sbi

sbi, 1 year ago

The TL;DR:
i) Due the need to do primary control (frequency response), a 100% renewable grid is impossible. You need either combustion engines or batteries to run your grid.
ii) Batteries are not great for storing significant amounts of energy, but they are very well suited for frequency response.

sbi, 1 year ago

Disclaimer: While I have been writing code in this domain for a decade, I am not an electrical engineer and mostly only learned what I needed, on the job. Electrical engineering is complicated, people spend years studying it, and there is no way I could cram everything needed into this thread even, if I knew it all—which I absolutely do not. So I have to oversimplify things. Dear electrical engineers, reading this might physically hurt you. I apologize.

sbi, 1 year ago

Traditionally, electrical energy is provided by big rotating masses. Steam, oil, diesel, or gas generators turn coils in magnetic fields, which creates an AC current in the coils. Their turning speed determines the grid's frequency. If too much energy is drawn from them, they get slower, if it is too little, they get faster. Machines have used centrifugal governors to regulate their speed at least since Watt invented his steam engine, so controlling rotation speed is a solved problem:

sbi, 1 year ago

Increase fuel usage when the frequency is going down, reduce it if it is going up, so you always provide exactly the power needed. This is generally called primary control (or frequency response): an immediate response to demand fluctuations, fast enough to be practically instantaneous. What's more, communicating this through the grid frequency allows generators to organize power distribution: As long as the frequency is off, all generators will try to compensate to the best of their abilities.

sbi, 1 year ago

Now let's add a bunch of photo-voltaic (PV) panels to the mix. Those are variable power providers: Their output changes according to supply, not to demand. The supply depends on latitude, time of day/year, cloud cover, and other factors. Since they cannot fully adapt their output to demand, they cannot create a grid on their own and have to sync to an existing grid, adapting their frequency and phasing to what others provide. You will need other generators to have formed a stable grid first.

sbi, 1 year ago

So your PV panels provide as much energy as they can and leave the rotating masses to sort out the rest.
But this is fine only if you have just a small amount of this unregulated power in your grid, and the generators can compensate for any fluctuations in variable supply. If you have enough PV to provide up to, say, 60% of your demand, with the actual amount varying between 20% and 60%, your generators need to provide between 40–80% of the energy. Then, things might get much more complicated.

sbi, 1 year ago

On a sunny day, only 40% of your fossil fuel generator capacity is needed, but if some dark clouds comes by, you might suddenly need 80% of it. And you need to be prepared, because such clouds can be here within minutes.
But rotating masses might need considerable time to startup and shutdown, so in order for a generator to quickly take over, it needs to already be running. However, generators cannot really idle, they can only provide as little power as possible, and that might considerable.

sbi, 1 year ago

The minimum power of all your generators might well be higher than 40% of your demand. You could curtail the output of your PV panels so that they can never produce more than what your idling generators do not provide for the current demand (plus some safety margin). But then you throw away power provided by the sun for free, and use fossil fuel instead. Or you could turn off some of the generators and risk not having enough generators running to compensate for sudden fluctuations.

sbi, 1 year ago

But there's more: Just like cars, power generators have their ideal operation point, where they are most efficient and where stress and wear is lowest—all of which translates into costs. So you need to balance a lot of factors. What you do in this situation depends on a lot of factors: What are the exact numbers we're talking here? What risks can we take? Is this a microgrid for a few weekend homes or the power supply for whole nation, with metropolitan areas, hospitals, traffic lights, trains?

sbi, 1 year ago

Enter batteries. Here, we use the term for chemical assets which can consume power from your grid by charging, and supply power to the grid by discharging. They charge/discharge only DC, though, while our grids are AC. So you need to connect a string of batteries to the grid through an inverter which converts between AC and DC. The combination of a bunch of batteries and an inverter is often called an "AC battery", and used as if it were a real singular asset.

sbi, 1 year ago

Important properties of batteries are their lifetime (number of charging/discharging cycles), the amount of energy they can store, the power with which they can charge/discharge, and the relation between the latter two, the C rate. The limitations of the inverter as well as the battery chemistry are responsible for all these factors. Nowadays, mostly Lithium ion batteries are connected to grids. They provide a high C rates and less storage capacity decrease per charge cycle than other batteries.

sbi, 1 year ago

As long as batteries have capacity to charge or discharge, they can do both at any power you want to (up to their maximum power), so they can actually replace your generators, create and maintain a grid, and control their power according to demand. So you can actually turn off those fossil fuel monsters!
And by charging, batteries can provide "negative power". Your minimum idling power is now negative, which means you can suck up excess PV power, and feed it back into the grid later!

sbi, 1 year ago

But you cannot charge a fully charged 5MWh battery any further and will only ever get 5MWh out of it. And once discharged, you need electricity to charge it again, rather than just refilling a tank. Plus, batteries have a much lower energy density than fossil fuel. The latter is why most batteries are not well suited for storing considerable amounts of energy: A 10MWh battery plant is the size of a school's gym, while a 1MW diesel generator plus 10h worth of fuel easily fits into a container.

A diesel generator in a container on a truck trailer.

sbi, 1 year ago

So you need to keep some of your generators as backup, because fossil fuel can be stored cheaply and almost without losses–but you get to turn them off for long stretches, significantly reducing fossil fuel use and mechanical wear on them. Using batteries, you can maximize you usage of your renewable assets, increase the renewable share in your grid, thereby helping to save this planet—and to top it all off you also save money. In fact, there's companies making a profit on this.

sbi, 1 year ago

Let me reiterate: By putting up wind and PV farms, and hooking up battery power plants to our grid, we can significantly reduce our fossil fuel usage at a profit within just a few years. This is proven technology, which has been in the field for years or decades, and comes at a relatively low safety risk. (Compare this to nuclear power plants, which are rather expensive, take a decade or two to build, and take whole nations as backup to get insured because of the risk of a meltdown.)

sbi, 1 year ago

For the last 10 years I have been paid for writing code which controls AC batteries (DC batteries plus inverter) or whole grids with fossil fuel, wind, PV, and batteries. I have seen small containers with a single 100kWh battery and 50MWh battery power plants. Some support tiny microgrids (like the backup power for a small hospital), some support islands cutting their diesel consumption down to 20%, and some of them provide primary control to some of Europe's biggest national power grids.

A big commercial battery power plant.

sbi, 1 year ago

Addendum: I have simplified things to the point where electrical engineers and battery chemists might feel physical pain reading this. I have not talked about generator and battery chemistry inertia, active/reactive/apparent power, inverters being able to do both grid-forming or -following, genset synchronization, black-starting grids, and many other stuff. I skipped over all this because I deemed it unnecessary to get the big picture (and partly because I know too little about it).
I apologize.

sbi, 1 year ago

@Paxxi Great to hear!

sbi, 1 year ago

@kevinrns @stux I think it's an European starling. ICBWT. They are well-known for their mocking ability.

sbi, 1 year ago

@2ck I could be wrong, though.

sbi, 1 year ago

@Migueldeicaza These are called "Midnight Commander-style" nowadays? Really? You people do not know that MC only copied that?

sbi, 1 year ago

@Paxxi @Migueldeicaza Oh, I didn't know! 😶

sbi, 1 year ago

@bencurthoys You made me look up the age of one of my nieces. She's more than a decade now and wasn't even born back then. sigh