Renewable energy resources can be replaced quickly (within a lifetime) by natural processes but can sometimes be depleted when their rate of use exceeds their rate of replacement. By comparison, fossil fuels such as petroleum and coal, require millions of years of natural processes to be produced. Therefore, petroleum and coal are considered a nonrenewable energy resource as they deplete Earth's resources for thousands of generations to come. Five main renewable energy sources exist: solar, wind, hydropower, biomass, and geothermal. Human societies have used renewable resources to meet their energy needs throughout history. Renewable energy is a reliable energy source for many residential and commercial applications, including heat generation, electricity generation, and vehicle use. Each renewable energy resource has inherent qualities that make it more suitable for some applications than others. The efficiency of converting renewable energy sources to useable energy varies according to the source and/or technology used. The availability of renewable energy varies; some renewable resources are in constant supply, while others are intermittent, meaning they available comes and goes (i.e. wind doesn't blow all the time and the sun doesn't shine all the time). Intermittent energy can be stored for future use in batteries.
Renewable energy systems can be centralized or decentralized. A centralized energy system is one in which large amounts of an energy resource are converted from one form into another form in one location. A decentralized energy system is one in which small amounts of an energy resource are converted from one form into another form in many locations by individuals or small groups of consumers.
There is a difference between primary sources of energy and secondary sources of energy. Primary energy sources are those that are either found or stored in nature. The sun is a primary energy source and the principal source of Earth's energy. Energy from the sun is stored in other primary energy sources such as coal, oil, natural gas, and biomass, such as wood. Solar energy is also responsible for the energy in the wind and in the water cycle (the hydrologic cycle). Other primary energy sources found on Earth include nuclear energy from radioactive substances, thermal energy stored in Earth's interior, and potential energy due to Earth's gravity. Secondary energy sources are produced from primary energy sources using technology. For example, we produce electricity - a secondary source - by burning coal in a power plant or by using photovoltaic cells to harness solar energy. We can also produce alcohol fuel from crops.
One output of renewable energy systems is electricity, which you will read more about in the next few sections. Electric production is measured in watts, kilowatts (1,000 watts) or megawatts (1,000,000 watts) (units of power), including electricity produced by renewable energy systems. Energy is multiplied by time. Electricity is measured in kilowatt-hours (units of energy). One kilowatt-hour of electrical energy equals 3,413 Btu (British thermal units). One Btu equals the amount of energy it takes to raise the temperature of one pound of water one degree Fahrenheit (the approximate amount of energy stored in a match tip). A therm is also a unit of energy and may be familiar to you since this is what the energy contained in natural gas is measured by (it may be listed on your monthly energy bill). One therm equals 100,000 Btu. (To learn more about units of measurement for energy, visit the online module: Energy Education: Concepts and Practices.)
Consider taking your class on a fieldtrip to a renewable energy site or having an expert come to your classroom to discuss renewable energy. A list of ideas can be found on our Guest Speaker and Field Trip page.
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