Clean Tech 101

Types of Renewable Energy - An Overview

Geothermal Energy
Solar Energy
Wind Energy
Ocean Energy


Hydropower is the most mature and largest source of renewable power. Hydropower plants convert the energy in flowing water into electricity. The most common form of hydropower uses a dam on a river to retain a large reservoir of water. Water is released through turbines to generate power.

No air emissions are produced by hydropower plants. However, hydropower can affect wildlife habitats and the water quality. As a result, hydropower plants are now being designed and operated to minimize the impact on the local environment. Some of the hydropower plants are diverting a portion of the flow around their dams to mimic the natural flow of the river. Although this improves the wildlife's river habitat, it causes a reduction in the power plant's output. In addition, fish ladders and other approaches, such as improved turbines, are being used to assist fish with migration and lower the number of fish killed.


Bioenergy is the energy that comes from organic matter, such as plants. Many industries, such as those involved in construction or the processing of agricultural products, can create large quantities of unused or residual biomass, which can serve as a bioenergy source.

Biomass has less sulfur than coal. Therefore, less sulfur dioxide, which contributes to acid rain, is released into the air. In addition, using biomass reduces nitrous oxide emissions.

Converting biomass into gas is a process known as gasification. Using gas turbines, these gases can be used to generate electricity. Methane gas produced during the decay of biomass in landfills can also be used to generate electricity or for other industrial processes. Biomass can also be heated in the absence of oxygen to chemically convert it into a fuel oil called pyrolysis oil. Pyrolysis oil can be used for power generation and as a feedstock for fuels and chemical production.

Biomass can also be converted into a liquid fuel referred to as biofuel through a conversion process. An example of biofuel is ethanol. The current largest source of ethanol is corn. Some cities use ethanol as a gasoline additive to help meet air quality standards. Another example of biofuel is biodiesel, produced from fats of vegetables and animals can be used as fuel for vehicles or as a fuel additive to reduce emissions.

Geothermal Energy

The core of the Earth which is 4,000 miles below the surface, can reach temperatures of 9000°F. This generated heat is known as geothermal energy. Geothermal energy radiates outwards from the core and heats the surrounding areas. The potential of geothermal energy in the uppermost 6 miles of the Earth's crust amounts to 50,000 times the energy of all oil and gas resources in the world.

Geothermal power plants use the underground steam or hot water from wells drilled a mile or more into the earth. The steam or hot water is piped up from the well to drive a conventional steam turbine, which powers an electric generator. Typically, the water is then returned to the ground to recharge the reservoir and complete the renewable energy cycle.

There are three types of geothermal power plants:

  1. Dry steam
  2. Flash steam, and
  3. Binary cycle.

Dry steam plants draw from steam reservoirs, whereas both the flash steam and binary cycle plants draw from hot water reservoirs. Flash steam plants typically use water at temperatures greater than 360°F. Unlike both steam and flash plants, binary-cycle plants transfer heat from the water to a so-called working fluid. Therefore binary cycle plants can operate using water at lower temperatures of about 225°F to 360°F.

Solar Energy

Solar energy uses the unlimited power of the sun to produce heat, light, and power. Solar electricity or photovoltaic technology converts the light from the sun directly into electricity. Solar electricity has been a prime source of power for space vehicles. It has also been used to power small electronics such as calculators, watches and lighting. Solar energy can be used to heat water for your home or your swimming pool.

During the last decade, a strong solar electric market has emerged for powering urban grid-connected homes and buildings as a result of advances in solar technology along with global changes in electric industry restructuring.

Solar electric systems primarily consist of three main components: modules that convert sunlight into electricity; inverters that convert that electricity into alternating current so it can be used by most household appliances; and possibly or sometimes batteries that store excess electricity produced by the system.

The heat from the sun can also be used to generate electricity. This is known as solar thermal electric systems. This is used mainly in large-scale power plants for powering communities and cities.

Concentrating solar power technologies convert solar energy into electricity by using mirrors to focus sunlight onto a component called a receiver. The receiver transfers the heat to a conventional generator, such as a steam turbine that generates electricity. There are three types of CSP systems: power towers (central receivers), parabolic troughs, and dish/engine systems.

Wind Energy

Wind energy has been the fastest growing source of energy in the world since 1990. A wind turbine works the opposite of a fan. Instead of using electricity to make wind, like a fan, wind turbines use wind to make electricity. The wind turns the blades, which spin a shaft, which is connected to a generator and makes electricity. Wind turbines are highly sophisticated power systems that capture the wind's energy by means of new blade designs or airfoils.

Ocean Energy

Two types of energy can be produced by the ocean: thermal energy from the heat of the sun, and mechanical energy from the tides and waves. Ocean thermal energy can be used for many applications, including electricity generation. Electricity is generated by using either the warm surface water or boiling the seawater to turn a turbine, which starts a generator.

Using tidal and wave energy to produce electricity usually involves mechanical devices. A dam is typically used to convert tidal energy into electricity by forcing the water through turbines. Meanwhile, wave energy uses mechanical power to directly start a generator, to produce electricity.

Source: AZoCleantech
Last Update 24th December 2007

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