Page 326 - Handbook of Energy Engineering Calculations

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tabulated clear-day solar insolation, Btu/(h · ft ) (W/m ). The value of p = 0.3
+ 0.65(S/100), where S = average sunshine for the locality, percent, from an
ASHRAE or government map of the sunshine for each month of the year. For
January, in Glasgow, MT, the average sunshine is 50 percent. Hence, p =

0.30 + 0.65(50/100) = 0.625. Then i = 0.625(1478) = 923.75, say 923.5
A
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Btu/(day · ft ) (2913.7 W/m ), by using the value found in step 1 of this
procedure for the daily clear-day solar insolation for January 21.

4. Determine the effect of a changed tilt angle for the collector
Most south-facing solar collectors are tilted at an angle approximately that of
the latitude of the location plus 15°. But if construction or other

characteristics of the site prevent this tilt angle, the effect can be computed by
using ASHRAE tables and a linear interpolation.
Thus, for this 48°N location, with an actual tilt angle of 48°, a collector tilt
angle of 74° will produce a clear-day solar insolation of i = 1578[(74 −
T
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68)/(90 − 68)](1578 − 1478) = 1551.0 Btu/(day · ft ) (4894.4 W/m ), by the
ASHRAE tables. In the above relation, the insolation values are for solar
collector tilt angles of 68° and 90°, respectively, with the higher insolation
value for the smaller angle. Note that the insolation (heat absorbed) is greater
at 74° than at 48° tilt angle.

Related Calculations. This procedure demonstrates the flexibility and utility

of the ASHRAE clear-day solar insolation tables. Using straight-line
interpolation, the designer can obtain a number of intermediate clear-day
values, including solar insolation at times other than those listed, insolation at
collector tilt angles different from those listed, insolation on both normal
(vertical) and horizontal planes, and surface daily total insolation. The

calculations are simple, provided the designer carefully observes the direction
of change in the tabulated values and uses the latitude table for the collector
location. Where an exact-latitude table is not available, the designer can

interpolate in a linear fashion between latitude values less than and greater
than the location latitude.
Remember that the ASHRAE tables give clear-day insolation values. To
determine the actual solar insolation, the clear-day values must be corrected
for dust, water vapor, and clouds, as shown above. This correction usually