most efficient collector for solar water heating
code...@gmail.com
heater. Can you tell me what is more efficient? A flat collector
(assuming it's insulated from convection) or a parabolic trough
covering the same area (with it's collector insulated also)?
thanks
Robert Scott
It depends on the kind of weather you have. A parabolic trough is good for
nothing if the sun is diffuse, as on a very hazy, partly cloudy day. But a flat
panel can still pick up some useful heat on such days. However if you have
bright direct sun to work with, then the parabolic trough may be better only
because it has less hot surface area which can leak heat to ambient, and can
therefore generate higher temperatures.
Are you thinking about efficiency in terms of BTUs per square foot? That may
not be the best measure of efficiency, unless you truly are space-constrained
and money is no object. Perhaps a better measure is BTUs per dollar. If the
choice is between a 30 square foot paraboloic trough versus a 60 square foot
flat plate collector (which costs less than the 30 square foot paraboloic
trough, especially if you count tracking hardware), then I think the flat plate
will win the BTUs per dollar contest.
Robert Scott
Ypsilanti, Michigan
Morris Dovey
In general, a flat collector will deliver more usable heat than a same
size parabolic trough.
--
Morris Dovey
DeSoto Solar
DeSoto, Iowa USA
http://www.iedu.com/DeSoto/
azuredu
> It depends on the kind of weather you have. A parabolic trough is good for
> nothing if the sun is diffuse, as on a very hazy, partly cloudy day. But a flat
> panel can still pick up some useful heat on such days.
The diffuse radiation is greatly exagerated by solar panel vendors.
According to my luxmeter measurements, diffuse radiation rarely
exceeds 150W/m^2. On the other hand, a typical flat panel collector
loses about 4W/m^2.K. If you do it yourself, the loss will probably be
higher. Counting optical efficiency, you hit zero output as long as
the water temperature is more than 30C above the environment. So it's
only of some use during the summer.
Even then, the contribution of diffuse radiation is probably at most
0.5kWh/m^2 for a favorite day for that. What can you do with that toy
energy? And should you compare this with the cost of the panel?
code...@gmail.com
Thanks for your suggestions. To provide more information, I'm in Mid-
Florida. The sun here is either on or off for short periods during a
passing storm, year round. I did a small experiment and built the
solar cooker at this link. In December I can bring the water in the
black jar to a boil!
http://solarcooking.org/plans/funnel.htm
Does this sway the suggestion? I'm thinking that pumping water through
the collector pipe of a parabolic trough will get it pretty hot in the
Summer time!
azuredu
>
> Does this sway the suggestion? I'm thinking that pumping water through
> the collector pipe of a parabolic trough will get it pretty hot in the
> Summer time!
I am field testing my new home made parabolic trough. We are at winter
solstice, and I have got a 570W/m^2 thermal output for a sun at a mere
22.8 degrees from the horizon. When dry burning without water in the
tube, it heats up to 200C in just a few seconds! Had to put it off
quickly for things were getting melted.
Sorry but the details are still not yet ready. Maybe in 2 to 3 weeks.
Just a picture for the time being. It will cost much less than a flat
panel!
code...@gmail.com
200C (not F)... that's awesome. I'm thinking the costs are going to be
much lower overall too. I tried the link to the JPG picture, but it
did not work. Can you try again?
andrew
>> http://wims/xiao/solar/pic/collector1.jpg
>
> 200C (not F)... that's awesome. I'm thinking the costs are going to be
> much lower overall too. I tried the link to the JPG picture, but it
> did not work. Can you try again?
http://wims.unice.fr/xiao/solar/pic/collector1.jpg
AJH
code...@gmail.com
thanks... one quick question: Why the blue?... have you not peeled the
coating off the plexiglass yet?
David Williams
-> solstice, and I have got a 570W/m^2 thermal output for a sun at a mere
-> 22.8 degrees from the horizon.
If the parabola is aimed at the sun, it will make little difference
how high the sun is above the horizon.
dow
azuredu
>
> thanks... one quick question: Why the blue?... have you not peeled the
> coating off the plexiglass yet?
Reflection of the sky by the cover.
azuredu
>
> If the parabola is aimed at the sun, it will make little difference
> how high the sun is above the horizon.
It is the radiation of the sun that is weaker, with less energy. Due
to a longer distance the light has to travel thru the atmosphere,
therefore more absorptions and so on.
azuredu
> 200C (not F)... that's awesome.
If you are looking for temperature limit, 200C is nothing. I have
reached 320C in an earlier model with much lower performance, making a
big hole in the cover. If I let go this one, it should make 450C if it
does not melt down before.
And when the evacuated receiver is ready, 800C is the virtual limit.
Virtual because the receiver will implode well before that.
This is because this small DIY model is beating all the current
industrial products in terms of performance, huge or small.
code...@gmail.com
In terms of your parabolic curve, are you choosing to have a curve
where the receiver is on the the same plane as the top edges of the
curve, that is to say if you placed a large enough flat ruler across
the assembly it would touch the the two edges of the curve and the
receiver, for performance or convenience reasons?
I have seen designs with a more open curve, such as the one in this
link:http://dvice.com/archives/2008/08/solar_icemaker.php
David Williams
-> >
-> > If the parabola is aimed at the sun, it will make little difference
-> > how high the sun is above the horizon.
-> It is the radiation of the sun that is weaker, with less energy. Due
-> to a longer distance the light has to travel thru the atmosphere,
-> therefore more absorptions and so on.
It depends on how clear the atmosphere is. If it's nice and clear,
there won't be much absorption even if the sun is fairly low in the
sky.
dow
azuredu
> It depends on how clear the atmosphere is. If it's nice and clear,
> there won't be much absorption even if the sun is fairly low in the
> sky.
Even on a clearest day, the atmosphere absorbs at least 20% of the
radiation reaching it when the sun is at zenith. This absorption rate
increases rapidly when the sun is near horizon.
RF
though I may have missed it.
I guess it would be necessary, unless you have a
slave constantly moving
the reflector.
What would they cost, including motors?
From the discussion it appears that the parabola
is more cost effective for water heating. How
would they compare for space heating?
code...@gmail.com
You might consider a Hydronic system for heating. That way you could
combine solar and back up from propane if needed. Here's an idea to
get you started:
http://www.solarhaven.org/HPArticle.htm
Also, there are some ingenious mechanical trackers, I've heard of but
not yet seen specs. on. Here's one possible link to a water powered
tracker:
http://www.builditsolar.com/Projects/OddProjects/odd_project_refs.htm
RF
Thanks CC - the second link is a great resource.
Enjoy the holidays :-)
azuredu
> I haven't seen any mention of a sun tracker,
> though I may have missed it.
> I guess it would be necessary, unless you have a
> slave constantly moving
> the reflector.
>
> What would they cost, including motors?
It didn't show in the picture, but I promiss you all the details, with
the schematics and an open source program, in a few weeks. A Chinese
factory is readying a product which will be sold online very soon,
probably for less then 10$ retail price. Of course including the
stepper motor and gears.
>
> From the discussion it appears that the parabola
> is more cost effective for water heating. How
> would they compare for space heating?
Solar space heating is much harder than water heating, because the
equipment is only used a few months in the year, and these are the
most difficult months of the year. I am afraid that my parabola is the
only solution that can really bring solar space heating cost down to
below that of fossil fuels, especially for high latitude regions.
Details of the economical analysis will come out with all the rest.
The economic criteria for space heating is that the collector should
not cost more than 250$/m^2 installed, and should remain very
efficient during the winter. Oh I've forgotten: I don't count on
government subsidies in the cost analysis.