THAYER SCHOOL OF ENGINEERING ENGS-171: INDUSTRIAL ECOLOGY. Homework 1 SOLUTIONS. Answers will vary with the student. Here is a possibility.

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1 THAYER SCHOOL OF ENGINEERING ENGS-171: INDUSTRIAL ECOLOGY Homework 1 SOLUTIONS 1. (10 points) Suppose that you live in Suva, Fiji, and you are getting married. Your spouse (husband or wife) insists that, for the banquet after the ceremony, all the food must be locally produced. Compose the menu of this banquet. In particular, what would you provide as drink to those who don t drink alcohol? Answers will vary with the student. Here is a possibility. Menu: - Vegetable soup with mushrooms - Fish with stir-fried vegetables (or salted fish) (or caviar) - Pork sausages, served with tomato sauce and French fries - Carrot cake for dessert. Drinks: - Beer, of course - Carrot juice or V8-type vegetable juice for those who do not drink alcohol.

2 2. (10 points) It seems to be a foregone conclusion that bicycling to work is better for the environment than driving to work, but is it really true? Think about it: Muscle energy expanded from the bicyclist s legs comes from food intake, and the bicyclist must therefore eat an additional portion of food to have the energy to pedal the bike. Not only is food production impacting the environment, but the human metabolism (conversation of food energy into muscle energy) is also quite inefficient. So, how does bicycling compare with driving according to the numbers? For the comparison, consider a 5-mile distance. (This distance actually does not matter since impacts will be proportional to it, and the ratio of bicycling to driving becomes independent of distance.) For the bicyclist, take: - Average speed: 16 mph (= 26 km/h) - Power expanded while bicycling: 163 Watts ( - Efficiency of energy conversion in human metabolism: 20% (previous source; also: energy.htm) - Energy conversion: 1 kcal (kilocalorie) = 4185 Joules - Average greenhouse gas emission for food production and distribution (not counting cooking): 4.4 g of CO 2 per kcal (Weber & Matthews, Environ. Sci. Technol. 2008, 42, ). Then, for the motorist, take: - Average speed: 45 mph - Vehicle fuel consumption: 29 mpg ( - Greenhouse gas emission of combusting gasoline: 8.8 kg of CO 2 per gallon ( Compare the greenhouse gas emission for each mode of transportation. Bicyclist: For the 5-mile ride, the bicyclist riding at 16 mph takes 5/16 = hours = minutes = 1,125 seconds. Since power is energy per time, the energy spent is power x time, or 163 W x 1,125 s = 183,375 J. (Recall: 1 Watt = 1 Joule per second.) To produce this energy via a 20% metabolism efficiency, the energy intake of food must be 183,375 J / 0.20 = 916,875 J. In terms of kilocalories, this amounts to 916,875 J / (4185 J/kcal) = kcal. The CO 2 emission to produce and transport this food is kcal x 4.4 g CO 2 /kcal = 964 grams of CO 2. Motorist:

3 Covering the 5-mile drive at 29 miles per gallon, the motorist burns 5/29 = gallons of fuel. (Note: Speed does not matter, only indirectly through the mpg factor.) The rate of CO 2 production is then gal x 8.8 kg/gal = kg of CO 2. Comparison: The ratio of greenhouse gas emission is: bicyclist motorist kg kg 63%. Thus, bicycling is 37% better than driving. Two people sharing the ride: In this case, the comparison ought to be between one motorist and two bicyclists. The ratio is 2 bicyclists 1 motorist kg kg In this case, two people in the same car beat the two bicyclists by 21%. Added information (not asked of the students): It is interesting to calculate the break-even mpg value that a car ought to have to equate the CO 2 emission of a bicyclist. The answer is 45.6 mpg. So, if your car has a fuel efficiency exceeding 45.6 mpg, you are better off driving your car than bicycling to work! The Toyota Prius, at 50 mpg ( meets this condition.

4 3. (10 points) Consider the object depicted below, consisting of 11 parts (numbered 0 to 10). Your recycling company has collected a very large quantity of them. Although all their components are still in good shape, these objects can no longer be used in their present form. How far should you carry the disassembly in order to maximize your profit? How much profit can you make per 8-hour day? And, per 1000 units? Below is a chart listing the time to remove each part, once it can be reached. The time of 1.25 seconds is the time to physically slide a part away after it has been freed from screws. A 2-second delay is caused when moving from one object to the next. Manual labor is at the rate of $12/hour (incl. benefits), and one must also factor an overhead charge of $4.50/hour. Uncaptured parts must be trashed at the stated disposal fee. Re-usability Recyclability Removal time Value after disassembly Disposal fee Part Description Material 0 Plate PS plastic Recyclable 1.25 sec Plate PS plastic Recyclable 1.25 sec Plate PS plastic Recyclable 1.25 sec Component Composite Waste 2.00 sec Plate PS plastic Recyclable 1.25 sec Component Steel Re-usable 1.25 sec Screw Steel Re-usable 6 sec Screw Steel Re-usable 6 sec Screw Steel Re-usable 6 sec Screw Steel Re-usable 6 sec Screw Steel Re-usable 12 sec 3 2 To solve this problem, we first need to express the labor cost in convenient units: $12 $4.50 Labor cost 1 hour c s cents/second

5 Then, we proceed step by step and explore how cost/profit per object and cost/profit per time vary as a function of amount disassembled. 1. Do nothing Throw the whole thing away: No action No labor cost Cost of disposal: 8 x x x 4 = 16 Net profit per object: -16 /object Net profit per time: -16 /0s = - /s 2. Remove top screws 7 and 8, then slide plate 1 away: Time taken = 2s (to place object on table) + 6s + 6s s = 15.25s Recovery = = +8 Discarding the rest = -13 Net profit per object: = /object Net profit per time: /15.25s = /s 3. Remove screw 10 and then extract component 5: Time taken = 15.25s (previous time) + 12s s = 28.5s Recovery = 8 (previous amount) = +86 Discarding the rest = -9 Net profit per object: = /object Net profit per time: /28.5s = /s 4. Remove bottom plate 4, which is now loose: Time taken = 28.5s (previous time) s = 29.75s Recovery = 86 (previous amount) + 4 = +90 Discarding the rest = -8 Net profit per object: = /object Net profit per time: /29.75s = /s 5. Remove lateral screw 6 and then take side plate 0 away: Time taken = 29.75s (previous time) + 6s s = 37.00s Recovery = 90 (previous amount) = +97 Discarding the rest = -6 Net profit per object: = /object Net profit per time: /37.00s = /s 6. Remove lateral screw 9 and then take side plate 2 away: Time taken = 37.00s (previous time) + 6s s = 44.25s Recovery = 97 (previous amount) = +104 Discarding the rest = -4 (component 3 left alone)

6 Net profit per object: = /object Net profit per time: /44.25 s = /s At this stage, component 3 is left alone and needs no time to be removed. Since its residual value is 0, there is no point in recycling it, and it is to be discarded. This cost was already factored in Step 6 above. So, we have completed disassembly if we get to Step 6. We now take note of what we found. On a per-object basis, the value recovered keeps going up as we proceed through disassembly, to reach an ultimate value of /object. On a per-time basis, however, we note that the value tops at /s at the end of Step 4, after removing the most valuable piece (inner component 5) and letting the bottom plate (plate 4) fall off. So, if we want to maximize the profit for 1000 objects, we go for total disassembly, and the profit is: c $ objects: 1000 objects $ object 100c But, if we want to maximize the return for an 8-hour day, we stop at Step 4, and the profit is: c 3600s $1 8 hours: hours $ s 1hour 100c