Improve heat pump performance with black paint?
aibaxter
| Posted in General Questions on
Hi all!
I’ve been thinking about heat pumps and wondering if one could (or if anyone has tried) to place a heat pump on a black surface (or even paint the outside of the heat pump with black paint) to generate solar heat gain so it can work better at lower outside temps.
What would this look like? Maybe just paint walls of the house around it or the unit itself black.
I understand that, of course one could do it, but would it help?
Does this make sense?
I look forward to your thoughts!
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Replies
I doubt it would make enough difference to notice, and you risk getting paint into the coil which would probably reduce efficiency by restricting airflow.
Black also helps to radiate heat away (which is why heatsinks meant to cool semiconductors are usually black). I don’t know for certain, but you might cancel out some of all of any gain you’re trying to achieve.
Snow covering any black objects would negate your efforts too.
Bill
This reminds me of the idea of placing an AC condenser in shade to improve efficiency. The improvement is maybe a few percent at most: https://www.energyvanguard.com/blog/will-shading-your-air-conditioner-save-you-money
The heat pump just isn't a very big object to act as a solar collector. It works by pumping thousands of CFM of air through itself all the time. That air is constantly being replenished by the wind and not just sitting stagnant nearby. I supposed maybe you could get a decent effect if the condenser was sitting in the middle of a huge patio painted black, but I don't think that would make much economical, environmental, or practical sense.
Hypothetically, it would improve performance in a heating-dominated climate. Conversely, white paint would be beneficial in a cooling-dominated climate (if the unit is in sunlight, at least). Perhaps someone can do the math out to figure out exactly how much energy could be saved, but right away I can see a couple things that would limit this plans effectiveness:
1) In a mixed climate, the winter efficiency gains and summer efficiency losses could easily cancel each other out.
2) Heat pumps are generally installed on a vertical wall, which is not the best angle for solar energy absorption.
3) The heat pump would have to be on a south-ish wall.
4) There is less available sun in winter, when the heating loads are highest.
5) If you have solar panels, the times you have the most energy available are when your heat pump would be saving the most energy, relatively speaking.
I would also be concerned that painting a heat pump would void its warranty.
Overall, while black paint may help, I would place higher priority on making sure the heat pump is in an area where it can have a lot of air exchange, so that it can function as it was intended to.
That's an interesting thought experiment, but I doubt it would help.
In the part of the heat pump cycle that applies here, we'll have the refrigerant having recently gone through the evaporator and so the result vapor+liquid is extremely cold. In order for it to be able to pick up any heat from the outside (I am assuming we are talking about air-source heat pumps), the air that blows across the refrigerant needs to be notably warmer than the refrigerant itself is. That latent heat is thus transferred to the refrigerant which goes on to the compressor to be super-heated again, and thus release the heat it just picked up.
So in that part of the cycle, what we really want is for the outside AIR to be as hot as possible. Painting the unit itself black may heat up the air inside of it a little, but I can't imagine it would be very much at all. Especially since we're talking about the air being blown across the system and the instant that fan turns on, any stagnant heated air will be long gone.
For this to work, there would likely need to be some way to pre-heat the air at a much larger scale. Like maybe if you were to dig a long trench through the ground that was deep enough to hit temperature stasis and use that heat to pre-heat the incoming air. That's sounding an awful lot like a ground-source heat pump, though. Heh.
At the 500cfm+ of the typical outdoor unit there's really no easily measurable performance boost from turning the sheet metal into an "unglazed solar collector" by painting it black, or setting it on a black asphalt pavement, etc. It's just too much air.
I have to admit I am kind of charmed by the idea that one could have seasonal outfits for one's heat pump. White after Labor Day, though? Hmmm.