ESS141 INTRODUCTION TO ASTRONOMY
YCP SPRING
NAME________________
DATE_________________
LABORATORY EXERCISE – “Measuring Solar Intensity”
INTRODUCTION: The intensity of solar radiation outside the earth’s atmosphere is nearly
constant, with annual changes in the earth-sun distance accounting for a minor ± 3.5% variation.
At air mass 0 and at 0 degree angle of incidence, the solar constant equals 428 Btu/hr/sq. ft.
(1,353 watts per square meter). The British Thermal Unit, or Btu, equals the amount of thermal
energy necessary to increase the temperature of 1 lb. of water to 1 degree F.
The interaction between solar radiation and the earth’s atmosphere is complex. You
should be aware of some of the basic conditions which are involved. Clouds, water vapor,
ozone, dust and smoke all contribute to absorption, reflection, and scattering of solar radiation in
the atmosphere. The solar conditions at your particular geographic location will be uniquely
affected by the above atmospheric factors.
Two components of solar radiation reaching the earth’s surface are generally recognized.
The first is called direct or, alternatively, beam radiation. This is the solar radiation which
arrives in parallel rays, and is characteristic of days with clear blue skies when the sun is highly
visible.
The second component reaching the earth’s surface is called diffuse or, alternatively,
scattered solar radiation. This is the solar component which reaches the earth from all parts of
the “sky dome” after it has been scattered by water vapor in clouds, and by dust and other air
pollution. It is the dominant form of solar energy which is available on a cloudy, overcast day.
Radiation levels on this type of day are characteristically 1/5 to 1/6 of what they are on a clear
day. Even on a clear day, however, as much as 10% of the total solar energy received at the
earth’s surface will be scattered, diffuse solar radiation.
PURPOSE: The objective of this activity is to record solar radiation intensity with a solar
radiation meter for our area at about solar noon over a two month time period.
EQUIPMENT and MATERIALS: Solar radiation meter, watch, laboratory handout, graph
paper, colored pencils and location.
PROCEDURE: Take the radiation meter outdoors to an open area unobscured by buildings or
trees, if possible.
When you record solar radiation intensity with the meter, you should also record the time,
an altitude of the sun, and describe the sky conditions to the best of your ability, (a
meteorological handbook with pictures of typical cloud types will be helpful here, but not
necessary). Work outdoors in as clear an area as possible. Do not record solar radiation levels
behind a glass window.
Included with your lab handout is a set of clear-day solar radiation intensity values for
this latitude. These values are assumed for standard, average sky conditions at sea level.
Consequently, as you measure solar radiation directly with your instrument and compare the
results with the values in the tables, you may find some differences. Also, these tables do not
include ground reflection. If you are working at an altitude significantly above sea level and
there is fresh snow on the ground, your values could be as mush as 30% to 40% higher than
those reported in the tables.
1. Aim the solar cell on top of the meter toward the sun. While keeping the meter in this
position, record the value on the scale meter. This meter reading indicates the instantaneous
value of solar radiation incident upon a surface parallel to the top of your meter. Be sure to
record the time.
2. Later, repeat the above experiment at other incident angles (at other times throughout the
day). It is recommended that you do not attempt this experiment at incident angles greater than
30 degrees as the accuracy of your meter will fall off rapidly. How do the values recorded
compare with the values recorded when the sun is high in the sky at its zenith?
3. Repeat the experiment on cloudy days and compare your recorded values with the solar
energy received on clear days.
4. Complete the attached chart and compare the readings. What was the fraction of solar energy
received on day #1_______________, on the cloudiest day________________, on the clearest
day________________?
5. On a sheet of graph paper plot the amount of solar energy received hourly on a horizontal
surface against the time of day for the fall equinox, and both winter and summer solstices. Be
sure to use clear-day solar radiation values from the table with the latitude nearest your own.
Plot the solar intensity (Btu/hr/sq. ft.) on the vertical axis and time on the horizontal axis.
6. Assuming a solar constant of 428 Btu/hr/sq. ft., how much solar radiation would be received
hourly by a satellite with a 2 sq. ft. solar panel oriented at a 30 degree angle incident to the sun’s
rays?___________________ *See attached example at the end of the Lab.
7. From the work you have done so far, what would you consider to be the best tilt angle for a
fixed south-facing surface in order to collect the maximum energy from the sun in winter months
at your particular location? What would be the optimum tilt for collecting the maximum energy
from the sun on a year-round basis?______________________________________.
8. Using the solar radiation intensity values you recorded on both cloudy and clear days, when
the sun was approximately at its zenith, determine the fraction of solar radiation intercepted by
the atmosphere at these times. What was the fraction transmitted?______________________.
Example - sample radiation readings:
 Clear day 266 Btu/hr/sq. ft.
 Cloudy day 115 Btu/hr/sq. ft.
The fractions intercepted and transmitted by the atmosphere will be directly related to the
solar constant outside the earth’s atmosphere, this value being 428 Btu/hr/sq. ft.
CLEAR DAY
Fraction transmitted = 266 = 0.62 = 62%
428
Fraction intercepted = 100 – 62 = 38%
CLOUDY DAY
Fraction transmitted = 115 = 0.27 = 27%
428
Fraction intercepted = 100 – 27 = 73%
9. Using the solar radiation intensity values you recorded on both cloudy and clear days at 0
degree angle of incidence, determine the fraction of solar radiation intercepted by the atmosphere
at these times. What was the fraction transmitted?____________.
10. Below shows a plot of the amount of solar energy received hourly on a horizontal surface
against the time of day for the fall equinox and both winter and summer solstices. Be sure to use
the clear day solar radiation values from your table with the latitude nearest your own. Plot the
solar intensity Btu/hr/sq. ft. in the vertical axis and time on the horizontal axis.
Example – solar radiation intensity values from 40 deg. N. latitude
11. Repeat the above using solar radiation values for a surface which is perpendicular to the
sun’s rays for noon each day.
MEASURING SOLAR INTENSITY
Table 1
NO.
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
Date
Time of Day
Sky
Conditions
Readings
Btu/hr/sq. ft.
Sun Angle
from Horizon
6. Assuming a solar constant of 428 Btu/hr/sq. ft., how much solar radiation would be received
hourly by a satellite with a 2 sq. ft. solar panel oriented at a 30 degree angle incident to the sun’s
rays?
Answer:
The basic rule for calculation of solar radiation on tilted surfaces is:
In this problem, the intensity on a surface perpendicular to the sun’s rays is 428 Btu/hr/sq. ft.
The angle between the solar panel and a surface perpendicular to the sun’s rays is 30o.
Therefore, the energy incident on the panel is:
428 Btu/hr/sq. ft. × 2 sq. ft. × cos 30 o = 428 × 2 × 0.866 = 741.3 Btu.hr