Energy Model Worksheet 6

Date Pd Energy Storage and Transfer Model Worksheet 6: U.S. Energy Consumption and Supply

1. Looking at the table from the U.S. Energy Information Administration website, a. Which energy sources could be increased to meet our energy needs?

b. Which energy sources will be forced to decline during your lifetime? Why?

c. Conventional hydroelectric and geothermal energy sources are essentially “maxed out” in the U.S. Why?

d. How does energy conservation (using energy efficiently) impact these numbers?

©Modeling Instruction – AMTA 2013 1 U8 Energy - ws 6 v3.1 2. Coal will be an essential source in the years to come. The U.S. has huge, but finite, coal reserves. Name several of the major drawbacks to extensive coal use:

3. Petroleum Humans have really figured out how to use petroleum, but it won’t last long at the rate we’re using it. Here’s a dramatic way of diagramming petroleum use. The total area of the rectangles represents the estimated total amount of oil past and present on earth. Each square represents the amount of oil used during the indicated time period. In the 1970’s, it was predicted that we would exhaust our oil supplies by 2000 because of the previous two decades of 7% annual increases in oil consumption. Oil use pre-1953 Oil use 1963-73 Oil use 1953-63 (7.8% growth rate) (7% growth rate) Oil use 1985-95 (had the 7% growth rate continued.) Oil use 1975-85 (had the 7% growth rate continued.)

In fact, the oil crisis of the 1970’s helped to curb the growth rate in oil consumption: Oil use pre-1953 Oil use 1963-73 Oil use 1953-63 (7.8% growth rate) (7% growth rate) Oil use 2000- 2035 (growth rate ~ 2.2%) Oil use 1973-2000 (3% growth rate)

Oil won’t run out in 2035, however, because consumption will not remain high and then suddenly stop when oil runs out. Instead, consumption will be forced to decline as oil becomes scarce (and therefore expensive.) The 2006 growth rate in petroleum consumption declined to 0.7%.1 A graph of the quantity of a resource vs. time is a bell-shaped graph called a Hubbert curve. It is estimated that the world has consumed 1 trillion barrels of oil in our entire history of use. If 1 trillion barrels of oil remain, (as most researchers2 estimate) where are we on the Hubbert curve? (Sketch a graph below.)

oil usage

now time

1 BP Statistical Review of World Energy 2007 2 for example: Colin Campbell and Jean Laherrere, 1998 Scientific American article; Ken Deffeyes, Hubbert’s Peak, 2001

©Modeling Instruction – AMTA 2013 2 U8 Energy - ws 6 v3.1 4. Biomass is both an energy source and a food source. The fundamental conflict is that as population increases, more of the earth’s surface must be directed toward food production, and to efficiently produce food, modern agriculture requires 80 gallons of gasoline or its equivalent [in the form of fertilizers and pesticides] to produce each acre of corn3. (1 gallon of gasoline = 1.3 x 108 Joules) a. Calculate the fossil fuel energy input to produce an acre of corn:

American corn production averages 130 bushels/acre. There are 25 kg of corn per bushel and 85 Calories per 77 grams of corn. (1 Calorie = 4186 J) b. Calculate the food energy output from one acre of corn.

c. Therefore, the quip is made that modern agriculture is the process of turning petroleum into food. The problem is much worse when people eat beef rather than cornbread. Each pound of beef requires sixteen pounds of corn feed. Burroughs always has a variety of meats available at lunch. How can this be reconciled with our school philosophy?

5. Windpower4 Wind energy produced 19.5 billion kWh of electricity in 2008. However, that is still less than 1% of U.S. electricity generation. By contrast, the total amount of electricity that could potentially be generated from wind in the United States has been estimated at 10,777 billion kWh annually—three times the electricity generated Installed US wind capacity as of Jan 09 (in Megawatts) in the U.S. today.

So you want to build a wind farm . . . would you make money? A 50-MegaWatt wind farm has the capability to produce 50 million Joules per second, but would average 35% of full production capacity because of wind variations. Construction would cost about $50 million ($1 million per MW). a. Running at 35% of full capacity, how much energy (in kWh) would the wind farm produce each year? 3 David Pimintel et al., “Food Production and the Energy Crisis,” Science 182, 448 (Nov. 2, 1973) 4 all data from the American Wind Energy Association, www.awea.org

©Modeling Instruction – AMTA 2013 3 U8 Energy - ws 6 v3.1 b. If you can sell the energy for 4 cents per kWh, what is your annual gross revenue?

c. Which does wind energy provide a limited supply of, energy or power? What implication does this have for our energy consumption?

6. Solar BP Solar advertises that “a 600 sq. ft. array [of photovoltaic cells] … rated about 3.3 kW . . . would generate about 3640 kWh per year. Our highest efficiency technology (also the most expensive) would generate about double this. The typical cost for a single residential system is about $10,000 per kW.”5 a. Using the solar array advertised, determine the cost per kilowatt-hour over a 30-year period of use.

b. Compare the energy output of this solar array to the energy needed by a water heater using 400 kWh per month.

It could be argued that using passive solar energy might decrease our energy needs by more than what might be gained by solar electrical generation. A water heater, for example, uses 375 – 525 kWh per

5 http://www.bpsolar.com/ContentDetails.cfm?page=135

©Modeling Instruction – AMTA 2013 4 U8 Energy - ws 6 v3.1 month.6 “Solar water heating systems typically reduce water heating costs by 50% to 80% over minimum efficiency electric resistance or gas-fired water heaters.”7 Common passive solar water heating systems cost $2000. c. Suppose your $2000 solar water preheating system saved you from buying 200 kWh per month over 30 years. What is the effective cost per kWh for the solar-heated water?

More on Solar: Despite the abundance of solar energy, solar is tricky to harness for electrical generation. Typical single crystal silicon cells usually average about 14 percent [efficiency].8 Solar cells are typically difficult, expensive, and energy intensive to make. I have found very little quantitative information about how much energy it takes to make a photovoltaic cell. (Find this information, and you will be rewarded handsomely.) It is possible that many solar cells never collect as much solar energy as the energy required to make the cell.

Non-conventional Hydropower: Tidal energy – Coastal dams could trap water at high tide and then let the water out through a generator at low tide. Undersea turbines could generate electricity as the tide goes in and out. Thermal gradient energy – Uses the temperature difference between the warm surface of the ocean and cold depths of the ocean.

Nuclear energy: Fission – energy is released as large atoms decay into smaller, more tightly bound atoms. The decay is induced by the addition of neutrons, which make the atom unstable. The decaying atom releases several neutrons, therefore creating a self-sustaining nuclear reaction. Fusion – Energy is released as small atoms fuse together to make bigger atoms. This is the Sun’s energy source. Although much research has been done with significant success, we are at least 25 to 50 years away from commercial fusion power.

Hydrogen is often cited as an energy alternative, but a source of hydrogen is needed. Currently, fossil fuel or solar energy is used to separate water into hydrogen and oxygen. There is hope that through bioengineering a microorganism could be developed to do the job.

6 Ames, Iowa city government, http://www.city.ames.ia.us/ElectricWeb/energyguy/appliances.htm 7 www.energystar.gov 8 Steve Hester, Solar Electric Power Association Technical Director www.solarelectricpower.org