INTRODUCTION TO ENVIRONMENTAL SCIENCES

LECTURE NOTES

Alternative Energy Resources

Given all the problems and uncertainties with fossil fuels and nuclear fission, what are our other choices for producing energy in the future? No one single answer, but many possibilities. What's best depends on application and the development of new technologies. The future will be different, exactly how is unclear.

I. Solar Energy In All Its Forms

Can be used to produce low quality (space heating) heat, high-quality heat, and electricity. Could meet 50-80+% of U.S. energy needs by end of this century or sooner. Fully renewable, much lower level of pollution and environmental impacts, creates jobs, and improves national security.

Major drawback of solar energy at present time is its inequitable distribution, less effective on cloudy days, not available directly at night, and relatively high cost. However, as technology improves, costs will come down and problems can be solved.

A) Using solar energy to heat homes and water

1) Passive solar heating systems capture sunlight directly within the structure. No mechanical systems are used to convert to heat energy. Use of solar energy designed into the structure. Can use thermal mass to retain heat. Can provide up to 70% of a typical building's heating needs. Extra cost of solar designs usually recovered in 3-7 years. May need a backup energy system for unusually cold or cloudy climates or areas where sunlight is not available more than 60% of the time.

2) Active solar heating systems use a mechanical system (usually a pump or fan) to distribute heat. Often have solar collectors on roof, which are very effective for heating water in sunny areas. Usually combined with passive systems.

Regardless of whether a passive or active system is used, energy source (sun) is free, net energy yields are moderate (active) to high (passive), pollution is low (no CO2), and the technologies are available and easy to install. However, we need to insure the solar legal rights of the user. Access to sunlight must be guaranteed.

B) Using solar energy to cool houses

Passive solar designs can be used to reduce heat input in summer. Overhangs, awnings, and trees can be used to block the sun. Earth tubes circulate air cooled by the ground. Could provide a typical house with 60% of its cooling needs.

C) Using solar energy to generate high temperature heat and electricity

Solar thermal systems concentrate sunlight and raise water or oil to a very high temperature. Heat can be used directly or to make steam for electricity. Systems use heliostats or other types of reflectors (parabolic dishes) to concentrate sunlight. Solar cookers used in developing countries to replace fuel wood for cooking.

In order to make large amounts of electricity, large areas have to be set aside. However, pollution is minimal. Generating costs are still high relative to fossil fuels.

D) Using solar to make electricity directly

Can be done using photovoltaic cells. Made of thin silicon wafers usually enriched with boron or lithium. A DC current is produced when sunlight strikes them. They are strung together in a panel and a number of panels are placed on the roof.

Typical conversion rates are 15-20%. One square meter of solar panels produces about 50 watts of electricity. A typical American home uses 1500 watts of electricity. Need 30 square meters (about 300 square feet) or about the size of a typical roof. Can make them to look like roof shingles.

Electricity produced can be stored in batteries (inefficient) or used immediately. Costs are presently quite high. Will come down as technology and efficiencies improve. If panels make more electricity than needed, can sell it to utility. They are excellent for remote locations where no other electricity is available. May provide 25% of world's electricity by 2050.

II. Electricity From Water

Can be produced by converting the kinetic energy of moving water or by utilizing the heat stored in water.

A) Hydropower

Large-scale hydropower utilizes a dam to store water in a large reservoir. Water falls through dam and turns a turbine attached to a generator. There are many advantages and disadvantages to producing energy in this way.

Small-scale hydropower utilizes a small dam with no reservoir. Natural river current turns generator. Good for on-site uses and fewer environmental issues.

In pump-storage generation, water is pumped up to a reservoir during times of low electricity demand. Water flows back down during times of high demand to generate electricity. In reality, this is a sort of mechanical battery.

Hydropower generally has moderate to net energy yields and low operating and maintenance costs. No air or water pollution. Dams also help in flood control and providing water for irrigation. However, large reservoirs destroy habitats, block fish migration, trap sediment, can cause earthquakes and landslides, and, if breached, produce devastating floods. Often now seen as more of a negative than positive.

B) Tidal and wave energy

Tidal currents passing through inlets, bays, and estuaries spin turbines. Need to be reversible so electricity can be generated on both the incoming and outgoing tides.

Large waves also can be used to move vanes or baffles that turn a turbine.

Both methods are restricted to relatively few sites worldwide. Not a generally applicable technique for generating electricity.

C) Using the heat in water to generate electricity

The large temperature difference between deep, cold bottom waters and warm, surface tropical waters can be used to drive a turbine. Ocean Thermal Energy Conversion (OTEC) plants use cold water to condense ammonia to a liquid and warm surface water to vaporize it, turning a turbine. Not presently economically viable and only a few pilot plants worldwide (one in Hawaii).

Similar systems using saline or freshwater ponds can be used for space heating or generating electricity. May supply local sites with, heat, hot water, and some electricity. Probably not going to become a major supplier of energy.

III. Electricity From the Wind

Now the world's fastest growing source of energy. Produces worldwide over 10,000 megawatts from over 25,000 individual wind turbines.

Typical installations have large number of turbines in a windy area (wind farms). Potentially could provide all the electrical power needs of the U.S.

No pollution, moderate to high net energy yield, and surrounding land can be used for other purposes. Can be unsightly and produce noise and may kill migratory birds. Need a backup system when winds are not blowing. It is not equitably distributed.

Costs are much less than nuclear and almost competitive with fossil fuel plants. Prices should come down as technology improves.

IV. Energy From Biomass

This is the original energy source for humans (burning wood). Use organic materials, such as crops, trees, waste, garbage, and liquid and gaseous biofuels (cooking oil) derived from them to produce energy. Also known as unfossilized fuels. Used widely in developing countries (wood and animal waste for heating and cooking).

Much of the raw materials are renewable in that they can be grown (trees, crops, dung, etc) or are produced as the result of other human activities (garbage). Still produce CO2 as a pollutant. Also, the indiscriminant cutting of trees leads to soil loss, flooding, desertification, and habitat destruction. Using crops as fuel also diverts resources (soil, fertilizers, water) from the production of food.

Can convert biomass to biogas, ethanol, and methanol. Can be burned directly or combined with other fuels (gasohol; gasoline with 10-23% ethanol). Can also burn our organic garbage to make electricity. However, produces large amounts of toxic ash and gases. All biomass products produce CO2 when burned.

V. Geothermal Energy

Uses heat stored in rocks and underground fluids to heat building, water, and produce electricity. Found in dry steam (steam only), wet steam (steam and water), and hot water (water only) systems. Steam and water can be extracted directly or water can be injected, heated, and then returned to the surface.

Systems eventually depleted of heat. Typically last 50-75 years, then need to drill in new areas. Usually found in geologically active areas near plate boundaries with many associated geologic hazards (earthquakes and volcanoes). Will probably have restricted applicability, but where available it may be very important (Iceland).

Often a problem with disposal of toxic brines brought to the surface. Can't be disposed of easily. Needs to be re-injected. However, very little CO2 emitted.

VI. Fusion Power

Fuse isotopes of hydrogen (deuterium and tritium) to make helium. This is the same process that powers the sun. Need very high pressure and temperatures (millions of degrees). Technologically very hard to do and maintain. Can be done with very high-strength magnetic fields or lasers.

At the present time, we can only maintain a fusion reaction for a few second and just about break even in terms of the amount of energy used to cause fusion and the amount of energy produced by the fusion reaction. However, if the process can be made to work, it will provide a nearly inexhaustible supply of energy (hydrogen can come from water) with little radioactive pollution. Fusion will probably not be technologically feasible until the second half of this century, if at all.

VII. Hydrogen as a Fuel

Derived by splitting water (essentially an inexhaustible resource) molecules into hydrogen and oxygen or from hydrogen rich fuels, such as alcohol or methane. However, energy is required to do this. Can be supplied by many sources, including solar or wind. Some algae in sulfur- and oxygen-free environments convert water and sunlight to hydrogen and oxygen; may be able to "grow" hydrogen gas.

When hydrogen is burned about the only pollutants produced are water vapor and some nitrogen oxides. Can use hydrogen in place of gasoline, natural gas, and other liquid and gaseous fuels. Cars can run on it very easily and at little additional cost.

Can also use hydrogen in fuel cells, which take hydrogen and oxygen (from air) and combine them to form water and electricity and heat. Used in the space shuttle for years. Can be used as batteries in cell phones, computers, etc. Larger fuel cell units could power electric/hybrid cars or provide all the energy for a home.

VIII. A Sustainable Energy Future?

Have many alternatives to the present way we make and use energy, each with their own advantages and disadvantages. Which ones will be used in the future is still uncertain. Some may never be technologically feasible or economically viable.

Most likely scenario is a mix of renewable energy sources within a decentralized distribution system combined with improved energy efficiencies. Large centralized utilities may be replaced with local energy producers utilizing whatever energy source make the most sense (environmental, abundance, economic) for that location. Individual homeowners or neighborhoods may generate their own energy.

Will probably require government intervention. Can do this in many ways, including legislating more stringent efficiency standards, purchasing efficient vehicles and devices, subsidizing renewable fuel and energy efficiency research and development, stop subsidizing nonrenewable fuels, provide tax credits for fuel efficient cars, houses, and appliances and tax surcharges for inefficient ones, taxing coal and oil use, and requiring full-cost pricing. Some of these ideas are not politically possible at the present time in the United States.