A newsletter for alumni, students, and friends of the Mechanical Engineering Department at the University of Wisconsin-Madison
Following is a revised version of a letter sent to President Clinton by four professors in the ME Solar Energy Laboratory:
The recent conference in Kyoto, Japan has increased worldwide awareness and concern over the potential global warming situation caused by combustion of carbon-based fuels. The U.S. has tentatively agreed to reduce emissions of carbon dioxide, the major global warming gas, to 7 percent below 1990 levels by the year 2015. In 1995, the U.S. produced 4.8 billion tons of carbon dioxide and every year our rate of production increases about 1 percent. Consequently, we are already about 15 percent above the proposed goal. The question remains as to how this reduction can best be accomplished.
Improving the efficiency of vehicles is seen as a major remedy. An automobile that is driven 12,000 miles per year and averages 25 miles per gallon emits about 4.3 tons of CO2 per year. The U.S. has more than 100 million personal cars which produce about 430 million tons of CO2 every year.
A number of alternative technologies have been proposed to reduce carbon dioxide emissions from vehicles, but these alternatives will require additional research and development. These improvements are likely to change the nature of power, weight, driving range, and cost of the vehicles. Doubling the mileage of cars to 50 mpg will still result in more than two tons of CO2 per year for every car.
But it is surprising that more attention has not been focused on domestic water heating. A typical household with an electric water heater consumes about 6,400 Kwh per year for water heating. If we assume that the electricity is generated by a typical power plant with an efficiency of 30 percent, then the water heater generates about eight tons of CO2. The surprising result is that the CO2 produced by an average electric water heater is almost double that produced by a modern automobile.
There are more than 35 million electric water heaters in use in the U.S. There are about 58 million natural gas and oil-fired water heaters in use, each contributing about two tons of carbon dioxide per year. The annual total carbon dioxide produced by residential water heaters is approximately equal to the carbon dioxide produced by all personal cars in the U.S.
Solar domestic water heating is a well-established, cost-effective technology that will significantly reduce carbon dioxide emissions. A typical solar water heating system can supply 50 percent of the annual water-heating load, even in the Madison climate. If half of all households in the U.S. employed solar water heaters, the effect on carbon dioxide production would be about the same as doubling the average mileage of all cars. This reduction could be accomplished with no changes in lifestyle. The cost of doubling car mileage is uncertain, but estimates usually begin at $5,000 and would save about $250 per year in gasoline costs. The cost of installing a solar domestic water heater is about $2,500 and saves about the same amount of money per year as the gasoline savings.
We often see newspaper stories about automobile emissions and the potential for CO2 reduction. When and where have you seen an article on the benefit of a solar domestic hot water system? The President, in his State of the Union Address in January, proposed a $4,000 credit (in the year 2003) for any car that gets double the mileage of the average car of similar size. He also proposed a $1,000 credit for solar water heating.
Meeting the Kyoto target by 2015 will require a 270 million ton per year reduction in CO2 emissions over business as usual. This can be achieved if one-third of the water heaters are converted to solar and one-third of the automobiles achieve double the mileage by 2015. The solar option is ready today. The automobile option may be available in the near future. Let's get started in making these changes.
W. A. Beckman *
J. W. Mitchell *
S. A. Klein *
D. T. Reindl
ME Newsletter is a periodic publication of the University of
Wisconsin-Madison, Department of Mechanical Engineering. Correspondence should be sent to the address below.
Editor: Gail Gawenda
Designer: Lynda Litzkow
APRIL 1998 Contents