The main renewable energy source
Hydropower Generates Electricity Hydropower is the renewable energy source that produces the most electricity in the United States. It accounted for 6% of total U.S. electricity generation and 60% of generation from renewables in 2010.
Hydropower Relies on the Water Cycle Understanding the water cycle is important to understanding hydropower. In the water cycle:
Solar energy heats water on the surface, causing it to evaporate.
This water vapor condenses into clouds and falls back onto the surface as precipitation(rain,snow, etc.).
The water flows through rivers back into the oceans, where it can evaporate and begin the cycle over again.
Wind is simply air in motion. It is caused by the uneven heating of the Earth's surface by the sun. Because the Earth's surface is made of very different types of land and water, it absorbs the sun's heat at different rates. One example of this uneven heating can be found in the daily wind cycle.
The Daily Wind Cycle During the day, the air above the land heats up more quickly than the air over water. The warm air over the land expands and rises, and the heavier, cooler air rushes in to take its place, creating wind. At night, the winds are reversed because the air cools more rapidly over land than over water.
In the same way, the atmospheric winds that circle the earth are created because the land near the Earth's equator is heated more by the sun than the land near the North and South Poles.
Wind Energy for Electricity Generation Today, wind energy is mainly used to generate electricity. Wind is a renewable energy source because the wind will blow as long as the sun shines.
The sun has produced energy for billions of years. Solar energy is the sun’s rays (solar radiation) that reach the Earth. This energy can be converted into other forms of energy, such as heat and electricity.
In the 1830s, the British astronomer John Herschel famously used a solar thermal collector box (a device that absorbs sunlight to collect heat) to cook food during an expedition to Africa. Today, people use the sun's energy for lots of things.
When converted to thermal (or heat) energy, solar energy can be used to:
Heat water — for use in homes, buildings, or swimming pools
Heat spaces — inside homes, greenhouses, and other buildings
Heat fluids — to high temperatures to operate a turbine to generate electricity
Solar energy can be converted to electricity in two ways:
Photovoltaic (PV devices) or “solar cells” change sunlight directly into electricity. Individual PV cells are grouped into panels and arrays of panels that can be used in a wide range of applications ranging from single small cells that charge calculator and watch batteries, to systems that power single homes, to large power plants covering many acres.
Solar Thermal/Electric Power Plants generate electricity by concentrating solar energy to heat a fluid and produce steam that is used to power a generator. In 2010, solar thermal-power generating units were the main source of electricity at 13 power plants in the United States:
Biomass is organic material made from plants and animals (microorganisms). Biomass contains stored energy from the sun. Plants absorb the sun's energy in a process called photosynthesis. The chemical energy in plants gets passed on to animals and people that eat them.
Biomass is a renewable energy source because we can always grow more trees and crops, and waste will always exist. Some examples of biomass fuels are wood, crops, manure, and some garbage.
When burned, the chemical energy in biomass is released as heat. If you have a fireplace, the wood you burn in it is a biomass fuel. Wood waste or garbage can be burned to produce steam for making electricity, or to provide heat to industries and homes.
Converting Biomass to Other Forms of Energy Burning biomass is not the only way to release its energy. Biomass can be converted to other useable forms of energy, such as methane gas or transportation fuels, such as ethanol and biodiesel.
Methane gas is the main ingredient of natural gas. Smelly stuff, like rotting garbage, and agricultural and human waste, release methane gas — also called "landfill gas" or "biogas."
Crops like corn and sugar cane can be fermented to produce ethanol. Biodiesel, another transportation fuel, can be produced from left-over food products like vegetable oils and animal fats.
How Much Biomass Is Used for Fuel?
Biomass fuels provided about 4% of the energy used in the United States in 2010. Of this, about 46% was from wood and wood-derived biomass, 43% from biofuels (mainly ethanol), and about 11% from municipal waste. Researchers are trying to develop ways to burn more biomass and less fossil fuels. Using biomass for energy may cut back on waste and greenhouse gas emissions.
The word geothermal comes from the Greek words geo (earth) and thermo (heat). So, geothermal energy is heat from within the Earth. We can recover this heat as steam or hot water and use it to heat buildings or generate electricity. Geothermal energy is a renewable energy source because the heat is continuously produced inside the Earth. Geothermal Energy Is Generated Deep Inside the Earth
Geothermal energy is generated in the Earth's core. Temperatures hotter than the sun's surface are continuously produced inside the Earth by the slow decay of radioactive particles, a process that happens in all rocks. The Earth has a number of different layers: