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As many people know, many of the renewable energy technologies generate fluctuating power dependent on availability of the power generating source e.g. wind and solar et al. Balancing power must be provided, to compensate fluctuations and to balance consumption and generation, guarantee the energy system operates reliable.

It sounds quite challenging for me. I would like to know how it's managed today. Is there advanced electrical or mechanical control systems implemented nowadays to minimise the negative impact on the grid? I.e. what is the common practice in markets with high wind energy penetration e.g. Germany, Spain, Denmark?

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There are very few grids with high penetration of those exogenously-variable renewables that have up til now provided no inertia to the grid - wind and PV. What matters is not the penetration in a particular market, nor in a particular country, but rather what the penetration is on a particular synchronous grid - the area over which the frequency is synchronised. So although Denmark has a high penetration of wind (sometimes more than 100% of demand - they export the surplus), that's not what's important for managing the grid.

East Denmark belongs to a large Nordic synchronous grid, that covers Norway, Sweden and Finland too. And on that grid as a whole, wind & PV have pretty low penetration, even when Denmark has more than 100% wind.

West Denmark belongs to the main continental European synchronous grid, which goes from Portugal to Greece to Poland. And on that grid, wind & PV have pretty low penetration too, even when Denmark has more than 100% wind.

So, for pretty much all grids, managing existing renewables penetrations is very easy: some other plants get turned down or off (typically coal or gas) when the wind or PV is available, and everything else carries on just the same.

The highest penetrations are currently found on the Single Irish Grid, which covers the Republic of Ireland, and Northern Ireland. There, there are times when wind could meet more than 50% of demand. Those times are infrequent, but they do happen.

The next highest to my knowledge is Britain, which has (at time of writing) not exceeded 49% wind + PV. I would expect it to exceed that some time in the next year or two, but to date, no special management techniques have been required in Britain.

Whereas Ireland has had to curtail some wind to keep penetration down to 50%. That is to say, they've had to spill some wind to manage the grid. They're looking at implementing measures now to raise that to 70%. Hydro Quebec have been looking at building frequency reserve into wind generation, to allow penetration to rise. Britain's National Grid is just completing trials of the same thing (the NIC project- “Enhanced frequency control capability" - efcc), and there should be a report out on that soon; the early sounds coming from the project team have been positive.

  • Regarding Denmark: Simply said, an excess energy goes to "other grids" (is exported) when it's more than 100%? So intermittent energy production from RE sources is too small in comparison to synchronous grid? – stardust Oct 21 '16 at 11:06
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    Yes, they export it. But Denmark is not a synchronous grid. It's two small parts of two very large synchronous grids. And on those grids, wind gets nowhere near 50%, let alone 100% – EnergyNumbers Oct 21 '16 at 11:08
  • Regarding England: They use traditional power dispatching techniques - people monitoring the power draw? – stardust Oct 21 '16 at 11:13
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    @stardust Britain's grid is balanced by a combination of automated systems and people in a control centre, dispatching mostly supply and some demand: by market prices and by direct instruction. – EnergyNumbers Oct 21 '16 at 12:15
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    @stardust at the moment, that's all I've learnt, from people involved in the project. They may be controlling frequency by using variable-speed rotors, or by some fancy power electronics. I'll update this answer with a link once the report is published. And it might be worth posting a new question here asking how wind turbines can provide frequency-response services, if we don't yet have a question on that. – EnergyNumbers Oct 27 '16 at 12:29
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I know that there a some Startups working on innovative solutions for the renewable energy load-shifting conundrum (maybe add "load-shifting" to the tags?), out of which the one that stuck most to my head was compressed air energy storage, also known as CAES.

In peak energy production when it is foreseeable that there will be a surplus, this excess energy will be used to pump compressed air into underwater tanks/ballons with a compressor. When there is an energy shortage, this transformed energy can be turned into electricity again by using turbines.


Bonus Excursion:
At a recent hackathon in Groningen, the Netherlands, called H4ck Energy my team conceptualised liquefied natural gas (LNG) powered freight trucks being additionally loaded with electric batteries, e.g. the Tesla powerwall to function as mobile load-shifting plants. Being charged at the truck stations, the legally binding downtime for recess at highway truck stations, combined with the renewable energy intermittency, would create a synergy.

Energy Surplus = Charge standby trucks, Energy Shortage = Discharge standby trucks.

("Watt, the truck"-Concept Animation here)

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The currently tenable options I see:

Move the energy to where it's needed

  1. Moderately large synchronous networks are tied together with DC lines. This allows further movement of power without requiring synchronization.

Store the energy for later

  1. Someone comes up with a chemical process that can be easily turned on/ turned off when you have/ don't have surplus power. A battery is one example. Electrolysis may be one. Someone may be able to come up with a way to make methanol this way.

  2. Compressed air storage may be viable, especially with volumes large enough that the air doesn't cool significantly while in storage. This has some efficiency issues.

  3. Pumped hydro works well, but there are not many suitable sites.

Where to store it?

If you are going to store energy for a while just where is optimum:

  1. At the point of use. Makes all kinds of sense for battery, or anything that is equivalent to a battery. (Battery = any box that you pump electricity in when you have lots, and pull it out when you are short.)

  2. At the point of generation. E.g. you have a wind farm that makes hydrogen when it has surplus power. hydrogen is burned or run through fuel cells when it's calm. The advantage here is that the wind farm is now 'dispatchable'

  3. At centralied locations. This makes the most sense for large systems that make methonol or other feedstocks.

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