When you place a turbine into a stream (without a dam and a duct for the water to flow through), doesn’t most of the water just move around the turbine (and thus perform no work)? Take wind turbines-obviously, they move,but how much of the potential wind power is wasted because the wind is free to blow around the blades?
Many years ago, a professor of mechanical engineering had an idea to place (water) turbines in the cape cod canal (due to the difference in high tide times at the ends, there is continuous flow of water through the canal). Would such turbines capture much of the potential energy of the water flow, or would the water simply bypass the turbines.
Has anyone buily wind turbines with “dams”? Do you realize more energy output this way?
No, or else there would be an observable region of non-flowing fluid around the turbine (or a vacuum, I guess, if the water was actually intelligently avoiding the turbine).
I don’t think so, but putting a whole farm of properly-spaced turbines in place should do the same thing - flow that misses one will interact with the neighbour instead.
The surrounding fluid acts to partly prevent fluid just diverting around the turbine- it’s not empty space, so in order to go around the turbine, it has to displace something - or the surrounding flow goes faster, but:
[ul]
[li]That’s normal - flow through the turbine should be slower than normal, if we’re extracting work[/li][li]Another turbine in can be placed in the marginally faster flow to take advantage of that.[/li][/ul]
Not sure what you mean by wind turbines in dams but hydro energy is pretty common, google Snowy River Scheme. But yes some of the kinetic energy is transferred to the turbines and some is still contained in the water.
There’s a finite amount of work available in such a system - the more turbines you put in the canal, the more you impede the flow - and the output of all the turbines drops a little - eventually, you could reach a point where there are so many turbines in the canal that the water flows too slowly to operate any of them efficiently, or actually, that you damp out the tidal cycle completely and the water level stays close to the mean tide level all the time.
An interesting concept known as Betz’ Law says that the theoretical maximum efficiency of a wind turbine is 59.3% (that is, it can extract no more than that percentage of the energy in the wind it receives).
Well designed real-world wind turbines achieve something like 75% of this - IOW about 45% efficiency.
Have they noticed any effects on weather downwind or wind turbines?
If they are producing electricity, they are obviously extracting energy from the air, and that should change weather patterns (unless the effects are too small to notice).
Yes. The effect is more due to turbulence, rather than from slowing the wind as energy is extracted…
Interesting…how is it that the latest America’s cup boats can sail 3x as fast as the wind that propels them?
Some researchers believe that there is a limit to how much energy can be extracted from the environment. Not all the energy is “free” - much of the energy from the sun striking the earth is used to drive weather patterns, the water cycle, plant growth etc. So the extraction of too much energy from environmental sources may well impact weather patterns. However, the limits that they are estimating are about half global energy consumption - a limit for environmental sources that we are nowhere near at this stage.
Energy extraction is not directly related to speed of the wind. The wingsail extracts energy from differential between the moving wind and the static water, then uses it to move the boat - because the wingsail is big, it can extract a lot of energy and thus drive the boat fast, and once the boat is foiling, it has very little drag and goes very fast.
There was a lot of discussion about “downwind faster than the wind” (relating to wind powered carts) that may be instructive - search on youtube.
The easiest way to visualise this (IMO) is:
Cut a lemon in half and extract one of the pips.
Squeeze the pip between your forefinger and thumb
The pip will ‘squirt’ out at a speed greater than the speed your fingers closed together.
Your forefinger is the force of the wind, your thumb is the boat’s keel in the water - the pip is the boat.
As others noted, maximizing speed is not achieved by maximizing energy extraction ratio.
Betz’ Law is only interesting to stationary turbines, that just sit there and wait for the air to flow through them. Such turbines cannot extract all the energy from the air, because that would stop the air relative to them, and then the air could not leave the turbine. So there is a limit on the amount of energy a stationary turbine can extract, based on true wind speed and turbine size.
A wind powered vehicle is not stationary, so there is a third factor: it’s velocity. The faster it moves, the more air it interacts with, so there is no upper limit on the amount of extracted energy. Betz’ Law still applies in the rest frame of the vehicle, but that is a moot point because that frame is not inertial. As the vehicle accelerates, the amount of available energy grows in its rest frame. So the Betz fraction doesn’t limit the amount of extracted energy here.
Here is more:
http://orbit.dtu.dk/fedora/objects/orbit:55484/datastreams/file_3748519/content
Also known as the wedge analogy, visualized here:
Near the surface, all obstacles (trees, hills, buildings) are extracting energy from the air all the time. And that energy is just “wasted”. A small wind turbine will affect the wind just like a tree would. But if you build them taller and bigger, you get into layers of air that are usually unaffected by ground obstacles.