Minnesota’s Energy Needs, in Natural Units of Renewable Energy
Louis Schwartzkopf, Minnesota State University, Mankato
The question:
How can we make the large quantities of energy we consume in the
U.S. intelligible to the average student and citizen?
Data:
What do these data mean?
•In 2002, Minnesota consumed 62,364 gigawatt hours of electricity
(from the 2004 Quadrennial Report at the Mn. Dept. of Commerce
website).
Nothing by themselves unless we compare them with other
meaningful figures.
•Minnesota used 2,560 million gallons of gasoline for transportation
in 2000 (from the U.S. Energy Information Administration).
•Wind turbines for electricity
•The residential demand for natural gas in Minnesota in 2001 was
125,984 million cubic feet (from the 2004 Quadrennial Report at the
Mn. Dept. of Commerce website).
Wind for electricity
Case study:
Express these energy data in natural units of renewable energy:
•Ethanol for gasoline
•Biodigesters for natural gas
Ethanol for gasoline
Biodigesters for natural gas
Renewable energy targets for
Blue Earth County, Minnesota
Blue Earth County
•Minnesota used 2,560 million gallons of gasoline for
transportation in 2000 (data).
Data
Minnesota: Pop. 4,919,479 (2000 Census)
Blue Earth County
Population: 55,941 (1.14% of Minnesota’s population)
Area: 765 sq. mi.
Major city: Mankato, pop. 32,507
Land use: agriculture 83%; mostly corn (36%) and
soybeans (34%)
•Blue Earth County electricity consumption in 2001 was
739,000 MWh (from the Minnesota Utility Data Book)
•This is energetically equivalent to 4,500 million gallons
of ethanol (ethanol: 74,000 Btu/gallon; gasoline:
130,000 Btu/gallon).
•The residential demand for natural gas in Minnesota in
2001 was 126,000 million cubic feet (data).
•Assume 1.14% of this for Blue Earth County, so we’d
need 51.3 million gallons of ethanol on an energy
equivalent basis.
•1 million cubic feet of natural gas contains 1.03 billion
Btu’s of heat energy, so the demand was for 130,000
billion Btu’s of heat energy.
•This is the amount of ethanol produced by one
medium-sized ethanol plant.
•Assume 1.14% of this demand for Blue Earth County,
so 1,480 billion Btu’s.
•One 1.5 MW wind turbine operating at 33% capacity
produces 4.336 GWh of electricity in a year.
•One Microgy biodigester can produce electricity at a
rate of 775 kW, or 64 billion Btu’s of natural gas in a
year.
•We’d need 170 wind turbines to supply all the
electricity for Blue Earth County.
Presented at the AAPT Summer Meeting
Syracuse, NY, July 2006
Support from Minnesota State University, Mankato, while on
sabbatical leave is gratefully acknowledged.
•We’d need 23 biodigesters to supply the residential
natural gas for Blue Earth County.
Conclusion:
Comments:
1. Wind turbines could supply the electricity for Blue
Earth County (setting aside questions such as
intermittency).
1. Energy targets in renewable energy units indicate the
magnitudes of the energy transformations toward a
sustainable economy.
2. Ethanol could replace the gasoline consumed in Blue
Earth County (setting aside questions such as net
energy gain and other uses of corn).
2. Energy targets are not by themselves plans, but can
indicate which plans are feasible.
3. Blue Earth County does not have the dairy herds to
support biodigesters for natural gas.
3. This method is easily generalizable to other states or
regions.
4. Physicists make comparisons like this naturally, as a
result of our training; we should help our students
and our fellow citizens to make such comparisons.