Renewable energies are sources of energy that renew themselves constantly through a natural process and seen on a human time scale, and never run out. There are three primary sources of renewable energy are solar radiation, geothermal heat, ocean power from tides. These three sources can be used either directly or indirectly in particular forms such as biomass, heat, and fuel.
Solar energy is one of the cleanest energy resources that does not compromise or add to global warming. The sun radiates more energy in one second than people have used since the beginning of time. Solar energy is often called “alternative energy” to fossil fuel energy sources such as oil and coal. Solar systems for electricity generation are based on the concentration of sunlight radiations. There are three different types of solar power systems.
Parabolic trough
The parabolic system is composed of long parabolic mirrors of thickness 4 mm which have high transmittance properties due to the sliver layer, a receiver tube that runs the length of the mirror, tracking support structure, and drive components that control the movement of a collector with respect to the position of the sun throughout the day. The working of this system includes the focusing of sunlight at a focal point from where energy is transferred to a linear receiver tube also called a heat collection element which is made up of stainless steel. This tube act as a special absorber and transfer heat to heat transfer fluids which can be high-temperature oil or a mixture of water with ethylene glycol. The heat from this fluid is utilized to convert water into steam which drives the turbines.
Solar power tower
This system consists of large field mirrors called heliostats that track the sun. A tower collects light from mirrors onto a central receiver at the top of the tower. Tower height ranges from 300 to 650 feet. Heat transferring fluid in this system consists of either water or molten nitrate salt and that can be heated to 500 0C, the heat transferring fluid is then sent to convert water into steam to run turbines. The advanced system uses molten salt that has the advantage of higher heat energy and storage, which allows the system to continue producing electricity during cloudy weather as well as during nighttime.
Parabolic dish technology using Stirling motor
This system is composed of a large mirrored dish, a series of tubes filled with heat transfer medium either hydrogen or helium, and a sterling engine. In this system heat from collectors via heating fluid is transferred to the engine. The engine uses the heated fluid to move the piston and generate mechanical power which is then used to run generators.
These are also called indirect solar power systems, one other system that uses direct sunlight to produce electricity is via photovoltaic effect using photovoltaic cells also called silicon cells.
Photovoltaic cells
Solar PV modules are solid-state semiconductor devices with no moving parts that convert sunlight into direct-current electricity. The basic principle underlying the operation of PV modules dates back more than 150 years, but significant development really began following Bell Labs’ invention of the silicon solar cell in 1954. The first major application of PV technology was to power satellites in the late 1950s, and this was an application where simplicity and reliability were paramount and the cost was a secondary concern. Since that time, enormous progress has been made in PV performance and cost reduction.
The efficiency of energy conversion depends mainly on the PV panels that generate power. The practical systems have low overall efficiency. This is the result of the cascaded product of several efficiencies, as the energy is converted from the sun through the PV array, the regulators, the battery, cabling, and through an inverter to supply the ac load. Weather conditions also influence the efficiency, which depends non-linearly on the irradiation level and temperature. For example, a cloud passing over a portion of solar cells or a sub-module will reduce the total output power of solar PV arrays. Under certain cloud conditions, the changes can be dramatic and fast. The wide acceptance of PV power generation depends on the cost and on energy conversion efficiency. Attempts have, however, been constantly made to improve sun-tracking systems to increase the efficiency to make solar energy attractive. In current technology conditions, the utilization of tracking PV systems is an optimum selection of enhancing system efficiency and reducing cost.
The capacity factor for the parabolic trough system is observed 25%, for the power tower system is 34% and for the dish engine system is 31 %. The efficiency values for the parabolic trough system are 15-18%, the power tower system has a value of 14-19%, for dish Stirling system value ranges from 18-23% and efficiency values for photovoltaic cells are up to 15%. The availability of cheap and abundant energy with minimum environmental and ecological hazards associated with its production and use is one of the important factors for the desired improvement in the quality of life of the people. The growing scarcity of fossil fuels has raised global interest in the harnessing of solar energy.
