Multifacet parabolic solar concentrator

Step 9 Measure performance

The whole project is now complete. You can now use the device for anything you please. You can measure the performance of the device by putting water into the heat-exchanger, mounting it in place, and measuring the temperature rise. How fast the temperature rises is an indication of the amount of energy being transferred per unit time. You can alternatively measure the amount of time it takes to boil a given amount of water or measure how much water boils per hour.

I finally abandoned this design and disassembled it mainly because I was not able to track the sun accurately enough for it to be practical. It sure was a lot of fun though.

Comments

[sindaear]

I really like this solar boiler project. I especially like your method of adjusting the mirrors to focus them. I've wanted to build one of these for a long time now. Unfortunately I live in an apartment and don't have anywhere to build it. I was wondering what your cost was for the aluminum angle pieces. Although it might make it slightly more difficult in drilling the holes, I was wondering if steel conduit might work better for providing strength and be cheaper as well (as little as $1.96 per 10ft). Also painting the heat exchanger flat black would help it's efficiency.

[jarney1]

I'm most gratified that you liked the project. Thank you. It was a most interesting and rewarding project. I don't have much room myself. That's why I built it so that it can be disassembled and stored when not in use. My wife is particularly happy that it's not taking up permanent room in the back yard. I don't recall exactly how much the aluminum angle braces cost. The most significant cost was the chrome plated mirror pieces. All in all, I believe I recall that I spent some $300 on the project. More accurately than that, I simply don't recall and didn't keep very good records. As far as painting the heat exchanger, I recall doing some experiments with that. It is a common misconception that simply painting something like this black will make it more efficient. It does indeed help for low temperature devices but works not at all for higher temperatures. It turns out that ordinary paint from the hardware store will burn readily under the power of a solar concentrator. Basically, it just burns up in the first 10 minutes or so leaving the copper bare again anyway. It seems that a better way to improve efficiency is to add an insulator to the back side. Something like fiberglass works well. Of course, the paper backing of the fiberglass should be removed to avoid fire. What you're aiming for is not allowing the excess heat to radiate from the back side.

[avid0g]

One of the best selective absorbers is oxidized nickel.  Since stainless steel contains a good bit of nickel, it turns out that it does a fine job itself.  You simply make the absorber out of stainless steel, heat it until it turns a dark purple, and you are done: a selective absorber that has an absorption/emission ratio of about 11 - excellent!  And, of course, it does not blister or peel.  Bare copper has an absorption/emission ratio of 3.  There are oxidizing chemical treatments that will turn copper black - and they are not paints.  They do not have good abrasion resistance, but that is not an issue here, yes?

If the absorber surface temperature stays low - like at or below the boiling point of water - it does not need to be a selective absorber as boiling hot objects do not radiate much infrared energy.  On the other hand, selective absorbers do not absorb infrared energy as well as non-selective absorbers do.  Since sunlight at the surface of the earth has about 40%-45% of the thermal energy in the infrared range, you typically want a normal absorber like carbon black.

If you are dealing with temperatures close to the boiling point of water you have many choices for paint, but if you need really high temperature paint, try Pyromark High Temperature Paint. It is available in 800, 1200 and 2000 degree F ratings.

One of the inefficiencies of heat transfer is the temperature drop across the thickness of the metal barrier; another is mixing the fluid enough to bring all of it in contact with the hot media.  I feel that the best, most efficient way to solar heat water, especially with concentrator mirrors, is to place the absorber in the water.  That is, pump the water in transparent tubes into the absorber surface at the focal point.  One excellent media is an open-cell black ceramic sponge (looks like an aquarium aerator stone); another is to put a very concentrated (stable) black dye in the water.  (maybe use both techniques?)

[STBro]

Great point about placing the absorber in the water. That is definitely the way to go.

I would add to the absorber discussion that the emissivity values are for thermal radiation and emissivity varies with both frequency and temperature. For solar power from most reflective concentrators, the majority of radiation is visible, not thermal. So glass is a great transmitter of visible radiation but very poor for thermal radiation. A glass mirror that looks shiny bright in the visible range looks almost completely black when viewed (with a thermal imager) in the thermal range. For concentrators that use glass mirror reflectors, nearly 99% of the reflected power is in the visible frequency range because the mirrors absorb most of the thermal radiation. However, this design uses metal reflectors and these are efficient in both visible and thermal bands. Ideally, the translucent material should transmit both visible and thermal frequencies. In addition, the receiving face of the translucent material should be flat and normal to the incoming beam. One possible solution would be to use a multichannel polycarbonate panel used for greenhouse walls. The polycarbonate has excellent visible and thermal transmission. Its important to get the window-grade polycarbonate.

[Lew8]

The material they use in big sized solar concentrators is carbon. although will probably not be able to get your hands on big carbon tubes etc, you could try coating the copper in carbon by putting the copper tubes under and oxygen deficient flame. I have seen that acetylene produces a lot of black deposit on things when no oxygen is mixed with it, maybe you could just use a candle to coat it? The only thing i suspect may happen with this is that once the carbon gets hot enough, it will combine with the oxygen in the surrounding air and simply float of in the form of co2. still, worth a try?

[phddas]

I wounder how to avoid letting the shade from the heat exchanger block the sun shinning on the reflectors. i.e. the focal point needs to be located where it will not obstruct the sun rays arriving on to any part of the reflector(s).

[avid0g]

 It really is not necessary to build parabolic dish reflectors that are fully symmetric.  For example, the new satellite receivers are not fully symmetric, so the single arm and receiver do not cast a "shadow" on the dish.  Speaking of shadows, that is how you pick where to omit the mirrors.