# Harvesting wind energy for vehicles

Theoretically, what would the possibilities/drawbacks be of harvesting wind energy created by the momentum of the vehicle to charge that vehicle's batteries?

Imagine that the vehicle had a dual power system: a traditional electric system that initially started and powered the vehicle with charged batteries, and then a wind turbine system that harvested the wind created by the vehicle's momentum and used this energy to provide additional charge to the batteries, and thus lengthening the battery life...any ideas on how this system would be utterly impossible?

• It works without the batteries, youtube.com/watch?v=jyQwgBAaBag, with a wiki en.wikipedia.org/wiki/…
– John
Sep 7, 2022 at 20:40
• Any usable wind here would be exploiting motion of air relative to the ground (an external source of energy), not motion of air relative to your vehicle's motion relative to the ground (which you created yourself).
– Wyck
Sep 8, 2022 at 2:51
• Wind created by the vehicle is a waste effect. The best way to lengthen the battery life would be to improve the aerodynamic properties of the vehicle so it produces less wind. This is evident in currently available EVs. Sep 8, 2022 at 9:02
• @John It's important to note that that vehicle is being powered by the existing energy source of fast wind moving in the opposite direction of the vehicle, whereas what OP is proposing is something that parasitically draws energy back by creating drag, harvesting energy from moving air it itself generated by moving forward. Without 100% efficiency, it's just a drag parachute. Sep 8, 2022 at 12:57
• I've seen trains with wind turbines on some of their cars. They're used to power sensors for monitoring tank pressures, etc. in cars that are a half mile behind the engine (thus powering them directly is impractical). Something like that would be feasible, where the wind energy is being fed into a separate system. Feeding energy back into the same system that generated it, though, is a first-law problem.
– bta
Sep 8, 2022 at 22:42

# Conservation of Energy:

We don't have perpetual motion because you can't get more energy than what you put in. All forward momentum of a vehicle is derived from the car being propelled forward. I suppose in theory you could have a car with a sail or that goes down a REALLY big slope (like a mountain) that gets enough extra energy this way to be worth something like this, but it isn't likely.

The forward momentum of the car is powered by the engine, so the wind on the car is almost exclusively due to forward momentum exerted by the engine. You could use a system like this to brake the vehicle (but it would likely be not worth the cost to operate due to the added drag). This would really be a much less efficient version of the braking system on hybrid vehicles. In that case, braking reduces momentum by turning the breaking into a generator.

So unless you have a substantial external source of wind (which would be slowing the vehicle due to wind resistance, defeating the function of this system) I can't see how it would be worth it.

You could certainly design a sail for a vehicle. If you had a steady wind on your world, it MIGHT be possible to have a wind turbine that could be deployed when the car was stopped. This would allow the battery to be charged with wind power at this point. I doubt it would be worth the energy cost of hauling around the turbine, but if it was small/light enough, or the winds steady enough, then maybe.

• @server_unknown I haven't done the math or the engineering, but I'm 95% sure it would be less efficient. If there were a openable/deployable air intake, it would help address the issue (somewhat) but is still added weight. Sep 7, 2022 at 18:13
• It is actually possible to build a vehicle which is a windmill connected through gearing to a drive train directly. A relative of mine built a snowmobile version and experimented with it in Antarctica, where there is no shortage of wind the majority of the time. However, it is highly inefficient and relies on a very strong wind blowing across the ground, as stated, trying to use the wind of the vehicle's passage in still air is an attempt at perpetual motion. Sep 7, 2022 at 22:25
• @server_unknown would trying to harvest energy created by the moving tires/wheels themselves result in the same issues? You don't seem to grasp a key point here. Moving wheels don't create energy. What moves them? The engine. Wheels by themselves want to stay stationary, it takes energy from the engine to move them. Now if you're somehow generating energy from the movement of the wheels, then the only thing you're doing is reclaiming some of the energy that the engine put in them. With extra steps. [continued] Sep 8, 2022 at 7:42
• @server_unknown This will be, by definition, less efficient than simply deploying less energy to the wheels in the first place. Aka making them turn slower. Remember the two fundamental laws - conservation of energy states we cannot create or destroy energy, only change its form. And ... I don't know what it's called, but basically no mechanism/system is 100% efficient, and every transformation of energy will result in some losses to heat in the environment. Sep 8, 2022 at 7:45
• @server_unknown - What you need is to find some external source of energy - that is, energy that isn't output by the car's engine, and harvest that. A strong wind, solar power, whatever. But it has to be external, unrelated to the engine. Sep 8, 2022 at 7:47

The harvest apparatus needs to be external to the vehicle.

https://www.shell.com/inside-energy/turbine-turns-traffic-into-energy.html

If I have a windmill on my hat, when I run the windmill slows me down. It offers additional wind resistance and I have plenty already. The windmill recaptures a fraction of the energy I put in.

But suppose the windmill was off to the side. As I run past, the air I displace turns the windmill. That air has already been accelerated by me. The windmill slows down the air that I have already moved, recapturing some of the energy I put into that air. It is not extra work for me.

People have tried this: wind turbines that capture energy from wind produced by moving vehicles as depicted above.

Do not give up on the windmill hat, though. I see it as an update on the propeller beanie. You could wear it and stand in windy places. Not too near traffic I hope. The battery you charge could power an LED on your cravat, lighting your face from below in a mysterious way. And if it were a strobe LED you could adjust the speed to make it look like the windmill was turning very slowly which is the dream we all dream of.

The drawbacks are that it would be a pointless waste that reduces your range.

The energy in wind generated by the vehicle's motion comes from the vehicle's batteries. Thermodynamics dictates that if you try to extract energy from the wind and feed it back into the batteries, you will get less than you started with. It's basically just a very inefficient regenerative braking system. And you wouldn't want to drive a car with the brakes on all the time, would you?

You're better off doing nothing.

Now, if you want to try to exploit the energy of external wind sources--that is, air motion relative to the ground--that's an entirely different matter! Sailing works just fine.

Not impossible, but it will lose you energy.

The practical efficiency of wind turbines in ideal conditions reaches a maximum of 50%. The drag on the vehicle from the turbine in the air, would need to be compensated for by using extra power from the battery - more energy than you get from the turbine.

Taking into account the energy lost in the drive motors of the vehicle (say 40%) you'd need a turbine which provides 140% of the energy you get from driving the car fast. This is of course the impossible dream of perpetual motion.

## It is possible. IF you are traveling directly with the wind AND you have generators on the wheels as well.

but it makes it nearly impossible to turn and needs perfectly flat ground. It requires so much extra material and mechanics on the car its not worth the tiny gain in energy.

It works without the batteries even, you can sail a "car" downwind faster than the wind.

To see the principle in practice look here youtube.com/watch?v=jyQwgBAaBag the blackbird is a wind powered vehicle that can travel faster than the wind. It feels like it should violate the conservation of momentum but it does not. It can however only generate a tiny amount of additional force it is essentially an engine that works on the difference in relative velocity and it tops out at around 2.8 times the wind speed.

A wiki contain links to all the relevant papers and explanations.

and great video breakdown of how you can sail faster than the wind.

## After the OP's Edit

No it is not possible to harvest the energy generated by the cars motion, while it is under power. Any attempt will only cause the cars batteries to drain faster.

• This would be a wind-powered vehicle harvesting energy it gets from existing wind, there are many of those examples. It's not an electric vehicle harvesting energy (above and beyond what it expends) from the airspeed created by its own electrically-powered movement. Sep 8, 2022 at 18:53
• @AlexandreAubrey I think you misread what the car is doing, the car I mentioned is energetically the same, the car is moving faster than the wind pushing it. or to put it another way, in a 20mph wind this car is traveling at around 55mph. you could drive it with wind at the same speed as the wind and still generate electricity off the rotor.
– John
Sep 8, 2022 at 20:10
• I understand what the car in your example is doing. I understand how it can drive faster than the wind pushing it. But it's still wind-powered. I don't think that's what OP is asking about. My understanding of the question is "can I hook a windmill to my electric car for unlimited/extended range?". An answer of the type "wind powered vehicles can generate electricity" is not relevant. Sep 8, 2022 at 22:58
• @AlexandreAubrey don't worry about it before the OP's last edit it sounded like thy wanted to harvest the regular wind now they want perpetual motion.
– John
Sep 8, 2022 at 23:18
• The "you have to travel with the wind" is wrong in any way. Oct 24, 2022 at 8:50

### Land based

There are several things that make wind regeneration on a land vehicle a non-starter.

• Wind turbines are non-linearly more efficient the larger the cross sectional area. Street legal cars/trucks have very small permitted cross sectional areas.
• When trying to cruise, the wind turbine should offer zero resistance else it will be costing energy. This is difficult.
• It is additional system with moving parts. It is going to be additional maintenance and expense beyond an electric drive chain. Big factor in limited efficiency devices.
• Pure electric drive trains are trivial to have regenerative breaking with almost no additional components.
• Typically operate in speeds that minimizing air friction is worth engineering time, wind turbines defeat this.

### Water vehicle, non sail. ie diesel-electric.

• It is possible to have large cross section wind turbines.
• Problems with wind turbines being more that they can interfere with intended operation. ie block crane access for container ships.
• Mass of additional equipment has less impact on efficiency.
• Some ships do have wind turbines. Who pays for operating costs vs construction costs affects existence. ie a holding company that commissioned and holds the ships but doesn't pay for operating costs won't care if they have turbines.
• Typically operate a speeds where wind can assist, providing power while cruising.
• There are limitations to breaking due to minimum speeds required for steering control.

### Sail based vehicles.

With these systems, primary power would be wind, the electric drive are auxiliary if present. Regenerative breaking via wind is not feasible due to sails and turbines being not really compatible, they would be occupying the same space.

### Summary

Land vehicles, no, too impractical, multiple reasons.

Larger water vehicles. yes. These will become more common for ships that they won't interfere with function whenever energy costs are high and those who pay operating costs have a say in ship features.

a wind turbine system that harvested the wind created by the vehicle's momentum

You've just reinvented a simple perpetual motion machine. Unless momentum (actually the relative velocity between the vehicle and air, not momentum which is the absolute velocity of the vehicle, but that's a separate misunderstanding) comes from somewhere other than the vehicle's fuel source, you're actually going to get back less energy than you put in.

If the vehicle gets its energy from somewhere else though, or if the relative velocity of the air versus the vehicle is mostly due to the wind, then this absolutely does work. Wind turbines have been used on boats for decades. For a sailing boat, all the energy comes from the wind anyway, so there's no fuel. For boats with engines, if they're relatively slow (e.g. barges) then the wind is likely to be going faster than the boat and the air resistance due to the turbine is negligible.

For power boats or anything moving fairly fast, the boat is typically going faster than the wind and a wind turbine won't get back the energy you put in from air resistance. In that case you're better using some of your fuel to run a generator, which will be a more efficient way of producing power.

As an interesting alternative technology, it's worth mentioning that SkySails have a kite-based generator system whereby an electronically-controlled power kite is steered into the fast-moving higher-altitude airstream. With the kite under electronic control, a motor-generator can wind in the tether with the kite in a minimal-traction state, then put the kite into a maximum-traction state to pay out the tether whilst using the motor-generator as a generator. Over the wind-in/pay-out cycle, you get more energy than you use, and this is becoming popular for powering cargo ships. Note that the same basic principle still applies though - the energy comes from the wind, not from the speed of the vehicle.

Charging the batteries while the car move would add a lot of drag. Unless you have high wind in a favourable direction charging while moving would just take energy from the car movement and due to the inefficiencies of the generator the balance would be negative. The right conditions with right wind speed and directions are quite rare, so a wind turbine wouldn't be so useful.

However

Cars usually remain parked most of the time. The classic commuters case is the car that travels two hours back and fourth to the office and stays parked 8 hours in front of the office. Then the same car might travel less than two hours to the shopping mall or the restaurant and then remain parked the whole night in front of the house. Closed parking space is quite expensive and many cars spend most of the day parked outside. Those cars might harvest a little bit of energy from the wind using a small turbine that can be retracted in the boot while travelling.

Be it solar or wind the average car has a lot more time to harvest energy while parked than while moving. That is a period that should not be ignored.

Update after the question update

Harvesting energy from the momentum means taking kinetic energy out of it and slowing down the vehicle, it would be a braking system, but that would be solved simply with regenerative braking, no need for a wind turbine.

Ok ... this is a super controversial topic because (as others have pointed out) conservation of energy will prevent you from generating more energy from your windmill than you put in.

... but 1 ...

What you can do is affect the aerodynamics of your vehicle with a turbine in the front: