We discussed renewable energy sources when we were talking about a generation. Let's concentrate on wind and solar. What role does renewable energy play in the United States? Up until the mid 1800's, wood supplied nearly all of the nation's energy needs. As more consumers began using coal, petroleum, and natural gas, the United States relies less on wood as an energy source. Today, the use of renewable energy sources is increasing especially bio-fuels, solar, and wind. Most recently, about 10 percent of total US energy consumption was from renewable energy sources, or about 10.2 quadrillion British thermal units. One quadrillion is the number one followed by 15 zeros. Renewable energy plays an important role in reducing greenhouse gas emissions. When renewable energy sources are used, the demand for fossil fuels is reduced. Unlike fossil fuels, non-biomass renewable sources of energy such as hydro-power, geothermal, wind, and solar, do not directly emit greenhouse gases. The consumption of bio-fuels and other non-hydroelectric renewable energy sources, more than doubled during the last decade mainly because of state and federal government mandates and incentives for renewable energy. Why don't we use more renewable energy? In general renewable energy is more expensive to produce and to use than fossil fuel energy. Favorable renewable resources are often located in remote areas, and it can be expensive to build power lines from the renewable energy sources to the cities that need the electricity. In addition, renewable sources are not always available. Clouds reduce electricity from solar power plants. Days with low wind reduce electricity from wind farms. Droughts reduce the water available for hydro-power. Let's talk about wind energy a little bit. Wind is caused by uneven heating of the Earth's surface by the sun. Because the Earth's surface is made up of different types of land and water, it absorbs the sun's heat at different rates. One example of this uneven heating is the daily wind cycle. During the day, air above the land heats up faster than air over water, warm air over land expands and rises, and heavier cooler air rushes in to take its place creating wind. At night, the winds are reversed because air cools more rapidly over land than it does over water. Let's briefly talk about energy from the sun. The sun has produced energy for billions of years, and is the ultimate source for all of the energy sources and fuels that we use today. People have used the suns rays, solar radiation for thousands of years for warmth and to dry meat, fruit, and grains. Over time, people develop devices to collect sun energy as thermal or heat energy. An example of an early solar energy collection is the solar oven, a box for collecting and absorbing sunlight. In the 1830's, British astronomer John Herschel, used a solar oven to cook food during an expedition to Africa. People now use many different technologies for collecting and converting solar radiation into useful heat energy for a variety of purposes. We use solar thermal energy systems to heat water for use in homes, buildings or swimming pools, heat the inside of homes, greenhouses and other buildings, heat fluids to high temperatures in solar thermal power plants. Solar photo-voltaic systems convert sunlight into electricity. Solar photo-voltaic or PV devices or solar cells, change sunlight directly into electricity. Small PV cells can power calculators, watches, and other small electronic devices. Arrangements of many solar cells in PV panels and arrangements of multiple PV panels in PV arrays, can produce electricity for an entire house. Some PV power plants have large arrays that cover many acres to produce electricity for thousands of homes. The two main benefits of using solar energy are; solar energy systems do not produce air pollutants or carbon dioxide. Solar energy systems on buildings have minimum impact on the environment. The main limitations of solar energy are; the amount of sunlight that arrives at the earth surface is not constant. The amount of sunlight varies depending on location, time of day, season of the year, and weather conditions. The amount of sunlight reaching the square foot of the earth's surface, is relatively small so a large surface area is necessary to absorb or collect useful amount of energy.
