There are two main types of solar energy technologies—photovoltaic (PV) and concentrating solar power (CSP). You're likely most familiar with PV, which is utilized in panels. When the sun shines onto a solar panel, photons from the sunlight are absorbed by the cells in the panel, which creates an electric field across the layers and causes electricity to flow. The second technology is concentrating solar power, or CSP. It is used primarily in very large power plants and is not appropriate for residential use. This technology uses mirrors to reflect and concentrate sunlight onto receivers that collect solar energy and convert it to heat, which can then be used to produce electricity.

Solar photovoltaic (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.

Interested in a solar installation for your property or business? Learn more about Solar installation here.

Wind turbines work on a simple principle: instead of using electricity to make wind—like a fan—wind turbines use wind to make electricity. Wind turns the propeller-like blades of a turbine around a rotor, which spins a generator, which creates electricity. The amount of wind electricity generation has grown significantly in the past 30 years. Advances in wind energy technology have decreased the cost of producing electricity from wind, and government and other incentives in the United States and in other countries have encouraged growth in wind power.

• Horizontal Axis
  Horizontal-axis turbines have blades like airplane propellers, and they commonly have three blades. The largest horizontal-axis turbines are as tall as 20-story buildings and have blades more than 100 feet long. Taller turbines with longer blades generate more electricity. Nearly all of the wind turbines currently in use are horizontal-axis turbines.
  
  
• Vertical Axis
  Vertical-axis turbines have blades that are attached to the top and the bottom of a vertical rotor. The most common type of vertical-axis turbine—the Darrieus wind turbine, named after the French engineer Georges Darrieus, who patented the design in 1931—looks like a giant, two-bladed eggbeater. Some versions of the vertical-axis turbine are 100 feet tall and 50 feet wide. Very few vertical-axis wind turbines are in use today because they do not perform as well as horizontal-axis turbines.

• Wind Power Plants (Wind farms)
  Wind power plants, or wind farms, are clusters of wind turbines that produce large amounts of electricity. A wind farm usually has many turbines scattered over a large area. One of the world's largest wind farms, the Horse Hollow Wind Energy Center in Texas, has about 430 wind turbines spread over about 47,000 acres. The project has a combined electricity generating capacity of about 735 megawatts (or 735,000 kW).

Hydroelectric power is generated by moving water. Water flows through a pipe, sometimes called a penstock, that then turns turbines spinning a generator. Because the source of hydroelectric power is water, hydroelectric power plants are usually located on or near a water source. The volume of the water flow and the change in elevation—or fall, and often referred to as head—from one point to another determine the amount of available energy in moving water. In general, the greater the water flow and the higher the head, the more electricity a hydropower plant can produce.

Understanding the water cycle is important to understanding hydropower. The water cycle has three steps:

1. Solar energy heats water on the surface of rivers, lakes, and oceans, which causes the water to evaporate .
2. Water vapor condenses into clouds and falls as precipitation—rain and snow.
3. Precipitation collects in streams and rivers, which empty into oceans and lakes, where it evaporates and begins the cycle again.
   
   

The amount of precipitation that drains into rivers and streams in a geographic area determines the amount of water available for producing hydropower. Seasonal variations in precipitation and long-term changes in precipitation patterns, such as droughts, can have large effects on the availability of hydropower production. Another important consideration with hydropower is the environmental consequences of disrupting natural ecosystems and aquatic habitats. Dams deteriorate over time, so when considering building new infrastructure, many projects will include damage funds. Damage funds are calculated monetary values that have considered the degradation of infrastructure and the future cost to repair or deconstruct existing infrastructure, such as a dam.

Geothermal resources are reservoirs of hot water that exist at varying temperatures and depths below the Earth's surface. Mile-or-more-deep wells can be drilled into underground reservoirs to tap steam and hot water brought to the surface for use in a variety of applications, including electricity generation, direct use, and heating and cooling. In the United States, most geothermal reservoirs are located in the western states.

Benefits of Geothermal Energy

• Renewable—Through proper reservoir management, the rate of energy extraction can be balanced with a reservoir's natural heat recharge rate.
  
  
• Baseload—Geothermal power plants produce electricity consistently, running 24 hours per day / 7 days per week, regardless of weather conditions.
  
  
• Domestic—U.S. geothermal resources can be harnessed for power production without importing fuel.
  
  
• Small Footprint—Geothermal power plants are compact; using less land per GWh (404 m²) than coal (3642 m²) wind (1335 m²) or solar PV with center station (3237 m²).*
  
  
• Clean—Modern closed-loop geothermal power plants emit no greenhouse gases; life cycle GHG emissions (50 g CO₂ eq/kWhe) are 4 times less than solar PV, and 6 to 20 times lower than natural gas. Geothermal power plants consume less water on average over the lifetime energy output than the most conventional generation technologies.

Biomass is organic material that comes from plants and animals, and it is a renewable source of energy. Biomass contains stored energy from the sun. Plants absorb the sun's energy in a process called photosynthesis. When biomass is burned, the chemical energy in biomass is released as heat. Biomass can be burned directly or converted to liquid biofuels or biogas that can be burned as fuels.

Examples of biomass and their uses for energy

• Wood and wood processing wastes—burned to heat buildings, to produce process heat in industry, and to generate electricity
• Agricultural crops and waste materials—burned as a fuel or converted to liquid biofuels
• Food, yard, and wood waste in garbage—burned to generate electricity in power plants or converted to biogas in landfills
• Animal manure and human sewage—converted to biogas, which can be burned as a fuel
  
  

Converting Biomass to Energy

• Solid biomass, such as wood and garbage, can be burned directly to produce heat. Biomass can also be converted into a gas called biogas or into liquid biofuels such as ethanol and biodiesel. These fuels can then be burned for energy.
• Biogas forms when paper, food scraps, and yard waste decompose in landfills, and it can be produced by processing sewage and animal manure in special vessels called digesters. Ethanol is made from crops such as corn and sugar cane fermented to produce fuel ethanol for use in vehicles. Biodiesel is produced from vegetable oils and animal fats and can be used in vehicles and as heating oil.