It’s finally hot.
I planned this post to coincide with the summer’s first hot weather, so I had originally expected to tackle this topic nearly a month ago, and yet here we are. The purpose of this post is to describe my idea for a “Peltier blanket,” a device that I hope will serve as an inexpensive and efficient substitute for in-home air conditioning. That might sound like a complicated topic, but I don’t expect this post to be overly technical. Since I haven’t actually built the device yet (because I was waiting for hot weather to really drive the necessity), this will just be an overview of the idea and the context behind it. Once I build it, I’ll write another, more technical post describing what I did and how well it worked.
What Is a Peltier Blanket?
The Peltier blanket is a concept for an active cooling blanket; you can think of it as the inverse of those electric warming blankets that have been around for ages and ages. The mechanism is much like a water-cooling technique sometimes used for computer CPUs (which I think is similar in principle to car radiators), so the concept is by no means novel. This is merely a novel (so far as I know) application of a familiar technique, but sometimes that’s all it takes to make something useful.
To effectively describe what (I hope) the Peltier blanket does, let me first give context on the problem that inspired it: it’s hot. I grew up in Georgia in the American South, where it’s always hot, so people have adapted by building air conditioning into every permanent structure and most temporary ones. However, I now live near Seattle, where it’s traditionally never hot; and while large and newer buildings in this area tend to have air conditioning, older residential buildings don’t. I’m currently sitting outside in a breezy patch of shade because my apartment is distractingly uncomfortable. Because it’s hot.
It was even hotter last year — so hot, in fact, that in desperation I built myself a homemade air conditioner from a small electric fan, a Styrofoam cooler, and a bunch of water bottles that I would chill in the freezer during the work day. The resulting kludge wasn’t very powerful — it actually could bring down the temperature of a large (say, 200 square foot) room, but only by a few degrees — but that wasn’t how I wanted to use it anyway. I built a small, enclosed structure out of cushions on the couch, pointed my air conditioner into a hole in one end, and slept my first cool, comfortable night in weeks.
But that, of course, isn’t a Peltier blanket. The blanket idea arose from a later conversation I had with my dad. I described my homemade air conditioner to him, and he thought it was hilarious; but, being an engineer himself, he felt obliged to point out that running this unit was actually making my apartment as a whole hotter, not colder. My freezer is indoors, and (thermodynamics being what they are) a refrigeration machine must always produce more heat than it refrigerates. A freezer doesn’t make cold by destroying heat; it simply moves heat, taking all the heat that was in the freezer compartment and moving it somewhere else, and that process produces even more heat. This introduces the concept of efficiency, which will become important in a moment. For now, though, just keep in mind that no matter what, in order to make something cold, you always have to make something else even hotter.
So my little air conditioning scheme was making my apartment as a whole hotter, not colder. That was fine, however, because I didn’t want my apartment to be cool; I wanted me to be cool. Consequently, if my apartment at large became warmer, that didn’t matter so long as the, say, ten cubic feet of air surrounding me was a comfortable temperature. That was when the idea for an active cooling blanket began to form in my mind, and that was when I began the Internet research that eventually led me to Peltier elements.
The Peltier Effect
The Peltier effect is a phenomenon that occurs when electricity is run through a certain type of circuit, because electricity is very, very weird. A Peltier element is a small, common, inexpensive electronic component that uses the Peltier effect to function as a heat pump. When you run an electric current through a Peltier element, one side of the element will become hot while the other side will become cold.
Because they have no moving parts and require only electricity to function, upon learning of these things I was immediately curious as to why they aren’t already used for air conditioning. The reason, I soon discovered, is that Peltier elements are wildly inefficient compared to the more common compressor-based technologies. I’ve heard claims that a typical Peltier element is about 5% efficient, whereas compressor-based air conditioners can be around 70% efficient in the right circumstances. (I have neither the inclination nor the expertise to double-check those numbers; as a software engine and a writer of fiction, I only know about things that don’t really exist.)
Thus, if you tried to use Peltier elements for something like household air conditioning, you would generate staggering amounts of heat in exchange for a relatively modest amount of cooling. Instead of the steady stream of hot air from a traditional compressor, a Peltier bank attempting to cool a house would probably produce melted circuitry and massive gouts of flame. However, if you just want to cool something comparatively small and you don’t mind if everything else in the vicinity gets a bit warmer, a Peltier element might be just the thing.
A Blanket for Cooling
So, with the idea of cooling only as small a space as possible, I came up with my initial design.
Yes, that drawing was done in Microsoft Paint. I sincerely apologize.
The idea is very simple: a Peltier element removes heat from a liquid coolant, which is pumped through a flexible hose into a specially designed blanket. The blanket is insulated on the top; the coolant tubes are affixed underneath, with a thermally conductive sheet forming the bottom of the blanket. Thus, as cool liquid is pumped through the blanket, heat will be removed from whatever is underneath the blanket and transferred to the external cooling unit, which will disperse it into the surroundings.
And that’s it. It’s almost disappointingly simple, so much so that I continue to wonder whether this has already been tried and rejected a dozen times before, and I’ve just failed to find the evidence of it. Either way, I have yet to build it myself — I’ll probably attempt that sometime in the next week or two — so I don’t know whether there are glaring problems that I’ve somehow simply missed. I suppose I’ll just have to find out.
And when I do, that will be the topic of another post.
–Murray