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Linear Fresnel reflectors

Discussion in 'Photovoltaics' started by Nick Pine, Jan 2, 2004.

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  1. Nick Pine

    Nick Pine Guest

    Here's a program to calculate angles and spacings of horizontal strip
    reflectors that might bounce sun down on to a 4' wide drainback water-
    filled poly film duct on an attic floor. The duct might have standard
    PV panels underneath. The strip reflectors might be 1" foil-faced
    polyiso board, which might have a layer of metalized mylar greased to
    the surface for greater reflectivity.

    This is a near-vertical Fresnel version of a parabolic north attic roof
    that's reflective underneath, behind a steep south transparent attic roof.


    10 PI=4*ATN(1)
    20 AXISELD=3'axis elevation above horizon (degrees)
    30 AXISEL=PI*AXISELD/180'axis elevation above horizon (radians)
    40 FL=4'focal length (feet)
    50 HEIGHT=1'strip reflector height (feet)
    60 N=6'number of strip reflectors
    70 PRINT"strip #","bottom (ft)","top (ft)","spacer (in)","tilt (deg)"
    80 PRINT
    90 A=ATN(HEIGHT/FL)'initial est angle to focus (rad)
    100 FOR STRIP=1 TO N'reflector strip # (from bottom)
    110 TILT=(A-AXISEL)/2'reflector tilt from vertical (rad)
    120 D=HEIGHT*SIN(TILT)'spacer (feet)
    130 TH=TB+HEIGHT*COS(TILT)'upper strip edge height (feet)
    140 AH=ATN(TH/(FL-D))'new estimate
    150 IF ABS((A-AH)/A)>.001 THEN A=AH:GOTO 110'iterate to 0.1%
    160 SPACER=12*D'spacer width behind top of strip (inches)
    180 TB=TH+D*TH/(FL-D)'next bottom edge height (feet)
    190 NEXT STRIP
    200 CONC=N*HEIGHT/FL'concentration ratio
    210 OHB=100*D/FL'overhead sun blocking (%)
    220 PRINT
    230 PRINT"height=";TH,"conc=";CONC,"ohb=";OHB;"%"

    strip # bottom (ft) top (ft) spacer (in) tilt (deg)

    1 0 .9951384 1.181832 5.651995
    2 1.020259 1.996933 2.576726 12.39952
    3 2.110213 3.05942 3.775822 18.3398
    4 3.320631 4.239043 4.747493 23.30492
    5 4.70433 5.592752 5.508333 27.32439
    6 6.317759 7.179152 6.095264 30.52662

    height= 7.179152 conc= 1.5 ohb= 12.69847 %

    Note the gap between the top edge of each strip and the bottom edge
    of the strip above. For instance, strip 4 ends 4.2' above the floor
    and strip 5 begins 4.7' above the floor, to avoid shading the bottom
    of strip 5 by the top of strip 4. The gap might be painted black.
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