26 September, 2022


Stochastic & Interruptible Electricity Sources

By Kumar David

Prof. Kumar David

As the country moves towards increased generation from renewable sources the public will need to learn a little bit about some peculiar aspects of wind and solar power. I will call them the stochastic (S-word) and the interruptible (I-word) nature of both. They are related but for the engineer the implications are different. The S-word says the wind may blow like crazy all night but come morning it’s becalmed and may not pick up again till god knows when. (“The wind bloweth where it listeth, but thou canst not tell whence it cometh and whither it goeth”: John 3.8). More troublesome is the I-word, quick and intermittent fluctuations between furious force and stately calm. The chaps in the Control Room tear their hair out when a large quantum of wind power freaks out suddenly. Imagine if one Vitoria-equivalent of wind power splutters on and off like a car with a petrol block. I will explain how the control people deal with it.

Similar things happen with solar; it’s a beautiful sunny day but quickly darkens with rain. It’s clear morning but clouds drift across and cut the insolation on the panels. Oh hell!

Shall I compare thee to a summer’s day?

Thou art more lovely and more temperate:

Fresh winds do shake the darling buds of May,

And summer’s lease has all too short a date:

And often is his gold complexion dimm’d;

By chance, or nature’s changing course untrimmed:

~ Lines from Sonnet XVIII

The S-word (stochastic is a fancy way to say probabilistic) is used to emphasise the headaches of the planner not the operator. Planners don’t tear their hair in panic as operators do, they go bald slowly; Buddhika’s hairless pate shimmers like a polished egg. Look at it in a simple way. Say we have two 100 MW wind farms, one in Mannar, the other Pooneryn. Take Mannar first: Full power output is 100 MW when wind blows with force. Say the mean is 60, hence between zero and 100 it is distributed with a probability like a bell-shaped curve with a peak at 60 and cuts off completely at 100. Pooneryn will be similar but here’s the jinx. The two may not be correlated (occur simultaneously) or only partly correlated. As the apostle John perceptively pointed out, the wind listeth as it wishes.

Proposed gigantic 10 GW array covering 30,000 acres in Australia’s
Northern Territory; expected to yield over 20 TWh

Planner have a good idea how much energy to expect each year from each farm but cannot hope for a steady 120 MW all the time. True they mostly think of the energy availability per annum but they must also tell management to give operators the right tools at each location to ride over severe fluctuations. If there are a large number of wind-farms distributed across the island, planners can estimate how much energy to expect each year or season, but riding through large fluctuations is a different story. I worked on this topic 40 years ago but those braincells are all dead. But don’t panic, there are bright young lads and lasses in CEB planning who know how to tackle these ambiguities.

The 140 MW Luz del Norte solar plant in ever cloudless Atacama occupies 1200 acres

Operational concerns – the I-word – seem to agitate the CEB these days as more windfarms are slated to come on grid in the next three or four decades (unless the economic debacle of a penniless government shrivels up money for food, medicine, electrification and renewables). The wind that shakes the darling buds of May, and the lovely sun of a summer’s day, are as fickle as my lady’s changing countenance untrimmed.   Imagine you are the control engineer leaning back in your dispatcher’s chair sipping your well-earned morning coffee after a busy night reading Tom Mix comic books. You are gloating over 600 MW of wind and solar that you are dispatching, and then the gods strike! A large part of it goes up in metaphorical smoke at a moment’s notice.  If wind or solar output fall quickly by a hefty amount it is the same as the forced outage of a big unit like Victoria or Norochcholi and the shock to the system is similar.

Germany suffers horrendous outages of power whenever the wind gods go on a lightning strike (no pun intended) and the system not only in Germany but throughout the interconnected Northern European grid experiences severe stability problems. Now wind farms are being shredded; see for example:

“Thousands of wind turbines in Germany will be shredded next year”

“Germans fall out of love with wind power” – Financial Times (UK)

“The growing mismatch between Germany’s renewables capacity and the strength of its electricity network is leading to curtailment, crazy pricing and challenges for neighbouring nations” greentechmedia.com

Yes of course wind and solar dispatch in the CEB system will never rise to 27% of total capacity as in Germany but the concern does highlight something important. Even if these plants are only 10% to 15% of the total system, operators must be provided with tools to cope with the large swings endemic to I-word generators. I try to make my stuff intelligible to lay readers but am unfairly accused of spewing technical gobbledegook all over. Let me try to simplify how to cope with I-word fluctuations. The response has to be fast, as fast as the wind shakes the darling buds of May. One must act in seconds; firing up gas-turbines or synchronising fast-hydro is not workable, it’s too slow. Two available approaches are a huge amount of excess inertia spinning on the system all the time or massive stand-by battery capacity.

If when driving a heavily laden truck you suffer intermittent petrol blocks, the truck may stutter but it will ride through and keep going thanks to the momentum. In the electricity system it could be heavy generator-rotors delivering little power but spinning along just for a honk. Excess rotating inertia smooths out fluctuations in frequency (comparable to the speed of the truck). There’s a limit to this game; then the fall back is gigantic batteries, fast electronics and inverters (converting battery DC to system AC) when big frequency drops are sensed. This is similar to the uninterrupted power supply (UPS) that backs up a computer when the mains is truant.  The problem of course is that a 100 MW UPS needs a battery stack as big as a warehouse and the price tag reaches millions of dollars; comparatively the control-electronics and inverter cost peanuts. If you ever get rich enough to afford a Tesla imagine buying batteries equivalent to those in 1000 Tesla cars to hold hands with a 100 MW wind farm! That’s big money and foreign exchange at that, sitting on the reserve bench just in case one of the eleven blokes on the field breaks a leg. Capital costing of wind power must include the price tag this ancillary stuff.

I have devoted the column thus far to the stochastic and interruptible nature of wind and solar power. Let me sign off with a few comments on energy; the 70% renewable-source electricity by 2030 fairy story. The economy shrank by 3.6% in 2020 and will do worse in2021; growth will not resume in 2022. The economic meltdown and Foodless Emergency will have knock-on consequences, and electricity growth will decline to below 5%. The government has no dollars to sanction import of food and medicines, so forget about funding big RE expansion! Economy, industry and energy growth may all go belly-up if things go on like this. The CEB recently said it will need only 28TWh in 2030; seventy percent of this is 19.6TWh. Let me stay with this for the purposes of this column. Major hydro now gives us 3.9TWh but we have pretty much used up all these resources. To round things off let’s say major-hydro can be pushed up to 4.6 TWh by 2030. Wind, solar and mini-hydro will then have to chip in an additional 15 TWh the other half from wind. Speculate then that the prodigies crowding round President Gotabaya expect 7.5 TWh of this to shine from solar. Then these wonks must envision a 10,000-acre Vista of land, replete with say 3.75 GW (3750 MW) of installed solar Splendour. I am rubbing my eyes! And for wind power, I see the Viyahmaga Sancho Panzas advising his Excellency Don Quixote, tilting at windmills.

[In the best locations, a one GW solar array may yield up to 3000 GWh (3 TWh) annually and needs about 3000 acres. Downgrade energy production by a factor of 1.5 and you will get my 3.75 GW RE installed capacity estimate – you can change the derating factor and recalculate. Mother Lanka is blessed with so much rainfall, so many overcast days and such ample passing cloud that “summer’s lease has all too short a day” unlike Atacama, Gobi, Rajasthan or the Australian outback which are pristine for the solar engineer].

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Latest comments

  • 4

    “But don’t panic, there are bright young lads and lasses in CEB planning who know how to tackle these ambiguities.”
    Yes, and there are those at the very top who sleep soundly in the belief that they know about everything from compost to renewables to turmeric.
    I think Prof. KD’ s detailed descriptions of the inner workings of power grids mean nothing to to those who get all their info from YouTube and FB. Sometimes I think it would have been better if the Green Revolution and vaccines never happened. That would have neatly terminated at birth the likes of Ratana Hamuduruwo and Padeniya.

    • 2

      old codger
      “That would have neatly terminated at birth the likes of Ratana Hamuduruwo and Padeniya.”

      By the same token we could say ‘that would have neatly terminated at birth a guy like Prabhakaran who terrorized people in this country for three decades.’

      • 1

        You seem upset. Are you a supporter of eating totally organic cow dung?

  • 0

    Wind and solar power represent the two fastest-growing sources of power in the US. and EU.
    Since nuclear energy is out of question for a country like Sri Lank,a we have to go in a big way for solar and wind power.

  • 0

    There is nuclear energy next door in India. All we have to do is run a cable across. But will our stupid nationalists borrow electricity from Tamil Nadu?
    Not over our dead bodies.

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