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: In 2002, Minnesota consumed 62,364 gigawatt hours of electricity (from the 2004 Quadrennial Report at the Mn. Dept. of Commerce website). Minnesota used 2,560 million gallons of gasoline for transportation in 2000 (from the U.S. Energy Information Administration). 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). What do these data mean? Nothing by themselves unless we compare them with other meaningful figures. Express these energy data in natural units of renewable energy: Wind turbines for electricity Ethanol for gasoline Biodigesters for natural gas Case study: Renewable energy targets for Blue Earth County, Minnesota Blue Earth County 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%) Wind for electricity Blue Earth County electricity consumption in 2001 was 739,000 MWh (from the Minnesota Utility Data Book) One 1.5 MW wind turbine operating at 33% capacity produces GWh of electricity in a year. We’d need 170 wind turbines to supply all the electricity for Blue Earth County. Ethanol for gasoline Minnesota used 2,560 million gallons of gasoline for transportation in 2000 (data). This is energetically equivalent to 4,500 million gallons of ethanol (ethanol: 74,000 Btu/gallon; gasoline: 130,000 Btu/gallon). Assume 1.14% of this for Blue Earth County, so we’d need 51.3 million gallons of ethanol on an energy equivalent basis. This is the amount of ethanol produced by one medium-sized ethanol plant. Biodigesters for natural gas The residential demand for natural gas in Minnesota in 2001 was 126,000 million cubic feet (data). 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. Assume 1.14% of this demand for Blue Earth County, so 1,480 billion Btu’s. 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 23 biodigesters to supply the residential natural gas for Blue Earth County. Comments: 1.Energy targets in renewable energy units indicate the magnitudes of the energy transformations toward a sustainable economy. 2.Energy targets are not by themselves plans, but can indicate which plans are feasible. 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. Conclusion: 1.Wind turbines could supply the electricity for Blue Earth County (setting aside questions such as intermittency). 2.Ethanol could replace the gasoline consumed in Blue Earth County (setting aside questions such as net energy gain and other uses of corn). 3.Blue Earth County does not have the dairy herds to support biodigesters for natural gas. Presented at the AAPT Summer Meeting Syracuse, NY, July 2006 Support from Minnesota State University, Mankato, while on sabbatical leave is gratefully acknowledged.