Author Topic: AC and DC  (Read 3674 times)

Brammofan

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AC and DC
« on: June 17, 2011, 08:52:05 AM »
I was showing the Enertia at a motorcycle show a couple weekends ago (I was the only electric) and this guy started asking me questions about the electrical system. I'm never very good at answering these questions because, while I can produce a spec sheet, I don't have the underlying electrical engineering-type knowledge for their sometimes very specific questions. I've been asked the following question before, and was at a loss as to how to respond.
Him: I assume it's a DC motor.
Me: Actually, it's a permanent magnet, brushless AC motor.
Him: AC? So you charge the batteries with AC, the charger converts it to DC, then the (something) has to convert it back to AC for the motor? Sounds like the most inefficient electrical system imaginable.
Me: uh.... it's fun to ride (or something like that).

So, I asked Roger at Brammo for some help, and here's what he said:

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1.       We start with the desire to store electrical energy. Since nobody has found a cost effective way of doing this, the best we can do is to instead store electro-chemical energy. Our best attempts to store electro-chemical energy result in batteries that produce direct current electrical energy or “DC”.

2.       We have an electrical energy distribution system that provides alternating current or “AC”. There are interesting technical and political reasons why throughout the world AC and not DC electricity is distributed. Most people agree the most compelling reason for the choice of AC is that it is less deadly when accidental exposures occur.

3.       Low voltage is not a health hazard to humans. High voltage is deadly.

4.       Our motorcycles operate at 76 volts DC in order to provide acceptable high speed performance at the lowest, and therefore safest possible operating point.

5.       Regardless of whether we choose to use a DC motor or an AC motor, we need a motor amplifier to control the speed of the motor and this forces us to convert the 76 volts from the battery into a variable voltage that drives the motor at various different speeds.  This amplifier is known as the motor controller.

6.       It mostly doesn’t matter whether we are converting between AC and DC since we are forced to convert from high voltage to low voltage to charge the battery. Then we are forced to convert from a fixed low voltage to a variable voltage based on speed and power control of the motorcycle. So you can look at it as 2 conversions: AC to DC for storage, then DC to AC for propulsion, and this looks foolish perhaps. If you instead look at it as 2 conversions: high voltage to low voltage for storage, then low voltage to variable voltage for propulsion, it might appear more sensible? Who cares whether we change the format between direct and alternating current, there’s no meaningful efficiency loss in doing this.

In another email, Roger said something that really resonated with me, and it reflects what I have come to understand as the part of Brammo's mission that differentiates them from the rest of the pack:

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Every successful consumer product should be a delight to use; none should be a harrowing challenge. This is also true of the high performance ICE sport bikes. The Enertia is not meant to compete against these high-performance sport bikes; it should be delightful to use in its intended application.

(I also posted this on Brammofan.com, if you're interested).
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FreepZ

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Re: AC and DC
« Reply #1 on: June 17, 2011, 12:56:07 PM »
One advantage of an AC motor is that is's burshless (although there are brushless DC motors out there too). The problem with brushes is that over time they wear out, and if you run the motor very hard, you could end up melting them.

And if you're going to talk about inefficient transfers of energy, take a look at an ICE.
Chemicals used to heat up a gas, to make a piston move up and down. How efficient can that be? Oh yeah, less than 40%. :P
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protomech

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Re: AC and DC
« Reply #2 on: June 17, 2011, 03:51:05 PM »
And if you're going to talk about inefficient transfers of energy, take a look at an ICE.
Chemicals used to heat up a gas, to make a piston move up and down. How efficient can that be? Oh yeah, less than 40%. :P

The best ICE may have full load efficiency in the 30-35% range, part load (ie the typical case) is significantly lower. And the average ICE is weaker still.

Lest electric-drive fans be tempted to be smug, keep in mind that approximately 70% of the US's electrical energy comes from combustion - coal and natural gas. Electric vehicles that plug into the grid - in the very general sense - are still largely energized from carbon fuel combustion. And while the best best co-generation plants have thermal efficiencies in the 50-60% range, and electrical efficiencies in the 35-40% range.. the average power plant is weaker.

Even things like solar are not "free". Panels last 25, 30 years with slight reductions in output over their lifetime. Current poly-silicon panels require large amounts of energy to produce - about 25% of their expected total lifetime output last I checked - and generally that energy isn't being produced by solar panels.. yet.
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Brammofan

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Re: AC and DC
« Reply #3 on: June 17, 2011, 07:20:07 PM »
Geeze, protomech... such a party pooper.  ::)
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Deadly Silent Ninja

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Re: AC and DC
« Reply #4 on: June 18, 2011, 09:34:42 AM »
They have already developed 75% efficient solar films and paints, which should be hitting the market in the next couple of years. A researcher I know is working on the same technology and he said that they can get 90% (of course, yet to be seen).
It may not be a "right now" advantage, but one difference between solar and other renewables is that they are only beginning to evolve, while ICE are not going much farther.
Imagine in five years, instead of installing expensive solar panels, you can simply paint your house and it will produce free energy directly to your Brammo...

FreepZ

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Re: AC and DC
« Reply #5 on: June 18, 2011, 12:50:26 PM »
(I apologize if this is getting off topic.)

They have already developed 75% efficient solar films and paints, which should be hitting the market in the next couple of years.

Really? I thought that upper limit on efficiency was more like 45%. 75% would be fantastic, but we're not there yet. There is something called the Shockley–Queisser limit, that says that single bandgap solar cells are limited to 31-41% power efficiency. Next generation cells are trying to overcome that limitation by making the cell multi-bandgap.

But efficiency is less important than cost. If the cost of a solar installation can get below $1 per watt, installing solar becomes a lot more appealing. Heck, if I could paint some electricity generating goop my house for a couple of thousand bucks and get enough power for my entire house, I would jump on it.
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protomech

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Re: AC and DC
« Reply #6 on: June 20, 2011, 06:57:42 AM »
(I apologize if this is getting off topic.)

They have already developed 75% efficient solar films and paints, which should be hitting the market in the next couple of years.

Really? I thought that upper limit on efficiency was more like 45%. 75% would be fantastic, but we're not there yet. There is something called the Shockley–Queisser limit, that says that single bandgap solar cells are limited to 31-41% power efficiency. Next generation cells are trying to overcome that limitation by making the cell multi-bandgap.

And good poly-silicon, like Gavin has on his roof, is probably more like 13%.


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But efficiency is less important than cost. If the cost of a solar installation can get below $1 per watt, installing solar becomes a lot more appealing. Heck, if I could paint some electricity generating goop my house for a couple of thousand bucks and get enough power for my entire house, I would jump on it.

Yeah. I probably have 1500-2000 sq feet (140 - 185 sq meters) of unshaded south-facing roof surfacing. I use about 30 kwh per day, averaged over the year; 4.5 average hours of sun per day means I need around 7kw installed. I have far more room than I need for polysilicon, so a less efficient cheaper-per-watt panel would suit me just fine.

The treehugger link is talking about just panel costs. Currently $5/kw installed is about as good as you can do, and that's providing installation labor yourself. If the panels drop in price that'll definitely help out. Once the unsubsidized payback point hits 10 years or so then you'll see massive adoption of solar power..
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Brammofan

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Re: AC and DC
« Reply #7 on: June 20, 2011, 08:39:25 AM »
Loving this discussion about solar.  Has anybody tried or compared or even heard of solar shingles?  My house is due for a new roof (come on, hail storm!) and I have been thinking about my options.
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EmpulseRider

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Re: AC and DC
« Reply #8 on: June 20, 2011, 01:03:21 PM »
I currently have a 2.94kWh system up on my roof. Just your standard polycrystalline type. Paid a good $3/watt (subsidized/installed) last year. Since I am gonna be purchasing 2 EVs over the next year I thought I would upgrade my system to make some room. The new upgrade will add another 1.44kW. I have already have the panels and am just waiting on the Enphase micro-inverters and the racking hardware. After subsidies and installation the upgrade SHOULD total in just over $1/watt. Difference is I am ordering the equipment online and paying a contractor I know to do the install. Seems to me that the companies installing this stuff make a KILLING. One thing to think about when ordering equipment online, none of the solar install companies will install it for you, so be sure to have your own resources or the technical know-how to install the system yourself.

Also, keep in mind that typical inverters crap out in about 10-12 years so there is that expense as well. Just when you think your panels have paid for themselves, boom, your inverter goes belly up. There's another $3k. Enphase's new inverters carry a 25 year warranty so hopefully they live up to their claims.

Oh, and to get an idea of what you will need to power your EV take a look at the calculator on my website: http://www.empulsebuyer.com/pvCalculator.php
This shows you ROI on your panels if you offset the price of gas with free electrons to get you from point-a to point-b.

Brammofan

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Re: AC and DC
« Reply #9 on: June 20, 2011, 01:17:18 PM »
That's a cool ROI chart, EB.  Maybe it's just me, but the link to "Solar Power Insolation for U.S. Cities" is not resolving.
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Gavin

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Re: AC and DC
« Reply #10 on: June 20, 2011, 01:30:36 PM »
3.3 kWh of monocrystalline panels (Sharp panels/ 235w model...it think all Sharp panels are Mono) on my roof....the prices keep getting better**...my system should pay for itself in 8 years and I also have Enphase micro-inverters...so I hope they last those 25 years.


**my system was 19k last year...would have been 27k 2 years before that...hopefully the prices keep dropping...add in the rebates and my system was 11k and the local electric company buys all my energy...haven't paid an electric bill since my panels have been up and PNM has been sending me a check every month (use to pay about 900 bucks a year in electricity...now PNM pays me about 400 a year...so panels paid for in about 8.4 years....

Gavin

« Last Edit: June 20, 2011, 01:52:29 PM by Gavin »

EmpulseRider

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Re: AC and DC
« Reply #11 on: June 20, 2011, 01:41:17 PM »
Yeah that location got dropped for some reason. Here is another resource: http://www.freesunpower.com/radiation.php

I would change the link but my web host hosed my account up pretty good.

EmpulseRider

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Re: AC and DC
« Reply #12 on: June 20, 2011, 01:54:16 PM »
3.3 kWh of monocrystalline panels (Sharp panels/ 240w model...it think all Sharp panels are Mono) on my roof....the prices keep getting better**...my system should pay for itself in 8 years and I also have Enphase micro-inverters...so I hope they last those 25 years.


**my system was 19k last year...would have been 27k 2 years before that...hopefully the prices keep dropping...add in the rebates and my system was 11k and the local electric company buys all my energy...haven't paid an electric bill since my panels have been up and PNM has been sending me a check every month (use to pay about 900 bucks a year in electricity...now PNM pays me about 400 a year...so panels paid for in about 8.4 years....

Gavin



That is solid! I keep getting bills for about 7 dollars (franchise/connect fees)... lame. I have fine tuned my house's efficiency since I got the panels though and my net meter is running backwards about 100kWh last few months. But I wont get a check unless im negative at the end of the year. Sounds like you got the hookup down there!

Gavin

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Re: AC and DC
« Reply #13 on: June 20, 2011, 01:55:16 PM »
http://rredc.nrel.gov/solar/calculators/PVWATTS/version1/

a site to figure out what you can product from Solar in your area....

luckily

 A) where I live is hella sunny...high desert...310 sunny days a year...8 inches of rain--and not even that much most years.

 B) my house faces true North South...so my South facing roof is perfect for panels.

Gavin

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Re: AC and DC
« Reply #14 on: June 20, 2011, 02:04:09 PM »

That is solid! I keep getting bills for about 7 dollars (franchise/connect fees)... lame. I have fine tuned my house's efficiency since I got the panels though and my net meter is running backwards about 100kWh last few months. But I wont get a check unless im negative at the end of the year. Sounds like you got the hookup down there!




yeah, PNM is pretty good...All my solar goes FIRST through a new meter that counts every electron those panels produce...PNM pays us for that total number even though some of it gets used by us and not back to the grid (though most does go to the grid except summer when the pool filter runs during the day).

Then there is a second meter that is spinning backwards during the day like a mofo...that keeps track of the power we use...

So the panels make our consumption either minimal or actually negative numbers. But even on the months that we use more than we make, well PNM pays us for everything we make, so it is always a check to us (complicated i guess...if me make 600 kwh a month (PNM pays us about 75 bucks for that) and use 500, well PNM see us as using 100 kwh and charges us about 10 bucks...so we clear 65 bucks...if we make 600 and use 1000, PNM see 400 and charges us about 50 bucks...so we clear 25 bucks....)

Gavin