Sustainable Energy - Homework

by Max Dunn

Homework 5 Essay - Sustainable Economy based on Eucalyptus

Questions

Lets analyze the sustainable economy based on a Eucalyptus plantations with some far out assumptions.

(a) The current electric power consumption is reduced by 30% through better efficiency.

(b) Eucalyptus is burned in 35% efficient power plants.

(c) Additional transportation by electric car, train and bus replaces gasoline energy. Assume ½ as much gas as is used today with the same 35% efficiency.

(d) How many acres are needed to meet this goal? How much is available in the Midwest?

Discuss each assumption a bit with whatever comments or additional assumptions needed.

Answers

(a) Reducing electricity consumption is a worthwhile goal no matter what form of energy is used to generate the electricity. However, reducing electricity consumption by 30% will be a difficult goal to achieve and will require a lot of sacrifices and lifestyle changes. 

For instance, the US produces about 4,000 TWh of electricity each year. As we saw in Homework 1, if every household in the US replaced 10 light bulb with CFLs we would save 115 TWh nationwide, or about 3% of the total. Even if people made extreme changes and reduced the electricity used in their houses by 50%, this would only save 17% of all electricity since about 1/3 of all electricity produced is used for the residential sector.

(b) A normal efficiency of a single-cycle power plant will be at least 28% while a combined-combined plant is 60%. However, biomass often contains moisture and moist biomass produces electricity at only a 20% efficiency. So it would be important to make sure the eucalyptus is dry and then burn it in an efficient power plant to achieve at least 35% efficiency. 

(c) At 24 MPG which is the national average, gas cars have an efficiency of about 17%. This can be seen from the fact that a gallon of gasoline contains 36 kWh of energy so it is using 1.5 kWh of gas equivalent each mile, but the actual power required to move the car is only about 0.25 kWh per mile which leads to the 17% efficiency. So in order to achieve 35% efficiency, a car would have to get about 50 MPG.

Since personal transportation in the US uses about 40% of our oil, and we use 20 million bpd, if we save 1/2 of the oil currently used for personal transportation by increased use of electric car, train and bus, then we could save 8 mbpd or almost 3 billion barrels per year.

(d) However, we would need to produce additional electricity to compensate for this. A barrel of oil contains 1612 kWh of total energy which can produce 564 kWh of power at 35% efficiency. So to produce enough electricity to replace the 3 billion barrels of oil would require generation of an additional 1,690 billion kWh (TWh) of electricity each year. 

Since it is estimated that an acre of eucalyptus would produce 36,600 kWh of biomass a year which would produce 12,800 kWh of electricity at 35% efficiency, it would take about 132 million acres to produce this much electricity. This is less than the size of Kansas, North Dakota and Missouri.