Air Con in a Warming Climate
I saw a comment on a post somewhere a few days ago, I'm afraid I have no specific recollection of exactly where, but it was a post (video?) around the topic of Americans having rooftop solar, the strange reticence around it, and the benefit it could have both on the personal and the global level. A commenter said something to the effect of "I have solar and I've been blasting air con all summer and my electricity bill has been like $8 a month".
On its own this is probably just standard hyperbole, but it's been bouncing around in my brain like a maths puzzle, the more I think about it the more I come draw towards a conclusion - solar powered air conditioning is just... completely free right? Environmentally, I mean.1
This does involve disregarding two major factors:
- The production impact (chemicals, installation energy, material extraction) as pretty much every manufactured object has that as a fixed cost massively amortised over a long service life
- The displaced energy usage of other appliances (lets just assume for a second that the solar is not supplementary but completely meets the house's needs)
Every joule of energy that is taken in by a rooftop solar panel is a joule that would have otherwise been spent as heat either absorbed by the roof (then passed into the house or back into the atmosphere) or dispersed back into the air to be absorbed elsewhere in the environment, still as heat. That's the whole idea - even if the translation of sunlight into electricity is inefficient with 80% of it going to heat, that's where the sunlight was going anyway!
The relationship between this conversion and most appliances is kind of abstract though - "my roof got less warm and now I can watch a movie on the TV" and such. The most obvious "natural" application for rooftop solar energy is for electric heating - you're essentially moving that heat energy away from the roof and into the living space where it is more useful - even if you lose 80% of the energy in the solar panel, that other 20% is in where you want it. 2
The big revelation I've been having over the last few days is that using that same energy for air conditioning is almost alchemical in its redirection of energy. You are taking what would otherwise be waste heat energy warming your living space or the outer environment, and turning into energy cooling your living space and venting the heat into the outer environment - in the non-immediate-term, the environment is entirely unaffected by that transaction, the heat would have got there eventually no matter what. 3
What makes this interaction so cool to me is that as your demand for air-con rises, so typically does the capacity for your solar panels to fuel it - more heat typically means more sunlight. For heating this relationship is inverted, and you'll probably have to rely on energy storage for off-peak grid energy to supplement your solar to get "good value" electric heating compared to gas, but I haven't looked into the numbers on that. At least having the solar panels presupposes the presence of some batteries?
Anyway I think this way of combining a famously attainable environmentally-friendly technology with a notoriously environmentally-unfriendly one to synthesise a completely neutral whole is kind of beautiful. A real triumph of modern engineering.
I really fucking hope I've not misunderstood something fundamental here.
Comments
If you're American or from another country that deals with air-con regularly, this may already have been obvious to you, but in Scotland we're far more concerned with heating than we are with cooling, but for the few weeks of the year where we melt it's nice to have options. We're in the process of moving much more to air-source heat pumps to handle that in recent years, and mechanically they can do both heating and cooling if they're set up correctly, so this is a pretty important thing to understand if you want it done right.↩
Worth noting here that electric heating is a 100% efficient process, as the waste product of inefficient electricity usage is... heat. If you're using a heat pump, that efficiency is even greater - they don't just heat the air by "applying joules of heat", they use a mechanical process (the same one a fridge uses) to pull heat out of one place and put it elsewhere, so the actual "heating efficiency" is greater than 100%. You might spend 1000KJ of energy to raise the heat energy in a space by 3000KJ (numbers out of thin air, but 300% is not an unreasonable number).↩
Cooling with a heat pump works the same as cooling, just in the other direction. Honestly, heat pumps are so sick.↩