Apparent Energy Gain

louis · 2021-05-31 17:18:29

An interesting video about a wind powered vehicle that can go downwind faster than the wind speed...

https://www.youtube.com/watch?v=jyQwgBAaBag

Reminds me of debates I had some time ago with "conservation of energy fanatics" who would simply not countenance the idea that you could have a boat that had a wave energy contraption attached which could harness energy from its own bow wave. I wasn't putting the idea forward as a practical method of transport but as a theoretical concept for energy gain. I remember in the end I resorted to things they couldn't possibly deny e.g. that there could be wave machines at the side of the river taking up the energy of the bow wave and relaying it back to the boat as microwave energy. At that point they would say all you have is a more efficient engine lol.

Calliban · 2021-06-01 10:54:40

Good video. Remember that wind speed increases with height. The propeller mounted on top of the vehicle is exposed to a higher wind speed than the drag sock mounted on the fuselage. The fuselage is travelling through a turbulent boundary layer, whereas the propeller is at least partially raised above the boundary layer. Hence, it appears that the vehicle is moving faster than the wind, which it is, if you measure the wind speed within the boundary layer close to the ground. But not if you measure the wind speed at the height of the propeller. The propeller is in this instance functioning as a sail.
https://en.m.wikipedia.org/wiki/Boundary_layer

The analogy to tacking is not strictly appropriate - the effect being observed here is due to increasing wind speed with height. Though you could conceivably tack with a land sail vehicle. The speed can in fact exceed the speed of the wind (though not dramatically), provided that excess drag and friction are both kept low. The concept does not in any way violate the conservation of energy. The kinetic energy gained by the vehicle is exactly equal to the kinetic energy lost by the air, plus some turbulent energy losses that will ultimately raise air temperature. I'm not quite sure where the reference to wave power comes into this.

The problem with powering oceanic vessels with the wind (or waves) is limited speed. Both concepts work from a purely mechanical viewpoint and indeed the former conveyed most of the world's long distance freight before the development of steam engines. But within a few decades of the development of coal powered steam ships, wind had all but disappeared as a practical energy source for long distance freight and passenger transportation. Why? The same economic forces that resulted in the rapid phase out of wind, hinder is reintroduction today. Low speed. Practical speed for a sailing vessel is in fact only a fraction of wind speed, due to the drag acting on the hull and the limit of thrust impulse that can be applied as a fraction of buoyant forces - thrust exerts a turning force that acts to push the keel under water.

Reducing speed has a very strong negative impact on transport economics, because the fixed capital costs and operating costs of the ship, must be amortised over a smaller number of tonne-miles or passenger miles delivered annually. Revenue is therefore proportional to average speed. Given that profit is only a small proportion of revenue, a small decline in average speed will seriously eat into profitability. Using wind will of course eliminate fuel costs. But for that advantage to outweigh the increase in other operating costs and capital costs, would suggest that fuel costs would need to be high indeed for wind power to break even.

One way of mitigating this is kite power. Wind speed increases with height, due to boundary layer effects. A kite can therefore access wind speeds far above those experienced by ordinary sails. We could use this to augment propulsion in diesel powered ships right now - it has been talked about since the 70s. Also, the kite does not need to be mounted to a tall mast, so the turning moment isn't as limiting. I'm not sure if it is practical to tack with a kite sail.

Last edited by Calliban (2021-06-01 11:15:06)

louis · 2021-06-01 13:11:37

The reference to wave energy was the question of whether you can use the energy from the bow wave of your own ship to add to propulsion. Many commenters I found disputed you could even though to me it's pretty obvious you can. It might be marginal but you could add to your speed. I recall now that the discussion grew out of an earlier discussion about whether lorries could capture some of the energy from their own air turbulence to add to the power and speed of the vehicle. Again , I think the answer is yes.

Calliban wrote:

Good video. Remember that wind speed increases with height. The propeller mounted on top of the vehicle is exposed to a higher wind speed than the drag sock mounted on the fuselage. The fuselage is travelling through a turbulent boundary layer, whereas the propeller is at least partially raised above the boundary layer. Hence, it appears that the vehicle is moving faster than the wind, which it is, if you measure the wind speed within the boundary layer close to the ground. But not if you measure the wind speed at the height of the propeller. The propeller is in this instance functioning as a sail.
https://en.m.wikipedia.org/wiki/Boundary_layer
The analogy to tacking is not strictly appropriate - the effect being observed here is due to increasing wind speed with height. Though you could conceivably tack with a land sail vehicle. The speed can in fact exceed the speed of the wind (though not dramatically), provided that excess drag and friction are both kept low. The concept does not in any way violate the conservation of energy. The kinetic energy gained by the vehicle is exactly equal to the kinetic energy lost by the air, plus some turbulent energy losses that will ultimately raise air temperature. I'm not quite sure where the reference to wave power comes into this.
The problem with powering oceanic vessels with the wind (or waves) is limited speed. Both concepts work from a purely mechanical viewpoint and indeed the former conveyed most of the world's long distance freight before the development of steam engines. But within a few decades of the development of coal powered steam ships, wind had all but disappeared as a practical energy source for long distance freight and passenger transportation. Why? The same economic forces that resulted in the rapid phase out of wind, hinder is reintroduction today. Low speed. Practical speed for a sailing vessel is in fact only a fraction of wind speed, due to the drag acting on the hull and the limit of thrust impulse that can be applied as a fraction of buoyant forces - thrust exerts a turning force that acts to push the keel under water.
Reducing speed has a very strong negative impact on transport economics, because the fixed capital costs and operating costs of the ship, must be amortised over a smaller number of tonne-miles or passenger miles delivered annually. Revenue is therefore proportional to average speed. Given that profit is only a small proportion of revenue, a small decline in average speed will seriously eat into profitability. Using wind will of course eliminate fuel costs. But for that advantage to outweigh the increase in other operating costs and capital costs, would suggest that fuel costs would need to be high indeed for wind power to break even.
One way of mitigating this is kite power. Wind speed increases with height, due to boundary layer effects. A kite can therefore access wind speeds far above those experienced by ordinary sails. We could use this to augment propulsion in diesel powered ships right now - it has been talked about since the 70s. Also, the kite does not need to be mounted to a tall mast, so the turning moment isn't as limiting. I'm not sure if it is practical to tack with a kite sail.

Calliban · 2021-06-01 16:40:28

There is nothing impossible about recovering energy from ship bow waves or indeed from traffic induced air currents. The point is not whether it is possible or not in some idealised or abstract situation. It is more a question as to whether it is desirable compared to other options, when the energy return is compared to the energy and money that need to be invested to make it work. Designing a vehicle based on new propulsion technology is horribly complicated. A balanced design needs to balance capital cost, engineering complexity, ease of maintenance, reliability, down time, operating cost (both fuel and parts replacement), safety, speed, weight, comfort, aesthetics, range and so on. Hopefully, at the end of the design process, you have a product that enough people want to buy and can afford to buy, for you to have a market that covers initial investment costs. Plenty of good ideas have ended up going nowhere, because they failed to achieve the best balance of customer needs. Nothing about the design process is simple. And the fact that some idea or other may work at some level, provides no guarantee of success unless it can provide a definite lifetime performance advantage at a competitive cost, without pushing up things like operating cost, weight or reducing system reliability. This is why no one is using wave powered ships. It is why wind powered cargo ships belong to history (at least for the time being). These things end up failing - either they are too slow, too unreliable, too labour intensive, etc. Getting them to work at some functional level provides no guarantee of success.

To give an example, diesel engines exhaust at temperatures of up to 500°C and dump around half of their total heat output into exhaust gases. A waste heat engine using a closed vapour cycle could boost engine efficiency by 30-50%. And it is definitely technologically achievable, so it would seem like a no brainer to go ahead and build it into a design. But it would add weight (which would increase friction). It would add to capital cost. It would need to be maintained. It would effect acceleration. So even though it would boost efficiency and is fully achievable, it ends up not featuring in new design vehicles.

Last edited by Calliban (2021-06-01 16:53:08)

louis · 2021-06-01 16:55:48

People have begun looking into air turbulence on and around motorways and it is quite significant in energy terms. Not suprisingly when you think that a large part of the energy of moving a vehicle forward involves pushing air out of the way. If we had electric roads for instance and an efficient means of capturing the air turbulence energy and then returning it to the electric roads (induction charging of EV batteries) that make a real contribution. The good thing about motorway air turbulence is that it is there all the time and is not dependent on the wind.

Calliban wrote:

There is nothing impossible about recovering energy from ship bow waves or indeed from traffic induced air currents. The point is not whether it is possible or not in some idealised or abstract situation. It is more a question as to whether it is desirable compared to other options, when the energy return is compared to the energy and money that need to be invested to make it work. Designing a vehicle based on new propulsion technology is horribly complicated. A balanced design needs to balance capital cost, engineering complexity, ease of maintenance, reliability, down time, operating cost (both fuel and parts replacement), safety, speed, weight, comfort, aesthetics, range and so on. Hopefully, at the end of the design process, you have a product that enough people want to buy and can afford to buy, for you to have a market that covers initial investment costs. Plenty of good ideas have ended up going nowhere, because they failed to achieve the best balance of customer needs. Nothing about the design process is simple. And the fact that some idea or other may work at some level, provides no guarantee of success unless it can provide a definite lifetime performance advantage at a competitive cost, without pushing up things like operating cost, weight or reducing system reliability. This is why no one is using wave powered ships. It is why wind powered cargo ships belong to history (at least for the time being). These things end up failing - either they are too slow, too unreliable, too labour intensive, etc. Getting them to work at some functional level provides no guarantee of success.

SpaceNut · 2021-06-01 18:21:03

Its in the math of the design as the water is not a wheel on a salt flat nor is a sail a prop blade that can change angle of attack..
Its not self evident that the prop is converting the spin to motion of the wheels via mechanical or through electrical conversion or that is just the force of the wind pushing the vehicle forward as in a soap box derby car.

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#1 2021-05-31 17:18:29

Apparent Energy Gain

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