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Old 02-10-2008, 02:46 AM   #11
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The extra power drain won't be felt. There is a huge excess of energy at night due to the fact that once fossil fuel powered generators fire up for peak power at about 6pm it is more efficient to keep them running rather than cutting them and starting them again, EV charging provides a small consistent load that allows them to run efficiently. I have read reports saying that millions of cars can be supported by this night time excess without needing more plants or even much more fuel (I think it was submitted to appeal the CAFE EV regulation backslide and I have read it in several places but I can't remember). A plug in hybrid would probably use at most about 10kWh, which could be covered by about a 2kW peak array if you wanted a green option. Electric drive even with dodgy power plants is still twice as efficient as gasoline to mechanical energy. Plug in hybrids make a lot of sense
The Tesla data is somewhat misleading - Tesla makes the assumption that their vehicles are propelled by electricity generated by natural gas.

http://en.wikipedia.org/wiki/Fossil_fuel_power_plant

These guys - while I am skeptical of Wiki I tend to trust the factual stuff since it's often "refereed" by the public - claim that at best a natural gas peaking plant has demonstrated 60 percent conversion efficiency.

In contrast a coal fired plant demonstrates about forty percent conversion efficiency at best. Nukes are often on par with coal fired plants in terms of their heat to electricity efficiencies.

Tesla motors is assuming the best possible efficiency of electric power conversion from fossil fuel. In reality, given that the vast bulk of electricity in the US (and mostly everywhere else but France) is coal generated the Tesla's "efficiency" is probably close to about two thirds as good as claimed, or marginally better than a Toyota Prius.

Moreover, the "peaking" plants use natural gas. Putting an extra strain on natural gas supplies is not really a prudent idea. Many of us heat our homes with natural gas and do not appreciate anything that would make our lives and cost of living rise. Especially the elderly who often struggle to make ends meet. Peaking plants still generate "greenhouse gases" so their sacrifices are moot.

We could debate the conversion efficiency and probable mix of sources for any one consumer. Reality is that the battery capacity of a hybrid is not terribly great and 10kWhr is not a lot of energy.

10kWhr is a "drop in the bucket". My Yaris motor has an 80kW output at peak. Which means if I had an electric battery power units and demanded the same power I'd get less than a minute of peak power from 10kWhr of charged power.

The Toyota Prius motor, incidentally, has an output of 52kW. Which means if you charge the batteries you're providing the Prius with about a minute of peak power.

I'll reiterate - the purpose of batteries in a hybrid is to buffer vehicle energy, not to substitute for the gasoline motor.

In exchange for this very modest benefit of pre-charge you'd be putting strain on natural gas supplies.

That all being said, I have no personal objection to people putting plugs on their hybrids. I think the marginal savings would be minimal but if it makes you all feel better and gives you a sense of empowerment have at it.

Gene
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Old 02-10-2008, 02:53 AM   #12
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Good post, Gene.
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Old 02-10-2008, 03:01 AM   #13
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I just consulted here... http://en.wikipedia.org/wiki/Toyota_Prius

The battery capacity of Toyota Prius is about 1.7kWhr. This capacity reinforces my assertion that the purpose of the batteries in a hybrid is to buffer vehicle energy.

If someone wished to make a more electricity oriented hybrid they're free to do so. I cannot see the EPA giving you a hard time for "tampering" with an electric power system, as they might do if you were to tweak or modify the car's motor or emission control system.

Please keep in mind that your constraints would be power capacity of the battery packs versus having to haul them around. Another constraint is cost of the batteries. You will have to optimize with respect to weight, efficiency and total cost.

I am sure that Toyota Engineering, which is known to be thorough, "gamed" many scenarios for capacity before arriving at their 1.7kWhr figure. They probably did not release their scenarios and which one predominated their calculations. Perhaps your own personal needs do not agree with these scenarios?

It is possible that for some here a higher capacity would yield better results for you in terms of mileage and overall fuel efficiency. As we say at work sometimes the devil is in the details and the details are in the numbers.

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Old 02-10-2008, 03:05 AM   #14
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Good post, Gene.
Thanks, Dave.

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Old 02-10-2008, 03:15 AM   #15
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Thanks also to Matt for quoting the Tesla site. I'm going over it. I work with motor controls myself and know a bit about what they're doing. They seem to be doing a fine job.

A 200kW motor comes to about 268Hp, which is pretty stout when you have a light weight car. Tesla Motors was ruthless in reducing weight and making the vehicle as simple as possible.

A hybrid, by contrast, has both the best and the worst of both worlds and one must carefully constrain each technology to get a best compromise. Toyota did a really impressive job.

I regret that the US lacks the power generating capacity to allow every one of us to use Tesla Motors's technology. I think that electric vehicles will be a key form of transportation in the future, though we will need to grow the "grid" a great deal to accommodate their energy consumption.

I also wish that Tesla Motors was a bit more honest in their presentation of efficiencies. They do not do themselves any service by assuming natural gas inputs, not when most of the US and elsewhere uses coal to generate electricity.

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