Given the steep decline of American manufacturing, many policy-makers and advocates have touted the promise of green energy to infuse new life into the nation's manufacturing base. The industries profiled below have a long way to go before they achieve the might of manufacturers past, but for some they represent a bright future in terms of innovation and jobs.
1. Solar panels
Solar power comes in two main varieties, photovoltaic solar panels, which convert sunlight into electricity, and concentrating solar power, which uses mirrors to generate heat. Like many green technologies, solar power has relied heavily on government subsidies. In the case of Solyndra and a few other big solar energy firms, those public funds didn't prevent bankruptcy. Questions have arisen about how well the companies were managed, and many experts have also pointed out how stiff foreign competition is when it comes to solar power.
2. Wind turbines
Building wind turbines -- huge steel structures with numerous components -- is a lot like building cars, which makes the industry appealing to former auto workers. The technology still poses some challenges: wind doesn't blow steadily, for example, and sometimes the windiest areas are the farthest from the big cities that need energy. Today nearly 2 percent of the nation's electricity comes from wind turbines, a figure that could jump to 20 percent by 2030, according to the U.S. Department of Energy.
3. Lithium-ion batteries
These energy-saving batteries are often used to power electric cars and plug-in hybrid vehicles, which has prompted big government subsidies and much hope for the growth of the industry. That's especially true in Michigan, a state that has been ravaged by the decline of manufacturing and the downsizing of the auto industry. Michigan's governor, Jennifer Granholm, predicted last year that electric cars and their batteries could create 62,000 jobs over the next decade.
4. Geothermal heat pumps
The mechanics involved in geothermal power are actually pretty simple: a series of pipes called a loop, installed underground or underwater, captures the Earth's heat. Fluid in the pipes carries the heat into the house, where its temperature rises even higher thanks to an electrically driven compressor and a heat exchanger. Ductwork, the same kind used to distribute forced-air systems, blows the heat into different rooms.
5. Hydrogen fuel cells
Fuel cells, which combine hydrogen and oxygen to produce electric current, are an emerging area of green technology. According to the National Renewable Energy Laboratory, run by the U.S. Department of Energy, most of the hydrogen power in the United States relies on the steam reforming of natural gas -- a type of fossil fuel. But the Laboratory reports that it is continuing its research into greener solutions.
Ethanol is an alcohol made from sugars and starches that can be used as an alternative to fossil fuels. Ethanol isn't the environmental golden child it was even a few years ago. At the end of 2011, a broad coalition of groups and politicians backed a measure to let a longstanding ethanol tax break expire. Still, the industry estimates that it can currently produce six billion gallons of corn ethanol, and may be able to produce 15 billion gallons by 2015.
7. Smart grid components
Smart grid technology basically computerizes the electrical grids we rely on to bring electricity into homes and businesses, making it easier to integrate alternative energy sources like wind and solar. It also relies on technology that uses no fossil fuels, for example the so-called "flywheel," a spinning rotor that captures rotational energy.
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