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-requ...@lists.re-
wrenches.org wrote:
Message: 3
Date: Wed, 22 Jul 2009 14:29:29 -0400
From: "Tom Lane" <t...@ecs-solar.com>
Subject: [RE-wrenches] tubes versus flat surfaces
To: <re-wrenches@lists.re-wrenches.org>
Message-ID: <00e401ca0afa$5b06e240$1114a6...@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/>
_______________________________________________
List sponsored by Home Power magazine
List Address: RE-wrenches@lists.re-wrenches.org
Options & settings:
http://lists.re-wrenches.org/options.cgi/re-wrenches-re-wrenches.org
List-Archive: http://lists.re-wrenches.org/pipermail/re-wrenches-re-wrenches.org
List rules & etiquette:
www.re-wrenches.org/etiquette.htm
Check out participant bios:
www.members.re-wrenches.org
