Relying on the renewables for power: challenge of still, dark days

wind turbines


On Boxing Day in 2020, we were mostly indoors due to the lockdown over the Christmas period in the United Kingdom; nature hardly helped alleviate the gloom during the difficult time either in the hour of national need; on the contrary, its fury developed into a monstrous storm, knows as Storm Bella, which relentlessly battered the British Isles.

Being true to the cliché, the dark clouds did have a silver lining as far as the camp of renewables was concerned; the winds of Storm Bella, with gusts up to 100 miles per hour, played its role to make history in the United Kingdom; more than 50% of the electricity needs of the country came from wind power on Boxing Day – for the first time ever.

Although it is a cause for celebration as far as the renewable sector is concerned, the inevitable challenges were not eclipsed by the technological feat; to put it simply, it was a random wind speed that did the trick rather than the human quick thinking in harnessing the wind power on this occasion. Storms are few and far between.

Once Storm Bella lost its momentum, so did the wind turbines, reducing the proportion of power from the stormy winds to where it was before the Boxing Day. Of course, in 2020, there were still power stations which burned fossil fuels to make up for the loss of power from the wind turbines.

In short, we had something to fall back on for a rainy day.

In another development, it was reported that in September last year, the electricity system operator was forced to increase the use of gas-powered power plants by 20% in order to compensate for a slump in electricity generation by renewables; the low wind speed was a major factor for the decision.

That means if storms of tornado calibre can cause power surge in wind turbines, the inverse is not just hypothetical; there are days with low winds and wind turbines do not generate enough power to fulfil the energy needs of an industrialized nation; there is a mathematically measurable caution that you cannot throw into the winds.

The power generated by a wind turbine is given by the relatively simple formula, P = πρCp(r.r)(v.v.v) , where Cp, r and v are density of air, power coefficient, length of the blades and the speed of the wind respectively.

There is just one factor in the equation that the human intervention can change in order to maximize the power of a wind turbine – the length of the blades; it has a limit, though; otherwise, the strain on the vertical body where the heart of the turbine, the generator, is placed,  going to be unbearably enormous.

Wind speed is beyond our control and is at the mercy of unpredictable weather patterns.

When the winds hit the blades, the kinetic energy, KE, of the former turns into the electrical energy of the generator inside a wind turbine; it’s, however, 59% of the energy carried by the winds; the rest is lost as heat and sound – inevitable.

The 59% limit, known as Betz’ Limit, estimated by Albert Betz, a German Physicist, is the ideal power that we can harness from the winds at most; in reality, it is even less; the power coefficient determines the actual power generated while taking into account the Betz’ Limit.

With a wind turbine that consists of 20m long blades, being subject to a power coefficient 0.2, we can generate around 2.4MW of power provided that wind blows at a sustainable speed of 72 km/hour; even for offshore wind turbines, this speed remains to be an aspiration.

I created an interactive animation to give some idea while illustrating how the power of a wind turbine is determined by wind in an ideal environment – on or offshore; it also shows physics behind the conversion of energy, from the kinetic energy of the wind into electrical energy of the generator.

Although wind farms spoil the landscape, harnessing winds in order to generate electricity is a step in the direction.

As far as the United Kingdom is concerned, it’s relatively easy as the country is blessed with a relatively shallow sea belt around the British Isles. In addition, thanks to the decades of involvement in the offshore oil fields, especially in Scotland, there are professionals with the right skills to help the expansion of wind farms beyond the shore limits.

It, however, does not mean we are out of the woods yet; time and again, analysts raise the predictable issue of turbulent times with respect to the anticipated fluctuations in electricity generation, when wind doesn’t blow and the sun doesn’t give enough light – the dark, still days, when it comes to relying on solar power.

In order to mitigate the impact, these analysts want to focus on daily picture, rather than cumulative picture that comes out after relatively long periods of time, as a better way of gauging, both the reliability and inevitable risks involving the renewables.

Since no weather model can ever predict the onset of still, dark days ahead in an accurate way, it’s very unrealistic to rubbing our hands with glee to dream of the end of fossil fuels in generating electricity; it will stay relevant for years to come.

When the well-known electric vehicle makers say in public that there will not be enough electricity to power them, the reality is no longer the elephant in the room.



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