J. H. Evans a n d L. G. Pedersen The University of North Carolina Chapel Hill 27514
A Simple and Inexpensive Solar Energy Experiment
The quest of e n e w alternatives is beginning in earnest now that manv heavilv industrialized countries see the end of cheap petroleum-energy (1). An energy alternative that promises a hright future of research for many areas of basic science is solar energy. The solar constant for the earth ( 2 )is 1353 WIm2 (428 Btu/ft2 hr) and the goal of current research is to capture as much of this energy as possible for doing useful work. The hulk of the power needs of modern society is provided by electricity. The two most exciting approaches in whichsolar encryy may supply elertricity are vta phot~~\r~ltitic (or gnlvanir~ and thermal runverhion pr~xt.ssc.;.The latter process, which inwlvrs turning sunlight into heat, ir o t m m t immedinte apit can utilize wnventional power plants. p l t o ~ h i l ~ hecause ty 'l'he (lynumos itre still turned hv heated steam just as if gas, oil, coal, and uranium u,err heing used tu generate the heat. A 10-megawatt demonstration facility in\,ulving it tr,ilvr tuwrr and a hattery of sun-tracking mirrors is scheduled for completion a t Barstow, California, in 1980-81. The former process, that is ohotovoltaic. was develoned to a nartiallv successful level h i the 1960's space effort, 6ut e1ect;icity sdprovided is currently a very expensive $15/W (3). Present research is exploring ways to reduce the cost of the solar cells. Two areas for which researchers have high expectation deal with doped amorphous silicon cells (4) and semiconductor liquid junction cells (5).At the present these cells are less than 10%efficient whereas the highly expensive solid state silicon cells are hetween 10 and 18% efficient (3). In this article we give details for an experiment which utilizes the current solid state technology to demonstrate (1) electrochemical generation of HZ, (2) direct generation of electricity for pumping H z 0 and (3) energy conversion efficiency. The experimental module costs about $100 and can he used repeatedly. The essential pieces are a small solar panel ($80), platinum electrodes ($15) and a tiny electrical pump ($4). T h e availability of an analog multimeter, a decade resistance box.. a olastic dishnan and a buret is assumed. The object of the experiment is to generate measurable amounts or H,electrorhem~callvr~lectricttvderwed from the solar panei) a t an inert p l a t i n k cathodeover a significant fraction of a dav. Two orientations of the panel relative to the sun are usedLone a t a fixed angle, the oiher a t the setting which optimizes the voltage. Also measured is the amount of water that can be pumped a given distance for the two orientations of the panel a t the same time.
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to the solar cell to comolete the circuit. The solar cell is suooorted . . hv a clamp on a r~ndstnnd3- 1% nn invrrwd hum u,h~ch1s placed uter the imrwrird cathodr 'I'he tl.! hlwrntrd ~s~cdlrctrd and n untcd tor 10 min. at each hour with the solar panel oriented so as to maximize measured voltage. Immediately thereafter the panel is readjusted to a fixed angle which should correspond to the sun's angle at midday. Again HZproduction is counted for 10 min. After this measurement the solar cell leads are connected to a miniature electrical pump (Edmund Scientific part No. 50,345,l.Sto3V). Water is pumped from a flat pan with the water level fixed at a height of about 30 em and the volume is measured in a large cylinder. This measurement needs only to be done for 1 minhr for each of the two panel orientations. The power output of the cell at different load resistances was measured by substituting a decade resistance hox fur the eleetro'chemical cell. The measurements were done at midday when the output was highest and were made by switching a multimeter from series (to measure amperage) to parallel (to measure voltage) with hoth the decade resistance box and the solar panel. Results (1) A plot of power versus load resistance curve is shown (8) in Fiaure 1. A maximum Dower of 0.54 W occurs a t ahout 10 ohm: This is approximately half of the advertised power for the cell. Voltage increases with load resistance. (2) Figure 2 shows an Hz production histogram where 10 min. readings have been multiplied hy 6 to get a m l H z h r value which is d o t t e d versus time of dav for hoth orientations. The maximized angle production is 0.546 Ifday; the fixed angle production is 0.452 Ilday. These numbers correspond to 5.08 X 10:lJand 4.22 X 10:'J, respectively, based on the minimum free enerev for dissociation of HqO. (3) The water . o u m.~ i n"data a is siGlarly displayed in ~ i g u r e3. ' Each hourly one minute reading is multiolied hv 60 to give a Der hour number. For the optim&ed angle, 114.61/day were pknped; for the fixed setting, 75.6 Ifday. These correspond to 3.8 X 1V2Jand 2.5 X lPJ, respectively, for the 34 cm height to which the Hz0 was pumped. Dlscusslon .Many l1.S. wmther stations record the location-dependerit solar ilux; a listing grf these i> given in IJ~tt'lit:and lh.kman's Irmk (21.In the Chapel Hill, North
