We've just started researching this little gem, so haven't acquired the lens yet...but checking around!
A Fresnel (the 's' is silent I think) lens is found in big screen TVs. Pick up a non-working rear projection TV off Craigslist, or wherever.
A guy on YouTube, fencekid, has quite number of videos relating to this. Check out the one about building the frame here. Then also check they way this guy recycles the TV box as a frame here (he rabbit-trails a bit but gets to the point).
Different sizes offer different results, fencekid again...see here.
Sounds like a great idea. It's very cold here in the Northeast and now I know what I can do to rid myself of boredom. I have some ideas. I just need to locate a Fresnel lens. Please keep us posted if you decide to build something.
This looks like it could work awesomely for heating a fish tank, would probably even need a temperature controlled timer to either cover the frenel or stop the pump (but then you have the heat exchanger not being cooled).
Of course like he says, you need to track the sun somehow to maintain focal for best heat, but might not need that just to keep the tank warmed.
At night of course you would dump a lot of heat from the day and run the risk of freezing, but might be a really nice experiment to run with graphs tracking temperature.
Yes, a Fresnel lens can achieve high temps if you don't mind adjusting it every ten to fifteen minutes. But I would be more inclined to consider a parabolic or "cusp" reflector as less adjusting is required and I prefer the lazy way of doing things. Google either one and see what comes up. Just a couple more options you may want to consider. Good luck.
You could easily use a computer and small motor with solar power to adjust the lens to track the sun
This might give you an idea of how much power you'll be dealing with. It's insane and you'll want to do your research before you set your tank, (or your fish) on fire.
Sun insolaiton is typical 1000W/sqm for overhead sun (90deg) - the lens only concentrate that power in to a very small area - thus the high temperature obtainable at the focus. i.e. the energy available to heat your water stays the same. You'll be better off using a _wind_ isolated_ coiled black irrigation small diameter PVC pipe (as in passive swimming pool heating - but with a transparent cover added) as it is better match to the desired working temperature of your AP system.
Also - check safety with lenses - that focus spot don't give your eyes a second change.