[RE-wrenches] tubes versus flat surfaces

Tom Lane tom at ecs-solar.com
Wed Jul 22 11:29:29 PDT 2009


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/> 

 

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