Clean Tech 101

Types of Renewable Energy - An Overview

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Dec 24 2007

As fossil fuel resources continue to decrease worldwide, more and more attention is being focused on harnessing power from renewable energies generated from alternative natural sources. Solar, wind, and hydropower are renewable energy sources that have become well established in industrial and domestic settings alike. Below is a breakdown of the types of renewable energy available.

Hydropower

Hydropower is the most mature and largest source of renewable power. Hydropower plants convert the energy in flowing water into electricity. Hydropower plants are most commonly in the form of a dam where water from a large reservoir is released through turbines to generate power.

Hydropower plants produce no air emissions; 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 hydropower plants diverting a portion of the flow around their dams to mimic the natural flow of the river. Although this improves the river habitat for wildlife, it causes a reduction in the power plant's output. In addition, approaches including fish ladders and improved turbines are currently used to assist fish with migration and reduce the number of fish killed.

In 2017, hydropower accounted for approximately 7.5% of electricity generation in the US; however, hydroelectricity is being surpassed by other sources of energy.

Bioenergy

Bioenergy originated from organic matter, such as plants, food waste or sewage. Many industries, such as agriculture, can create large quantities of unused or residual biomass which can serve as a bioenergy source. In other instances, stores of biomass material are grown specifically for use in bioenergy.

Bioenergy possesses a number of environmental benefits. Biomass contains less sulfur than coal, and therefore produces less sulfur dioxide which contributes to acid rain. 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 a biofuel is ethanol, the largest current source of which is corn. Some cities use ethanol as a gasoline additive to help meet air quality standards. Another example of a biofuel is biodiesel, produced from vegetable and animal fat, which can also be used as fuel for vehicles or as a fuel additive to reduce emissions.

Geothermal Energy

The core of the Earth is 4,000 miles below the surface and can reach temperatures of 9000 °F. This heat is known as geothermal energy and it radiates outwards from the core, heating surrounding areas. The potential of geothermal energy in the uppermost six 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:

Dry steam
Flash steam
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 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 approximately 225 °F to 360 °F.

Solar Energy

Solar energy uses the unlimited power of the sun to produce electricity. Solar electricity has been a prime source of power for space vehicles and is also used to power small electronics such as calculators, watches and lighting. Solar energy can also be used to heat water in domestic settings, or even to heat swimming pools. Transparent solar panels for use on windows have also been developed, decreasing the visual impact solar panels have on a property or landscape.

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 electricity into alternating current for use by most household appliances; and sometimes batteries to store excess electricity produced by the system. However, storing energy generated from solar panels can be expensive.

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

Concentrating solar power technologies convert solar energy into electricity 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, to generate electricity. There are three types of CSP systems: power towers (central receivers), parabolic troughs, and dish/engine systems.

The advantages of solar energy include low operating costs, although the initial cost of purchasing and setup is high, and zero CO₂ emissions; however the manufacture, transportation, and installation of solar panels do have associated emissions.

However, solar energy is weather dependent, with the efficiency of solar systems decreasing in cloudy or rainy weather, although energy is still generated in these conditions. Additionally, power is not collected overnight, which can affect the efficacy of systems in locations where daylight hours are shorter.

Wind Energy

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

A variety of sizes and power ratings can be produced, with the largest wind turbine capable of generating enough energy to power 1,400 homes.

Ocean Energy

Two forms of ocean energy can be utilized to produce electricity: 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 drive a generator, to produce electricity.

There is significant risk to damage of marine life and ecosystems from the machinery required to operate marine power systems. This includes injuries caused by the movement of tidal turbine blades, as well as the effects of underwater noise generated by the system. There are also concerns regarding the effect ocean energy systems will have on water quality.

Summary

Despite the clear advantages in renewable energy, including relieving pressure on fossil fuel resources and lowering the cost of energy bills for homeowners, there are concerns about how harnessing energy from the environment, be it wind, solar, marine, or biofuel, will affect wildlife in those different habitats. Additionally, the cost of installing many of these systems is high, although the running costs and maintenance costs are comparatively low against traditional energy generation methods.