Page 325 - Handbook of Energy Engineering Calculations

A south-facing solar collector will be installed on a building in Glasgow, MT,

at latitude 48°13′N. What is the clear-day solar insolation on this panel at 10
a.m. on January 21 if the collector tilt angle is 48°? What is the daily surface
total insolation for January 21, at this angle of collector tilt? Compute the
solar insolation at 10:30 a.m. on January 21. What is the actual daily solar

insolation for this collector? Calculate the effect on the clear-day daily solar
insolation if the collector tilt angle is changed to 74°.

Calculation Procedure:

1. Determine the insolation for the collector at the specified location
The latitude of Glasgow, MT, is 48°13′N. Since the minutes are less than 30,

or one-half of a degree, the ASHRAE clear-day insolation table for 48° north
latitude can be used. Entering Table 3 (which is an excerpt of the ASHRAE
table) for 10 a.m. on January 21, we find the clear-day solar insolation on a
2
2
south-facing collector with a 48° tilt is 206 Btu/(h · ft ) (649.7 W/m ). The
daily clear-day surface total for January 21 is, from the same table, 1478
2
2
Btu/(day · ft ) (4661.6 W/m ) for a 48° collector tilt angle.
2. Find the insolation for the time between tabulated values

The ASHRAE tables plot the clear-day insolation at hourly intervals between
8 a.m. and 4 p.m. For other times, use a linear interpolation. Thus, for 10:30
a.m., interpolate in Table 3 between 10:00 and 11:00 a.m. values. Or, (249 −
2
2
206)/2 + 206 = 227.5 Btu/(h · ft ) (717.5 W/m ), where the 249 and 206 are
the insolation values at 11 and 10 a.m., respectively. Note that the difference
can be either added to or subtracted from the lower, or higher, clear-day
insolation value, respectively.

3. Find the actual solar insolation for the collector
ASHRAE tables plot the clear-day solar insolation for particular latitudes.
Dust, clouds, and water vapor will usually reduce the clear-day solar
insolation to a value less than that listed.
To find the actual solar insolation at any location, use the relation i = pi ,
A
T
2
2
where i = actual solar insolation, Btu/(h · ft ) (W/m ): p = percentage of
A
clear-day insolation at the location, expressed as a decimal; i = ASHRAE-
T