Wind turbine system recycles 'spillage' to improve energy efficiency

Wind turbine system recycles 'spillage' to improve energy efficiency
Jie Cheng, doctoral student in electrical engineering, stands next to the partial prototype of a wind turbine system that research suggests could yield 8.5 percent more electricity than conventional counterparts. Credit: Troy Fedderson/University Communications

A new prototype from the University of Nebraska-Lincoln's Jie Cheng could give a second wind to turbines that currently waste Mother Nature's strongest breaths.

Cheng, a doctoral candidate studying electrical engineering, has designed a system that could improve wind turbine efficiency by capturing and storing surplus for later use as electricity.

In a recent study based on historical wind data from rural Springview, Nebraska, Cheng compared the prototype's performance against a conventional wind turbine for a week. Cheng found a 250-kilowatt system would yield an additional 3,830 kilowatt-hours of electricity per week – about 16,400 extra kwh a month.

That extra juice would account for more than 18 times the monthly energy use of the average American household, according to the U.S. Energy Information Administration. The system would also marginally cut total energy costs while significantly reducing fluctuations in power generation, the study reported.

Physicists have shown that only 59.3 percent of the kinetic energy from wind can be captured by a rotor, with modern wind turbines designed to approach this limit. However, these turbines also have a capacity for generating electricity that corresponds to a given wind speed—a speed far slower than what nature regularly whips up.

Wind turbine system recycles 'spillage' to improve energy efficiency
Jie Cheng, doctoral student in electrical engineering, stands next to the partial prototype of a wind turbine system that research suggests could yield 8.5 percent more electricity than conventional counterparts. Credit: Troy Fedderson/University Communications

When the wind exceeds this speed threshold, turbines adjust their blade angles to maintain a constant energy output and prevent potential mechanical damage. The resulting "mechanical spillage" means that turbines fall well short of utilizing their full capacity.

Cheng's system helps resolve this problem by converting and directing the spillage to an air compression tank, where the excess energy remains until wind speeds dip enough to pull the turbine back beneath its optimal capacity. At that point, the tank kicks in to regenerate the electricity.

That way, Cheng said, the system addresses the wasted potential energy normally lost during strong winds as well as the energy deficit that turbines typically suffer during relatively calm periods.

"The biggest problem for is that it's not a reliable energy resource," Cheng said. "Even if there's not enough wind to generate electricity, the community still needs it. If we can (scale up) this system, it could improve reliability by producing electricity even when there's no wind."

Cheng said vast swaths of open land and strong, consistent winds that characterize the Cornhusker State have made it ideal for testing his system and could make it a good place to begin incorporating the design.

"We have a lot of energy here, and we live among grasslands, so there's little environmental (interference)," he said. "I see a lot of potential for this, especially in Nebraska."

Cheng is collaborating with Lincoln Electric System, the American Public Power Association and UNL's NUtech Ventures office to explore the market viability and introduction of his design.

The Journal of Power and Energy Engineering published Cheng's recent study, which he authored with adviser Fred Choobineh, a Blackman Distinguished Professor of Engineering.


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Citation: Wind turbine system recycles 'spillage' to improve energy efficiency (2015, September 17) retrieved 23 August 2019 from https://phys.org/news/2015-09-turbine-recycles-spillage-energy-efficiency.html
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Sep 17, 2015
So it's basically a CAES based battery.

The thing is, compressed air storage has abysmal efficiency because of compression heat loss and decompression pressure loss. When you compress the air, the tank gets hot and your energy spills. When you release the air, the tank goes cold and the pressure is reduced.

For small scale systems, the efficiency is barely above 10%. The slower you manage to compress/decompress, the more efficient it is, which is why the efficiency goes up when you scale the system up relative to the input.

It's good thinking, but it's thinking too small. CAES starts to make sense if you hollow out a volume the size of a small apartment building, or seal off an old mine for the air tank. Other proposals have been to sink large concrete spheres under the sea and use the water as counter-pressure.

Sep 17, 2015
The thing is, compressed air storage has abysmal efficiency

So?
Not having it wastes 100% of the spillage. If having it wastes 90% then you've just gained 10% extra energy production for very little spillage and alleviated some of the intermittency issue (which is a far greater bonus than that 10%).
And since air compression is a fairly robust/maintenance free it should pay for itslef rather quickly (especially since you can sell the excess energy when prices are at a premium).
CAES starts to make sense if you hollow out a volume the size of a small apartment building

Well whaddaya know: the hollow parts of the wind energy towers are just about that volume (and then some). No extra construction needed.

Sep 17, 2015
So?


The point of the thing is, that at the high wind speeds the power variation in gusty winds becomes too great for the turbine to handle. If it tried to utilize the power, it would drop out of sync with the grid and shut down, so it settles to throttle to a level "beneath the waves". The pump allows an extra load to absorb more power, but you will still eventually run into mechanical limits because the gusts bend and rattle the blades and bearings.

So there isn't -that- much more energy to be captured. In this case, 22 kW extra in a 250 kW system.

Well whaddaya know: the hollow parts of the wind energy towers are just about that volume (and then some). No extra construction needed.


It also has to withstand the mechanical stress of being repeatedly inflated and deflated. Otherwise you got a DeHaviland Comet situation just waiting to happen.

Old decommissioned salt mines have been used in the past.

Sep 17, 2015
If having it wastes 90% then you've just gained 10% extra energy production for very little spillage


Also, in larger wind farms or regions with lots of wind power, the other turbines already capture the energy spilled over.

The better the individual turbines capture energy, the more they slow down the wake they leave behind, so it's not strictly necessary to optimize individual turbines to maximum efficiency rather than minimum price and minimum maintenance.

Rather, it would be cheaper and more effective to have a larger CAES battery that is served by multiple turbines through the grid. The efficiency goes up and the price goes down because you don't need to duplicate effort and maintain so many moving parts.

Sep 17, 2015
(especially since you can sell the excess energy when prices are at a premium).


That too depends on the size of the system. At this small scale, you have to sell it in minutes between the wind gusts or otherwise you have no more capacity left to catch more.

The power of a wind turbine increases in V^3 so it naturally produces enormous amounts of energy in very short timescales, which is the reason why the gust power variations becomes so great. A 1 m/s difference or gust at 12 m/s is eight times greater in power than the same gust at 6 m/s average wind speed. In other words, when you're in a storm, the waves are greater.

Fortunatelly, that means you can use adiabatic compression which means the pressure vessel is insulated and the heat is not lost between cycles, and that allows for a greater efficiency.

As the system is scaled up from minutes to hours, you can't maintain adiabatic conditions anymore and the efficiency drops. You need to go bigger still.

Sep 17, 2015
It's a good thing Eikka did not tell us they were not economically feasible before we built them.

We might have missed the 2.5 cents/kWh wind power.

Sep 17, 2015
It's a good thing Eikka did not tell us they were not economically feasible before we built them.

We might have missed the 2.5 cents/kWh wind power.


Oh, they already have negative prices for wind power. They pay you to take it. Just visit Denmark in the winter.

http://www.reuter...20140109
On December 24, 2013, when industrial and business power demand dropped sharply, the price of German power for intra-day delivery fell to an average of -35.45 euros per megawatt-hour (MWh) between 0000 and 0600 in the morning, touching lows of -62.03/MWh halfway through that period.

German power traders said prices fell this low due to huge wind power production, at times exceeding 20 gigawatts or two thirds of capacity, that was coupled with mild weather.


Unfortunately, that doesn't mean the electricity is actually that cheap. It's being paid by subsidies.

Sep 17, 2015
Nukes get greater subsidies, and would not even exist without them, yet the cost to produce (not sell), power from Vogtle Three is already up to thirteen cents, and still climbing.

Sep 17, 2015
In Denmark and Sweden, strong winds also caused negative prices on Christmas Eve, with contracts in western Demark falling to -6.28 euros/MWh as wind farms generated near full capacity.

The Elbas intraday market for the Nordic, German and Estonian markets offered by the Nordpool Spot exchange saw the average price drop to a low of -30 euros/MWh during one hour on December 25.


This is why battery systems like CAES are desperately needed.

The situation is already ridiculous, because governments are paying wind power producers subsidies for every kWh made, so they keep making those kWhs despite the fact that they have to pay the utilities to take it, so that the utilities could cover losses on poweplants that sit burning fuel at idle while the wind blows.

So you're paying the government to pay the wind power producer to pay the utilities for wind power that nobody needs.

Sep 17, 2015
Nukes get greater subsidies, and would not even exist without them, yet the cost to produce (not sell), power from Vogtle Three is already up to thirteen cents, and still climbing.


Bullshit.

It takes a good old Texas ten gallon hat to pull those figure out of.


Sep 18, 2015
What Jie Cheng, the article's author, and most commenters here apparently fail to understand is that a properly designed wind turbine does not simply spill wind energy when its generator has reached maximum power. The entire system, the foundation, the tower, the bearings, the gearbox, and the blades are designed for that maximum designed power level. Were this not so, it would be simple enough to simply install a larger generator to up the design power level.

Turbine manufacturers and the operators who buy them have long ago identified the optimum performance criteria for economical operation. There may be utility in the system described, but it is not in the wind sector.

Sep 21, 2015
Were this not so, it would be simple enough to simply install a larger generator to up the design power level.


The towers and turbines are designed to withstand over 22 m/s winds, but they generally throttle at 12 m/s not because of mechanical concerns, but because the power variation in gusty winds above that level would put too great a strain on the transmission system.

If you have a ~1 GW wind farm operating at the 12 m/s peak and the wind speed increases by 1 m/s for five minutes, you would have a sudden power surge of around 300 MW which is equivalent to bringing a full size gas turbine powerplant on and off, or as if around 100,000 households suddenly became disconnected from the local grid at the flick of a switch.

You just can't put that into the grid.

Well, you -can- but you're risking cascade failure and systemic blackout like what happened in Germany in 2006.

Sep 21, 2015
You will have to learn to control your inverter outputs.

Or, maybe batteries will save you, the kind they already have in Germany for reactance control.

Sep 21, 2015
You know... if people listened to the naysayers like Eikka double guessing every design decision we'd still be riding horses.

If it wasn't worth doing then no one would be doing it.

There's money to be made in them there wind turbines, and only those who build them will make it. You haven't come up with one good reason why they're not a great idea.

So far your front runner is because some stupid birds might fly into them, as if domestic cats don't kill many thousands of times more birds or that birds fly into windows far more than wind turbines.

But don't let facts ruin your rant. Carry on. Tell us how burning coal is good for your lungs.

Sep 22, 2015
If it wasn't worth doing then no one would be doing it.


That would be true if the governments weren't paying for it to be done anyways.

Try the free market without subsidies if you want to make that argument.

So far your front runner is because some stupid birds might fly into them


I think you're confusing me with someone else.

My main complaint is that they're unreliable and inconvenient in their intermittency, and as a result highly expensive as an energy source. As such they aren't a solution to our energy needs and problems, instead creating more problems than they solve, yet we are being forced to pay ever-increasing amounts of public money to the few private companies that build them.

Sep 22, 2015
You will have to learn to control your inverter outputs.


The inverters can't just "magically" slip several hundred megawatts of extra power on and off the grid without so much as a moment's notice.

Or, maybe batteries will save you, the kind they already have in Germany for reactance control.


At $300-500 per kWh capacity, the few extra megawatts you get on average are going to be super expensive - and that's going to be right when the wind is blowing hard and the prices are falling towards zero due to oversupply.

It just doesn't work until there are enough batteries to shift the output to the next day.

And as I already explained, catching all the wind at one turbine is unnecessary because you have many turbines anyways that catch whatever spills over. That's why it's absolutely pointless to try to use these complex and costly systems to optimize individual turbines.

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