# Homework Help: Solar panel and regenerative braking system

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1. Sep 29, 2014

### huzzi.123

1. The problem statement, all variables and given/known data
For my ongoing project, I'm thinking of using Solar panels and Regenerative braking system as two main sources to charge the batteries that will run the motor and in turn the accessories of the car.

2. Relevant equations
I've to think of how can the above sources increase the overall efficiency of the car?

3. The attempt at a solution
1) The use of battery pack can reduce the overall weight of the car and thus decreases the aerodynamic drag.
2) Using solar energy as an alternative to non-renewable resource such as petroleum that means the car won’t emit dangerous gases while using solar power to run the accessories.

Wondering if the above statements sound legit?

2. Sep 29, 2014

### Staff: Mentor

They sort of sound true, but on closer inspection ....

1) the battery pack gives a reduction in weight in comparison to what alternative??

2) minimizing dangerous gases may be true, but hasn't been of concern to most petrol-engine drivers all these years---except in mine shafts and enclosed spaces such as inside buildings and refrigerated storage sheds. Oh wait, you are talking about having to run the engine to power, say, the aircon while parked?

3. Sep 29, 2014

### huzzi.123

reduction in weight compared to regular combustion engine cars
but it still sounds technically correct? Because all I'm concerned about is increasing the efficiency of a car

4. Sep 29, 2014

### CWatters

You should probably start by defining what you mean by "efficiency"

Miles per Gallon
Energy out/Energy In?
$per mile? The fuel for a solar powered car is free so would that mean you consider a solar powered car infinitely efficient? On the other hand batteries are expensive and wear out. Currently they add a lot to the running cost of an electric car. 5. Sep 29, 2014 ### huzzi.123 Efficiency in the sense that it can overcome the losses mentioned in the link below: http://www.consumerenergycenter.org/transportation/consumer_tips/vehicle_energy_losses.html [Broken] Last edited by a moderator: May 7, 2017 6. Sep 29, 2014 ### OmCheeto Weight is not listed in the equation for drag, and therefore, has no effect. from wiki: FD = 1/2 ρ v2 CD A FD is the drag force, ρ is the density of the fluid v is the speed of the object relative to the fluid A is the cross section area CD is the drag coefficient – a dimensionless number.​ 7. Sep 29, 2014 ### NascentOxygen ### Staff: Mentor Electric cars are being constructed to be smaller than most petrol-engined cars, but these small production electric cars can't cruise the non-stop distance of their petrol rivals on one "tank". Their advantages lie elsewhere. Lighter weight means less energy is needed to get the vehicle up to speed and up hills. 8. Sep 29, 2014 ### huzzi.123 Makes sen Makes sense. But does solar car with (regenerative braking system) help reduce idling losses (when it is not running) or any engine losses? 9. Sep 30, 2014 ### OmCheeto If you're talking about a hybrid electric-gasoline vehicle, and the gasoline engine is not running, due to the start-stop technology listed in http://www.consumerenergycenter.org/transportation/consumer_tips/vehicle_energy_losses.html [Broken] earlier then the gas engine losses are zero. As long as you don't count the energy required to restart the gasoline engine. Which from some guy at car talk: varies quite a bit. But if you have 750 watts worth of solar panels on your roof, that range of energy can be collected in between 1/2 second to 1/2 minute. As for your idea in general, I like it a lot. I know a guy who's alternator broke just as he was leaving work one day. Instead of spending$100+ bucks to get towed home, he temporarily installed 150 watts worth of solar panels to his car. Believe it or not, it worked. The trip was about 13 miles, and took about 30 minutes.

Of course, it was a sunny June weekend day when he did this. Had it been a typical November cloudy weekday during rush hour, he would have called a tow truck.

ps. He has since replaced the car. It was the mother of all lemons. It was towed home on two other occasions, and should have been two more times. But the guy is a bit of a nut.

Last edited by a moderator: May 7, 2017
10. Sep 30, 2014

### huzzi.123

Sounds good but doesn't electric motor needed to run a car? So solar panel will charge the batteries and batteries will in turn run a motor to convert electrical to mechanical energy. I haven't done much research on hybrid electric cars but do you think the idea I'm talking about refers to hybrid electric cars? I heard that regenerative braking system comes into play when the brakes are applied, the heat generated is converted into electrical energy so engine turns off and energy is saved but I was wondering if solar panel can contribute something to overcoming the losses mentioned in my link before?

Last edited by a moderator: May 7, 2017
11. Sep 30, 2014

### OmCheeto

It's your job to tell us what you are designing. A pure electric car will either be expensive and have a decent range, or cheap and almost no range. I calculated last week that a battery with a range of 25 miles would cost \$5000. A hybrid car like the Chevy Volt is in my opinion a very well matched system. It has a range of 35 miles in full electric mode. The average American between the age of 20 and 55 drives 41 miles per day, on average. (ref) I think you could probably outfit a Chevy Volt with 500 watts of panels, which if you left it in the sun for 8 hours, would generate 4 kWh of energy. Since the battery pack is 16 kWh, you can interpolate the "miles per kWh " as being roughly 2 miles per kWh. So 4 kWh will power the car for about 8 miles.
I'm not aware of any regenerative braking systems that convert heat into electricity. That would be very inefficient. They use generators to slow the vehicle while simultaneously charging the battery pack.
In a gas-electric hybrid, yes.
Yes. A solar panel will collect energy, which will either charge the battery, or run auxiliaries.

12. Sep 30, 2014

### CWatters

To my mind it makes little sense to put solar panels on a general purpose electric car. The amount of power you can collect on a trip will be very small compared to the amount you need to make a typical trip. Much better to use the space and weight that the solar panels would occupy to fit a slightly bigger battery. Build a much larger array of solar panels at home/work or some field to charge it.

Regenerative brakes don't convert the heat produced by conventional brakes. Many electric motors (such as those used to power an electric car) can be used "in reverse" as generators - so adding regenerative braking to an electric car might not add any significant weight just a bit of complexity to the motor control system.