Solar Energy Experiment for Beginning Chemistry Clyde E. Davis Humboldt State University, Arcata, CA 95521 Never hefore has there been a greater need to incorporate chemical applications of solar energy into the curriculum. Laboratory experiments a t the beginning chemistry level which make quantitative measurements of solar energy are few and far between. The following experiment illustrates how many chemical concepts such as light absorption, thermodynamics, and solid state photovoltaics can be incorporated into solar enerm education. This exueriment is wesentlv used the remaining part of the 3-hr laboratory period being used for calculations and comparison of results. The experiment is divided into two parts which are easily run simultaneously. The first part of the experiment consists of determining the percent efficiency of an insulated flat solar collector using a dye solution in one compartment and water as a control in the second compartment. A number of identical solar collectors were built to close tolerances (Fig. 1)which allowed the direct comnarisons of the efficiencies of solar absorption of different dye solutions. Eachgroup of 4-6 students uses a different dye solution: yellow, blue, red, and a mixture of all three which is black. The concentrations of all the dye solutions are adjusted to an absorbance = 1.0 at their X max.' The temperature of the dye solution and water are measured versus time a t 5-10 min intervals over a period of about 45 min. Simultaneously a pyranometer was used to determine the total light - enerm -.falling - on the collectors.2 From the experimentally measured volume of water or dye solution, the change in temperature (AT) over a specified time (min) and the total area of the insulated box (cm2),the number of Langleys received by the collector can he calculated as follows mL~,oX
+
Figure 1. Insulated solar coilector,equipped with 1-L culture flasks, lhermameters., stoppers. and stainless steel stirring rods. Campartmentsare lined with aluminized mylar.
I gH20 1cal x ---------X A TiT) mLHzO gH20 'C 0.1 cal g (box wt.) x ---- X ATi0Cl rnm X cm2 g
OYELLOW .BLUE n RED A BLACK
11
*c
The heat absorbed by the total contents of the box collector
Demonstration of this experiment was presented at the Third Forum on General Chemistry, Golden, CO, June 5, 1981. ' For me aosomance 01 1.0tne ole sc Aoni Mere prepare0 as lo OWS. 3 33 X 10 'g of FDA Ye ow ~5 4.48 X lo-- q FDA Red d 2 p1.s onc pcl rl of ha04 nas o ssolred in na!er then a lure0 lo 1 -, 1 74 X g/L of Bromthymol Blue, the black solution was prepared by mixing all three dyes in the above quantities followed by dilution to 1 L. The ovranometer used in this exoeriment was calibrated in units .. 01 -ang ays ckl cm- minl. arniio2 c from Bc Ion lnslrmenl Co . 1600 South C nl on St., Ba I more. MO 21224 Re.nl ve y nexpens ,e solar rao a1 On meters Cal 0latt.d In BT- f l . ma W ni .re wal aole from Crystal Productions, Box 12312, Aspen, CO 81612 The heat capaclty of the entire solar collector was estimated by two differentexperimental methods which were averaged. The culture bottles were filled with a measured volume of warm water (-4O0C), Dlaced in the insulated collector at room temoerature and a time versus iemperalue plol maoc lo dclcrm ne when lhirma.equ u r ~ mha0 %en eslao shw (no tcmpcralure cnange over 15 20 rn t i , kron~me cnmge n lsrnperalure ,T, .,-T ,,,, ,..,r, the Iota we ghl of me 2ox and bonles, the weight of water and heat capacity of water, the heat capacity of the collector can be calculated. The second method was done in an identical manner except cold water (4%) was used. ~
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158
Journal of Chemical Education
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~~~
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Figure 2. Typical heating curves for dye solutions on clear day at sea level.
except the water or dye solution is represented by the additive term. The total mass and heat capacity of the box excluding water and dye solution was given to the student because of time limitation^.^ The percent efficiency is calculated from the number of Langleys received by the water or dye and the average pyranometer reading in Langleys. Langleys (received by collector) X 100 Langleys ( w e . pyranometer) On a clear day, with the described collector design, the typical experimental results are shown in the table and Figure 2. Since the concentrations of the dye solutions have been adjusted so that the ahsorbance is equal to 1.0 a t the A max., the intensity of light removed from the availahle solar radia%Efficiency=
Percent Efliclencies of Solar Absorption Substance' Water
Black Dye Blue Dye (Bromthymolblue 2.61 X 1 0 V M) Red Dye (FDA Red #2 7.41 X 1 0 F M) Yellow Dye (FDA yellow #5 6.23 X M)
h Max (nm)
low efficiencv of the vellow dve. the swectral enerev distri%
Efficiency
-
21
615-395
32
615
28
490
26
395
22
All dye solvtions had an absorbance = 1.0 at their A max. Slack dye solution prepared by mixing the proper quantifies of yellow, red. and blue dyes fallowed by dilution to 1
liter.
a t 490 nm, this more than offsets the higher energy content of the lirht ahsorhed hv the yellow dye. The relative intensities of solarradiations a t iea level 490 kin (red dye) and 610 nm (blue dye) are close. The red dye should absorb about 21% more energy. However, the blue dye is slightly more efficient probably due to the small absorption peak of bromthymol blue a t 390 nm. In the second part of the experiment, water was electrolyzed by means of solar energy. One electrolysis apparatus powered by eight 3-in. diameter silicon photovoltaic cells5 wired in series was set up for class use. The cells are mounted on a hoard which is tilted 40" and are protected by an acrylic olastic window. Since the aowaratus was in continuous ower.. ition during the experiment, the students could collectthe data a t their convenience. The volume of hydrogen gas generated in a conventional electrolysis apparatus filled with 1 M sulfuric acid is measured over a swecifictime. At the same time, the pyranometer readings aremade. On a clear day up to 2 mL of H2 gaslminute are obtained. From the Ideal Gas Law, using water vapor pressure corrections, the number of moles of HZis calculated. Using the AG" of formation of water, the free energy content of the collected hydrogen is calculated in units of calories. From the total area (em2) of the photovoltaic cells. the time (minl and the observed ovranometer ~
~
energy of thecollected hyd;ogen (calories) and the availahle solar energy (calories). %Efficiency Figure 3. Absorption spectra of dyes: - - 6.23 X lo5 MFDA Yellow #5: - . - . 7.41 X 10' MFDA Red #2: - 2.61 X lo5 MBromthymol blue. tion is roughly the same for each dye. Small peaks and shoulders do make small contributions a t other wavelengths (see Fig. 3). T h e use of standard 1-liter culture bottles (Pyrex") eliminates any significant variations of pathlength. With the control of the above parameters, the observed differences in heating curves and calculated efficiencies for the different dye solutions are due to two factors: (1)the energy content of the ahsorhed light and (2) the relative intensity of solar radiation a t the surface of the earth.4 The differences between the black dye and the water (control) are explained easily since the black dye absorbs the major portion of the visible spectrum (615-395 nm). The yellow dye has the lowest efficiency (22%)or about the same as water. Planck's equation shows that the energy content of the ahsorhed light for the yellow dye is about 3.1 eV or 19%greater than the energy ahsorbed by the red dye solution (2.5 eV). In order to explain the
56.7 Kcal 1000 eal X -mole Hz burned Kcal X 100 Langleys (pyranometer)X (cell area) cm2 X min It is sururisine to most students that this wrocess for an alternative fuel from solar energy is only about 1% efficient. The above ex~erimentwrovides an excellent way of -pulling together many different topics such as light absorption, gas laws, the mole concept, and thermodynamics. Copies of the experiment and data sheets are availahle from the author on request.
moles Hz X
~
Acknowledgment
'Brill, Thomas 6.. J. CHEM. EDUC., 57, 259 (1980). Available from Edmund Scientific Co., 101 E. Gloucester Pike, Barrington, NJ 08007.
Applied Molecular Spectroscopy Short Course
parameters, applications, and spectral interpretation. The program includes basic theoretical considerations, hands-on instrumental training, and the interpretation of spectra. Four hours of lecture each morning will serve to present the theory, instrumentation,and applications of infrared spectroscopy. Some reference to Raman and ultraviolet spectroscopy will also be included. Each student will spend every afternoon working in the laboratory under the direct guidance and supervision of experienced technical personnel. The instructional staff includes memhers of the Department of Chemistry at Arizona State University augmented by guest lecturers from industrial laboratories. Enrollment in the course is limited and sufficientequipment is available to insure each student adequate time for personal operation of the instruments. The cost for the program is $450. For complete information,including descriptive brochure, please write Dr. Jacob Fuchs, Director, Applied Molecular Speetrascopy, Department of Chemistry, Arizona State University, Tempe, Arizona 85287.
Volume 60
Number 2
February 1983
159
