Solar power is generated by converting sunlight to either thermal or electric energy. Thermal energy (heat) from the sun has been used for thousands of years to heat water and spaces (homes, greenhouses) and to cook. To convert the sun's energy to electricity, photovoltaic cells (PV cells for short) are most often used, although solar power plants do exist. When light hits a PV cell, some reflects back, some passes right through and some is absorbed. The absorbed light causes electrons to flow in the PV cell, creating a current and a voltage (electricity), which can power everything from watches to calculators to cars and homes.

Using PV cells has several advantages over fossil fuels, primarily its minimal environmental impact because no water is required for cooling and no by-products result from the process. Since PV cells convert the sun's energy directly into electricity, making mechanical generator systems are unnecessary. PV cells can be installed in whatever size is needed. Solar power plants work almost the same as conventional power plants by using steam to turn a turbine and converting that mechanical energy into electricity with a generator. The only difference is that instead of burning a fossil fuel to produce the steam, sunlight heats a fluid that produces steam that turns turbines. By not burning fossil fuels, solar power plants have less environmental impact. The only disadvantages to using solar power are that the amount of sunlight hitting the Earth's surface is dependent on time of day, time of year, location, and the weather conditions, and that the energy provided to any one place at any one time by the sun is so small that large areas are needed for any practical use of the sun's energy.

A wind farm. click for copyright

Wind is a natural phenomenon caused by the sun's uneven heating of the Earth. We can take advantage of the kinetic energy associated with air molecules in the wind to generate electrical energy. The process to generate energy from the wind is similar to the process used in hydropower generation. The kinetic energy of the air is transferred to turbines to make them spin, which turns generators that produce electricity. Wind energy generation is an ideal source of energy in places with open space and frequently high winds. By combining many wind turbines in one location, engineers create wind farms.

Wind energy is preferred over fossil fuels because it produces no pollutants. The only complaints with wind power are its negative effect on bird populations and the sight of 20-story tall, 200 foot wide wind turbines in the middle of open space. Then again, some prefer the sight of wind farm over the alternative of fossil fuel-burning power plant smokestacks and pollution. And, newer wind turbines are less likely to kill birds because of their wider blades and slower spin speed. One other disadvantage is that the wind does not always blow, thus energy generation is not always possible, requiring supplementation with other energy sources until the wind picks up again.

The Hoover Dam on the Colorado River in Nevada produces great amounts of electricity for people in nearby states. click for copyright

People have been using energy generated from water (hydro power) for thousands of years for applications such as grinding grain and running sawmills. To do this, engineers design turbines that are spun by the flow of the water, which then creates electricity using a generator. One method is to place the turbines in a swiftly-flowing river where the fast moving water has enough kinetic energy to transfer it to the turbines and spin them. Another approach is to place the turbines under a flow of falling water, taking advantage of the water's transformation of potential energy into kinetic energy and transferring the water's kinetic energy to the turbines.

To accomplish both of these methods, dams are often built. When energy demand increases, the dam releases more water to spin the turbines and thus generate more energy. For example, the hydroelectric Hoover Dam in Nevada blocks the Colorado River in southern Nevada. When energy demand is high in places like Las Vegas and California, operators increase the flow of water through the dam so more energy is generated.

Hydropower is an efficient way to generate energy. It is a renewable source of energy because the Earth's water cycle continuously replenishes the water in the river the dam is blocking. Hydropower also does not pollute the water or the air. One criticism of the use of hydropower is that you must build a dam to generate the energy. By damming a river, we disturb the surrounding environment and animals' natural habitats. Engineers continually work to find solutions that minimize environmental problems caused by dams.

The largest renewable source of energy found in the world is biomass or biofuels. Biomass is simply organic material made from plants and animals. It can be as simple as burning wood or dung for heat. Other forms of biomass include crops such as corn, waste and alcohol fuels. Biomass contains stored energy from the sun, originally generated by plants through photosynthesis. When biomass is burned, this energy is released as heat (thermal energy). Wood and garbage can also be burned to produce the steam needed to drive steam-powered turbines. Burning biomass is not our only option for releasing the stored energy in biomass. Often crops such as corn and sugar cane are fermented to produce ethanol and other alcohol fuels. Biodiesel is fuel that can be created using left-over food products such as vegetable oils and animal fats.

Wood is an example of a biomass energy source. click for copyright

Biomass is considered a renewable energy source because more trees and crops can be planted. For thousands of years, wood has been the most commonly used type of biomass, most often for heating and cooking. Unfortunately, when burned, all biomass releases carbon dioxide, which is a greenhouse gas. However, the growing of plants for use as an energy source takes in about the same amount of CO2 during photosynthesis so the net production of the gas is small. Biofuels (ethanol and biodiesel) have advantages over gasoline because they are renewable, safe, biodegradable, and reduce the amount of air pollution. However, currently not all cars can use these fuels.

Just like the sun, the Earth generates energy at its core; we call this geothermal energy and use it as an energy resource. A geothermal power plant is like a regular power plant except that no fuel is burned to heat water into steam, because the steam comes directly from the Earth, tapped by deep wells. This is what makes geothermal energy a naturally occurring process, or a renewable resource. Volcanoes, hot springs and geysers are all examples of geothermal energy at the surface of the Earth. Engineers take advantage of geothermal energy through underground geothermal reservoirs that are made from ground water heated by magma as it creeps closer to the surface of the Earth. When this water is really close to the surface, engineers design systems that pump the water directly into homes and businesses for heating purposes. For deeper underground reservoirs, we drill very deep wells into the Earth and pump the hot water or steam to the surface to run turbines that produce electricity.

Geothermal power plant at The Geysers in northern California. click for copyright

Geothermal energy is a good alternative to fossil fuels because the direct use of geothermal energy has almost no negative impact on the environment and is continually available. A geothermal power plant releases almost no carbon dioxide pollution since fossil fuels are not being burned to produce the steam to turn the turbines.

Offshore drilling to find fossil fuel oil reserves. click for copyright

The oil we use for energy generation begins as crude oil found deep underground in reservoirs accessed through drilling. The crude oil is pumped out, transported and refined into petroleum. Almost 50% of harvested petroleum is used for gasoline and another 15% is used for diesel fuel. Other uses for petroleum include heating oil and jet fuel. Petroleum is also used in the production of many everyday items such as ink, crayons, bubble gum, dishwashing soap, deodorant, eyeglasses, records, tires, plastics and heart valves.

Despite the myriad uses of petroleum, many disadvantages exist in the production and use of this limited fossil fuel resource. The production and burning of petroleum release pollutants and carbon dioxide into the atmosphere, harming the environment, disrupting the Earth's' natural carbon cycle (and thus the planet's climate), and impacting the health of people worldwide. Drilling also affects marine and land habitats though the technology has improved towards reducing the negative effects of drilling. The world's reliance on oil is of concern because when we reach the point at which our use surpasses the production of crude oil, we will be forced to limit our consumption of oil and find alternative energy sources.

Natural gas is a fossil fuel, having formed from the remains of plants and animals that died millions of years ago. Like oil, natural gas is found deep underground in reservoirs that must be accessed by drilling. The biggest US uses for natural gas are for industry and electricity production. Natural gas is also an essential raw material for products such as paint, fertilizer, plastics, dyes, photographic film, medicines and explosives. Since natural gas is an odorless, tasteless, invisible gas, a chemical is added to it during production to give it a sulfur-like (rotten egg-like) smell so its presence can be detected. The burning of natural gas releases pollutants and CO₂ into the atmosphere (although less than other fossil fuels) and affects the surrounding environment. And, the risk always exists that leaking natural gas can cause explosions. The drilling process is disruptive to marine and land habitats although technology is improving to reduce the negative side effects of drilling.

Burning fossil fuels to generate electricity releases pollutants and CO2 into the Earth's atmosphere. click for copyright

Coal is another fossil fuel that consists mostly of carbon. Through surface or underground mining, coal is extracted from the Earth and most often burned in power plants to generate electricity. Coal is also an important industry resource in the production of steel, cement and paper. Surface mining consists of removing the topsoil, rocks and dirt above coal deposits. Once the coal is completely extracted, the rock, dirt and topsoil are all returned and the land is used for other purposes. For deposits that are hundreds of feet below the Earth's surface, the coal is extracted through underground mining methods and shipped to plants to clean it of unwanted materials (rock, dirt and ash) in order to increase its heating efficiency.

The burning of coal has many adverse affects on the surrounding environment. Like the other fossil fuels, when burned, it releases pollutants and CO₂ into our atmosphere, including sulfur, which when mixed with oxygen gas forms sulfur dioxide, which, when combined with rain, creates destructive 'acid rain."

In a nuclear power plant, atoms are split apart to generate electricity. click for copyright

Nuclear power uses uranium to produce electricity by taking advantage of the energy found in the nucleus of atoms. The bonds that hold atoms apart are extremely strong and can provide a lot of energy. Two different ways exist to extract energy from the nucleus of atoms. The first is nuclear fusion, which is the combining of atoms to form a larger atom; this is how the sun generates its energy. Nuclear fission is the splitting apart of an atom into smaller atoms; this is how nuclear weapons work and also how energy is created in nuclear power plants. The most common element used in nuclear energy generation is U-235, an isotope of uranium; this element undergoes fission relatively easily and is the only fissile element that naturally exists in large quantities. Nuclear power plants produce electricity by using the heat released from a chain reaction to heat water to produce steam to turn turbines and generate electricity.

Compared to fossil fuels, nuclear power plants are very clean. They produce no pollutants or CO₂, though processing the uranium releases a small amount of pollutants. Disposing of nuclear waste is the biggest environmental concern because the highly-radioactive parts of nuclear power plants pose dangers to humans, wildlife and plants. Currently, unwanted highly-radioactive waste is buried deep underground.

Accessing Prior Knowledge: Ask students the following questions and discuss as a class:

• What are some non-renewable energy sources? Make a list.
• What are some renewable energy sources and what makes them renewable? Make a list.

Compare/Contrast: Have students compare and contrast non-renewable vs. renewable energy sources. What are some characteristics both sources share? (Possible answer: They are used to generate electricity.) In what ways do they differ? (Possible answer: Most non-renewable energy sources emit pollutants and toxins into the air as a side effect of their production and use.)

Class Discussion: Ask students the following questions and discuss as a class:

• In what ways are one or more of the non-renewable energy sources similar to renewable energy sources? (Possible answers: Fossil fuels are replenished naturally via the carbon cycle, but it is not in a short period of time [millions of years]. Nuclear power plants emit little to no pollutants and toxins into the air.)
• Do you think it is best to spend money developing renewable energy sources and technologies, developing ways to burn fossil fuels more cleanly, developing ways to remove the pollutants from the air that are released by the burning of fossil fuels, or developing other ways to make non-renewable energy cleaner? Use what you have learned as support to argue for different answers.