SOLAR ENERGY ASSIGNMENT

Lab #6: SOLAR ENERGY Purpose To find the daily amount of solar energy reaching the earth’s surface and relate it to the daily amount of solar energy falling on an average house. Required Equipment/Supplies 2 Styrofoam (plastic foam) cup plastic wrap Graduated cylinder rubber band Water thermometer (Celsius) Blue and green food coloringmeterstick Procedure

Step 1:Fill a Styrofoam cup adding small equal amounts of blue and green food coloring to the water until it is dark (and a better absorber of solar energy).Then measure and record the amount of “Water” in your cup. Volume of water = __________ Mass of water = ____________ Nest the cup in a second Styrofoam cup (for better insulation).

Step 2:Measure the water temperature and record it.Cover the cup with plastic wrap sealed with a rubber band. Initial water temperature = __________

Step 3:Put the cup in the sunlight for 10 minutes.

Step 4:Remove the plastic wrap.Stir the water in the cup gently with the thermometer and record the final water temperature. Final water temperature = ___________

Step 5:Find the difference in the temperature of the water before and after it was set in the sun. Temperature difference = ___________

Step 6:Measure and record the diameter in centimeters of the top of the cup.Compute the surface area of the top of the cup in square centimeters. Diameter = ________________ cm Surface area of water = _____________cm2

Step 7:Compute the energy in calories that was collected in the cup.You may assume that the specific heat of the mixture is the same as the specific heat of water.Show your work. Energy = _____________________ cal

Step 8:Compute the solar energy flux the energy collected per square centimeter per minute.Show your work. A typical value obtained for the solar energy flux is 1.1 cal/cm2.min. Solar energy flux = _________________ cal/cm2.min

Step 9:Compute how much solar energy reaches each square meter of the earth per minute at your present time and location.Show your work.(Hint: there are 10 000 cm2 in 1 m2.) Since there are 10 000 cm2 in 1 m2 the solar energy flux obtained in Step 8 should be multiplied by 10 000 cm2/m2.A typical value would be 11 000 cal/m2.min. Solar energy flux = _________________ cal/cm2.min

Step 10:Use your data to compute the rate at which solar energy falls on a flat 6-m by 12-m roof located in your area at the time when you make your measurement.Obtain the answer first in calories per second then in watts.Show your work. The area of a 6-m by 12-m roof is 72 m2.This is them nukltiplied by the number of calories per square meter per minute obtained in Step 9.That quanitity is then divided by 60 s/min.A typical value is 1.1 x 104 cal/s.Finally this is multiplied by 4.18 J/cal to give the power in J/s or watts (W).A typical value is 45 000 W or 45 kW which is larger than the power likely to be consumed within the house. Solar power received by roof = __________ cal/s Solar power received by roof = __________ W

ANALYSIS/CONCLUSION Scientists have measured the amount of solar energy flux outside our atmosphere to be 2 calories per square centimeter per minute on an area perpendicular to the direction of the sun’s rays.This energy flux is called solar constant.Only 1.5 calories per square centimeter per minute reaches the earth’s surface after passing through the atmosphere.What factors could affect the amount of sunlight reaching the earth’s surface and decrease the flux of colar energy that you measure? How does this solar power compare with typical power consumption within the house?