I may be a little obsessed with the Fan Group's research into radiative cooling. (what can I say, I really hate excessive summer heat).
As the Greater Boston area is in the midst of a rather unpleasant summer heat wave, the mind turns to ways to help cool our human made environments. (A big thanks to Willis Carrier for inventing modern AC) While active heat pumping has the advantage of being incredibly fast, it does increase our civilization's energy demands, and until those energy sources are no longer adding greenhouse gasses, we are leaving ourselves in a nasty little feedback loop. One potential means of increasing how efficiently we cool our homes, would be to create a window unit that passively provides radiative cooling while still allowing light through. The design would have 3 major parts, the transparent cooling surface, heat pipes, and a small solar panel to power a fan. After a user puts their cooling panels into place, the houses heat would be transported from the inside through the heat pipes. To ensure that as much heat is being taken away from the house as possible, a small fan attached to a heat sink, similar to what you would see inside your computer, would blow air as necessary. Sounds great in theory, but let's try to figure out how useful this idea would be.
Each square meter of radiating material would provide roughly 850 watts of cooling or the equivalent to 2900 BTUs (about half the cooling of what a window unit made for a small room would provide. This means that for a small room (about 125 square feet), you would need about 2 square meters of cooling surface (I am making a distinction for reasons I will go into later). Without knowing how heavy all of the elements would be it would be pre-mature to comment on how unwieldy the mass of the system would be, that being said just that amount of area would be hard. If this magical room had 4 decent sized windows, you would still need each panel to be at least 0.5 m by 1 m (or about 19.75 inches by 39.5 inches, unless this thing is folded up before you open it, not easy to safely place out of your window). The reason the size of the panel needs to be at least half a meter by a meter, not exactly that size is a matter of how radiative cooling works. Black body radiators need to be pointed at something colder than they are, in the case of this technology, the cold of space, if the surface of the panel is perfectly smooth, your cooling window is now most likely pointed at your neighbor's house, probably picking up the heat being reflected and or emitted by the neighbor.
If the panel was to have a bunch of ridges creating a cool 3-D panel, probably a bunch of 45 degree slopes, you are only getting about 70% of the equivalent height of the array. so now instead of being half a meter by a meter, the panel needs to be 0.5 m x 1.5 meters to get the same effect. Now home owners have to spend the energy installing these far more massive panels, or putting in way more small panels to get a similar effect.
We are quickly running into a design that seems less and less appealing, which is what I came to realize, as excited as I am about passive window cooling units, they start seeming pretty silly as a primary means of cooling a home. Small seasonal units might still find a niche market, for home owners who want to minimize the amount of work their actual AC system needs to work, but the real market for passive cooling technologies would most likely come from larger businesses that want seasonal cooling capacity. Instead of designing a window unit that needs to be small and light enough for a home owner to put in, engineers should focus their efforts on creating two types of passive cooling installations, seasonal and permanent. Designing a building to permanently have passive cooling systems on the outside, would make the most sense as to be customized for the building's use case (read too difficult to properly be analyzed in this blog), the seasonal solution is a bit easier (as I am looking at the problem from my perspective). Imagine giant shutter looking structures. some intended to allow light in, others to maximize cooling. Businesses and organizations who require massive amounts of air-conditioning could lease these panels and have them place around their building at the beginning of the summer, drastically offsetting how much energy the would need to devote to air-conditioning. The challenge for this use case is the business atmosphere, realistically without either utility or government mandates to minimize peak energy consumption during the summer months, it could be difficult to inspire wide spread adoption of this kind of cooling technology. If society had the will power to invest in more passive cooling technologies, there would be less demand for peaking power plants (the most expensive types of electrical generators), reducing the overall cost of energy, which is generally a good thing.
A quick note on the passive window unit. While I don't think it makes sense for most American consumers, I do think the idea has merit in regions where power is less reliable or more expensive.
The idea behind this post is I generally only quickly outline an idea or act like I've seen the future and it must include my "brilliant" solution, I wanted to convey, at least in a small way, how I iterate through ideas and what problems I try to consider. I hope this provides a small sense on how I try to create solutions to problems.
As always, questions, comments, feed back what-have-you always welcome.
As the Greater Boston area is in the midst of a rather unpleasant summer heat wave, the mind turns to ways to help cool our human made environments. (A big thanks to Willis Carrier for inventing modern AC) While active heat pumping has the advantage of being incredibly fast, it does increase our civilization's energy demands, and until those energy sources are no longer adding greenhouse gasses, we are leaving ourselves in a nasty little feedback loop. One potential means of increasing how efficiently we cool our homes, would be to create a window unit that passively provides radiative cooling while still allowing light through. The design would have 3 major parts, the transparent cooling surface, heat pipes, and a small solar panel to power a fan. After a user puts their cooling panels into place, the houses heat would be transported from the inside through the heat pipes. To ensure that as much heat is being taken away from the house as possible, a small fan attached to a heat sink, similar to what you would see inside your computer, would blow air as necessary. Sounds great in theory, but let's try to figure out how useful this idea would be.
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| Fig 1 On the left a flat panel trying to radiate heat away on the right, the bumpy pattern points the heat towards space |
If the panel was to have a bunch of ridges creating a cool 3-D panel, probably a bunch of 45 degree slopes, you are only getting about 70% of the equivalent height of the array. so now instead of being half a meter by a meter, the panel needs to be 0.5 m x 1.5 meters to get the same effect. Now home owners have to spend the energy installing these far more massive panels, or putting in way more small panels to get a similar effect.
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| Fig 2: Cooling panels to go outside big buildings |
A quick note on the passive window unit. While I don't think it makes sense for most American consumers, I do think the idea has merit in regions where power is less reliable or more expensive.
The idea behind this post is I generally only quickly outline an idea or act like I've seen the future and it must include my "brilliant" solution, I wanted to convey, at least in a small way, how I iterate through ideas and what problems I try to consider. I hope this provides a small sense on how I try to create solutions to problems.
As always, questions, comments, feed back what-have-you always welcome.
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