Wind Energy

As moving air, wind is a nondepleting source of renewable energy. Wind can be of devastating power. Storms and hurricanes are proof of strong winds having the power to uproot trees and destroy homes and other infrastructure. That wind power can be used as a source of energy has long been known to humanity. The utilization of wind energy is not a new technology; the generation of electricity from wind power took place many years ago. Electricity generation (energy conversion) from fossil fuels is largely contributing to greenhouse gas (GHG) emissions, primarily carbon dioxide (CO2), which have been identified as the main driver of global warming and climate change. Wind is a clean energy source with the potential to replace other polluting, electricitygeneration technologies.

How much energy the wind is carrying depends on several factors: the amount of wind energy flowing through a given area, or area swept by a turbine (A) during a fixed time; the time to pass the turbine (t); the wind’s velocity before the turbine (v); and its density (ρ). The wind’s kinetic energy (Ekin) is calculated using the formula: Ekin = ρ/2 A v3 t. The wind speed (v) is normally measured in meters per second (m/s) using an anemometer (wind speed meter). Practical wind power density (WPD) maps are established to provide the mean annual power available per sq. m of swept area of a turbine for different heights in watts per square meter (W/m2) and serve as a reference as to the best location to install and operate wind turbines.

Wind is produced by differences in air pressure, which are the result of the unequal heating of the Earth’s surface by the sun (which in turn heats the air above it). This unequal heating of the Earth’s surface is the result of the difference between outgoing and incoming radiation at high and low latitudes of the Earth, which rotates around a shifted axis. Heated air expands and decreases in density.

Following the second law of thermodynamics, the air then flows from areas of high pressure to areas of low pressure until the air pressure is balanced (and the entropy is maximized). The higher the pressure difference and gradient, the stronger the wind that seeks to balance the difference. Because the Earth is rotating, the angular momentum is conserved with the wind, which is shifted along a longitudinal direction resembling a circular movement (Coriolis effect). On a local scale, the geographic and topographic properties of the Earth’s surface largely influence how uniformly and consistently the wind is flowing.

Offshore and onshore winds are generated along the shores of large lakes and ocean beaches. Those winds blow very regularly as they are created by the different heat absorption and storage properties of the land and water surfaces. Likewise, mountain-valley breezes arise from the unequal heating properties of the mountain-valley topography. Surface wind speeds tend to be lower when the movement of air is obstructed by geographical features, vegetation, or buildings. Therefore, coastal and offshore sites are ideal locations for wind-turbine installations.

Solar Energy

Solar energy is a renewable energy source in the form of radiant light from the sun. The term renewable solar energy refers to a broad range of technologies and techniques for capturing and utilizing this energy. In one hour, approximately 440 exajoules (EJ) of energy from the sun reach the Earth’s outer atmosphere. This is only slightly less than the estimated world population’s energy consumption from primary energy sources, such as biomass and fossil fuels, in an entire year. The Earth’s atmosphere, land surfaces, and oceans absorb a fraction of this incoming radiation, resulting in an increase in atmospheric, oceananic, and land mass heat, in addition to water evaporation.

These heat and evaporation processes in turn drive the Earth’s water cycle and produce atmospheric phenomena, including wind and weather patterns. Plants convert solar energy into chemical energy through the process of photosynthesis. Energy from the sun is critical to processes that sustain life on Earth. Directly capturing and utilizing even a small fraction of this vast energy source would offset fossil fuel consumption, which in turn would decrease the greenhouse gas (GHG) emissions that contribute to climate change.

Solar Energy Availability

Thermonuclear reactions in the sun result in the emission of radiation or light in all directions into space. The Earth intercepts a small fraction of this emitted radiation. At the top of the Earth’s atmosphere, the amount of radiated power from the sun remains fairly constant, at 1,361 watts per square meter (W/m2), varying only slightly throughout the year by plus or minus 3.5 percent because of the eccentricity of the Earth’s orbit around the sun. As this solar energy enters the Earth’s atmosphere, some wavelengths of this light are absorbed, scattered, or reflected back into space by different molecules, such as the gases and water vapor in the atmosphere. For example, a process known as Rayleigh scattering results in the blue appearance of the sky. On clear days, with little cloud cover, the radiated solar energy reaching a flat surface on the Earth can exceed 1,000 W/m2; however, on cloudy or overcast days, this power can be reduced to less than 100 W/m2. Cloud cover, aerosols, dust, smoke, and suspended water droplets all reduce the transmittance of the atmosphere and decrease the amount of solar radiation reaching a given location.

For terrestrial solar energy applications, the amount of energy that falls on a given surface is of primary concern. The orientation of a surface relative to the incoming sun’s rays must be considered when evaluating the solar energy available for a given application. The total solar radiation available to a surface at any orientation is a combination of two components: direct beam radiation and diffuse radiation. Because the incoming solar rays are nominally parallel, geometric relations can be used to estimate the amount of beam radiation available to a given surface. The surface’s available direct beam radiation is related to its latitude, the tilt of the Earth on its axis (which depends on the day of the year), the time of day, the slope of the surface, and the orientation of the slope in the east–west direction. Maximum beam radiation on a clear day occurs when the incoming solar radiation is perpendicular to the surface of interest. Diffuse radiation refers to radiation that is scattered and reflected from the atmosphere, clouds, and the ground to a particular surface.

The estimation of diffuse radiation availability depends upon the orientation of the surface, the scattering potential of the atmosphere, and the reflectance properties of the ground. On cloudy days, and when the surface is shaded from direct beam radiation, diffuse radiation is the primary source of solar energy availability.

Because the solar energy available at a specific location depends on many factors, historical data is often used to obtain more accurate estimates of energy availability during an average day. Solar energy availability can be estimated based on data collected from pyrheliometers and pyranometers. Pyrheliometers are instruments that track the sun and measure the available direct-beam radiation. Pyranometers obtain data on the total solar radiation or the diffuse solar radiation available to a fixed surface. Using historical data from such instruments located near a proposed installation site provides a more accurate measurement of the solar energy capture potential at that location. Such data can be modified using geometric relations and empirical observations to estimate the solar energy availability on a surface at any orientation.

Renewable Energy

According to the International Energy Agency, renewable energy is energy that is derived from natural processes that are replenished constantly. In its various forms, it is derived directly or indirectly from the sun, or from heat generated deep within the Earth. Included in the definition is energy generated from solar, wind, biomass, geothermal, hydropower and ocean resources, and biofuels and hydrogen derived from renewable resources. Renewable energy sources from natural energy sources such as sun, wind, waves, and tides have developed rapidly in recent years and have been used widely around the world as an alternative-energy system.

Renewable energy is considered one of the viable options to meet the challenge of achieving sustainable development and conserving natural resources that have been depleted because of the rapid growth in population, urbanization, and fossil fuel consumption while addressing the issues of energy security. Renewable energy has been widely considered an indispensable basis of sustainable energy systems, as electricity generation from renewable sources helps in reducing greenhouse gas (GHG) emissions, contributing to sustainable development and addressing climate change over the past decades as compared to electricity from conventional fossil fuels. 

Renewable-electricity generation capacity, including large hydropower, reached an estimated 1,140 gigawatt (GW) worldwide in 2008, with a total share of about 18 percent—of which 15 percent was from hydroelectricity and 3 percent from new renewables such as small hydropower, modern biomass, wind, geothermal, and biofuels. According to Renewable Energy Network 21 (REN 21), about 19 percent of global final energy consumption came from renewables in 2008, with 13 percent from traditional biomass used for heating, 3.2 percent from hydroelectricity, and a small percentage—about 2.7 percent—from the new renewables.

Apart from the environmental benefits, renewable energy has also been playing an important role in alleviating poverty and enhancing the development of remote regions through offering opportunities for work and social advancement of underemployed and disadvantaged segments of the population. However, their adoption has been mainly driven by impending environmental and energy security considerations arising from the use of fossil fuel–based energy (coal, oil, and gas) and the fact that fossil-based energy sources are finite.

Geothermal Energy

Geothermal energy is a renewable resource that is generated by natural processes within the Earth. Since prehistoric times, humans have enjoyed the recreational and perceived therapeutic properties of surface geysers and thermal hot springs. It has also been used to produce electricity on an industrial scale for more than 100 years. Geothermal energy provides a viable, sustainable, lowcarbon alternative to fossil fuels. Furthermore, the potential global energy production capacity of the Earth’s exploitable geothermal resources far exceeds both current and predicted future primary energy demand. However, in 2010, geothermal energy production comprised less than 1 percent of total global primary energy consumption.

The concept of geothermal energy raises some important questions: What is geothermal energy and under what conditions can it be exploited? What are some direct and indirect strategies for harnessing geothermal energy? What are some of the benefits and limitations of geothermal energy? And finally, what are the prospects for future development of this abundant and valuable energy source?

Geothermal Energy Basics

Over 99 percent of the Earth’s mass is extremely hot—above 1,830 degrees F (1,000 degrees C). In general, temperature increases with depth. The core of the planet, which is about 2,174 mi. (3,500 km) in radius, is composed of a solid iron inner core and a superheated liquid outer core that generates a magnetic field. The inner and outer cores are enveloped by another region called the mantle, which is about 1,800 mi. (2,900 km) thick. The mantle, in turn, is enclosed by an extremely thin and largely impervious outer crust (12–40 mi. or 20–65 km thick in continental areas, less in ocean regions), which in relative terms is analogous to the skin of an apple. In most places, the outer crust blocks a significant amount of heat from rising to the surface. However, there are some locations, such as a space between tectonic plates or active volcano systems, where the outer crust is unusually thin, permeable, or cracked, and heat can rise close enough to the planet’s exterior to allow human exploitation. Commercially exploitable resources are generally 1.8 mi. (3,000 m) or less below the Earth’s surface.

Besides heat and close proximity to the Earth’s surface, exploitable geothermal systems also require water and permeability. Hot water or steam is sometimes trapped in permeable rock formations, forming geothermal reservoirs below layers of impermeable crust. In some cases, these reservoirs can be accessed via geothermal wells; in other situations, the hot water or steam may seep to the surface naturally. It is also possible to artificially introduce water into a field of fractured hot rock where it is heated and then extracted by a production well.