More on Lapp's solar-assist Prius Classic.

[skids]

"I have hardly been able to do any comparison runs like with the system on and with the system off. I do know that on the few runs I've made, I've been able to meet or exceed the very best mileage that I've ever gotten in the car under ideal driving conditions in the past. Just on my drive into work, I‘ve been able to exceed those very best conditions, which was down to about 3.9 liters per 100 kilometers". He quickly calculated that to be equivalent to 58 US miles per gallon. This compares to his normal commuting fuel efficiency which averaged about 50 mpg.

While these estimates are admittedly "loose", he does have a better feel for what the system should do theoretically. He calculates the Prius uses 125 watt hours per kilometer in an urban driving environment. Given the amount of solar panels on the car and the current sun angle in eastern Ontario, he estimates the batteries will receive about 800 watt hours of energy over a four hour period. Dividing this by 125 watts hours translates into just over 6 kilometers of solar electric-assisted, gasoline-free travel. If your total commute is 20-30 kilometers, he explained, this can represent an additional 25 percent reduction in fuel consumption.

http://www.mixedpower.com/modules.php?name=News&file=article&sid=831

On a side-note, IIRC the Prius uses 100 watts of DC on the onboard electronics while the key is on... an important detail to know with that 125W "urban driving" figure.

[Throckmorton]

1. 125 watts is just 1/6 horsepower.

Somehow I think his math is flawed.

[NNadir]

2. It depends on how much he drives and when he drives.

If he drives 5 miles every three or four weeks in the peak of summer in the desert, and records his fuel efficiency during that period, he can really extend his apparent gas mileage since his solar cell will have lots of time to recharge.

If he only drives the car when the battery is fully charged by the solar cell, he can have infinite miles per gallon. This isn't particularly useful to a person who has a job, but if your whole day consists of diddling with toys, you can drive whenever it is convenient for your car as opposed to driving when it is convenient for your life.

Like most of this kind of marketing hype, it sounds good on its face but the practical import is less certain. If it worked well, it would already be standard practice. We've been hearing about this sort of thing for decades upon decades. The first solar car race was in Australia in 1987, almost 20 years ago and that race, in turn, came after almost two decades of endless talk.

http://news.nationalgeographic.com/news/2004/09/0927_040927_solar_car.html

It's rather telling that there is still no commercially available fully solar cars on the market almost 30 years since Ralph Nader informed us that the world would be 100% solar powered by the turn of the century - and he wasn't referring to the thirty first century. (Of course, one needs to account that Ralph, a scientific illiterate, apparently believes that a risk free world is possible. This apparently accounts for his God complex - he believes he should be immortal and so he has no need to specify which century about which he's speaking.) Even during modern solar car races, the cars are escorted by gasoline powered cars or in the case of the Sundancer and the Southern Aurora pictured in the link below, by big fat ugly SUV's.

You can see photographs of the great solar car race here:

http://www.wsc.org.au/2003/photographs/

Solar cars are still toys for rich boys.

[skids]

4. "If it worked well, it would already be standard practice."

Untrue.

[jpak]

6. You are correct

What we need are NUCLEAR powered automobiles like they drive in Shangri La...

:rofl:

[skids]

3. Watt-hours, not Watts.

Usually it takes a lot less than a whole hour to drive 1 kilometer.

[Throckmorton]

5. makes no difference in this case.

1/6 HP = 125 watts. 125 watt-hours is just the time rate of the work expressed in hours. I don't care if it took him a microsecond or a month to travel the Kilometer, he still is claiming that it only took 125 watts to move this distance.

Which, when you consider the curb weight of a Prius is 2890 pounds, is very efficient. About on par with a human walking that distance in 1/2 hour.

[skids]

7. Power != Energy

It takes 27 Watts to light my desk lamp.

It takes 27 Watt hours to light my desk lamp for one hour.

It takes 54 Watt hours to light my desk lamp for two hours.

It would also take 54 Watt hours to light two of my desk lamps for one hour.

He claims the Prius (on average) uses 125 Watt hours for one Km of electric-only travel. Let's say it took one minute to spend this energy. That would mean the 125 Watt hours would have to be expended at a power rate of 125 * 60 = 7500 watts. Or 10hp. 10hp is a reasonable amount of power for sustaining forward momentum of a vehicle against air resistance, which is what cars spend most of their time doing.

[Throckmorton]

8. No I submit that ten horsepower is not a reasonable number

for the total energy expended to move a 2800 pound vehicle 60 Kilometers, at any rate of speed.

"He calculates the Prius uses 125 watt hours per kilometer in an urban driving environment."

[skids]

9. One kilometer.

Not 60.

(Oh, and on edit, no number of horsepower can represent energy. HP is a measure of power, not energy.)