---

Physics 140: Greenhouse Effect
Spring 2008
Homework #5, due Wednesday, April 23, 2008

---

1) You use 100 gallons of home heating oil per month in the winter,
and you decide switch to a natural gas system to reduce your carbon
emissions. Show your work.
(a) (2 pts) How much natural gas will you burn each month? Assume the
two systems have the same efficiency, so that the energy content of
the two amounts of fuel will be the same. Remember that heating oil
has 139,000 Btu/gallon and natural gas has 100,000 Btu/therm.
Answer: 139,000 Btu/gallon x 100 gallons = 13,900,000 Btu
13,900,000 Btu x 1 therm/100,000 Btu = 139 therms
(b) (3 pts) By how many pounds/month do you decrease the monthly carbon dioxide emission? (one gallon of home heating oil leads to emission of 22 lbs of
carbon dioxide; 1 therm of natural gas leads to emission of 12 lbs
of carbon dioxide).
Answer: oil emission is 100 gallons/month x 22 lbs/gallon = 2200 lbs/month.
Natural gas is 139 therms/month x 12 lbs/therm = 1668 lbs/month
Decrease is 532 lbs/month.
(c) (1 pt) By what percentage do you decrease the monthly carbon dioxide emission
relative to its original value?
Answer: 532/2200 x 100% = 24%

2) The manager of a European company is considering a factory
renovation that will reduce carbon dioxide emissions by 400 tons
per year and cost $130,000. Without the renovation, each year
the company will have to buy emissions permits for 400 tons at
$30 per ton.
(a) (1 pt) How much will the company have to spend on emissions permits
each year?
Answer:  $30/ton x 400 tons = $12,000

(b) (3 pts) What is the payback time for the factory renovation,
based on the savings in emissions permit purchases?
Answer: $130,000/($12,000/year) = 10.8 years

3) You are trying to decide whether to drive or fly from New Jersey on a trip with two friends to Colorado this summer. The distance each way is 2000 miles. 
(a) (2 pts) How many tons of carbon dioxide will be emitted as a result of your trip if you drive in a car that gets 40 miles per gallon (the trip can be regarded as being all on the highway)? Assume that producing and burning a gallon of gasoline emits 25.3 lbs of CO₂ and no other greenhouse gases.
Answer:
4000 miles x 1 gal/40 miles x 25.3 lbs/gal x 1 ton/2000 lbs = 1.3 tons

(b) (2 pts) If three people drive together in a car that gets 30 miles per gallon, what is the fuel consumption of the vehicle in passenger miles per gallon?
Answer: 90 passenger miles/gal.

(c) (2 pts) How many tons of carbon dioxide equivalent will be emitted as a result of your trip if the three of you go by plane? The fuel consumption of domestic air travel is 42 passenger miles per gallon. Producing and burning a gallon of jet fuel emits 45.6 lbs of CO₂ equivalent (about half is CO₂ and half is other greenhouse gases).
Answer:
4000 miles x 1 gal/42 passenger miles x 3 passengers x 45.6 lbs/gal x 1 ton/2000 lbs = 6.5 tons

(d) (2 pts) How much more CO₂ equivalent is emitted by choosing to fly? What percentage is this of the average annual CO₂ emission for three US residents (recall that the annual per capita CO₂ emission is 20 tons).

Answer:
5.2 tons. 5.2/60 x 100 = 8.7%. This is a significant increase.