Fresnel to heat water

[woodsie]

I'm new on here.

Want to batch heat water at remote cabin.  Thinking of using a fresnel lens focused on a tube below a tank of cold water. (maybe 5 gallons) Want to set this up in the same config as a coffee maker, either hot going into separate tank, or recirculate until total 5 gallons is hot. Whattayathink?  

[ghurd]

Never tried it.  Aiming would be a problem as the sun moves.

A big fresnel lens is what? 6"? 12"?

There is only X amount of BTUs / area in sunlight. Focusing it moves most of the energy to a single spot, but it does not increase what is there.

Meaning a solar water heater (a simple black insulated box with a water pipe zig zagging through it) could have 10 times more area than the fresnel lens, with 10x the available BTUs, no aiming problems, better overall performance, and probably cheaper.

Have a look at "heat" under remote living at the top.  And maybe 'solar' under HB Electricity (things get put in there sometimes).

Just 2 cents

G-

[Dan M]

Total radiant energy from sunlight is about 750 watts per sq meter.

Somebody please correct that if it's off.

It would take a really big lens.  You might do better with a parabolic trough, but you still have to aim it.

The "black box" might be the best idea, but you need to cover it at night, otherwise on a clear night you can be radiating heat to deep space (this falls under strange but true).

A simple wood fired boiler might be the most convenient.  Simple like a 55 gal drum over a fire pit.  Thousands of watts in a modest campfire.

Have Fun,

-Dan M

[woodsie]

My idea was to heat just the area of the small tube. (say 3/8" dia.)  I don't know what temp would be generated from this focused beam, but I would suspect it to be much higher than an electric element like is used in a coffee maker. This hot water would then percolate out, bringing cold water to replace it.  and so on and so on.

My idea is to get hot water fast.  Coleman sells a propane unit like this for camping.  In 30 seconds, boiling water comes out the tube. Wouldn't the heat from the beam be hotter than the propane flame.  I may be on the wrong path, but sounds feasible to me.  As far as having to continuously  move the lens, I'm expecting it to again work as fast as a coffee maker.  They make 12 cups in what, about 5 minutes?  That's 7 minutes / gallon, of near boiling water.

[veewee77]

While you are right, the heat produced by a spot from a Fresnel lens is very high, it is a ver small spot.  If you are using one of those lenses that are about a foot square, you will be disappointed, even though it seems like it would boil the wter relatively quickly.  One thing you have to remember, heat rises, and unless you can large amounts of heat to a large enough chunk of water to 'flash' boil it (boil it rapidly before it has a chance to flow out by convection) or have a very small tube that wouldn't allow convection, it will take a pretty long time to heat that whole gallon of water.

Now, if your fresnel lens is say 4 1/2 feet x 3 or thereabounts, you have a chance of flashing water.  I have a lens this size that will melt newer pennies in about 15 seconds and old, solid copper ones in about 45 seconds.  You might have a chance with that one.  I am going to try the same thing with this one when I get a chance.

Have fun!

Doug

[woodsie]

So you think that a coffee maker works, because it is heating a large enough "slug" of water, with a big enough element or heat zone, that it near boils the water, B4 it pulls more cold water in?

[ghurd]

I do.  Thats why the glug, glug, glug.  And there is no pump in one.

Using the 750w/meter^2, that is about 75w/ft^2. A 12"x12" fresnel lens with good sun has available only about 7% of what my coffee pot uses.

G-

[woodsie]

OK.  Think I'll have to try it anyway.

[kurt]

http://www.howstuffworks.com/coffee-maker.htm

[pyrocasto]

I dont think a frensel lense would work very well, because as stated you would probably want to have it on a tracker, and also because of it's size. I think you would be better off with a big panel, or a concentrator.

[Aelric]

Just a couple of links for ya...

A man who built a solar water heater from a 55 gallon drum and some old car heat exchangers:

http://www.motherearthnews.com/arc/3683/

A simple design for a solar hot water heater (black box with zig zag tube):

http://www.motherearthnews.com/arc/4996/

A great reference site which includes detailed info on pressure regulation and overall design:

http://www.homepower.com/magazine/downloads_solar_thermal.cfm

Another design for a hot water heater:

http://www.motherearthnews.com/arc/2240/

And an article about heating your water by woodstove:

http://www.motherearthnews.com/arc/4199/

I think the idea of using a lens to heat the water is excellent if used with a tracking system, my thinking though is that this would require power to make the thing work.   As I understand it a fresnel lens looks kinda like a bullseye, many concentric circles and the result is a much more powerful beam.  Just wondering, what if you used multple lens' to wrap the entire small pipe you are heating, I think that with such a setup you would be good to go from any angle.  Anyway just a few thoughts.  If I am wrong I am sure I'll be corrected hehehehe :-)

[jimovonz]

Look, it is pretty simple (if you don't count the unit conversions - the amount of energy it takes to raise the temp of water is 4.2kJ/degC/litre in other words it takes a fixed amount of energy to raise the temp of a fixed amount of water by a fixed amount (got that fixed?). The amount of energy available from the sun is nominally 1000w/m2 (clear sky with sun directly overhead) but a typical daily figure (reasonably sunny climate) for insolation on a fixed flat surface optimally inclined is 5kWh/m2/day. A typical fresnel lens is 12 x 12" or approx 0.1m2 so figure on 0.5kWh/day falling on your lens. Because the efficiency of such systems is inversly proportional to the target temp, even if you insulate your target well, you will be lucky to get 50% efficiency. So you are getting 0.25kWh/day into your 5 gallons - this equates to 900kJ/day (1kWh = 3600kJ). If you ignore the heat lost from your storage container over the day, this energy is enough to raise the temp of your 5 gal (~20litres) of water from 15degC (59degF - typical ground water temp) to around 26degC (79degF) or what amounts to a cool shower. You would require roughly 4 of these 12x12" lenses to raise the temp of your 5 gal to what we here in NZ consider the minimum safe temp of 55degC (131degF to discourage bacteria growth) and that is with no draw off at all during the day.

The only time you should consider using a concentrator to heat water is when you need 80+degC (176+degF - steam for instance) as lower temp systems are much more efficient. I have 16m2 solar heating pannels and I figure on approx 80% efficiency due to the fact my target temp is only 30degC (86degF for radiant floor heating)

Go and make a flat panel collector.

[woodsie]

Thanks everybody.  I guess I was just overestimating the amount of heat the focused beam would generate.  I have considered the collector types, but I was looking for an on-demand, fast system.

[troy]

Heat gain is directly and inexorably tied to collector surface area.  So a Fresnel lens won't get you more heat in the water, just more temperature on that little spot.  A good analogy would be trying to heat your shower water with a match held under the copper pipe.  The flame of the match measures close to 900F at the tip of the flame, but your shower will be disappointingly cool anyway.  Big difference between total heat, and temperature.

And, as others have mentioned, a Fresnel must be actively aimed directly at the sun or it works even worse.

Good luck and have fun,

troy

[Dan M]

"So you think that a coffee maker works, because it is heating a large enough "slug" of water, with a big enough element or heat zone, that it near boils the water, B4 it pulls more cold water in?"

A coffee maker works primarily because of the 1000w (or larger) heating element.

A 1 sq ft lens or other collector will never "collect" more than about 70 watts of solar energy (at least here on earth).

Think BIG for the collector.

Have Fun,

-Dan M

[picmacmillan]

one of the folks on here, ifred was trhinking of doing what you are thinking of, but with an old satellite dish(those big ones)....i have 2 store bought solar hot water heaters...you may want to try and build, they are very simplistic...just some copper pipes, aluminum sheeting and insulation...all covered in glass...picture below

now did someone say slug?...slug is imperial 32.2 i am guessing (i forgot)...now i remember a kip is 1000 pounds...i am really rusty?? pickster

[Psycogeek]

reasons why concentration of the sun might be effective.

the exposed area could be smaller, reducing loss back to the environment.

you could achieve steam , that you could not possibly with non-concentrated sun, steam could be created that would form a steam bubble that would motivate the water.

these 2 things might offset the disadvantages of having to aim the lens somehow.

them coffee heaters are lotsa watts (as stated above) and you would have to have one big arse lens , or many lenses combined to collect the sun to concentrate it.

but the logic of the IDEA is what i like, to think of ways to motivate the fluid without pumps.

this trick works in the coffee heater to move the water quite a few inches without a pump, WHILE heating the water, 2 stones one boyrd.

so much of todays thought processes revolve around what we have seen, what we do, and old archaic ideas that were thought up when power was 2.5c , and death by CO2 was because you TRIED to kill yourself :-)

If we can take all Waste by products of things and utalize them, where then would we be?

will your CPU heat all your hot water? no but the waste heat from your whole house will.

just how much steam power could be made of the waste heat of an automobile engine? enough to run 2 more cylinders for free?

so i like the IDEA, even if it is not practical as presented.

[Kwazai]

http://www-personal.umich.edu/~bclee/lens.html#faq

I had seen some that were homemade using water ,hoop and plastic sheet- but the link doesn't work any more.

not sure how you would maintain a focal point-maybe a hooped pipe under neath it-so th foci would follow the pipe with the sun?

Mike