for every conversion it has an efficiency, i.e. losses.

One problem is that there's DC and then there's DC. Lots of different voltages. DC to DC conversion goes through an AC step and there are conversion losses both ways.
 
95% is a reasonably good efficiency for in the box thinking. But translate that to 100MW of power, (BTW in DC there is no power factor) but nevertheless, 95% efficiency would mean 5MW of power wasted. What voltage would you be distibuting at, and what are the lengths. There is still going to be lines losses with AC.
  1. Compare that 95% losses to typical transformer losses @ that power magnitude.
  2. No line losses on DC?
 

xformer

Senior Member
Location
Dallas, Tx
Occupation
Master Electrician
But if your going to look soley at $/Kw then why are we switching from coal, which has the cheapest cost, to predominantly gas turbines. Which didn't we already live through the gas turbine market crash. Becasue of efficiency? Hardly, the difference is like 38% for the newest coal technology to 42% for gas turbines. What about the effect/toll on the environment for gas capturing. You can't tell me it's carbon footprint, with the water, chemicals, and contamination to the ground water supply make it overall any better. Once the eco people start regulating it because of damage to environment, then what? Prices for gas will sky rocket. It will be a disaster.

Switching to DC. Think of all the devices out there that use DC internally. Think of all the conversion devices that could eliminated, which for every conversion it has an efficiency, i.e. losses. Simpler systems can be utilized. At what point do we as a society quit looking for ways of throwing money around to improve the mousetrap, when in reality we should step back and say, why not build a better mousetrap. What makes more sense for long-term health of the US?

In this country, I believe we are at a crossroads. The utility infrastructure is deteriorating, and has been for many years. As plants, equipment, and systems are replaced with newer "efficient" models we are simply buying into another 40 years of the same old, same old. it's time to think outside the proverbial box, and look for ways to rebuild from the ground up. Essentially re-invent concepts not simply recycle old ones and put on a new bow.

You can put lipstick on a pig, but it's still a pig! (to be PC - no offense to the pigs out there.............)

What about Tesla Coils? I think part of the problem with seeking alternate ways of generation is that no one can agree on what fuel type to use. Coal, Gas, Nuclear, seem to be the best three so far with solar, and wind bringing up the rear. Problem is no one wants new power plants in their back yard, and with construction continuing, more load being added to the grid without extra supply added we are forced to squeeze every electron out of our current system... with solar being added to augment the system.
 

mivey

Senior Member
It has been a while since I considered it but, for large power distribution, DC makes sense at very high voltage and few conversions. Otherwise, AC wins (to the best of my recollection).
 

iceworm

Curmudgeon still using printed IEEE Color Books
Location
North of the 65 parallel
Occupation
EE (Field - as little design as possible)
It has been a while since I considered it but, for large power distribution, DC makes sense at very high voltage and few conversions. Otherwise, AC wins (to the best of my recollection).

Yes. As is well known, DC has several problems:
1. circuit breakers don't have an easy time opening the circuit if there is any inductive load (and there always is some L)
2. and transformers don't work (unless one does a DC-AC, transform< AC-DC 9three places to loose efficiency)

I'm thinking King is planning to transmitt and distribute to residential at 500KVDC. Now that would keep the efficiency up. But the main CB might be bigger than the house.

Oh, that won't work. Too many different voltages needed.

Okay, distribute at:
1. 575V for motors and heaters
2. 240V for lighting and appliances
3. 42V for electronics and small loads (that is the same as the cars will be soon)

Now, use an SWER and it only takes 3 conductors - or 4 if you want a ground -- Arrg, that won't work. Too far to do the conversion at the substation and then distribute out.

I know. Put the converters at the house tap, right on the 500KV line. Then the three distribution feeders are short. I like that one ~:)-)

Tongue firmly in cheek. ice
 

iceworm

Curmudgeon still using printed IEEE Color Books
Location
North of the 65 parallel
Occupation
EE (Field - as little design as possible)
okay. Semi-serious this time.

It takes forward thinking to get out of the box and head off in a new, more promising way. We get stuck with this incremental change - till some one breaks out

Horses, buggys. buggy whips were at their peak when some one said, "Hay, maybe James Watt called it 'HORSEpower' for a reason." So, John Rockefeller boiled kerosene into gasoline, OTTO Diesel came up with the AUTO cycle, and Ferruccio started building cars. All because of that first forward thinker.

Small generation is remote areas is expensive. So Wally Hickle looked into beaming power (microwave maybe?) out to isolated villages. It didn't work - no current technology available. But that's okay. So was he a Crackpot or a Forward thinker? Not much difference - maybe marketing is the only difference.

Nick Tesla was certainly a crackpot - but he had good marketing. Look where he went with that. Oh, wait a minute - that's why we are where we are. All this AC and only selected long haul routes are DC.

Okay, maybe a little less than "semi-serious" ice
 

kingpb

Senior Member
Location
SE USA as far as you can go
Occupation
Engineer, Registered
I'm not proposing anything..... Believe me, if I had the answer I'd be rich and you all would be paying me to install my designs. :blink:

Some good points being made. Certainly the multiple different DC voltages are an issue, so maybe standardization is the first step. Forget IEEE, maybe a new organization called FTFAM - Forward Thinkers For Advancement of Mankind

I agree, many of the "crack-pots" were just forward thinkers, the man is always going to try and keep you down by labeling those that are different as weirdo's. :cry:

Ideas expressed today may be obsolete by the time the are uttered, but does that mean we keep quiet and march in line lockstep (Read George Orwell 1984) or do those brave enough question why, and push for change.

Seems as a society we are always thinking of ways to "improve" what we already have. When do you say, this just ain't working anymore, throw it out, and start fresh. OK maybe PV isn't the answer, maybe wind isn't the answer, maybe batteries for storage don't make sense. I'll tell you though fracking to get natural gas ain't the answer either.

We need jobs, we need infrastructure, and we need a new direction. The same ol', same ol' isn't working anymore.
 
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