What's the most efficient non-moving shape for collecting solar energy?

January 9, 2017 7:42 PM

I am working on a solar water heater, and I am trying to figure out this geometry problem. I know a parabola is the most efficient movable shape to collect the sun's energy, but what is the most efficient fixed shape? It will clearly change with date and lattitude, but given an average day, what shape focus the most energy onto a heating tube?

Is it a simple half cylinder? A parabolic cylinder? I'd love to see actual equations and a graph - I'm having a hard time working this out myself, and my google-fu is apparently useless on this question.

posted by juice boo to Science & Nature (6 answers total)

It's not just the collection shape, it also has to do with the net absorption of energy, and minimizing thermal losses. This is why flat rectangles, or long evacuated cylinders tend to win out, maximizing heating potential, limiting loss (surface area), in a form that can be manufactured.
posted by nickggully at 7:57 PM on January 9, 2017

Response by poster: Right, apologies for not making that clear, I am working with long evacuated cylinders. I am wondering about a reflector shape for this cylinder.
posted by juice boo at 8:02 PM on January 9, 2017

A parabola is still going to be nearly best. What a specular reflector does is throw the image of the emitter another direction and shape. As the sun moves through the day, a portion of that image will be thrown into the focus cylinder. People talk about point sources, but it's actually a disc. You also have to realize that the maximum heating potential will be near solar noon, so a fixed parabola at that will focus the most light within an hour each side of that.

But this can change depending on your prevailing weather conditions in the area, when it is most likely to be clear.
posted by nickggully at 8:20 PM on January 9, 2017

Best answer: A "compound parabolic concentrator" is a fair place to start. Here's a paper with accessible math: COMPARISON OF SOLAR CONCENTRATORS, ARI RABL.

A search term you might find useful is "non-imaging solar thermal collectors".

You might look into monthly performance for an entire year, indexed by latitude, to see if your engineering requirements can be met, rather than using an "average day".
posted by the Real Dan at 10:11 PM on January 9, 2017

Best answer: I am working with long evacuated cylinders.

For an array of evacuated cylinders, it's pretty hard to beat mounting them parallel to the Earth's axis (i.e. oriented north-south and tilted at an angle equal to your latitude away from the horizontal) with a non-specular flat reflector behind them and at least one tube's width of gap between them (the more gap you leave, the more space your array will take up but the less of the day the tubes will spend shading each other).

You can do complicated things with curved and/or tracking reflectors, but you pretty rapidly reach a point where what you gain from your fancy reflectors would be better spent on just adding tubes.
posted by flabdablet at 10:48 PM on January 9, 2017 [2 favorites]

Flabdablet has a great answer on the collectors. The other big consideration is hot water storage volume, insulation, etc. For domestic hot water you don't want to overheat the water and storing too much volume also means you need more collectors to keep it at the useful temperature. Also, in the spirit of helpfulness (though not answering your question), you may also consider using PV and a hot water heat pump.
posted by meinvt at 10:02 AM on January 10, 2017 [1 favorite]