1. Biofuels Biomass is a renewable energy source because its supplies are not limited. We can always grow trees and crops, and waste will always exist. Environmentally, biomass has some advantages over fossil fuels such as coal and petroleum. Biomass contains little sulfur and nitrogen, so it does not produce the pollutants that can cause acid rain. Growing plants for use as biomass fuels may also help keep carbon dioxide levels balanced. Plants remove carbon dioxide—one of the greenhouse gases— from the atmosphere when they grow.

2. Biofuels When biofuels are burned, they release carbon dioxide (CO 2 ), which is a major contributor to climate change. However, biofuel is made from crops that absorb carbon dioxide and give off oxygen. This cycle would maintain the balance of CO 2 in the atmosphere, but because of the CO 2 emissions from farm equipment and production of fertilizer and pesticides, biodiesel adds more CO 2 to the atmosphere than it removes.

3. Fossil Fuels Many advances have been made in protecting the environment since the passage of the Clean Air Act in 1970. Oil companies have redesigned their refineries to reduce emissions into the air and water. Gasolines have been reformulated to burn cleaner, dramatically cutting the levels of lead, nitrogen oxide, carbon monoxide, and hydrocarbons released into the air. Fossil fuels are readily available and produce large amounts of energy.

4. Fossil fuels Drilling for and transporting oil can endanger wildlife and the environment if it spills into rivers or oceans. Leaking underground storage tanks can pollute groundwater and create noxious fumes. Processing oil at the refinery can contribute to air and water pollution. Burning gasoline to fuel our cars contributes to air pollution. Even the careless disposal of waste oil drained from the family car can pollute rivers and lakes.

5. Geothermal Geothermal power plants can produce electricity as cheaply as many conventional power plants. Geothermal energy is a renewable energy source that does little damage to the environment.

6. Geothermal Geothermal steam and hot water do contain naturally occurring traces of hydrogen sulfide (a gas that smells like rotten eggs) and other gases and chemicals that can be harmful in high concentrations. Geothermal reserves are not readily available, and initial installation costs are high.

7. Hydroelectricity Hydropower is called a renewable energy source because the water on Earth is continuously replenished by precipitation. As long as the water cycle continues, we won’t run out of this energy source. Hydropower’s fuel supply (flowing water) is clean and is renewed yearly by snow and rainfall. Furthermore, hydro plants do not emit pollutants into the air because they burn no fuel.

8. Hydroelectricity Hydropower dams can cause several environmental problems, even though they burn no fuel. Damming rivers may permanently alter river systems and wildlife habitats. Hydro plant operations may also affect water quality by churning up dissolved metals that may have been deposited by industry long ago. Hydropower operations may increase silting, change water temperatures, and lower the levels of dissolved oxygen.

9. Solar Using solar energy produces no air or water pollution, and it is a free and widely available energy source.

10. Solar Manufacturing the photovoltaic cells to harness solar energy, however, consumes silicon and produces some waste products. In addition, large solar thermal farms can harm desert ecosystems if not properly managed.

11. Wind Wind energy offers a viable, economical alternative to conventional power plants in many areas of the country. Wind is a clean fuel; wind farms produce no air or water pollution because no fuel is burned.

12. Wind The most serious environmental drawbacks to wind turbines may be their negative effect on wild bird populations and the visual impact on the landscape.

13. Nuclear Nuclear power plants have very little impact on the environment. Generating electricity from nuclear power produces no air pollution because no fuel is burned. Most of the water used in the cooling processes is recycled.

14. Nuclear Like most industries, nuclear power plants produce waste. One of the main concerns about nuclear power plants is not the amount of waste created, which is quite small compared to other industries, but the radioactivity of some of that waste.

15. Hydrogen When hydrogen is burned, the only emission it makes is water vapor, so a key advantage of hydrogen is that when burned, carbon dioxide (CO2) is not produced. Clearly, hydrogen is less of a pollutant in the air because it omits little tail pipe pollution.

16. Hydrogen It still costs a considerable amount of money to run a hydrogen vehicle because it takes a large amount of energy to liquefy the fuel.