[RE-wrenches] tubes versus flat surfaces

[Jim Grundy]

Tom,

	You mentioned 90% as best performance for flat glass.  Do you have a  
corresponding number for tubes, in terms of losses due to  
reflectance?  Anyone else have any data to share?

Thanks,

Jim Grundy
NABCEP Certified Solar PV Installer
Vermont Solar Specialist Plumber PS-279
Elemental Energy, Inc.
1750 Clark Rd
E. Montpelier, VT   05651
802-476-3441 (p)
802-476-5680 (f )
802-272-8933 (c)


On Jul 22, 2009, at 6:15 PM, re-wrenches-request at lists.re- 
wrenches.org wrote:

Message: 3
Date: Wed, 22 Jul 2009 14:29:29 -0400
From: "Tom Lane" <tom at ecs-solar.com>
Subject: [RE-wrenches] tubes versus flat surfaces
To: <re-wrenches at lists.re-wrenches.org>
Message-ID: <00e401ca0afa$5b06e240$1114a6c0$@com>
Content-Type: text/plain; charset="us-ascii"

Glazed collectors: i.e.

evacuated tubes and flat plate collectors - the single one factor that
effects the collectors efficiency is most is the Angle of Incidence of
sunlight to the flat glass or curved glass cylinders. Typically in  
the best
low iron glass at perfect orientation to the sun - only 90% of the  
sunlight
gets through the glass while at least 10% gets reflected off the  
surface. No
matter how efficient the absorber is in capturing the solar thermal  
energy -
if only 90% get through - then 90% insolation x 90% efficiency means  
at a
theoretically best is 81% efficient  collector under optimal conditions.
Evacuated tubes have an obvious distinct disadvantage to a flat  
surface - as
the plane rotates away from the south. On east or west facing roofs, the
back side of the tube curvature will typically be greater than a 40  
degree
angle of incidence from September 21st to March 21st above 40 degrees
latitude. Rotating the absorber in the tube will not change the angle of
incidence to the glass. In the summertime above 40 degree latitude the
morning  and/or afternoon  sun will be north of a south facing  
absorber in
the glass tube. IT IS IMPOSSIBLE for a tubular surface  that covers  
the same
area TO CAPTURE AS MUCH SUNLIGHT AS A FLAT SURFACE -  as much of the  
surface
on one side  will always be curved AWAY from the sun. Even at direct 0
orientation at solar noon to a flat plate or evacuated tube facing south
only the 10 o'clock to 2 o' clock position on a tube is in excellent
orientation to the sun. TOTAL NONSENSE IS TUBULAR PEOPLE CLAIMING THE  
TUBES
TRACK THE SUN! Please take a geometry class! PLEASE NOTE THIS IS TRUE  
FOR
SOLAR ELECTRIC OR SOLAR THERMAL COLLECTORS THE ANGLE OF INCIDENCE TO THE
GLASS IS CRITICAL. NOTE TO FOLLOW - IT IS ILLEGAL TO TO PUT ANNEALED  
GLASS
ON A ROOF. Only tempered glass can legally be put on a roof.  Gator tom



Tom Lane

Energy Conservation Services

6120 SW 13th ST, Gainesville, FL 32608

(352) 377-8866  F (352) 338-0056

Give Your Sun A Job <http://www.ecs-solar.com/>


-------------- next part --------------
An HTML attachment was scrubbed...
URL: <http://lists.re-wrenches.org/pipermail/re-wrenches-re-wrenches.org/attachments/20090727/b6c1d37c/attachment-0004.html>