The Energy Relationships of Corn Production Thomas E. Van Koevering, Michael D. Morgan, and Thomas J. Ywnk Environmental Sciences 317, University of Wlsconsln-Green Bay, Green Bay, WI 54301 Recent calls for the improvement of science education have recommended that teachers put a greater emphasis on relating scientific principles to everyday life (1-4). Some have suggested that such an approach would provide students, nonscience majors as well as science majors, with the "scientific and technical knowledge to fulfill civic responsibilities, improve the students' own health and life and the ability to cope with an increasingly technological world" ( I ) . Moreover, most teachers would agree that the use of everyday examples raises the interest of students in science and thus improves their motivation for understanding scientific principles. T o meet this need for the teaching of more practical applications of scientific principles, we present for consideration the example of alcohol production from corn. During the late 1970's considerable public attention focused on the production and use of gasohol as a means of reducing our dependence on foreign petroleum. The use of gasohol has increased substantially in recent years, although public interest has waned. In 1980, less than 0.5% (1,900,000,000 L) of the gasoline sold contained 10% alcohol (gasohol). By 1985, gasohol sales were in excess of 4% (16,700,000,000 L) (5). The production of alcohol from corn lends itself well to illustrating the practical applications of scientific principles that deal with energy transformations and inefficiencies. Moreover, we use this study to illustrate to students that there is more than one way to approach and solve a problem. We achieve this objective by calculating and comparing the efficiency of energy transformations within a system (such as a plant's transformation of solar energy to chemical energy) from the perspectives of both an inputoutput analysis of a hlack box and a more detailed examination of specific transformations within the hlack box. Parts or allof this case study would be appropriate for a variety of courses because tracing the flow of energy from sun to corn plants to alcohol incorporates an understanding of information from biology, chemistry, and physics. We begin witha consideration of thesolar energy available for plant growth, then proceed to examine the utilization of solar energy by plants, and end with a study of the conversion of corn to alcohol. We also include suggestions for classroom and laboratory study. Solar Energy Avallable for Plant Growth T o begin this case study, students first can he asked to list the types of data that they would need to he able to calculate how efficiently corn plants transform solar energy into chemical energy (biomass). After some discussion they should realize that one of the first things that they need to determine is the amount of solar energy that strikes the corn plants during the growing season. Students also should realize that the amount of incident solar radiation varies with latitude. Moreover, the length of the growing season (freezefree period) also varies with latitude, topography, and proximity to large bodies of water. So that students can he aware of the role of those factors on crop yield, we recommend the use of a t least two locations to calculate the amount of solar
Table 1. Solar Energy Avallable for Plant Growth
May
June
July
August
September
Ames, Iowa (130&y m) (14) Langleysl 465 538 535 462 369 day 19 30 31 Days 31 19 3.70 Jlha month 6.75 6.93 5.99 2.93 (X St. Paul, Minnesota (90-day Langleysl 468 day Days 0 0 Jlha month
527
546
31 30 0.61 7.08
Total (lor growing season)
26.3
corn) (15)
480 31 5.97
337 0
0
19.7
energy available for plant growth durine the erowine season. A pr&tical consideration is the limitediumber of Ztes that record incident solar radiation on a continual basis. The location of such sites can be obtained by contacting your nearest office of the National Oceanographic and Atmospheric Administration (NOAA). We chose Ames, Iowa, (near the north-south center of the corn belt) and St. Paul. Minnesota.. (near the northern mar, gin of the corn belt). In Ames, many farmers grow corn that takes about 130 davs from eermination to maturitv (130-dav corn) (6). Farther horth Paul, many farmers &ow 9 6 das corn because of the shorter erowine season (7). T o calculate the total solar radiationthat is available for plant growth, we chose 12 Mav as the eermination date for the 130day corn and 1June foithe 90-day corn. The solar radiation data that are readily available from NOAA are in the form of daily averages for each month. Table 1 illustrates one way that students can organize their data. (Incident solar radiation is recorded in Langleys where one Langley equals 4.18 joules11 cm2 of receiving surface.) The following is a sample calculation for the month of May in Ames: 465 (Langleys/day)X (4.18 J/cmZLangley)X (19 days)
Efflclency of Utlllzatlon of Solar Energy by Corn Plants (The Black Box Model) Now that the students have determined the average amount of incident solar radiation on a corn field, they can calculate the efficiency of utilization of this energy by corn plants. One way to calculate this efficiency is to compare the total energy incident upon the corn with the energy content of the biomass produced. This is a useful approach, particularly if details of energy pathways are unknown or unavailable. Because we are concerned principally with the production of alcohol from shelled corn, our initial calculations will deal with the transformation of solar energy into shelled corn. Volume 64
Number 1 January 1987
11
To determine the energy content of the shelled corn produced on a hectare of land near Ames, we multiply the average yield (7240 k g h a ) ( 6 ) times the energy content of 1.00 kg of corn (15,600 kJ/kg) (8).This calculation yields a value of 1.13 X los kJ/ha. The efficiency of utilization of solar energy by corn plants during the growing season can now he determined by calculating the ratio of energy content of the shelled corn produced per hectare to the solar energy incident upon the corn plants.
Therefore, only 0.43% of the solar energy that is incident on a hectare of corn nlants over a erowine season is converted into chrrn~cillenergy that is stored in the shclled corn prtduied on that hectare of land. I'sina an averare yield of 5980 kdha (7) the energy efficiency of srhelled corn production nea; St. Paul is
The efficiencv of total above-around production can he drtcrrniued hY adding the energs.value ufthe cornstalks and corn cobs produced to that ofthe sht!llcd corn. (Because the determination of the root production is extremely difficult (9),such values are usually not considered.) The cornstalks and cobs have an energy content of about 13,900 kJ1kg (10) and the quantity of corn stalks and cobs by mass is about the same as the mass of shelled corn produced (6).Thus the mass of corn stalks and cobs produced on ahectare near Ames would he equivalent to the mass of 7230 kg of shelled corn. The energy content of this mass of cornstalks and cobs is 1.00 X 10s kJ for the cornstalks and cobs on a hectare. When we add this energy content to that of the shelled corn, we find the total ahove-ground energy output to he 2.13 X 108 k J h a . Such an addition increases the calculated energy efficiency of biomass production from 0.43 percent to 0.80%. One conclusion from this exercise that should he of interest to students is the very low efficiency by which plants convert solar enerev -.to biomass. We can take advantaee of this surprising result hy asking students what could account for efficiencv values of less than 1% in our enerav i n p u h output analysis. T o gain insights, we need to pro-iced-to a more detailed examination of the inefficiencies inherent in the energy pathways within plants. These inefficiencies are described in the next section. How Plants Use Solar Energy for Growth (The Energy Pathway Model) Bv tracine the flow of enerev from its canture bv areen to t& production of sugars, we can h s t r a i e iome complexities of energy transformation. Moreover, this example can well illustrate to students the need for drawing together principles and information from various disciplines if they are to understand energy flow in the environment. To trace the flow of solar energy, students should first recognize that like all energy-requiring reactions, photosynthesis requires a minimum threshold of energy to activate the process. Only visible light (400-700 nm) possesses enough energy per photon to activate the photosynthetic machine. Because onlv half of the solar enerav strikina a plant leaf lies in the hsihle portion of the sol& spectrum, onlv half of the incomina solar radiation can be potentiallv converted to chemical energy in the'form of sugar-an end product of photosynthesis. We can then ask how much of the visible light is absorbed by plant leaves. Because living leaves are not black, students can deduce that not all visible light is ahsorhed by leaves. In 12
Journal of Chemical Education
fact, plants absorb only 80% of the incident visible wavelengths. The lowest absorption, 70% and therefore the highest;eflection lies in the-green wavelengths-as we would expect because leaves appear green to us (11). Is all the energy ahsorhed by a leaf directed into photosynthesis? Students should recall that only chlorophyll molecules and other associated oiement molecules can caotnre . light for photosynthesis. Certainly, green plant cells contain hundreds of thousands of chlorophyll molecules, hut each plant cell also contains a nucleus, many organelles, a cell wall. and considerable water. These substances also absorb sola; energy, but they can convert it only to heat. In fact, 'only about half of the solar energy that is absorbed by a leaf is actually captured by photosynthetic pigments (11). From an energy efficiency standpoint, let us summarize the energy gains and losses to this point: (1)only half of the incident solar energy is photosynthetically active, (2) only 80%of this photosynthetically active energy is absorbed by leaves, and (3) only half of the energy absorbed by a leaf is absorbed by photosynthetic pigments. If we let the total incoming solar energy he equal to one, then the percentage of energy entering the photosynthetic process is equal to 1.0 X 0.5 X 0.8 X 0.5 = 20%. Thus only 115 of the initially available solar enerav -. that actuallv strikes a leaf enters the photosvnthetic pnlcess. \Vhat happens tu this energy? \$'hen chluruph\'ll absorbs solar energy, certain electrons are raised to a state of higher energy. ~cientistsknow that red light is the most efficient wavelength because its energy content is just a little more than that needed to excite an electron in a chlorophyll molecule. Blue light is less efficient because of its hieher enerev content. When blue lieht is absorbed by chlo~ophyllmzecules, the energy differential between it and the less-energetic - red lieht - is excess and is wasted as heat. Through a complex of biophysical and biochemical reactions, the energy in excited chlorophyll molecules is eventually transformed into chemical energy in the form of sugars. These reactions well illustrate the Second Law of Thermodynamics. Considerable energy losses occur in the stabilization of high-energy pigments and the many intermediate forms before the stable sugars are formed. For example, only about 35% of the energy absorbed in the red wavelengths (the most efficient wavelengths) is converted to chemical energy in sugars (11). Not all the sugar that is produced by photosynthesis goes into plant biomass (growth). Further energy losses occur when some of the sugar is funneled through the process of cellular rwpirntim, which provides energy for the synthesis of materials that are used in gruwth and repair. As in the reactions involved in photosynthesis, energy transformations during the reactions of cellular respiration are not 100% efficient, and some chemical energy is converted to heat energy and is lost to the plant. About 25 to 30% of the chemical energy that results from photosynthesis is lost as heat energy through the reactions of cellular respiration (12). Hence, students should gain an appreciation that plant production, like many other energy-utilizing processes, contains many points where inefficiencies occur. Even under ideal con&tibns, the maximum possible efficiency for converting total incoming solar radiation into stored photosynthetic-nroducts is about 5%. But solar intensitv changes during the day and ideal growing conditions rarely exist during much of the growing season. Therefore, scientists have found that the maximum sustained efficiency is often near 3% or lower (12). Interesting questions can be raised about the effects of varying air and soil temperatures, soil moisture, soil nutrients, and plant densities on those energy transformations. Such questions could be addressed in class discussions or, even better, in simple plant growth experiments.
-
Conversion of Shelled Corn to Alcohol (Black Box Model) We now focus on the energy efficiency of converting shelled corn to alcohol. Tahle 2 illustrates the mass components of shelled corn. Note that the industrial processes-by which shelled corn is converted to alcohol onlv utilize carbohydrates. Hence the energy that exists in thk protein fraction remains constant. Moreover, the carbohydrate component is a complex mixture of different carbohydrates, some of which do not undergo fermentation to produce alcohol. Thus students shoulddeduce that the energy content of alcohol produced by fermentation will be considerably less than the enerev content of the shelled corn. Tahle 3 illustrates the results of the fermentation process. The heat of comhustion of 1.00 ke of shelled corn is 15.600 kJ. This much corn produces 0.387 L of alcohol. Each iiter has a heat of comhustion of 23,380 kJ. Thus the heat of combustion of alcohol produced from 1.00 kg of corn is 9050 kJ (8).Therefore the fermentation of shelled corn to produce alcohol results in an energy yield efficiency of 58.0%. The energy that remains resides in the protein fraction, carhohydrates, and other organic compounds that did not undergo fermentation. Of course water has a zero heat of combustion. T o understand further the reasons for these inefficiencies we need to study the chemical reactions involved in fermentation and comhustion. Energy Avallable from Shelled Corn (The Energy Pathway Model) The heat content of the nonprotein biomass that remains has not been accurately defined because we still do not know how much heat was gained or lost during the fermentation reaction. Since fermentation is a spontaneous reaction in the presence of appropriate enzymes we might suspect that it is an exothermic process. In order to account for the energy involved in this reaction we must first determine how much carhohvdrate material was converted to alcohol. The quantity oi carbohydrate material that ferments to form alcohol can he calculated from the equation C,H,,O&)
-
2C2H,0H(I) + 2C02
(1)
81.9 kJ/mol of CsHlz06 used or 273 kJ for the 0.599 kg of C6H1206 consumed. This is the amount of heat released during the fermentation of one kilogram of corn. Table 4 is a revision of Table 3 adding the heat lost due to fermentation so we now can specifically identify the heat content of the remaining organic matter. Another interesting calculation is to derive the heat of combustion for ethyl alcohol using AHr values from the handbook and then compare this calculated value with the value of 1360 kJ1mol often cited in the literature. Students should find that the'calculated value agrees with the given value to within 0.5%. The corn product that remains after fermentation has several important characteristics. The mass of this product is 0.400 kg per 1.00 kg of shelled corn of which 0.130 kg is water and 0.090 kg is protein, leaving 0.180 kg of unfermented carbohydrates and other organic matter. The relative concentration of protein has increased from 9% in the shelled corn to 22.5% in the moist hyproduct and 33.3% in the drv. bvnroduct. This hiehlv nutritional hvnroduct can he -. used as livestock feed. ~ h A o energy k contknt of 1.00 kg of the hv~roduct(from Table 4) is 6310 kJ of which 4170 kJ is ener&from nonprotein sources. When compared to the energy of the original shelled corn (15,630 kJ/kg) the energy content of the byproduct is (15,800 kJ/kg) is somewhat greater. Another useful commercial bv~roductfrom fermeniation is the carbon dioxide produced.' Fosoll Fuel Inputs Our discussion thus far has only dealt with the conversion of wlar cmergy to corn toalcohol.~owever,it the producriun of alcuhol is to be successful, therr must be an input of fossil fuels into growing, hnrvesting, and proct-saing of corn. \Vithout appropriate soil cultivation and applications of fertilizer and ~esticides(which reauire iossil fuels for thrir uroduction and application), insufficient corn is produced to provide a sumlus that can he converted to alcohol. In addition. unless fossil fuels are used to provide heat for fermentation: the reactions ~ r o c e e dtoo slowlv to be efficient. Thus an analysis of eneIgy conversions shbuld also address the question of whether alcohol can he produced that has more fuel energy than that of the fossil fuels which go into corn production and fermentation. m e d
One kilogram of corn yields 0.387 L of ethyl alcohol. Using a density for ethyl alcohol of 0.791 gImL, the mass of the alcohol produced is 0.306 kc. Using 0.306 ke as the mass of is0.399 s u 1he product in eq 1 theam& of ~ ~ ~ ~ ~ ~ c & kg. (The trrmentation reaction also nwduces 0.292 kg.of CO,.) In order to determine the energy released from reaction 1, it is necessary to know the AHr (heats of formation) of the reactants and products. The AHr values for C2HSOH(1) (-277.4 kJ/mol) and COz (-393.1 kJ/mol) are readily available from chemistry textbooks and handbooks (13).The AHf value for C6Hlz06 can he calculated by using a heat of reaction of 2813 kJ/mol (AH) for the aerobic respiration reaction represented in eq 2. The value of AHr for HzO(l) is -285.6 kJImol, and AHr for 0 2 is 0.00 kJ/mol. C,H,,O,(s)
+ 60,(g)-
6COz(d + 6H20(1)
Table 3. Energy Components of 1.00 kg of Shelled Corn (Black Box Model)
Heat of combustion of alcohol Heat of combustion of water Heat of combustion of remaining organic matter Heat of combustion of shelled corn
0.306
6,950
0.130 0.564
0 6,650
1.0000
15.600
(2)
The value of AHf for that is calculated from eq 2 is -1259 kJ/mol. Going back to the fermentation reaction in eq 1 with all of the AHtvalues, the AHvalue for the reaction is
Table 4.
Energy Components of 1.000 kg of Shelled Corn (Energy Pathway Model) mass
Table 2. Mass Components of 1.000 kg of Shelled Corn ( 8 ) (kg)
Protein (16) Water Carbohydrates and other organic compounds
0.130 0.760
T"+d
1 nnn
0.090
Heat of combustion of alcohol Heat of combustion of protein (76) Heat of combustion of water Heat lass due to fermentation Heat of combustion of CO, Heat of combustion of remaining organic matter Heat of combustion of whale corn Volume 64
Number 1
(kg)
(kJ)
0.306 0.090
8.950 2.125 0
0.130
273 0.292
0
0.160
4.165
1.000
15,600
January 1987
13
Unfortunately, this question does not have an easy answer. A wide range of values has been published for fossil fuel reauired to ~ r o d u c ehieh corn vields. The hiehest ouhlishcd \:slue (19,:i00 k ~ / L o ~ & ~ hproduced) ol ispken h i the ACR Procrss Corporation (171. Thev hreak this cncrm requirement down into (1) direct on-faim energy (6,475 g / ~ ) , (2) fertilizer and chemicals (8,410 kJ/L), (3) transportation (2,200 k J L ) , and (4) capital equipment (1,790 kJ/L). The lowest value (7,420 kJ/L) is sueeested bv Commoner (18). Several researchers have c a l c u l ~ e dthe amount of required fossil fuel energy to he approximately 13,900 k J L (19-22). This value lies-about midway between the most favorable and least favorable values for the required input of fossil fuel enerev. One reason for the differences in these values is that not zi of the researchers included the same number of enerev sources in their data. or. as is noted in Table 5. thev assien &ues by using different criteria. Additional fossil fuel energy is also required to ferment and distill the shelled corn. Again, a wide range of published values exists from several independent sources. Commoner (18) and the ACR Corporation suggest that about 12,000 kJ/ L is required to fuel those processes while Scheller and Mohr (22) and Reilly (20)(independent of Scheller and Mohr) proposea considerably higher value of 39,700 kJ/L. Scheller and Mohr break their value down into (1) distilline" (12.500 . ,~~~ k J L ) , (2) evaporating (13,800k J L ) , (3)'purifying (5,170 kJ/ L) and (4) drvine the corn. alcohol. and bvnroducts (104.200 .. . , k J L ) . ~ o m m o n e stated r that the purifying of the alcohol is .unnecessary. It is evident that the answer to the question, "Can alcohol he produced in a manner that does not require the invut of more fossil fuel energy than is returned upon combustion?" depends upon the energy efficiency of the technology employed. The lower values reported reflect the kinds of energy efficiencies that can he possible if new energy distilleries are used. The higher values reported are ohtained by assuming that the alcohol will be produced in existing facilities, which are less efficient.
Table 5.
Range of Balance Sheets for Producing 1.000 L of Ethyl Alcohol
Least Favorable
Average
Most Favorable
input Energy to g r o w corn f r o m w h i c h ethanol Will b e distilled Energy to ferment and distill t h e ethanol. and t o a byproduct a n i m a l feed Total Input
19.8 (17)
to
14.5 (20,23)
7Xa(18)
36.5 (19,23) 39.6 (20,24
19.2 (21)
to 12.8 (17)
56.3 to 59.2
33.6
18.8 t o 20.3
11.0(18)
produce
output Energy from 1.000 L o f ethanol Energy value of f e e d grain byoroduct
23.4 (18)
23.4
44.4O ( 18)
13.9 ( 2 0
13.9
13.9
Total Output Net Energy
37.3 -19.5 to -21.9
37.3 -3.7
58.1 37.8 to 39.6
-
~~~~~
'Only two-thlrdsof theenergy neceora!ytoralsethe cornnsededtoprovlde 1.000Lof slmhol is considered input energy because only tw-mird~ of the corn i s oonsumsd by fermentation 119. "1~0hol r n p l a ~ ean ~ equivalent volume of Qsaoline, which has a higher heat of combustion. and energy is required to produce m e gaooline (IS).
2. Juhnson,K.L.:Aldridge, B. G. J. Call. Sci. Teock. 1984.16 20. 8. The National Commieaimon Excelieneein Education. A Nation a t Risk; U.S. Department oi~ducation:wa.hinmton. n c ,981~
alcommunbation. 1984. 7. Hicks, D., Deoartment of Aaronomv. University af Minnesota-Minneanolir. . .~ e r r u n a l communication. 1984. 8. David. M. L.: Hammaker. G. Buzenberg, R. J.; Wagner, J. P."Gasohol Econamic Feasibility Study-Final Report": prepared for Energy Research and Development Center. University of Nebraska, Lincoln. by Development Planning and Research Associates, Manhallan, KS, 1978: p 8. 9. G i l h d , R. M.;Tholne, J. H.: Hitz, W. D.:Fiaguinta,R.T. Science 1984,225,801. 10. Conuerse.J.,DepartmentofA~riculLuralEngineering.Uniw,rityofWiseonsin-Madison, personal communication, 1984. 11. Nobel, P.S. R~ophysicolPlant Physioiom and Ecolopy; W. H. heeman: New York. 1988. Phyriology, 2nded.: MaeMillan: New York. 1979. 12. Bidwell. R. G. 13. Weart.R.C..Ed.HondbookolChamislryandPk~~aics,66thed.:CRC:BoeaRalun,FL, ~
Conclusion This analysis of the use of solar energy by plants to produce a product that can yield alcohol serves as an excellent exampie of how theory a i d the real world do agree. Living plants and industrial processes do obey the laws of thermodynamics. Teachers can use this example to illustrate that an energy analysis from an external perspective of energy in versus energy out is consistent with the results obtained when an examination is made of the internal energy pathways. The analysis of the net gain (or loss) of fossil fuel energy in the erowine, harvestine. and fermentine of corn illustrates the discrepincies and ckroversies tharresult when differing assum~tionsand criteria are made. Hence i t serves as a valuahle lesson to students always to check the original sources of their data to determine the assumptions and methods used to obtain the data. Literature CRed I.The National Science Board Commission on Precollege Education i n Mathematin, science. T ~ C ~ ~ O I ~E Z~ Y u. c . , ~ ; ~ ~P ~ lorthe a i srt centurv: ~A pinnofi
14
Journal of Chemical Education
s.;
S.Plonl
198% ~~-
14. Iowa State Weather Station. Iowa State Univemity, Amps, IA, personal rommuniea-
~
~~~. ~~
17. ACR Corpurstion."Energy Balance",partofa preliminary pilot project application to the U.S. Department of Agriculture. April 1978. 18. Hearinskfore theSubmmmitttt 0nAgricultural Researchand General Legislationof iheCimmitteo nn Arricutture. Nutrition, and Fniestry. United States Senate,NincLy-sixth Congrem, FiratSerrion, July 23,1979, pp 126129. a y , 19, 1979. 19. NPUYo?& T i m e r . B u s i n e ~ ~ D a y , S ~ t ~ r dMay 20. Reilly. P. Pager presented a t Energy Conference. Iowa Farm Bureau, DOE Mainel. C%","k. " ", >O"",. 21. Pimentel, D.: Hurd, L. E.; Bellotti, A. C.; Forster, M. .I.; Oka. I.N.: Sholes. 0. S.: ~ ~ ~ ~ Whitmsn, R. 1913.182.4B449. 22. Scheller, W.: Mahr, B.Paperpresented atthe 171st Nationel Meeting d t h e American Chemical Society, New York.7 April 1976. 21. BsrtlerviliaEnergy ResearchCenter,Operated hy theDe"anm~nfafEnecg~,reporfed
..,,.,
J.Scianre
