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.

Why not place the air conditioner next to a large electric space heater?

Many heat pumps already have a mechanism for deliberately wasting energy for defrosting the coils. I bet that the same hardware with a different control algorithm could be convinced to heat out the outdoors without much net change in indoor temperature. (The solution involving the smallest amount of extra hardware is to run the system in reverse periodically. There are other solutions.)

Whether the negative energy price is enough to balance wear on the system and potential noise is a different question.

Yes, it's not much of an issue if you have free energy.

It only makes sense if you have ‘more than free’ energy you need to get rid of, because not getting rid of it causes problems. Similar to flaring natural gas, but for actual electricity.

This is not a common occurrence or situation, or shouldn’t be anyway, or someone is screwing up pretty badly somewhere.

With variable sources of electricity it can be cheaper to have capacity at a level that you sometimes overproduce than to have a capacity that produces at a lower level, and so mostly needs a backup source of power.

It’s actually quite common. You have base load generation stations and your highly variable solar and wind. There are often times when the power at a wholesale rate dips below zero. It’s too costly to turn off your base load plants and maybe both solar and wind are generating above normal.

Solar inverters can just not draw the solar current, and wind can generally just change the pitch on their rotors at the individual level. The only ones that generally can not help ‘over produce’ are baseload power stations as they have actual physical inertia in very large turbines and can’t respond as quickly to demand.

Baseload power stations sometimes over-produce on longer timescales than just a few seconds because they'd rather not turn them off for maintenance/operational reasons. E.g. imagine you have a big biomass boiler feeding a steam turbine. Turning it off for an hour or two means everything cools down, which is a thermal stress, reducing lifetime compared to keeping it at constant power.

But yes, certainly poorly managed solar/wind that doesn't have good mechanisms to turn off in response to lack of demand is mainly the issue. In the future, when control systems are better, I'm sure negative pricing will be much less common.

Right but keep in mind these events are generally short lived and depending on the market there may be reliability guarantees that keep these open or specific federal funding rates.

But like I said before when rates go negative you will typically see it in occurrences where you have abnormal conditions (wind and solar generating at the same time) or aggressive night winds. And it does not happen long enough to need to curtail generation.

Electricity prices around here (Austria) are negative around noon on most summer days. They pay you to waste all that solar energy people are feeding into the grid.

Is it really screwing up? If solar panels are cheaper than batteries, then you can over-provision the solar panels and then you won’t need to use the batteries as much, so you can probably get away with smaller installations.

My gut would expect it to approach $0 if full communication were possible, based on the instinct that most people would run their dishwashers if the energy cost was $0.

Solar panels don’t produce excess power that needs to be dissipated - just don’t invert the unneeded current, and that’s it.

‘Overproduction’ in this sense is from something like a spinning generator which starts to overspin, or an inverter which oddly starts to overvolt the output for some inexplicable reason.

That only works if the grid operator has control over the inverters, which they often don't have.

We currently have the situation where operators of solar farms of all sizes get a fixed amount of money for each joule they feed into the grid. Of course those people have zero interest in turning down their inverters when the sun is shining and there's already a surplus in the grid.

That isn’t actually structurally true - the way inverters work is they do high frequency analysis of the output power, and then ‘push’ energy in at various parts of the waveform. That’s how DC->AC inversion works. We’re taking much higher frequency than the AC waveform for any non-terrible-quality inverter.

While they can be configured to overvolt the outputs or drive frequency higher than they should if they have the excess energy, that is certainly not a normal setup and seems pathological, frankly. Which I guess goes to your point regarding solar operators - if they have an incentive to keep pushing power into the grid even if the grid doesn’t need it, they certainly can configure their equipment to do that. But that is also a clearly screwed up situation for the grid eh? And of all the generation providers, they are the ones with the least excuse to actually do that.

It’s trickier with something with moving parts (like wind generation), where there may be some element of physical inertia or direct physical coupling to the output power. But inverters are not that.

And my understanding is that all modern wind turbines do automatic blade feathering and a significant degree of voltage/frequency regulation to avoid blowing things up ‘downstream’. So with any luck it would be a minutes at most, type situation.

Still, 90% of the problem is likely with wind farms or the baseload eh? Unless the solar operators are doing this intentionally to screw their competition.

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.

HAAS CNC mills are famous for using electric stove elements for the spindle brake resistor: https://www.reddit.com/r/CNC/comments/1es1d01/someone_didnt_...

I'm not sure that this really is a completely off-the-shelf stove element. But obviously, the technology is basically identical to what you'd have on your stove.

It is cery common for Frequency Inverter for AC Motors to have a break resistor. It's expensive to push energy back to the grid with from an inverter. So in most case just a break resistor is used. Just if you break a lot and a long time it is worth to buy an inverter who can do it. If you have multiple axis in a machine, often they are coupled with DC, so the break energy is used by another drive.

Yup. On a much bigger scale it's common for the power supplies going to electric trains to do this too. They like to use regenerative braking because it's efficient. But the electricity grid can't always handle the extra power, so in some designs they have a backup mechanism to dissipate the power if needed; diesel-electrics usually just have a big bank of air-cooled load resistors on the roof.

A neat example of regenerative braking being important is the London Underground: they've had a persistent problem with high temperatures in the subway, of which a decent % is actually heat from trains braking. By using regenerative braking rather than putting that energy into the tunnels as heat, they can transport that energy outside the tunnels, keeping them cooler.

so you're installing a resistive heater? what is the economics of that?

500kw is nothing, that is 1/3000 of the nuclear power plant electric output.