bobbobwhite said:
As kids, we all tried the magnifying glass trick outside, using the sun's rays through the glass to burn paper. My question is, is this simple magnification used to increase solar heating of water in (rooftop) solar heating systems? If not, why not?
Potential downsides realized...perhaps it generates too much heat for plastic materials used typically? Metal perhaps could be used then, but metal is expensive and may not be cost effective? How about this magnification in low sunlight areas, such as Canada and northern Europe? Just thinking.
Here are some real-world observations.
You can use concentrators to ramp up the heat but you run into the problem of flashing your water into steam. Using very basic techniques such as black copper tubing on a dark background sealed under a pane (or two) of glass or acrylic will allow your water temperatures to easily reach 160-170 with very little effort. Considering that water temperatures over 125F are a great way to scald yourself there really isn't much reason to go much higher except when you consider thermal storage principles for late night or early morning usage.
But let’s say you do decide to use a surplus Fresnel lens. Many are available in fairly large sizes and I have two that are in this size range. You have 2 basic types of Fresnel (pronounced Fra-nel) lenses - spot and linear.
On my spot lens I have no idea how hot it can get and I slagged a temperature probe at 2500F at the focal point (it's max rating). Obviously a 1 x1 inch area is such a limited area that it isn't of much use and actively tracking the sun could be considered a problem. However there are folks that make pretty nifty sun trackers using 4 green LEDs and shading techniques. Whichever green LED is in the sun produces the most voltage and that can be tied into a simple circuit using MOSFETs to move things around. That's way beyond anything I've ever done but it is possible and the trackers usually sell for just a couple dollars.
A linear Fresnel lens usually has a focal point roughly 1 inch wide and several inches long when you are using a larger lens. This is a little easier to use for heating a strip and the focal temperatures usually range from 600-800F - still more than you need but at least you aren't melting as many things. That can be somewhat tempered by bringing the item slightly out of the focal point and closer to the lens. You have a wider area to work with and lower temperatures to deal with ... but it is still not very useful unless you wanted a small "hot water on demand" system that did not have to maintain a large flow for overly long periods of time.
We've used a concentrator to boost temperatures but found it wasn't needed. You can build a very simple system that does exactly what you need without the added complexity.
However, concentrators are nice if you are working with used motor oil as a temporary thermal storage medium where your temperatures won't be exceeding 400-500F. There are thermal oils with higher ratings but used motor oil is cheap and will usually suffice for basic experiments. We use this sort of setup to convert recycled biomass into biochar (terra preta). The collected heat from the concentrator helps kick start a process where wood gasses are expelled from the biomass which are then burned to take the process up to a higher temperature level. The biochar is then incorporated into our fields. The technologies used for this system have been around for generations and there is an abundance of material on the web if you want to research further.
If you do decide to experiment with oil, make sure there is absolutely no/zero air in the system or it will combust as it starts to reach it's maximum temperature rating. Very bad ju ju.
Sorry for getting off track. But you don't need a concentrator for heating water. heliostats (a bunch of mirrors pointing at a central point) are complicated and expensive), parabolic troughs are a possibility but are still way more than you need.
If you do want to add a little more radiant energy to your setup then we've also used reflective Mylar on fold-outs that bounce additional sunlight onto our solar water heaters. It's cheap and easy to do. You can get a large roll of 7mil reflective Mylar (or any other brand) for not much. We've purchased 4ft by 50ft for around $120. You can probably find it for cheaper but we were in a hurry and didn't do much shopping or much in the way of price comparisons.
Just remember that when heating water for domestic use, more heat/energy input is not always better.
H
