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Comment by leashless | Hacker News Reader

Something to note. If you put solar panels on the roof of a building, and plug them directly at 12V or 24V into an appropriate air conditioner (no batteries, inverters etc) the efficiency gains can be startling. Potentially useful for a lot of work places.

https://www.researchgate.net/publication/323883629_DC_Applia... for example

The match between daytime AC loads in workplaces and the output from solar panels is close enough to work in many areas.

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I wonder how much residential energy usage ends up being DC vs AC these days, with computers, USB devices, LEDs, etc. I remember reading something about converting homes to have DC outlets, wonder if that's not too far away.

What would be magical about 12 or 24 volts that leads to an efficiency gain?

I think it's less that 12 or 24 is more efficient than mains, but rather that not having to convert DC -> AC -> DC and also not having to step the voltage up and back down is more efficient. That's 2x 'rounds' of conversion you don't have to do anymore.

(though I'm not an electrician, I'm just guessing)

does someone actually make an compressor that runs off DC directly? It'd have to be a universal motor from what I understand. That seems really unreliable.

https://www.hotspotenergy.com/DC-air-conditioner/ for example

the spec sheet for that says "Panasonic BLDC" which is just an induction motor with an inverter making AC from DC power

At the scale used for building AC? Probably but I wouldn't know where to even look.

At smaller scales? Absolutely. You'll find them in almost every enclosed vehicle in the USA at various sizes.

I believe most vehicle AC compressors run directly off of the serpentine belt (12v power comes from the engine anyway, why send it through the alternator and battery?) or, in the case of electric cars, off of the ~400v traction battery. The compressor draws around 1-2kW at full tilt, so in all cases it's best to run it directly off of the main power source with as few intermediate conversions as possible.

Ok, I was basing this on searching for DC air conditioning compressors with results that referenced automotive and mobile uses. Possibly what I was seeing were retrofit kits?

While I think Teslas somehow drive the compressor via an electric motor, it's almost certainly brushless.

In traditional ICE cars it's belt driven off the engine. There is no electricity e involved to rotate it, just to engage a small clutch.

If the AC system is running on DC internally (e.g. DC fans instead of AC) then you have DC --> AC --> DC conversion and should probably figure about 10-15% loss at each of those. For convenience, call it a 25% loss of efficiency overall so not insignificant.

Edit: AC has advantages in transmission over distances, but if you're looking at both solar panels and AC units on the same roof distance is probably not a major factor.

the fan in an AC unit doesn't consume any reasonable fraction of the power. It's the compressor that takes power.

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