What is renewable energy?

What is renewable energy?

Renewable energy is energy generated from natural resources, such as sunlight, wind, rain, tides and geothermal heat which are naturally replenished.

Energy sources are said to be nonrenewable if they cannot be replenished in a short period of time. Fossil fuels such as coal, natural gas and oil are nonrenewable.

Commonly used renewable energy sources are:

1.    Solar

2.    Wind

3.    Bioenergy:

a.    Wood and wood waste

b.    Municipal solid waste

c.     Landfill gas and biogas

d.    Ethanol

e.    Biodiesel

4.    Hydropower

5.    Geothermal

Solar

Solar is probably the most common and economical among renewable power sources. The energy that reaches the earth via solar radiation can be converted into electricity and heat. 

Solar energy can be converted into electricity in two ways:

1.    Photovoltaic (PV devices): A photovoltaic (PV) cell, commonly called a solar cell, is a device that converts sunlight directly into electricity. Individual PV cells are grouped into panels and arrays of panels that can be used in a variety 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.

2.    Solar thermal/electric power plants generate electricity by concentrating solar energy to heat a fluid and produce steam that is then used to power a generator.  Solar thermal power generation works essentially the same as power generation using fossil fuels, but instead of using steam produced from the combustion of fossil fuels, the steam is produced by heat collected from sunlight. Solar thermal technologies use concentrator systems to achieve the high temperatures needed to produce steam.  There are three main types of solar thermal power systems:

a.    Parabolic trough;

b.    Solar dish;  

c.     Solar power tower;

When converted to thermal energy, solar energy can also be used to heat water for use in homes, buildings, or swimming pools; to heat spaces inside homes, greenhouses, and other buildings; and to heat fluids to high temperatures to operate turbines that generate electricity.

WIND

Wind is caused by the uneven heating of the earth's surface by the sun. Because the earth's surface is made of different types of land and water, it absorbs the sun's heat at different rates. 

During the day, the air above the land heats up faster than the air over water. This warm 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.

Today, wind energy is mainly used to generate electricity, although water pumping windmills were once used throughout the United States.  Wind turbines are most commonly installed in wind farms either inland or offshore. Wind is not restricted to huge projects; if you have space you can install a small wind turbine on to generate power for your home.

Bioenergy

Biomass is organic material that comes from plants and animals, and it is also a renewable source of energy.  Organic matters recovered from alive or recently alive creature, like trees, fuels from alcohol are examples of biomass.

Wood energy is derived both from harvested wood as a fuel and from wood waste products. Waste energy can be generated from municipal waste, manufacturing waste, and landfill gas.

Burning biomass is only one way to release its energy. Biomass can be converted to other useable forms of energy like methane gas or transportation fuels like ethanol and biodiesel.

Methane gas is a component of landfill gas or biogas that forms when garbage, agricultural waste, and human waste decompose in landfills or in special containers called digesters.

Crops such as corn and sugar cane are fermented to produce fuel ethanol for use in vehicles.

Biodiesel, another transportation fuel, is produced from vegetable oils and animal fats. Biodiesel fuel has chemical characteristics similar to petroleum-based diesel, so it can be used as a direct substitute for diesel fuel. Biodiesel fuel can also be blended with petroleum diesel in any percentage without reducing vehicle fuel economy.

Hydropower

Hydropower is the largest renewable energy source for electricity generation in the United States. In 2015, hydropower accounted for about 6% of total U.S. electricity generation and 46% of electricity generation from all renewables.

1.    Hydroelectric power is produced from moving water which moves a water turbine.  The volume of the water flow and the change in elevation (or fall) from one point to another determine the amount of available energy in moving water.  Swiftly flowing water in a big river carries a great deal of energy in its flow. Water descending rapidly from a high point, like in a fall, also has substantial energy in its flow.

2.    Tidal Power: Tides are caused by the gravitational pull of the moon and sun, and the rotation of the earth. Near the shore, water levels can vary up to 40 feet as a result of tides. The movement of water as a result of tidal forces can be used to produce energy.

3.    Wave Power: Waves are caused by the wind blowing over the surface of the ocean. There is tremendous energy in ocean waves. One way to harness wave energy is to bend or focus the waves into a narrow channel, increasing their power and size. The waves can then be channeled into a catch basin or used directly to spin turbines that generate electricity.  Many other methods of capturing wave energy are being developed. These methods include placing devices on or just below the surface of the water, and anchoring the devices to the ocean floor.

4.    Ocean Thermal Energy Conversion: Energy from the sun heats the surface water of the ocean. In tropical regions, surface water can be much warmer than deep water. This temperature difference can be used to produce electricity.

No large-scale operation of Ocean Thermal Energy Conversion exists today because of the many challenges, technical as well as economical, associated with the technology.

Geothermal

Geothermal energy comes from deep inside the earth.  The slow decay of radioactive particles in the earth's core, a process that happens in all rocks, produces geothermal energy.

The earth has a number of different layers:

1.    The inner core is solid iron and is surrounded by an outer core of hot molten rock called magma.

2.    The mantle surrounds the core is about 1,800 miles thick. The mantle is made up of magma and rock.

3.    The crust is the outermost layer of the earth. The crust forms the continents and ocean floors. The crust can be 3 to 5 miles thick under the oceans and 15 to 35 miles thick on the continents.

Some applications of geothermal energy use the earth's temperatures near the surface, while others require drilling miles into the earth. There are three main types of geothermal energy systems:

Direct use and district heating systems, which use hot water from springs or reservoirs located near the surface of the earth.

Electricity generation power plants. Geothermal electricity generation requires water or steam at high temperatures (300° to 700°F). Geothermal power plants are generally built where geothermal reservoirs are located within a mile or two of the earth's surface.

Geothermal heat pumps use the constant temperatures near the surface of the earth to heat and cool buildings. Geothermal heat pumps transfer heat from the ground (or water) into buildings during the winter and reverse the process in the summer.

Environmental Impacts of the Renewable energy Technologies

All power generation techniques or sources have some impact on our environment.  

Many of the chemical compounds in the earth’s atmosphere act as greenhouse gases. When sunlight strikes the earth’s surface, some of it radiates back toward space as infrared radiation (heat). Greenhouse gases absorb this infrared radiation and trap its heat in the atmosphere, creating a greenhouse effect that results in global warming and climate change.  In the United States, greenhouse gas emissions come primarily from the burning of fossil fuels in energy use.

Most renewable resources such as solar, wind and hydropower produce electricity without emitting greenhouse gas. Others such as burning biomass and ethanol are considered carbon neutral because the plants that are sources of the feedstocks for making them absorb carbon dioxide (CO2) as they grow. Geothermal power plants do not burn fuel to generate electricity, so the levels of air pollutants they emit are low.

The environmental impact of renewable energy is highly specific to the source, for example in some parts of the world, large areas of natural vegetation and forests have been cleared and burned to grow crops biodiesel or ethanol. The negative environmental impacts of this land clearing and burning may be greater than any potential benefits of using biodiesel or ethanol. Growing plants for fuel is a controversial issue because some people believe the land used to grow energy crops should be used to grow food crops instead.

In the case of hydropower, a dam that creates a reservoir may obstruct fish migration and can also change water characteristics. These changes may have negative impacts on native plants and animals. A reservoir may also result in the relocation of people. The impacts of a dam and reservoir can change the environment over a much larger area than the area covered by a reservoir.