Study provides new insights into fluctuations of wind energy, with implications for engineering and policy

January 2, 2017, Okinawa Institute of Science and Technology
Professor Mahesh Bandi is head of OIST's Collective Interactions Unit. Credit: Greta Keenan

The amount of energy generated by renewables fluctuates depending on the natural variability of resources at any given time. The sun isn't always shining, nor is the wind always blowing, so traditional power plants must be kept running, ready to fill the energy gap at a moment's notice. Because the grid has no storage, and unlike coal or nuclear, there is no control over the fluctuating production of renewable energy, the energy they produce has to be consumed straight away, or risk collapsing the electrical grid. On particularly windy days, for example, surges in power generated by wind turbines have been known to overwhelm the electrical grid, causing power outages. To avoid this, operators of large power plants sometimes resort to paying consumers to use electricity on particularly sunny and windy days when there is too much excess power in the system, in order to balance the supply and demand of energy at the grid.

Dealing with the peaks and troughs of intermittent renewable energy will become increasingly challenging as governments try to phase out of more stable coal-powered energy sources in the coming decades. In order to mitigate or manage these in , we need to understand the nature of these fluctuations better. Professor Mahesh Bandi, head of the Collective Interactions Unit at the Okinawa Institute of Science and Technology Graduate University (OIST) has used turbulence theory combined with experimental plant data to explain the statistical nature of wind power fluctuations in a single-author paper published in Physical Review Letters.

Wind speed patterns can be depicted as a wind speed spectrum on a graph. In 1941, Russian physicist Andrei Kolmogorov worked out the spectrum of wind speed fluctuations. Subsequently, it was shown that the spectrum for wind power follows the exact same pattern. However, until now, it was simply assumed that these spectra were identical due to the relationship between power and speed, where power equals wind speed cubed. But this proved to be a red herring. Professor Bandi has shown for the first time that the spectrum of wind power fluctuations follows the same pattern as wind speed fluctuations for a different reason.

Kolmogorov's 1941 result applies to measurements of wind speed made at several distributed points in space at the same time. But wind power fluctuations at a turbine are measured at a fixed location over an extended time period. The two measurements are fundamentally different, and by carefully accounting for this difference, Professor Bandi was able to explain the spectrum of wind power fluctuations for an individual turbine.

We can think of turbulence as a ball of air, or an 'eddy', of fluctuating wind speed. Long time-scale, low frequency eddies can span hundreds of kilometers. Inside these large eddies are shorter time-scale, high frequency eddies that might span a few kilometers. Therefore, if all of the turbines in the same wind plant fall within the same short and long time-scale eddies, the energy they produce fluctuates as if the entire plant were one giant turbine. This is exactly what Professor Bandi found when he looked at the wind power fluctuations of all of the turbines in a wind plant in Texas.

In fact, even geographically dispersed wind plants can exhibit correlated fluctuations in power if they fall within the same short and long time-scale eddies. However, as the distance between wind plants increases, their start to decouple from each other. Two geographically dispersed wind plants might encounter the same long time-scale wind speed fluctuations whilst encountering completely distinct shorter time-scale fluctuations.

In the past, some scientists have underestimated the problem of turbulence, arguing that the power produced by geographically dispersed wind turbines in windy and calm locations at any one point in time will average out when they reach a centralized grid. However, Professor Bandi's findings show for the first time, that this phenomenon, known as 'geographic smoothing', only works to a certain extent.

The power generated by geographically dispersed turbine plants averages at high frequencies, because while one plant might fall within the short time-scale eddy, the other might not. In other words, the surge in power output at one plant is averaged out by a trough in power output from another, far-away plant at high frequencies. But because the plants still fall within the same long time-scale eddy, the power they produce will have correlated fluctuations at low frequencies. A surge in power at one wind turbine plant will coincide with the surge at a far-away plant within the same long time-scale eddy, meaning that the power they feed to the grid cannot be averaged out. This means that there is a natural limit to how much one can average fluctuations in wind power; a limit beyond which fluctuations can continue to wreak havoc on the grid. Using data from 20 wind plants in Texas and 224 wind farms in Ireland Professor Bandi showed that this limit exists in reality.

"Understanding the nature of fluctuations in wind turbine power has immediate implications for economic and political decision making," says Professor Bandi.

Due to the variability of renewables, coal-fired providing back-up energy are kept running in case of sudden power outages, meaning that more energy is produced than needed. This means that 'green' energy is still contributing to carbon emissions, and there is an associated cost of maintaining reserve energy, that will only increase as the proportion of renewables increases in the years to come. The discovery of a limit in geographical smoothing, articulated by Professor Bandi, will enable better estimates of the operative amount of reserves that needs to be maintained.

This discovery will also impact environmental policy. By considering the limit for averaging fluctuations of , combined with the availability of different renewable resources such as sun, wind and waves in a particular area, policy-makers will be better equipped to work out optimal combinations of different sources for specific regions

"Understanding the nature of fluctuations for could also open up other avenues of research in other fluctuating systems," says Professor Bandi.

Explore further: Re-thinking renewable energy predictions

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WillieWard
2.3 / 5 (3) Jan 02, 2017
"The sun isn't always shining, nor is the wind always blowing, so traditional power plants must be kept running, ready to fill the energy gap at a moment's notice. Because the grid has no storage, and unlike coal or nuclear, there is no control over the fluctuating production of renewable energy, the energy they produce has to be consumed straight away, or risk collapsing the electrical grid."
"Due to the variability of renewables, coal-fired power plants providing back-up energy are kept running in case of sudden power outages, meaning that more energy is produced than needed. This means that 'green' energy is still contributing to carbon emissions"
"Suggesting that renewables will let us phase rapidly off fossil fuels in the United States, China, India, or the world as a whole is almost the equivalent of believing in the Easter Bunny and Tooth Fairy." - Dr. James Hansen(climate scientist)
Carbon-free nuclear power is the only way to stop climate change.
Seeker2
5 / 5 (2) Jan 02, 2017
Carbon-free nuclear power is the only way to stop climate change.
Try geothermal backup. There's enough going on under Yellowstone to drive the world and each watt of energy we pull out of there could delay the next eruption. It's what they call a twofer. Plus all the birds you're going to save from wind turbines and the radiation accidents and oil spills and carbon pollution you're going to prevent.
Seeker2
4 / 5 (1) Jan 02, 2017
Plus all the birds you're going to save from wind turbines...
Well that might not be such a bad thing around airports come to think about it.
Colbourne
4.7 / 5 (3) Jan 02, 2017
Hydro is a good backup power source and when there is too much power in the system you simply pump the water back up for future use. Even in areas where there is small rain fall this can be carried out as the water is in a closed loop system.
Seeker2
4.5 / 5 (2) Jan 03, 2017
Home power generation should be harder to hack I would think. It's a matter of national security.
humy
3.4 / 5 (5) Jan 07, 2017
The problem of energy supply from renewables being fickle has a very simple solution; just introduce a supergrid;

https://en.wikipe...per_grid

and then just perhaps combine that with just a bit of off-the-grid storage only if necessary, problem solved!
And yet this link didn't even consider this obvious well-known possible practical solution to this problem; why not?
I think the supergrid is the best solution.
Eikka
3 / 5 (4) Jan 07, 2017
The sun isn't always shining, nor is the wind always blowing, so traditional power plants must be kept running, ready to fill the energy gap at a moment's notice.


That is a false narrative.

Renewables such as solar and wind have such low capacity factors that the traditional powerplants have to be kept on more than off. It is more truthful to say that the conventional powerplants are on most of the time, producing the bulk of the energy, and the renewables barge in occasionally to take over the grid - and then go away just as quickly as they came.

That is a completely different problem to having a "gap" in production.

It means the conventional powerplants have to scamper off and return very quickly to accomodate the short bursts of power from renewables, which they largely can't, which makes the renewable power an unwanted extra and a surplus on the grid. The more there is, the more difficult it is to find some place to put it.
Eikka
3.7 / 5 (3) Jan 07, 2017
An example:

http://www.eia.go...main.png
Texas (ERCOT) hourly wind generation output (March 2014)
Source: EIA


The graph shows the hourly fluctuation of the whole system, which is less severe than of individual wind farms, but still has peaks and throughs that rise and fall up to 8-9 Gigawatts of power over the span of 24 hours and less, and individual spikes that go up 1-2 GW over the span of less than 6 hours.

The output is not steady with occasional gaps, it's all gap with power spikes like a porcupine's back. That makes it many times more difficult to build an energy storage system to deal with the variation, because it has to take in huge amounts of energy in short bursts and then release it gradually over a long period. Conventional hydroelectric dams don't have enough capacity for that.
Eikka
3.7 / 5 (3) Jan 07, 2017
Therefore, if all of the turbines in the same wind plant fall within the same short and long time-scale eddies, the energy they produce fluctuates as if the entire plant were one giant turbine.


That finding isn't entirely new. Previous studies over wind turbine output find that turbines spread over large geographical areas correlate in output up to 600 miles apart.

That is to say, entire countries/states behave like singular giant turbines. Same issue with solar power - everyone in the same geographical time zone gets sunlight on the same hour and each zone is 1000 miles wide, so balancing the load out by transmission requires ultra-long transmission lines that are vulnerable to breakdowns and accidents or terrorism and acts of war. These become the critical points of infrastructure, without which the whole system collapses.
Eikka
3.7 / 5 (3) Jan 07, 2017
One vulnerability of long transmission lines is geomagnetic storms.

https://en.wikipe...tructure

AC grids are more vulnerable to the induced very low frequency currents because transformers can't pass DC, but they also have means to filter and block it. DC grids don't mind so much, but they can't distinguish between genuine loads and geomagnetically induced currents by their frequency, so a good solar storm can blow up equipment and cause overload conditions at the ends of a long transmission line.

For cases like these, a state or a country must maintain a full reserve of conventional power generation capacity for the entire local demand, plus spares. Well, if they're wise. You can rely on imports.

Adding renewables merely lowers the utilization factor of these powerplants so you're paying money for not using them. The fuel you save is itself quite cheap; around 2/3 of the system cost is the infrastructure itself.
Seeker2
1 / 5 (1) Jan 07, 2017
Adding renewables merely lowers the utilization factor of these powerplants so you're paying money for not using them. The fuel you save is itself quite cheap; around 2/3 of the system cost is the infrastructure itself.
Adding renewables reduces peak demands and allows less, cheaper, and more efficient power generation and more reliable transmission. When the grid does go down at least you should have power when the sun shines or the wind blows.
Seeker2
not rated yet Jan 07, 2017
I think the supergrid is the best solution.
Sure seems that way. If you lose the grid you still have your own renewables.
Eikka
3 / 5 (2) Jan 07, 2017
Adding renewables reduces peak demands and allows less, cheaper, and more efficient power generation and more reliable transmission.


How does it do that when the peak output does not match the peak demand?

When the grid does go down at least you should have power when the sun shines or the wind blows.


When the large transmission links go down, all the renewables are useless because they need the grid for load balancing. The large wind turbines dotting the fields and the solar farms synchronize to the grid - they're not suitable for so called "island operation" where they're cut off from the larger power system.

If you try to bootstrap them up by using some small local power station, once they start up the wind turbines overpower the local grid and the frequency and voltage starts to go all over the place with the wind gusts - until something else blows up and the local grid too collapses.
Eikka
2.3 / 5 (3) Jan 07, 2017
Even home solar systems go down when the grid goes down, because again, the inverters are grid-synchronizing. If the grid goes down, the inverters need to go down and stop backfeeding the grid and causing an oversupply condition.

Or if there's a fault in the same sub-branch and the power company needs to cut power, the inverters drop automatically so your solar panels wouldn't feed an electrical fire in the neighbor's house.

It's a different case for people with proper off-grid systems with battery backed inverters and isolation switches, but most people who have solar systems have them built on the cheap to output power to the grid and collect the net metering benefits.
Seeker2
not rated yet Jan 07, 2017
Adding renewables reduces peak demands and allows less, cheaper, and more efficient power generation and more reliable transmission.
How does it do that when the peak output does not match the peak demand?
Pull power from the grid. That's what it's there for.
Seeker2
not rated yet Jan 07, 2017
When the large transmission links go down, all the renewables are useless because they need the grid for load balancing.
You're right. If you can't build a suitable stand-alone renewable forget it.
Seeker2
not rated yet Jan 07, 2017
...most people who have solar systems have them built on the cheap to output power to the grid and collect the net metering benefits.
So keep the cheap systems off the grid.
Eikka
2.3 / 5 (3) Jan 08, 2017
Pull power from the grid. That's what it's there for.


Does the electricity just come out of the pylons?? When the output doesn't match demand, there's no power in the grid to pull from. You get a blackout.

You're right. If you can't build a suitable stand-alone renewable forget it.


It's not a question of can't build, but who's gonna afford it?

So keep the cheap systems off the grid.


So stop giving out subsidies for them. That's the problem. There's all these incentives that are doing exactly the wrong things - giving money to people and companies to build more renewables without any care or responsibility about how they're actually going to work and co-operate with the rest of the system.

It can't go on like this - the whole system is going to implode one way or the other. The government can't keep taking people's money and throwing it down a well, because all the money that is wasted is actually energy and resources that are wasted.
Eikka
2.3 / 5 (3) Jan 08, 2017
The bottom line is, the energy economy is like the Tsiolkovsky's rocket equation - if you want your rocket to go up higher, you need to put in more fuel, and then you need to put in slightly more fuel to lift the fuel, and more fuel to lift the fuel to lift the fuel...

So in the same way, if you have to pay more money for your energy, you have to work (produce) more to earn that money, which means you're going to use more energy, which you then have to pay by using more energy... it escalates.

The current raw inputs of energy to the economy - fossil fuels - are seriously cheap compared to all the renewables. A kWh of coal or gas are 1-3 cents where each is cheap. Gas in Calfiornia, coal in China or Eastern Europe etc. A barrel of oil at ~$50 costs 3 cents a kWh.

Solar power in Dubai sells for 6 cents a kWh, and that's pretty much the cheapest there is. The US government is paying 23 cents a kWh to solar producers, and the actual customers yet more on top of that.
Eikka
3 / 5 (2) Jan 08, 2017
Alternatively, you could save energy to avoid working more, but if you can't save energy by increasing efficiency then you have to scale back in all aspects of your living.

To make a simplified example of it, if energy prices in general double, your house has to shrink into half; those who were already living poorly will be homeless on the streets.

Something needs to be done to make renewable energy seriously cheap, much cheaper than it is now, but all the means by which it could be utilized on a larger scale in increasing amounts actually cost more - not less.
Seeker2
not rated yet Jan 08, 2017
Solar power in Dubai sells for 6 cents a kWh, and that's pretty much the cheapest there is.
I saw somewhere in the world it was down to 3 cents but can't find the link now. In the meantime it's headed to 4 cents per https://cleantech...-4ckwh/. Also I understand Saudi Arabia is going solar so they can export more oil.
Seeker2
not rated yet Jan 09, 2017
deleted
WillieWard
not rated yet Jan 09, 2017
...down to 3 cents...
3¢/kWh! Who cares? If cost-effective batteries/energy storage does not exist or is prohibitively expensive.
It matters to the fossil fuel barons because it's coal and/or natural gas/fracking that keep lights on when sun is not shinning or wind is not blowing or during prolonged droughts .
Saudi Arabia is going solar so they can export more oil.
"The competition between renewables and fossil fuel is an illusion. They actually support each other."
https://uploads.d...a1f1.jpg
Eikka
3 / 5 (2) Jan 09, 2017
I saw somewhere in the world it was down to 3 cents but can't find the link now.


Those are predictions, not actualized prices. 3 cents can't be the full price - propaganda sites usually report just the power purchase agreement (PPA) price and leave the subsidies out to make it seem cheaper.

The low prices in Dubai/Saudi-Arabia at the moment are due to predatory pricing competition between local and foreign investment companies. Basically, the local guys are making backroom deals with the government, and publicly bidding lower than profitable to drive the foreign competitors out.

Prices are also being driven low by China which is dumping solar panels on the market with export subsidies - making their taxpayers pay for the shipping so the industry can collect the profit - and illegal dumping of silicon tetrachloride wastes, and REE mining wastes, in the environment, and the use of cheap coal power for energy to save on manufacturing costs.
Eikka
3 / 5 (2) Jan 09, 2017
Or as I've pointed out before, the reduction of high purity Silicon from sand involves a chemical reaction that uses carbon as a reducing agent. That is to say, coke made out of coal.

Alternative processes and alternative sources of carbon exists, but they're more expensive to use, so solar panels are in practice manufactured by Chinese coal, and the Chinese solar panels are dominating the market because people who install solar panels try to maximize profit in the absence of any audits or quality standards with the subsidies they're being paid.

Nobody's really asking whether it helps - the government is just paying people to build more.

Large corporations are investing in renewable energy simply because it gives them massive tax breaks, and in doing so they don't care whether it actually causes more carbon to be emitted or more energy wasted, so automatically the cheaters win.

That's realpolitik for you.
Seeker2
not rated yet Jan 09, 2017
Large corporations are investing in renewable energy simply because it gives them massive tax breaks, and in doing so they don't care whether it actually causes more carbon to be emitted or more energy wasted, so automatically the cheaters win.

That's realpolitik for you.
Good point. So how about oil and gas subsidies?
Zzzzzzzz
not rated yet Jan 10, 2017
wet manure is always heavier than dry manure - this page is getting really heavy......

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