Saturday, August 20, 2022

No Sun, No Wind, Now What? Renewable Energy Storage

[This is a transcript of the video embedded below. Some of the explanations may not make sense without the animations in the video.]

Solar panels and wind turbines are great – so long as the sun shines and the wind blows. What if they don’t? You could try swearing at the sky, but that might attract your neighbor’s attention, so I’ll talk about the next best option: storing energy. But how? What storage do we have for renewable energy, how much do we need, how expensive is it, and how much does it contribute to the carbon footprint of renewables? That’s what we’ll talk about today.

I’ve been hesitating to do a video about energy storage because in all honesty it doesn’t sound particularly captivating, unless possibly you are yourself energy waiting to be stored. But I changed my mind when I learned the technical term for a cloudy and windless day. Dunkelflaute. That’s a German compound noun: dunkel means “dark” and “flaute” means “lull”. So basically I made an entire video just to have an excuse to tell you this. But while you’re here we might as well talk about the problem with dunkelflaute…

The renewable energy source that currently makes the largest contribution to electricity production is hydropower with about 16%. Wind and solar together contribute about 9%. But this is electric energy only. If you include heating and transport in the energy needs, then all renewables together make it to only 11%. That’s right: We still use fossil fuels for more than 80% of our entire energy production.

The reason that wind and solar are so hotly discussed at the moment is that in the past two decades their contribution to electricity production has rapidly increased while the cost per kilo-Watt hour has dropped. This is not the case for hydropower, where expansion is slow and costs have actually somewhat increased in the past decade. This isn’t so surprising: Hydropower works very well in certain places but those places have been occupied long ago. Solar and wind in contrast still have a lot of unused potential, and this is why many nations put their hopes on them.

But then there’s the dunkelflaute and its evil brother, cold dunkelflaute. That’s when the sun doesn’t shine and the wind doesn’t blow, and that happens in the winter. It’s a shame there aren’t any umlauts in the word, otherwise it’d make a great name for a metal band.

It’s no coincidence that Germans in particular go on about this because such weather situations are quite common in Germany. The German weather service estimates that it happens on the average twice each year, that the power production from wind and solar in Germany is less than 10% the expected average for at least 2 days. Every once in a while these situations can last a week or longer.

Of course this isn’t an issue just in Germany. This figure shows the average monthly hours of dunkelflaute for some European countries. As you can see, they are almost all in the winter. A recent paper in Nature Communications looked at how well solar and wind can meet electricity demand in 42 countries. They found that even with optimistic extension scenarios and technology upgrades, no country would be able to avoid the problem.

The color in this figure indicates the maximum reliability that can be achieved without storage. The darker the color, the worse the situation. As you can see, without storage it would be basically impossible to meet the demand reliably anywhere with wind and solar alone. Even Australia which reliably gets sunshine can’t eliminate the risk, and Europe is more at risk than North America.

The situation might actually be worse than that because climate change might weaken the wind in some places and make dunkelflaute a more frequent visitor. That’s because part of the global air circulation is driven by the temperature gradient between the equator and the poles. The poles heat up faster than the equator, which weakens the gradient. What this’ll do to the wind isn’t clear – the current climate models aren’t good enough to tell. But maybe, just maybe, banking on stable climate patterns is not a good idea if the problem you’re trying to address is that the climate changes. Just a thought.

Ok, so how can we deal with the dunkelflaute problem? There are basically two options. One is better connectivity of the power grid, so that the risk can be shared between several countries. However, this can be difficult because neighboring countries often have similar weather conditions. A recent study by Dutch researchers found that even connecting much of Europe wouldn’t eliminate the risk. And in any case, this leaves open the question whether countries who don’t have a problem at the time could possibly cover the demand for everyone else. I mean, the energy still has to come from somewhere.

And then there’s the problem that multi-national cooperation doesn’t always work as you want. Instead of being dependent on gas from Russia we might just end up being dependent on solar power from Egypt.

The other way to address the problem is storing the energy until we need it. First, some technical terms: The capacity of energy storage is measured in Watt hours. It’s the power that the battery can supply multiplied by the discharge time until it’s empty. For example, a battery system with an energy capacity of 20 Giga Watt hours can power 5 Giga Watt for 4 hours before it’s empty. This number alone doesn’t tell you how long you can store energy until it starts leaking; this is something else to keep in mind.

At the moment, the vast majority of energy storage is pumped hydro which means you use the energy you don’t need to pump water up somewhere, and when you need the energy, you let the water run back down and drive a turbine with it. Currently more than 90 percent of energy storage is pumped hydro. Problem is, there are only so many rivers in the world and to pump water up a hill you need a hill, which is more than some countries have. Much of the increase in storage capacity in the past years comes from lithium ion batteries. However, they still only make a small contribution to the total.

To give you a sense of the problem: At present we have 34 Giga Watt hours of energy storage capacity worldwide, not including pumped hydro. If you include pumped hydro, it’s 2 point 2 Tera Watt hours. We need to reach at least 1 Peta Watt hours, that’s about 500 times as much as the total we currently have. It’s an immense challenge.

So let us then have a look at how we could address this problem, other than swearing at the sky and at your neighbor and at the rest of the world while you’re at it. All energy storage systems have the same basic problem: if you put energy into storage, you’ll get less out. This means, if we combine an energy source with storage, then the efficiency goes down.

Pumped hydro which we already talked about has an efficiency between 78 percent and 82 percent for modern systems and can store energy for a long time. The total cost of this type of storage varies dramatically depending on location and the size of the plant, but has been estimated to be between 70-350 dollars per kilo Watt hour of energy storage.

Pumped hydro is really remarkable, and at least for now it’s the clear winner of energy storage. For example, in Bath County Virginia, they store 24 Giga Watt hour this way. But pumped hydro also has its problems, because for some regions of the world, including the united states, climate change brings more drought and you can’t pump water if you don’t have any.

A similar idea is what’s called “gravitational energy battery” which is basically pumped hydro but with solids. You pile concrete blocks on top of each, store the gravitational energy, and when you let the blocks back down, you run a dynamo with it. Fun, right? These systems are very energy efficient, about 90%, and they store energy basically indefinitely. But they’re small compared to the enormous amounts of water in a reservoir.

The Swiss company EnergyVault is working on the construction of one such plant in China which they claim will have 100 Mega Watt hours energy storage capacity. So, nice idea but it isn’t going to make much of a difference. I totally think they should get a participation trophy for the effort, but keep in mind we need to reach 1 Peta Watt hour. That’d be about 10 millions of such plants.

A more promising approach is compressed air energy storage or liquefied air energy storage. As the name suggests, the idea is that you compress or liquefy air, put it aside, and if you need energy, you let the air expand to drive a generator. The good thing about this idea is that you can do it pretty much everywhere.

The efficiency has been estimated to lie between 40 and 70 percent, though it drops by about zero point two percent per day due to leakage, and that’s the optimistic estimate. The costs lie between 50-150 dollars per kilo Watt hour, so that’s a little less than pumped hydro and actually pretty good. This one gets the convenience award. The McIntosh Power Plant in Alabama is a very large one, with capacity of almost three Giga Watt hours.

Another option is thermal energy storage. For this you heat a material, isolate it, and then when you need the energy you use the heat to drive a turbine, or you use it directly for heating. You can also do this by cooling a substance, then it’s called cryogenic energy storage.

The problem with thermal energy storage is that the efficiency is quite low; it typically ranges from only 30 percent to 60 percent. And since no insulation is perfect, the energy gets gradually lost. But being imperfect and losing energy is something we’re all familiar with, so this one gets the sympathy award.

In this video we’re looking into how to store solar and wind energy, but it’s worth mentioning that some countries use thermal energy storage to store heat directly for heating which is much more efficient. The Finnish company Helen Oy, for example, uses a cavern of 300 thousand cubic meters to store warm seawater in the summer which gives them about 11.6 Giga Watt hours. That’s a lot, and the main reason is that it’s just a huge volume.

As I mentioned previously, most of the expansion in energy storage capacity in the past decade has been in lithium-ion batteries. This one’s the runner-up after pumped hydro. They have a round trip efficiency of 80 to 95 percent, and a lifetime of up to 10 years.

But we currently have only a little more than 4 Giga Watt hours in lithium ion batteries, that’s a factor 500 less than what we have in pumped hydro. It isn’t cheap either. The cost in 2018 has been estimated with about 469 dollars per kilo Watt hour. It’s expected to decrease to about 360 in 2025 but this is still much more expensive than liquefied air.

And then there’s hydrogen. Sweet, innocent, hydrogen. Hydrogen has a very low round trip efficiency, between 25 and 45 percent, but it it’s popular because it’s really cheap. The costs have been estimated with 2 to 20 dollars per kilo Watt hour, depending on where and how you store it. So even the most expensive hydrogen storage is ten times less expensive than lithium ion batteries. In total numbers however, we currently have very little hydrogen storage. In 2017 it was about 100 Mega Watt hour. I suspect though that this is going to change very quickly and I give hydrogen the cheap-is-neat award.

Those are the biggest energy storage systems to date but there are a few fun ones that we should mention, for example flywheels. Contrary to what the name suggests, a flywheel is neither a flying wheel nor a gymnastic exercise for people who like being wheeled away in ambulances, but it’s basically a big wheel that can rotate and that stores energy because angular momentum is conserved.

Those flywheels only store energy up to 20 Mega Watt hours for a couple of minutes, so they’ll not solve the dunkelflaute problem. But they can reach efficiencies up to 95 percent, which is quite amazing really. They also don’t require much maintenance and have very long lifetimes, so they can be useful as short-term storage buffers.

There are also ultracapacitors which store electric energy like capacitors, just more of it. They have a high efficiency of 85-95 percent, but can store only small amounts of energy, and are ridiculously expensive, up to 60,000 dollars per kilo Watt hour. 

The difficulty of finding good energy storage technologies drives home just how handy fossil fuels are. Let me illustrate this with some numbers. A kilogram of gasoline gives you about 13 kilo Watt hours, a kilogram of coal a little less, about 8 kilo Watt hours. A lithium ion battery gives you only 0 point 2 kilo Watt hours per kilo gram. A kilo gram of water at one kilometer altitude is just 2.7 Watt hours, that’s another factor thousand less.

On the other hand, 1 kilo gram of Uranium 235 gives you 24 Giga Watt hours. And one kilogram of antimatter plus the same amount of matter would produce 25 Tera Watt hours. 25 Tera Watt hours! With a ton of it we would cover the electric needs of the whole world for a year.

Okay, so we have seen energy storage isn’t cheap and it isn’t easy, and we need a lot of it, fast. In addition, putting energy into storage and getting it back out inevitably lowers the efficiency of the energy source. This already doesn’t sound particularly great, but does it at least help with the carbon footprint? After all, you have to build the storage facility and you need to get those materials from somewhere, and if it doesn’t last long you have to recycle it or rebuild it.

A paper in 2015 from a group of American researchers found that carbon dioxide emissions resulting from storage are substantial when compared to the emissions from electricity generation, ranging from 104 to 407 kilo gram per Mega Watt hour of delivered energy.

This number probably doesn’t tell you anything, so let me put this in context. Coal releases almost a ton of carbon dioxide per Mega Watt hour. But the upper limit of the storage range is very close to the lowest estimate for natural gas. And remember that you have to add the storage on top of the carbon dioxide emissions from the renewables. Plus, the need to store the energy makes them less efficient.

In the case of lithium-ion batteries, the numbers strongly depend on how well you can recharge the batteries, that is, how many cycles they survive. According to a back-of-the-envelope estimate by the Chemical Engineer Robert Rapier, for 400 cycles the emissions are about 330 kilo gram carbon dioxide per Mega Watt hour but assuming 3000 cycles the number goes down to 70 kilo gram per Mega Watt hour.

A few thousand cycles seem possible for current batteries if you use them well. This estimate roughly agrees with a report that was published about two years ago by the Swedish Environmental Research Institute. So this means, depending on how often you use the batteries, the carbon footprint is somewhere between solar and natural gas.

How big the impact of storage is on the overall carbon dioxide emissions of wind and solar then depends on how much, how often and for how long you put energy into storage. But so long as it’s overall a small fraction of days this won’t impact the average carbon-dioxide emissions all that much.

Let’s put in some numbers. A typical estimate we’ve seen used in the literature is something like 10% of days that you’d put energy into storage. If you take this, and one of the middle-of-the-pack values for energy storage and assume it’s 80 percent efficient, then the carbon footprint of wind would increase from about 10 to about 30 kilogram per Mega Watt hour and that of solar from about 45 to about 65. So, they are both clearly still much preferable to fossil fuels, but the need to store them also makes nuclear power increasingly look like a really good idea.

What do we learn from this? At least for me the lessons are that first, it makes sense to use naturally occurring opportunities for storage. Our planet has a lot of water, and, unlike me, water has a high heat capacity. Gravitational energy doesn’t leak, location matters, and storing stuff underground increases efficiency. Second, liquid air storage has potential. And third, there’s a lot of energy in uranium 235.

Did you come to different conclusions? Let us know in the comments, we want to hear what you think.

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