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which solar thermal system is the best? - Page 2

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Posted by harry on July 16, 2006, 5:25 pm
 

The advantage of evac. tubes is that they can produce higher
temperatures than flatplates  when the the light levels are lower.  The
thing to remember is you can only gather heat when you collector
temperature is higher tan your heat store temperature. Ergo your evac
tubes can function for longer periods producing hotter water, This due
to the "concentrating" effect of the mirrors in them and the reduced
losses  due to the insulating vacuum. Don't forget your collectors are
simultaneously losing heat as well as collecting it.
*******************************

schooner wrote:


Posted by Gary on July 16, 2006, 6:10 pm
 
harry wrote:

Hi Harry,
I hear this argument a lot, but if its true, why does it not show up in the SRCC
numbers?

Another thing to think about is that while evac tubes do a better job in getting
collecting energy on cloudy days, there is not much energy to get on cloudy
days.  That is, the energy that gets through the clouds to arrive at your panel
is generally less than half the sunny day amount, and it can be much, much less
than that.  Just scan through a TMY weather file if you want to verify that.
It seems to me its very important for a collector to do well on sunny and partly
cloudy days (when there is some energy to collect), and not as important to be
good on cloudy days, when there is not that much energy to work with????


It would sure be nice to see some side by side comparisons or simulations with
real numbers.

Gary



www.BuildItSolar.com
gary@BuildItSolar.com
"Build It Yourself" Solar Projects









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Posted by Jeff on July 17, 2006, 6:29 am
 Gary wrote:


It seems that way to me also, up to a point.

Here's day totals for latitude 32N, south facing, 45 degree, on January 21.

7AM    0   5PM
8AM   103  4PM
9AM   190  3PM
10AM  251  2PM
11AM  288  1PM
12AM  300


 From 10 - 2PM when the collector is near it's peak instantaneous
efficiency: 1678 BTUs, in mid efficiency 380 BTUs and on the tails only
206 BTUs. You have 7 hours when you are getting good efficiency.

   That's fairly typical of what you see at other times of the year.
Figures from "The Passive Solar Energy Book" by Edward Mazria. This book
has pages and pages of figures, BTW.

So, it looks to me that losing 10 - 25% in the mids, as you might get
from a flat versus evacuated tube, isn't so much.

   Now lets orientate that so it's either SE, or SW and what you will
see is that you are getting half as much time or so at good insolation
levels.

   It looks to me that if you are dealing with delat temperatures of say
75 F and you have a good collector orientation, that flat plates are
hard to beat. If you are needing higher rises or have less optimal
aiming, then evacuated tubes will really fly.

I've been thinking of designing down to about .35 F-hr-Ft2/BTU or so at
about 40% collector efficiency. That seems like a good point to me and
that's about where Mike's designs fall (for efficiency at that delta T).
That's 70F delta operating to ambient for 200 BTUs/hr-ft2.

   From:
<URL:
http://72.14.209.104/search?q=cache:J7uinzaKlZAJ:scholar.lib.vt.edu/theses/available/etd-11698-16389/unrestricted/Chapt_2.pdf+solar+insolation+cloudy+sunny&hl=en&gl=us&ct=clnk&cd $
 >
                  MID   LOW multiplier
Sunny           %110.0 %100
Mostly Sunny    %95.0 %90
Partly Sunny    %82.5 %75
Partly Cloudy   %67.5 %60
Cloudy          %52.5 %45
Overcast        %37.5 %30
Rain/Snow       %20.0 %20

use low for december only

On a partly cloudy day you'll still get 3hrs 200+ BTU's/hr ft2
insolation. That's roughly half of the days useable insolation.

  I could be wrong, of course! I haven't seen the instantaneous
efficiency graphs for any evacuated tube collector. And I could be wrong
for other reasons!

   Jeff




Posted by Harry on July 19, 2006, 6:53 pm
 I fancy that commercially manufactured/installed solar panels (for the
purposes of generating hot water) are pretty dodgy financially. The
only way for it to be financially viable is to make something yourself.
 Ask your self, exactly how much is your hot water costing you and how
much does a commercial solar panel set you back?  The last time I
looked it was going to be a thirty year payback. The design life of the
panel was only around ten years! You also have to deduct the cost of
running the circ. pump (though this could be solar powered too, more
expense)
Maybe the answer is a combined system, ie a homemade flat panel to
preheat and a small commercial evac tubes system to get maximum
temperature. That is to circulate water through the flat panel first
and afterwards through the evac tubes.  What think you?
***********************
Jeff wrote:

http://72.14.209.104/search?q=cache:J7uinzaKlZAJ:scholar.lib.vt.edu/theses/available/etd-11698-16389/unrestricted/Chapt_2.pdf+solar+insolation+cloudy+sunny&hl=en&gl=us&ct=clnk&cd $


Posted by Jeff on July 19, 2006, 8:33 pm
 Harry wrote:


   I'm thinking more along the lines of solar heating, although I've
recently read that the Israelis require solar hot water heating on all
new construction.

   Here's what I gather:

1) Domestic hot water can be supplied from lower temp flat panels.
2) Solar heating using stand alone radiant heating requires higher
temps. You could preheat the water using solar and finish using some
other means. In essence you would be replacing half or so of the total
heating with solar. 40F Ambient + 70F solar gain + 50F electric or some
other heat -> 16OF output to radiator. Or...
3) Radiant floor heating fits in nicely with the temperatures you can
expect efficiently from home built flat panels.

   So, that makes me wonder two things:

a) Can radiant floor heating be retrofitted in existing structures and
what kind of losses would you have considering that the heat would also
be radiating into unheated spaces also.

b) Can you have better radiators that work more efficiently at lower
temps. It would seem to me that two auto type radiators back to back and
in series might work.

   It's easy to collect low quality heat (ie lower temp) cheaply and
efficiently. If you can use that heat then you are saving money and
using smaller collectors, although you would need a larger (water tank)
heat store. It has been mentioned making tanks out of pond liner.

   I have no practical experience so this is largely conjecture on my
part. Also, I'm on the lower end of the scale!

   Jeff




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