Renewable Energy Sources: A Comprehensive Overview
Renewable Energy Sources
Hydropower
A small number of dams were built specifically for hydropower generation. While hydropower generators do not directly produce emissions of air pollutants, hydropower dams, reservoirs, and the operation of hydropower electric generators can affect the environment. They can obstruct fish migration, change natural water temperatures, river flow characteristics, ecology, and the physical characteristics of the river.
Solar Energy
2.1 Transformation into Thermal Energy
Solar thermal energy can be used to heat water or air. It is most often used for heating water in homes and swimming pools. It is also used to heat the insides of buildings.
2.1.1 Solar Thermal Collectors
A solar collector absorbs solar radiation and converts it into heat (photo-thermal conversion). The high-efficiency solar collector implies maximum absorption of incident solar radiation with minimum thermal and optical loss.
A. Greenhouse
All greenhouses trap solar energy during the day, usually with the benefit of south-facing placement and a sloping roof to maximize sun exposure. Stones, cement and water, or water-filled barrels can all be used as simple, passive thermal mass materials (heat sinks), capturing the sun’s heat during the day and radiating it back at night.
B. Solar Distillation
The sun provides energy to evaporate freshwater from saline water. In solar distillation, the water vapor formed from the evaporation process condenses on a clear glass or plastic covering and is collected as freshwater in a condensate trough.
Solar Furnace
Solar furnaces can reach 1 MW and a temperature of 3300 °C. They are primarily used for experiments involving heat and melting metal.
Power Tower Systems
Power tower systems use a large field of Sun-tracking mirrors known as heliostats to focus sunlight onto a central receiver at the top of a tower. The receiver contains a heat-transfer fluid which is heated by the concentrated sunlight.
Parabolic Trough Systems
Parabolic trough systems use parabola-shaped reflectors to focus sunlight onto a tube that runs along the focal-line of the reflectors. A heat-transfer fluid (oil) inside the tube is heated and used to generate steam in interchange heat to drive a conventional turbine generator which then produces electricity.
Photovoltaic Solar Plant
Photovoltaic solar plants are made of silicon, a special type of melted sand. When sunlight strikes the solar cell, electrons (red color) are knocked loose.
Wind Energy
The winds that travel around the earth have two different driving mechanisms. The first mechanism is uneven heating of the earth by the sun: equatorial latitudes receive more of the Sun’s rays than the polar latitudes. Wind power is actually another form of solar power. The second mechanism is the rotation of the earth, which, through the Coriolis Effect, influences wind patterns.
1.1 Types of Wind Machines
There are two main designs of wind turbines that capture the wind’s energy.
The Horizontal Axis Wind Turbine (HAWT)
The Horizontal Axis Wind Turbine (HAWT) is rather simple. Much like medieval windmills used to grind flour, modern turbines harness wind by using large angled propeller blades to catch the wind. When the wind passes through the blades, it causes the entire blade assembly, known as a rotor, to spin around a central nacelle atop a tall tower. Inside the nacelle is housed a gearbox which converts the low-speed incoming rotational force into high-speed outgoing rotational force, powerful enough to run an electrical generator also housed in the nacelle.
It requires a wind speed of 5 m/s to start. The maximum efficiency is reached when the speed reaches 15 m/s. However, if the wind speed is too high, it will not start.
Vertical-Axis Wind Turbines
Vertical-axis wind turbine designs include the Darrieus, Giromill, and Savonius turbines. They all work on the same basic principle: the rotors spin around a central vertical tower, generating power for the gearbox and generator located at the base of the tower.
PWIND = 0.37 * S * V3
The aerodynamic efficiency is:
RENDTO = Euseful = Puseful
Ewind = Pwind
Biomass
Biomass is biological material from living, or recently living organisms such as trees, grasses, and agricultural crops. As an energy source, biomass can either be used directly or converted into other energy products such as biofuel. It is usually turned into higher heat power fuel because it has low energy efficiency and takes up a lot of space to store.
1.1 Direct Extraction
Direct extraction is based on the fact that some vegetable species produce hydrocarbons and products with a high amount of hydrogen in their metabolism. These products are known as biofuels. The process of obtaining them is through crushing and adding chemical products. Ethanol and Methanol are made from some plants, for instance, palm leaves and sunflowers. They are used in internal combustion engines.
1.2 Thermochemical Processes
Biomass is subjected to high temperatures, and irreversible chemical processes are carried out by heat (combustion). There are several types of processes:
- Burning Biomass: When biomass is burned with a large amount of air, heat is obtained, which is used for moving a turbine, and consequently, electric energy is generated.
- Gasification of Biomass: Gasification is a high-temperature process in which biomass is converted into a combustible gas or syngas. Gasification takes place at high temperatures (700 – 1500ºC) with the addition of heat or a controlled amount of air/oxygen (between 10 to 50% of the theoretical amount necessary). The air is blown at high speed through the burning combustible. Depending on the use of air or pure oxygen, there are two types of products.
- Pyrolysis: Pyrolysis is the thermal decomposition of biomass occurring in the absence of oxygen. It is the fundamental chemical reaction that is the precursor of both the combustion and gasification processes and occurs naturally in the first two seconds. The products of biomass pyrolysis include biochar, bio-oil, and gases, including methane, hydrogen, carbon monoxide, and carbon dioxide.
P = 9.8 * Q * H
E = P * t = 9.8 * Q * H * t(H)
SOLARQ = K * t * S