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Glazing options for solar panels

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Posted by AC Me on January 15, 2005, 10:24 pm
 
Hi all. New to this and finding the discussions here very informative.

Does anyone know if using low-e glass or double glazing, or both,
increases the efficiency or effectiveness of a flat panel unit?
Does a sheet of glass, or two if double glazed, have a focal length? If
so should the collector be fitted at this distance from the glass?

Mike


Posted by Gary on January 15, 2005, 11:22 pm
 
AC Me wrote:

Hi Mike,

The table below gives some values for solar heat gain coefficient
(SHGC) - which is the fraction of incident solar energy that is
transmitted through the window (eg an SHGC of 0.8 means 80% of the
incident solar energy gets through the glazing).  The more the better
for collectors.
And, the heat loss coefficient (U).  The lower the value of U, the
less of the heat absorbed by the collectors absorber is lost out the
glazing.  The heat loss from the collector is (U)(Glass Area)(Tcol -
Tambient) -- in cold weather, a typical collector will lose about half
the heat that comes in out the glazing -- so reducing U helps
collector performance significantly.

SHGC is always important, U is more important for cold climates, since
the colder it is, the more energy is lost out the glazing.

           single    double    double glaz low e    double glaz low e
           glaz      glaz      hi sol gain          lo solar gain
SHGC      0.86      0.76       0.71                 0.39
U         1.04      0.5        0.29                 0.25

Compared to single glazing, double glazing cuts down the solar energy
getting to the absorber from 85% to 76%, but also cuts the heat loss
out the glazing by half.  This would probably be a good trade,
especially if you live in a cold climate where heat losses out the
glazing are large.
Going to the type of low e, double pane glass that is designed for
high solar gain, the low e coating reduces solar energy getting to the
collector by another 5 points (76% to 71%), but also cuts the losses
out the glazing by quite a bit.
Be sure to avoid the last column, which is low e glass designed for
low solar gain, as it dramatically cuts the solar gain down - ie if
you get low e glass make sure it the right kind.

There is also glass made for collectors that has lower iron content,
and transmits about 90% of light - a bit more than regular glass.

Note also that as you go from single to double to double/low e the
price of the glazing is also going up!

This site has a lot of glazing info: http://www.efficientwindows.org
The Pilkington site also has a lot of info:
http://www.pilkington.com/the+americas/usa/english/default.htm

Another option is a single layer of Polycarbonate glazing (at about $
per sqft) transmits about 90% of solar energy, and has a U around 1.
Or, twinwall polycarbonate transmits about 82% and has a U around 0.55.

Using a collector in which the absorber surface has a "selective
coating" helps to reduce the losses from the collector.  The selective
coating improves U roughly the same amount as going from single glaze
to double glaze.

--
Flat glass does not have a focal point- so spacing does not matter much.

Gary




Posted by John Canivan on January 16, 2005, 7:17 am
 : quoted-printable


  AC Me wrote:
  > Hi all. New to this and finding the discussions here very =
informative.
  >
  > Does anyone know if using low-e glass or double glazing, or both,
  > increases the efficiency or effectiveness of a flat panel unit?
  > Does a sheet of glass, or two if double glazed, have a focal length? =
If
  > so should the collector be fitted at this distance from the glass?
  >
  > Mike
  >
  Hi Mike,

  The table below gives some values for solar heat gain coefficient
  (SHGC) - which is the fraction of incident solar energy that is
  transmitted through the window (eg an SHGC of 0.8 means 80% of the
  incident solar energy gets through the glazing).  The more the better
  for collectors.
  And, the heat loss coefficient (U).  The lower the value of U, the
  less of the heat absorbed by the collectors absorber is lost out the
  glazing.  The heat loss from the collector is (U)(Glass Area)(Tcol -
  Tambient) -- in cold weather, a typical collector will lose about half =

  the heat that comes in out the glazing -- so reducing U helps
  collector performance significantly.

  SHGC is always important, U is more important for cold climates, since =

  the colder it is, the more energy is lost out the glazing.

             single    double    double glaz low e    double glaz low e
             glaz      glaz      hi sol gain          lo solar gain
  SHGC      0.86      0.76       0.71                 0.39
  U         1.04      0.5        0.29                 0.25

  Compared to single glazing, double glazing cuts down the solar energy
  getting to the absorber from 85% to 76%, but also cuts the heat loss
  out the glazing by half.  This would probably be a good trade,
  especially if you live in a cold climate where heat losses out the
  glazing are large.
  Going to the type of low e, double pane glass that is designed for
  high solar gain, the low e coating reduces solar energy getting to the =

  collector by another 5 points (76% to 71%), but also cuts the losses
  out the glazing by quite a bit.
  Be sure to avoid the last column, which is low e glass designed for
  low solar gain, as it dramatically cuts the solar gain down - ie if
  you get low e glass make sure it the right kind.

  There is also glass made for collectors that has lower iron content,
  and transmits about 90% of light - a bit more than regular glass.

  Note also that as you go from single to double to double/low e the
  price of the glazing is also going up!

  This site has a lot of glazing info: http://www.efficientwindows.org
  The Pilkington site also has a lot of info:
  http://www.pilkington.com/the+americas/usa/english/default.htm

  Another option is a single layer of Polycarbonate glazing (at about $ =

  per sqft) transmits about 90% of solar energy, and has a U around 1.
  Or, twinwall polycarbonate transmits about 82% and has a U around =
0.55.

  Using a collector in which the absorber surface has a "selective
  coating" helps to reduce the losses from the collector.  The selective =

  coating improves U roughly the same amount as going from single glaze
  to double glaze.

  --
  Flat glass does not have a focal point- so spacing does not matter =
much.

  Gary
  Good points Gary. These are all important considerations. A few others =
that you may want to consider would include.
  1. cost        FRP is inexpensive. It's shipped in 4' wide rolls up to =
50' long.
  2. ability to reflect IR radiation. Glass does a good job at trapping =
heat so does fiberglass.
  3. weight      FRP is light
  4. durability FRP is tough

  If you're thinging of converting your entire roof to a solar thermal =
roof you might want to consider a special kind of fiberglass reinforced =
plastic designed to withstand uv degradation. The cost is about $ per =
foot and it's easy to work with. FRP also conducts heat less than glass =
so you can get away with single pane. It will degrade slowly with time, =
but if you apply a special coating every five years you can make it last =
a very long time. I recommend FRP for do it yourself projects.
  John    www.jc-solarhomes.com









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BORDER-LEFT: #000000 2px solid; MARGIN-RIGHT: 0px">
  <DIV>"Gary" &lt;<A
wrote in
  message <A
  =
05830795.58c82ec311badc9f9b160e86ff135830@1usenet</A>...</DIV>
  <DIV>AC Me wrote:<BR>&gt; Hi all. New to this and finding the =
discussions here
  very informative.<BR>&gt; <BR>&gt; Does anyone know if using low-e =
glass or
  double glazing, or both,<BR>&gt; increases the efficiency or =
effectiveness of
  a flat panel unit?<BR>&gt; Does a sheet of glass, or two if double =
glazed,
  have a focal length? If<BR>&gt; so should the collector be fitted at =
this
  distance from the glass?<BR>&gt; <BR>&gt; Mike<BR>&gt; <BR>Hi =
Mike,<BR><BR>The
  table below gives some values for solar heat gain coefficient =
<BR>(SHGC) -
  which is the fraction of incident solar energy that is <BR>transmitted =
through
  the window (eg an SHGC of 0.8 means 80% of the <BR>incident solar =
energy gets
  through the glazing).&nbsp; The more the better <BR>for =
collectors.<BR>And,
  the heat loss coefficient (U).&nbsp; The lower the value of U, the =
<BR>less of
  the heat absorbed by the collectors absorber is lost out the
  <BR>glazing.&nbsp; The heat loss from the collector is (U)(Glass =
Area)(Tcol -
  <BR>Tambient) -- in cold weather, a typical collector will lose about =
half
  <BR>the heat that comes in out the glazing -- so reducing U helps
  <BR>collector performance significantly.<BR><BR>SHGC is always =
important, U is
  more important for cold climates, since <BR>the colder it is, the more =
energy
  is lost out the
  =
glazing.<BR><BR>&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nb=
sp;
  single&nbsp;&nbsp;&nbsp; double&nbsp;&nbsp;&nbsp; double glaz low
  e&nbsp;&nbsp;&nbsp; double glaz low
  e<BR>&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;
  glaz&nbsp;&nbsp;&nbsp;&nbsp;&nbsp; glaz&nbsp;&nbsp;&nbsp;&nbsp;&nbsp; =
hi sol
  gain&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp; lo solar
  gain<BR>SHGC&nbsp;&nbsp;&nbsp;&nbsp;&nbsp; =
0.86&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;
  0.76&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;
  =
0.71&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nb=
sp;&nbsp;&nbsp;&nbsp;&nbsp;
  0.39<BR>U&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;
  1.04&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;
  0.5&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;
  =
0.29&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nb=
sp;&nbsp;&nbsp;&nbsp;&nbsp;
  0.25<BR><BR>Compared to single glazing, double glazing cuts down the =
solar
  energy <BR>getting to the absorber from 85% to 76%, but also cuts the =
heat
  loss <BR>out the glazing by half.&nbsp; This would probably be a good =
trade,
  <BR>especially if you live in a cold climate where heat losses out the =

  <BR>glazing are large.<BR>Going to the type of low e, double pane =
glass that
  is designed for <BR>high solar gain, the low e coating reduces solar =
energy
  getting to the <BR>collector by another 5 points (76% to 71%), but =
also cuts
  the losses <BR>out the glazing by quite a bit.<BR>Be sure to avoid the =
last
  column, which is low e glass designed for <BR>low solar gain, as it
  dramatically cuts the solar gain down - ie if <BR>you get low e glass =
make
  sure it the right kind.<BR><BR>There is also glass made for collectors =
that
  has lower iron content, <BR>and transmits about 90% of light - a bit =
more than
  regular glass.<BR><BR>Note also that as you go from single to double =
to
  double/low e the <BR>price of the glazing is also going =
up!<BR><BR>This site
  has a lot of glazing info: <A
  =
href="http://www.efficientwindows.org">http://www.efficientwindows.org<=
/A><BR>The
  Pilkington site also has a lot of info: <BR><A
  =
href="http://www.pilkington.com/the+americas/usa/english/default.htm">h=
ttp://www.pilkington.com/the+americas/usa/english/default.htm</A><BR><BR>=
Another
  option is a single layer of Polycarbonate glazing (at about $ <BR>per =
sqft)
  transmits about 90% of solar energy, and has a U around 1.<BR>Or, =
twinwall
  polycarbonate transmits about 82% and has a U around =
0.55.<BR><BR>Using a
  collector in which the absorber surface has a "selective <BR>coating" =
helps to
  reduce the losses from the collector.&nbsp; The selective <BR>coating =
improves
  U roughly the same amount as going from single glaze <BR>to double
  glaze.<BR><BR>--<BR>Flat glass does not have a focal point- so spacing =
does
  not matter much.<BR><BR>Gary</DIV>
  <DIV><FONT face="Arial Rounded MT Bold">Good points Gary. =
</FONT><FONT
  face="Arial Rounded MT Bold">These are all important considerations. =
A few
  others that you may want to consider would include.</FONT></DIV>
  <DIV><FONT face="Arial Rounded MT Bold">1.
  cost&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;FRP is =
inexpensive. It's
  shipped in 4' wide rolls up to 50' long.</FONT></DIV>
  <DIV><FONT face="Arial Rounded MT Bold">2. ability to reflect IR =
radiation.
  Glass does a good job at trapping heat so does =
fiberglass.</FONT></DIV>
  <DIV><FONT face="Arial Rounded MT Bold">3.
  weight&nbsp;&nbsp;&nbsp;&nbsp;&nbsp; FRP is light</FONT></DIV>
  <DIV><FONT face="Arial Rounded MT Bold">4. durability FRP is
tough</FONT></DIV>
  <DIV><FONT face="Arial Rounded MT Bold"></FONT>&nbsp;</DIV>
  <DIV><FONT face="Arial Rounded MT Bold">If you're thinging of =
converting your
  entire roof to a solar thermal roof you might want to consider a =
special kind
  of fiberglass reinforced plastic designed to withstand uv degradation. =
The
  cost is about $ per foot and it's easy to work with. FRP also =
conducts heat
  less than glass so you can get away with single pane. It will degrade =
slowly
  with time, but if you apply a special coating every five years you can =
make it
  last a very long time. I recommend FRP for do it yourself
  projects.</FONT></DIV>
  <DIV><FONT face="Arial Rounded MT Bold">John&nbsp;&nbsp;&nbsp; <A
  href="http://www.jc-solarhomes.com">www.jc-solarhomes.com</A>  =
</FONT></DIV>
  <DIV><FONT face="Arial Rounded MT Bold"></FONT>&nbsp;</DIV>
  <DIV><FONT face="Arial Rounded MT Bold"></FONT>&nbsp;</DIV>
  <DIV><FONT face="Arial Rounded MT Bold"></FONT>&nbsp;</DIV>
  <DIV><FONT face="Arial Rounded MT Bold"></FONT>&nbsp;</DIV>
  <DIV><FONT face="Arial Rounded MT Bold"></FONT>&nbsp;</DIV>
  <DIV><FONT face="Arial Rounded MT Bold"></FONT>&nbsp;</DIV>
  <DIV><FONT face="Arial Rounded MT Bold"></FONT>&nbsp;</DIV>
  <DIV><FONT face="Arial Rounded MT Bold"></FONT>&nbsp;</DIV>
  <DIV><FONT
face="Arial Rounded MT =
Bold"></FONT>&nbsp;</DIV></BLOCKQUOTE></BODY></HTML>

------=
Posted by AC Me on January 17, 2005, 7:28 pm
 Thank you John. I am building my own home at the moment and want to
make as energy efficient and effective as possible - so am doing a lot
of research.
I live in Ireland which can be a cold, damp place at the best of times
- so maximising solar energy use will be a challange.
I think you may have access to materials not available to me - or
perhaps I am just ignorant of those available. Am I to understand there
is UV resistant Fibreglass Reinforced Plastic that can be used as a
see-through pane for solar panel use? Any idea who manufactures this
meterial?

Mike.

Thanks for the info


Posted by AC Me on January 17, 2005, 7:16 pm
 Many thanks for the information Gary. The double wall polycarbonate
looks particularly interesting.
A few further questions, if you don't mind.
'Selective coatings' - what types are there? where to optain? would
this include 'TiNOX' that I keep tripping over in my research?

Mike
Gary wrote:

informative.

length? If

selective


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