Joe Bruin has a big lawn in front of his house that is 30 meterswide and 20 meters long. Josephine makes him go out and mow thegrass every weekend, so several years ago, he bought a fancy JohnDeere riding mower , which he loves to ride around his yard. Themower has a gasoline engine that is rated at 20 horsepower. To mowhis lawn each week, Joe needs to use the mower for about 30minutes, and the average power used by the mower during this periodis 5 horsepower. Joe has a very healthy lawn that generates a lotof lawn clippings, and after doing the EPSS 101 biofuels lab, Joegets the idea that 3 if he converted his mower to run on biofueldsinstead of gasoline, then maybe he could use biofuels generatedfrom his lawn clippings to power his mower
a. Calculate the weekly fuel energy requirements for Joe’sriding mower. (Hint. Don’t forget to include the efficiency of themower’s internal combustion engine)
b. Calculate the chemical energy content of the clippingsproduced by Joe’s lawn each week, assume solar insolation is 5.62kWH per square meter per day (Hint. Start with the amount ofsunlight the lawn absorbs each week and then use the photosyntheticefficiency of corn to calculate the chemical energy content of thegrass that grows each week)
c. Use your answer from Part b to calculate the mass of grassclippings produced by Joe’s lawn each week. Given that the densityof grass clippings is 667 pounds per cubic yard, how many 50-gallontrash cans would be required to hold the clippings? (Hint: Convertthe energy content you calculated for Part b to dry biomasscontent, and then assume that the grass clippings are 80%water)
d. Joe’s first idea is to use solar power to dry the grassclippings. Then, he would mod his riding mower to install aStirling engine in place of the gasoline engine, and then power theengine by burning the grass clippings. Compare the fuel energy thatcould be provided by burning the grass clippings in this mannereach week to the energy needs of Joe’s riding mower. Independent ofthe significant air pollution that would be generated by burningthe clippings, would this scheme work? Could the clippings powerJoe’s mower? Would there be any energy left over for otherpurposes?
e. Josephine works in a biology lab at UCLA and they haverecently come up with a magic strain of bacteria that convertscellulose (the dominant organic component of grass clippings) intoglucose. To create ethanol fuel, Joe and Josephine put the wetgrass clippings into barrels, and then add an equal mass of water,plus a cup of the magic bacteria, and then they let it sit for twoweeks while the cellulose is converted to glucose. Then, Joe takesthe resulting glucose solution and adds yeast to ferment thesolution to produce a 15% ethanol solution, which takes anadditional week. Then, every week, he burns additional grassclippings in a large still to distill the 15% ethanol solution tocreate a 95% ethanol solution. The resulting distillate isclean-burning fuel, which Joe uses to power his riding mower, whichhe has, of course, modified to run on ethanol. Ignoring the factthat the magic strain of bacteria doesn’t really exist, will thisscheme work? Would the clippings provide enough ethanol to powerJoe’s mower? Would there be any additional ethanol left over forother purposes, like powering Joe’s car, or to create alcoholicbeverages to 4 drink when Joe’s friends come over to watch UCLAfootball games on Saturday afternoons?
f. If Joe could find a way to use biofuels to power his mower,would this whole operation be sustainable? Discuss the needs ofJoe’s lawn for water and nutrients. Discuss the demands on Joe’stime. Do you think Joe would have enough time to run his homebiofuel operation and also have time to invite his friends over towatch football games on weekends?
