What might a simple formula be to calculate the output in BTU for a compound parabola

[frazelle09]

We finally have a parabolic hot water heater built and functioning and were able to determine an output of 4,500 BTU in 50 L of water over six hours of sun (a 20 degree Cent. rise in temp.).

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We now come to find out that there exists compound parabolic collectors which do not require motorized sun-following (electronics + motor). If we had the same area of exposure concentrated on about the same collection area (1" wide by 8' long) would the concentation be about the same?

From some of the literatue we understand that CP are more concentrating than the regular parabolic ones... but most of the papers on these collectors and very detailed and technical, somewhat beyond my hight school (many years ago) calculus - lol.

Well, any help would be appreciated. Have a great day!

[J.P.M.]

We finally have a parabolic hot water heater built and functioning and were able to determine an output of 4,500 BTU in 50 L of water over six hours of sun (a 20 degree Cent. rise in temp.).

Our Appropedia link

We now come to find out that there exists compound parabolic collectors which do not require motorized sun-following (electronics + motor). If we had the same area of exposure concentrated on about the same collection area (1" wide by 8' long) would the concentation be about the same?

From some of the literatue we understand that CP are more concentrating than the regular parabolic ones... but most of the papers on these collectors and very detailed and technical, somewhat beyond my hight school (many years ago) calculus - lol.

Well, any help would be appreciated. Have a great day!

Short answer: the concentration ratio will probably be less for CPC's , but there is not a simple way to describe that reduction quantitatively.

CPC's (Compound Parabolic Concentrators) are a compromise between pure parabolic trough concentrators and flat plate collectors with side mounted reflectors. CPC's have the advantage that less adjustment is necessary, perhaps only monthly or even seasonally as a function of something called the half acceptance angle, but have a lower concentration ratio meaning lower max. possible temps. - the no free lunch rule applies here.

Depending on temp. requirements for the application, CPC may be a better choice.

I'd suggest keeping in mind that unless you're after high temps. for solar air conditioning or some process heat application, simple flat plate solar is usually more appropriate for low temp. applications.

Sometimes you need to delve into the details. Respectfully: it may be you are confusing temperature (quality) with quantity of heat.

[frazelle09]

Wow, J.P.M., that was fast! Thanks for replying!

In answer to last part of your comment, we are looking for good heat production. We'd like to be able to provide hot water to agricultural workers who presently don't have any, so we need to produce it during the day and store it in insulated water tanks. After some 4 years of trying to get our present design to work, i'm really looking to get away from something that has to track the sun, as sexy as electronics and movable parabolic troughs appear.

Eventually we'd also like to get into air-conditioning as well, but we've got a lot of learning yet to do before we can hope to begin looking at this although if we had the equipment, we could really go to town in this area - Mexicali - average summer temps of some 110 F or more.

Thanks again for posting and we'll keep plugging away at it. Thanks again for your comment on the difference between heat and temperature.

Have a great afternoon!

[J.P.M.]

Wow, J.P.M., that was fast! Thanks for replying!

In answer to last part of your comment, we are looking for good heat production. We'd like to be able to provide hot water to agricultural workers who presently don't have any, so we need to produce it during the day and store it in insulated water tanks. After some 4 years of trying to get our present design to work, i'm really looking to get away from something that has to track the sun, as sexy as electronics and movable parabolic troughs appear.

Eventually we'd also like to get into air-conditioning as well, but we've got a lot of learning yet to do before we can hope to begin looking at this although if we had the equipment, we could really go to town in this area - Mexicali - average summer temps of some 110 F or more.

Thanks again for posting and we'll keep plugging away at it. Thanks again for your comment on the difference between heat and temperature.

Have a great afternoon!

You're welcome. For hot water for showers, hand washing and laundry, etc. batch heaters are one very appropriate and low tech, uncomplicated and thus less failure prone way to accomplish the task. Next most practical would be pumped flat plate with remote storage. See builditsolar .com for ideas. Domestic water heating is pretty low tech. I'd not make it more complicated than necessary.

[Philippeberard]

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