Comment by amluto
2 days ago
> However, because it is physically not possible to charge and discharge the battery at the same time, such trades have to be prevented.
The authors are observing that, if electricity prices are negative and your battery is not perfectly efficient, then you would like to charge and discharge simultaneously to get paid for wasting energy, but you can’t.
This is a silly limitation. Surely the power electronics or even just the control algorithms in a BESS could be slightly modified to consume power, get warm, and not transfer any current to or from the battery cells, effectively taking advantage of the BESS’s heat sink to sink excess power and sell that service.
More seriously, in a world with occasional negative prices, you would want your battery to be able discharge itself, without exporting power, in a controlled and power-limited manner so as to avoid overheating. And the optimization algorithms should factor this in. I wonder if real grid-scale BESS systems have this capability.
Not sure if it's discussed in the paper but apparently in Australia there have already been recorded instances of batteries charging with negative price electricity and then selling back that electricity at a still negative but closer to zero price and so profiting.
When I first heard it, it seemed wild that they couldn't hold on for the price to go back above zero, but I guess if we're talking high frequency trading it makes more sense. They might have bought and sold many times while the price is different levels of negative before switching to charging up in preparation for the later price rises.
And the round trip inefficiency helps too.
That's not as ridiculous as it sounds!
As you know, negative electricity prices mean that someone is willing to pay you to dispose of electricity they need to generate for some reason. For example, a conventional steam-turbine-based electricity plant might prefer to just keep running for a brief period of time when demand is low, rather than subject their equipment to a power cycle, which increases their maintenance costs. There's other, dumber, examples based on stupid contracts and badly designed solar... but this example is a reasonable one that exists for good engineering reasons.
The battery provider in this circumstance is profiting from their ability to accept power when demand to dispose of electricity is particularly high. When that need goes down, they can reasonably profit by dumping that energy on someone else who is also able to dispose of the electricity. But at a lower cost. E.g. imagine an big industrial refrigerated storage facility that can consume some excess energy by supercooling their refrigerators. But they can't consume unlimited excess energy, because at some point their warehouse just gets too cold, and they don't have unlimited refrigeration capacity anyway.
So in this simplified example, the battery storage service is getting paid a lot of money to quickly absorb a lot of energy, which they then dump more slowly to the refrigerated warehouse (and similar providers) as the surplus diminishes, in anticipation of another surplus in the near future.
> That's not as ridiculous as it sounds!
I'm not sure: why doesn't someone 'just' put up a few resistive heaters and fans to benefit from negative prices?
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Oxygen capture and liquid nitrogen seems like a great use of negative prices.
If it was so profitable, why wouldn't the electricity utility do it themselves? Certainly, they have the scale, infrastructure, and pricing power to do it.
Oh, that's right. This is supposed to be wealth transfer.
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> and then selling back that electricity at a still negative but closer to zero price and so profiting.
How is it not better to discharge the batteries instead? I guess if you don't have that hardware option integrated into the platform maybe, but otherwise...
I don't know why it rankles me to think that generated power should be fed into a dump load just to make the storage owners extra money. Even though it's inefficient at the system level, it shouldn't be harmful releasing energy that would have been eventually dissipated as heat anyways. And yet it still just feel wasteful to me.
I had to go search my bookshelf for this one:
Eric R. Laithwaite, Induction Machines for special purposes
>I don't know why it rankles me to think that generated power should be fed into a dump load just to make the storage owners extra money. Even though it's inefficient at the system level, it shouldn't be harmful releasing energy that would have been eventually dissipated as heat anyways. And yet it still just feel wasteful to me.
This is one of those efficient market things where you need to manage the market in order that wasteful things happen sometimes... but that waste is an opportunity.
If you and your competitor are both in the business of dumping energy into heat, you're going to compete with each other for access to that money.
Then one of you is going to try to find a way to make _more_ money with that energy and find something quickly scalable with not-too-high idle overhead costs to do with that energy besides just flowing through a resistor.
Negative prices are a sign of an inefficient market or just the lag time between a changing landscape of resources and someone to utilize them.
If there's a free resource someone's going to figure out how to use it, just let it hang out for a while and the problem fixes itself.
Especially with solar energy, this is just going to be a thing. There's a certain balance where overprovisioning is cheaper than storage and so you just do that. Then you wait for industry (or consumers) to figure out how to take advantage of the intermittent cheap energy.
> Then one of you is going to try to find a way to make _more_ money with that energy and find something quickly scalable with not-too-high idle overhead costs to do with that energy besides just flowing through a resistor.
Yes, exactly.
Which reminds me of the occasional story about how one native group or another was so in tune with nature, because they used every part of the (insert important animal here).
Modern economies obviously use all parts of the animal, for exactly the reason you outline.
> Especially with solar energy, this is just going to be a thing. There's a certain balance where overprovisioning is cheaper than storage and so you just do that. Then you wait for industry (or consumers) to figure out how to take advantage of the intermittent cheap energy.
Yes, though you also need to make sure that regulations don't get in the way. Or at least not too badly.
One example I can think of is forcing utilities to charge people by net-metering, forcing the utility to implicitly pay the same price for electricity as they charge. We don't do that for eg used car salesmen.
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My shallow understanding is that utilities and grid operators need to manage the supply/load ratio carefully to keep the grid's operating frequency in a very narrow band, centered around 50 or 60 Hertz. If supply outstrips demand, and assuming supply can't react [quickly enough], the operating frequency starts to rise as all the rotating masses connected to the grid gain momentum from the additional power. If the operating frequency increases too much outside of design parameters that could end badly.
So one solution is to incite demand (with negative rates) for folks to ramp up their use of electricity (into e.g., a dump load resistor bank), bringing demand back in line with supply, and bringing the operating frequency back under control.
I hate the waste, agreed. But it would be irresponsible of the operator to bank that extra supply energy into the momentum of spinning things owned by the consumers just so they could pull it out later by intentionally under-supplying. E.g., an aquarium's big water pumps designed to spin only so fast or produce so much pressure might not like being operated at 110% the rated speed at random times of the day.
related links:
https://fnetpublic.utk.edu/frequencygauge.html (you can watch the grid frequency fluctuate in real-time, here!)
https://en.wikipedia.org/wiki/Stuxnet
https://en.wikipedia.org/wiki/Utility_frequency
The grid connected thermostats, where the energy provider has (some amount of) control over when you heat/cool your house are pretty unpopular (I know people who have had their AC turned off during heat waves and were not very pleased). But this seems like an application of that that people would like? And most people would probably even be happy with just dramatically reduced/free heating/cooling and not actually needing to get paid. And of course it has the added benefit of actually using the energy in a useful manner, rather than just wasting it.
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> Surely the power electronics or even just the control algorithms in a BESS could be slightly modified to consume power, get warm, and not transfer any current to or from the battery cells, effectively taking advantage of the BESS’s heat sink to sink excess power and sell that service.
Unless you specifically design for it (specifically, with a dummy load), the efficiency of the system is inversely proportional to its ability to do this. You need a secondary system.
The power system can connect the battery terminal to in or to out, so if you switch both on at once you effectively bypass the battery. It's called shoot-through current and is generally considered a destructive process. If you can switch on and off fast enough you could limit it to a non-destructive level, but in practice most systems will not switch fast enough. They are designed to operate with the battery load, which is at minimum ~10x higher resistance than the transistor itself. In practice it is often 100s of times higher.
That's where the efficiency comes into it. If a power system is 98% efficient (pretty normal- this does not include power lost to heat in the battery itself), that means the electronics can only burn 1/50th as much power as normally passes through the system. Worse, when you put the switch into shorted position it will try to pass 50x its rated current. You need to switch much faster -certainly more than 50x faster- and that will probably put it outside its operating region.
It is relatively easy to just have a large resistor, but it is not very well suited to use battery power systems. Batteries are very low impedance, and the power system exists to transform to a lower voltage and higher current. Resistors are cheaper when they are higher voltage, so the power system is a hacky kludge.
The overall solution is just more batteries. Oversupply is a transient problem and always will be.
> Unless you specifically design for it (specifically, with a dummy load), the efficiency of the system is inversely proportional to its ability to do this. You need a secondary system.
Many multicell BMS already have this kind of "power shedding" capability. They use it for cell balancing - to equalize voltage between cells with slightly different characteristics. This is desirable despite the power waste, because it reduces wear, increases charging efficiency and allows battery packs to last longer.
Some battery packs are also designed to be able to dump enough power into heat to be able to keep the batteries warm during extreme cold.
The amount of power you can dump for balancing is just a fraction of the charge/discharge power (because it only needs to offset differences in self-discharge rate). So you still need a proper dummy load when you want to dump more.
Similarly, the heatsinking capacity of the battery is designed for charging/discharging losses (say 5% of charge/discharge power).
I work in the industry making hardware and software for large scale commercial and grid scale storage.
There are several challenges with this, safety, thermal runaway, and life cycle of the asset which has a limited amount of cycles.
Also the architecture of the system for the AC inverters and the DC side can come from very different places in the supply chain and aren’t as vertically integrated leaving you in a position where you can’t actually make this work without compromising something in the supply chain. That being said we are talking about a LOT of energy in these systems and to dissipate that much heat you’d need a load bank.
Our house have geothermal heating (heatpump conncted to 160m drilled hole, pretty common in Scandinavia). The heatpump supports having a coolant loop for cooling the house in the summer. Thus the heat pump pretty much exchanges heat from the house to the well (heating it up ever so slightly). It would certainly be possible to insert a resistive dummy load on that loop and just store that heat in the bedrock as well.
This! Or, if you don't have geothermal heating but have an electric water heater, maybe temporarily increase the temperature it goes to: maybe it's normally set to go to 65C, then when you detect that you have negative prices and your batteries are full and your water already hot, maybe heat the water to 70C and store that little bit of extra energy as heat! If you have thermostatic valves in your bathrooms, you won't even notice the difference except by the fact that your water heater now can apparently hold a little bit more water than usual :)
I have a heat pump for hot water and calculated this with an offered floating energy tariff. It is not economical because the high net tariffs are not floating but fixed per kwH and negative / very low prices are seldom here and only for a short period of time available.
Assuming regular negatives (more than once a day) you could also tie the heating to the grid prices with maybe an hour buffer around your high water usage times to make sure you are up to temp.
Modern water heaters will keep temp for a shockingly long period of time.
I know a country, the national train company turns lots of outside lights on (daytime), if the price is negative.
Yes that is the SBB in Switzerland but they do not turn the lights, instead they turn on the heaters for the track switches. Not sure if that is all rumours.
Great for the local insect population.
Wouldn't that discharge the battery and hence contribute to battery wear, by wasting a charge cycle?
I think they just don't understand how such a market would work. You'd trade futures on the power and if you hold the contracts to maturity you'd be on the hook for delivering/accepting the power. Everything before maturity happens on paper.
If you have free energy the obvious thing to use it for is carbon capture.
The big problem there is you have these intensely capital expensive capture plants sitting idle around 75% of the time. Also the processes may not gracefully start and stop though maybe you could smooth that out by building a huge battery bank along with the CC plant to effectively run a full duty cycle with 'free' energy. That bumps the capital costs up again though so the economics get tricky.
Yeah. Anything that's designed to use nearly-free or negative-priced energy from the grid needs to be cheap to build and easy to start and stop (The former being one of the main issues with the 'bitcoin mining as grid management' idea).
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The real low hanging fruit is energy use you were going to do earlier/later anyway but where timing isn't important.
Heating water, cooling water, pumping water, charging batteries, running power hungry machines.
It's half century old tech and usually the only thing missing is a financial incentive to do so.
I wonder if desalination would be another good use. But, yeah, it is probably just a matter of how fast the processes can absorb extra power.
District heating and cooling would be an excellent sink for the power.
Water needs a lot of energy to cool or heat, concentrated at a district, you could easily absorb a lot of energy at negative prices.
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The problem is desalination plants cost billions. You're not going to make money building one then running it the 1% of the time the price of electricity is negative.
Couldn't the battery just do, as an example, 1 minute long charge then discharge cycles?
For example, if the electricity price is -28€/MWh (like today in Germany), and your battery efficacy is 80%, you could get paid 28€/MWh charging, then only pay back 22€ discharging, generating a 6€/MWh profit.
The wholesale energy markets don't have sub 5-minute granularity anywhere that I'm aware of. In the US, 1-hour is standard in the day-ahead markets and 5-minutes is standard for the spot markets.
There is also the problem that your battery would likely degrade fast depending on the technology.
Balancing.. thats probably 1-0.5% of the BESS capacity. The impedance of LFE cells are so high when charged pretty small amounts of energy can slosh around before a protection disconnect, over voltage for example
Find area near shore, stick two big electrodes in water a mile apart, energize circuit when price is negative, profit!
I am in the power generation industry and I have honestly wondered why nobody does this. I figure getting the interconnect big enough to make meaningful money is both prohibitively expensive and a lengthy delay.
Or put them closer together and generate all of the chlorine gas, H2, and O2 a girl could want!
Supercapacitors ready to soak up power to charge batteries whenever rates stabilize.
Supercapacitors are much more expensive then batteries with much shorter lifespans to boot (years, versus decades).
The solution is actually what's called a "dummy load". Get paid to waste energy and heat up the planet a tiny bit more, gotta love it.
I know this is grossly pedantic, but not matter what that electricity is used for, it will end up "heating up the planet a bit more". Energy is a waterfall whose base is heat.
Fossil fuels contain energy that are not in the form of heat, so electricity from fossil fuels would heat the planet even ignoring greenhouse effect. If from renewables, however, the energy has been previously extracted from the environment, thus being neutral in terms of heating the planet.
Not that it matters, because the effect would be miniscule in any case.
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Technically if you power a laser shooting into space with solar panels you are cooling the planet, but you are ofcourse right in practice and on the scale of the universe!
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Can’t we power a big laser and point it at space or something instead? Anyone got a dumber idea?
There are so many things that are energy intensive and not really economically viable: co2 capture, crypto mining, "green" hydrogen, we could see a world soon where a large scale BESS would have an on-site dummy load that does something useful with that electricity
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I guess the problem with building a pure energy waster is that it could only operate every now and then, and it's not guaranteed to see negative prices in a few years from now. So, might not be all that profitable.
Obviously the complaint is about the changing atmospheric absorption properties as a side effect of the generation side, not the heat from using the power.
Either way I think people are overthinking it though.
Use the electricity to heat up a lump of iron to a very high temperature, than use electromagnets to fling it into space?
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Presumably the negatively priced energy came from solar panels, so those sun rays were going to heat the planet anyway. The same still happens with a dummy load, just with extra steps in between to convert to and from electricity.
With enough solar panels deployed, you could still argue that they change the albedo of the Earth and therefore it's temperature.
Related, do Solar PV panels need any extra equipment to curtail instead of feeding into the grid?
Aside from software integration to remotely control household PV systems, is there anything else needed to curtail during negative price events?
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Not necessarily. There's also reflection involved.
Now to figure out how much exactly you need to take into account the solar panel absorption spectrum & the albedo of the earth.
>"dummy load"
You mean crypto miner.
Surprisingly, not always.
If I buy a device for $100 that, given free electricity, will mine $500 of cryptocurrency in its useful life - I can easily lose money if I run it less than 20% of the time.
And I doubt electricity is negative priced >20% of the time.
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Could we use it for some kind of carbon capture process?
I think it's pretty clear with the constantly increasing durations of negative prices, so far we haven't found a way to do so profitably. Carbon capture or anything else for that matter.
Anything that would really love free energy also cost a lot to build and maintain/operate besides electricity. So much that a few hundred hours of free (or even better than free) energy a year is far from enough when you need >90% uptime to make sense. Maybe it makes you go from 95 to 85%, but still clearly it's far more than there are sunshine hours.
It's basically the idea behind things like hydrogen electroysis with excess energy.
The problem is that things that can use bulk energy productively like electrolysers, hydrocarbon crackers, smelters, AI training farms, etc. are very expensive and having them on warm standby but idle most of the time waiting for good grid weather makes for bad returns on the capital expenditure and operational costs.
Isn't this the basic description of what a gravity battery should provide?
No, because his situation is basically that the gravity battery is already sitting at its max height.
He's just trying to burn energy because a negative rate means he's getting paid to use it.
So sure - it's great to give that energy a functional use first (ex - charge his batteries) but eventually he runs out of functional ways to use the energy but could still be making money by using it.
Enter the desire for a dummy load.
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Good luck trying to get rid of such an amount of heat anyway near your batteries.
Some years ago, I helped with battery load tests in a nuclear power station. The constant test load was just a big (~500kW) heater. We burned the battery energy for 5 hours. So it's easy possible to do such things.
Exactly this.
And the same thing for residential scale is literally just a ceramic space heater running at ~1500w.
They're dirt cheap, usually have temp safety checks built in, work on a residentially sized circuit, and are available everywhere.
I needed a cheap and consistent load to do LFP battery testing, and I could spend $5,000 for a real test unit, or $21 for a ceramic heater that will do basically the same thing.
If you've already got the monitoring for the batteries/inverters, a heater is a GREAT load choice.
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500kw is nothing, that is 1/3000 of the nuclear power plant electric output.
Well in times of negative energy prices wouldn't it even be good if the air conditioning ran at full capacity?
Depending on how powerful your air conditioner is, it would rapidly start cooling down the building to a temperature which is too low to still be comfortable. You could maybe buffer this with more thermal mass, but then you are back in the game of storing energy and might as well just get extra batteries.
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In summer? If its not getting to cold for you.
In winter yes also if its not getting to warm for you, but also heating water is easy enough. But you don't need that much hot water
Potentially also cooling down your fridge more and your freezer. But that is not that much energy.
While that works, it would still be quite a waste. It would be a lot better to save it and discarge it later
Yes, it's not much of an issue if you have free energy.
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there are no circumstances under which it makes sense for energy prices to actually be negative. it is a sign of a market behaving very badly. propping up prices by intentionally wasting energy only treats the symptom not the cause.
It’s only ever brief and essentially a penalty to power generators for not turning off the generation in a reasonable time
In most markets, it happens because one player is being given some subsidy - ie. 'we will guarantee all power you sell sells for $X/MWh', or 'You get a contractual green bonus of $$$ if you produce XMWh this year'.
Those contracts mean the person producing the power is still incentivised to do it even with negative prices.