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Home » Energy » solar energy » Large Solar Energy S
Large Solar Energy S

Solar Generated Electricity 

Many power plants today use fossil fuels as a heat source to boil water. The steam from the boiling water rotates a large turbine, which activates a generator that produces electricity. However, a new generation of power plants, with concentrating solar power systems,

uses the sun as a heat source. There are three main types of concentrating solar power systems: parabolic-trough, dish/engine, and power tower. 

Parabolic-trough systems concentrate the sun's energy through long rectangular, curved (U-shaped) mirrors. The mirrors are tilted toward the sun, focusing sunlight on a pipe that runs down the center of the trough. This heats the oil flowing through the pipe. The hot oil then is used to boil water in a conventional steam generator to produce electricity. 


A dish/engine system uses a mirrored dish (similar to a very large satellite dish). The dish-shaped surface collects and concentrates the sun's heat onto a receiver, which absorbs the heat and transfers it to fluid within the engine. The heat causes the fluid to expand against a piston or turbine to produce mechanical power. The mechanical power is then used to run a generator or alternator to produce electricity. 

A power tower system uses a large field of mirrors to concentrate sunlight onto the top of a tower, where a receiver sits. This heats molten salt flowing through the receiver. Then, the salt's heat is used to generate electricity through a conventional steam generator. Molten salt retains heat efficiently, so it can be stored for days before being converted into electricity. That means electricity can be produced on cloudy days or even several hours after sunset. 


         

Some solar cells are designed to operate with concentrated sunlight. These cells are built into concentrating collectors that use a lens to focus the sunlight onto the cells. This approach has both advantages and disadvantages compared with flat-plate PV arrays. The main idea is to use very little of the expensive semiconducting PV material while collecting as much sunlight as possible. But because the lenses must be pointed at the sun, the use of concentrating collectors is limited to the sunniest parts of the country. Some concentrating collectors are designed to be mounted on simple tracking devices, but most require sophisticated tracking devices, which further limit their use to electric utilities, industries, and large buildings. 

Large Solar Energy Systems

The largest solar electric generating plant in the world produces a maximum of 354 megawatts (MW) of electricity and is located at Kramer Junction, California. The solar collectors concentrate sunlight to heat a heat transfer fluid to a high temperature. The hot heat transfer fluid is then used to generate steam that drives the power conversion subsystem, producing electricity. Thermal energy storage provides heat for operation during periods without adequate sunshine.

Another way to generate electricity from solar energy is to use photovoltaic cells; magic slivers of silicon that convert the solar energy falling on them directly into electricity.  The purpose of a solar collector is to intercept and convert a reasonably large fraction of the available solar radiation. For solar thermal systems this energy is  converted into thermal energy at some desired temperature and then, maybe, into electricity.

Large scale applications of photovoltaic for power generation, either on the rooftops of houses or in large fields connected to the utility grid are promising as well to provide clean, safe and strategically sound alternatives to current methods of electricity generation.  In photovoltaic systems, intercepted solar energy is converted directly into low voltage direct current electricity

The Solar Energy Conversion System

There are many different types of solar energy systems that will convert the solar resource into a useful form of energy. The following block diagram

                  

showing three of the most basic systems, i.e.,  the solar resource is captured and converted into heat which is then supplied to a demand for thermal energy (thermal load) such as house heating, hot water heating or heat for industrial processes. This type of system may or may not include thermal storage, and usually include an auxiliary source of energy so that the demand may be met during long periods with no sunshine.

If the demand (load) to be met is electricity (an electrical load) rather than heat, there are two common methods of converting solar energy into electricity. One method is by collecting solar energy as heat and converting it into electricity using a typical power plant or engine; the other method is by using photovoltaic cells to convert solar energy directly into electricity.

In general, if solar energy conversion systems are connected to a large electrical transmission grid, no storage or auxiliary energy supply is needed. If the solar energy conversion system is to be the only source of electricity, storage and auxiliary energy supply are usually both incorporated. If the thermal route is chosen, storage of heat rather than electricity may be used to extend the operating time of the system. Auxiliary energy may either be supplied either as heat before the power conversion system, or as electricity after it. If the photovoltaic route is chosen, extra electricity may be stored, usually in storage batteries, thereby extending the operating time of the system. For auxiliary power, an external electricity source is the only choice for photovoltaic systems.

The Solar Resource

The basic resource for all solar energy systems is the sun. Knowledge of the quantity and quality of solar energy available at a specific location is of prime importance for the design of any solar energy system. Although the solar radiation (insolation) is relatively constant outside the earth's atmosphere, local climate influences can cause wide variations in available insolation on the earth’s surface from site to site. In addition, the relative motion of the sun with respect to the earth will allow surfaces with different orientations to intercept different amounts of solar energy.

The following map shows regions of high insolation where solar energy conversion systems will produce the maximum amount of energy from a specific collector field size. However, solar energy is available over the entire globe, and only the size of the collector field needs to be increased to provide the same amount of heat or electricity as in the shaded areas. It is the primary task of the solar energy system designer to determine the amount, quality and timing of the solar energy available at the site selected for installing a solar energy conversion system.


                 

Solar Collectors


The solar collector is the key element in a solar energy system. It is also the novel technology area that requires new understandings in order to make captured solar energy a viable energy source for the future. The function of a solar collector is simple; it intercepts incoming insolation and changes it into a useable form of energy that can be applied to meet a specific demand. In the following text, we will develop analytical understandings of flat-plate and 

             


concentrating collectors, as used to provide heat or electricity. Each type is introduced below.

Flat-plate thermal solar collectors are the most commonly used type of solar collector. Their construction and operation are simple. A large plate of blackened material is oriented in such a manner that the solar energy that falls on the plate is absorbed and converted to thermal energy thereby heating the plate. Tubes or ducting are provided to remove heat from the plate, transferring it to a liquid or gas, and carrying it away to the load. One (or more) transparent (glass or plastic) plates are often placed in front of the absorber plate to reduce heat loss to the atmosphere. Likewise, opaque insulation is placed around the backside of the absorber plate for the same purpose. Operating temperatures up to 125oC are typical.

Flat plate collectors have the advantage of absorbing not only the energy coming directly from the disc of the sun (beam normal insolation) but also the solar energy that has been diffused into the sky and that is reflected from the ground. Flat plate thermal collectors are seldom tracked to follow the sun's daily path across the sky, however their fixed mounting usually provides a tilt toward the south to minimize the angle between the sun's rays and the surface at noontime. Tilting flat-plate collectors toward the south provides a higher rate of energy at noontime and more total energy over the entire day.

Flat-plate photovoltaic collectors contain an array of individual photovoltaic cells, connected in a series/parallel circuit, and encapsulated within a sandwich structure with the front surface being glass or plastic. Solar energy falls directly upon the photovoltaic cell front surface and produces a small direct current voltage, providing electrical energy to a load. Unlike thermal collectors however, the backside of the panel is not insulated. Photovoltaic panels need to loose as much heat as possible to the atmosphere to optimize their performance.

Like flat-plate thermal collectors, flat-plate photovoltaic collectors (panels) absorb both energy coming directly from the sun's disc, and diffuse and reflected energy coming from other directions. In general, flat-plate photovoltaic panels are mounted in a fixed position and tilted toward the south to optimize noontime and daily energy production. However, it is common to see flat-plate photovoltaic panels mounted on mechanisms that track the sun about one tilted axis, thereby increasing the daily output of the panels.

When higher temperatures are required, concentrating solar collectors are used. Solar energy falling on a large reflective surface is reflected onto a smaller area before it is converted into heat. This is done so that the surface absorbing the concentrated energy is smaller than the surface capturing the energy and therefore can attain higher temperatures before heat loss due to radiation and convection wastes the energy that has been collected. Most concentrating collectors can only concentrate the parallel insolation coming directly from the sun's disk (beam normal insolation), and must follow (track) the sun's path across the sky. Four types of solar concentrators are in common use; parabolic troughs, parabolic dishes, central receivers and Fresnel lenses.

                                  


A parabolic trough concentrates incoming solar radiation onto a line running the length of the trough. A tube (receiver) carrying heat transfer fluid is placed along this line, absorbing concentrated solar radiation and heating the fluid inside. The trough must be tracked about one axis. Because the surface area of the receiver tube is small compared to the trough capture area (aperture), temperatures up to 400oC can be reached without major heat loss.

A parabolic dish concentrates the incoming solar radiation to a point. An insulated cavity containing tubes or some other heat transfer device, is placed at this point absorbing the concentrated radiation and transferring it to a gas. Parabolic dishes must be tracked about two axes. 


                       


A central receiver system consists of a large field of independently movable flat mirrors (heliostats) and a receiver located at the top of a tower. Each heliostat moves about two axes, throughout the day, to keep the sun's image reflected onto the receiver at the top of the tower. The receiver, typically a vertical bundle of tubes, is heated by the reflected insolation, thereby heating the heat transfer fluid passing through the tubes. 

A Fresnel lens concentrator, such as shown in Figure 1.10d uses refraction rather than reflection to concentrate the solar energy incident on the lens surface to a point. Usually molded out of inexpensive plastic, these lenses are used in photovoltaic concentrators. Their use is not to increase the temperature, but to enable the use of smaller, higher efficiency photovoltaic cells. As with parabolic dishes, point-focus Fresnel lenses must track the sun about two axes.

Solar System- Hyberdization

Because of their thermal nature, all the solar thermal technologies can be hybridized, or operated with fossil fuel as well as solar energy. Hybridization has the potential to increase the value of concentrating solar thermal technology by increasing its availability and dispatchability, decreasing its cost (by making more effective use of power generation equipment), and reducing technological risk by allowing conventional fuel use when needed.


Like with any other power plant, solar power plant output must satisfy the demands of the utility market. During peak demand periods, kilowatt-hour prices are high and financial incentives are high for guaranteed supply. Solar plant input is limited by diurnal, seasonal and weather-related insolation changes. In order to cope with these fluctuations, the solar plant input may be backed up by fossil fuels, or the solar changes may be mitigated by a buffering storage system.

The Sun's Energy System Design

The sun, our singular

source of renewable energy, sits at the center of the solar system and emits energy as electromagnetic radiation at an extremely large and relatively constant rate, 24 hours per day, 365 days of the year.

The rate at which this energy is emitted is equivalent to the energy coming from a furnace at a temperature of about 6,000 K (10,340ºF). If we could harvest the energy coming from just 10 hectares (25 acres) of the surface of the sun, we would have enough to supply the current energy demand of the world.

However, there are three important reasons why this cannot be done: First, the earth is displaced from the sun, and since the sun’s energy spreads out like light from a candle, only a small fraction of the energy leaving an area of the sun reaches an equal area on the earth. Second, the earth rotates about its polar axis, so that any collection device located on the earth’s surface can receive the sun’s radiant energy for only about one-half of each day. The third and least predictable factor is the condition of the thin shell of atmosphere that surrounds the earth’s surface. At best the earth’s atmosphere accounts for another 30 percent reduction in the sun’s energy. As is widely known, however, the weather conditions can stop all but a minimal amount of solar radiation from reaching the earth’s surface for many days in a row. 

The rate at which solar energy reaches a unit area at the earth is called the "solar irradiance" or "insolation". The units of measure for irradiance are watts per square meter (W/m2). Solar irradiance is an instantaneous measure of rate and can vary over time. The maximum solar irradiance value is used in system design to determine the peak rate of energy input into the system. If storage is included in a system design, the designer also needs to know the variation of solar irradiance over time in order to optimize the system design.

The designer of solar energy collection systems are also interested in knowing how much solar energy has fallen on a collector over a period of time such as a day, week or year. This summation is called solar radiation or irradiation. The units of measure for solar radiation are joules per square meter (J/m2) but often watt-hours per square meter (Wh/m2) are used. As will be described below, solar radiation is simply the integration or summation of solar irradiance over a time period.

For system design 

optimization studies, it is considered better to use actual recorded weather databases. For the selected sites, we first examine the solar resource and the ability of various types of solar collectors to capture it effectively. Design tools are developed which integrate performance of isolated solar collectors, along with energy storage, into a larger system that delivers either electrical or thermal energy to a demand.

Evaluating sun’s radiation first outside the earth’s atmosphere and then on the earth’s surface, we then develop analytical models that may be used by the designer to estimate the solar irradiance at a specific site.

Site Selection Criteria

Solar technologies using concentrating systems for electrical production require sufficient beam normal radiation, which is the beam radiation which comes from the sun and passes through the planet's atmosphere without deviation and refraction. Consequently, appropriate site locations are normally situated in arid to semi-arid regions. On a global scale, the solar resource in such regions is very high. More exactly, acceptable production costs of solar electricity typically occur where radiation levels exceed about 1,700 kWh/m²-yr.

Solar electricity generation costs and feasibility of the project highly depend on the project site itself. A good site has to have a high annual beam insolation to obtain maximum solar electricity output. It must be reasonably flat to accommodate the solar field without prohibitive expensive earth works. It must also be close to the grid and a substation to avoid the need to build expensive electricity lines for evacuating the power. It needs sufficient water supply to cover the demand for cooling water of its steam cycle. A backup fuel must be available for granting firm power during the times when no solar energy is available. Access roads must be suitable for transporting the heavy equipment like turbine generators to the site. Skilled personnel must be available to construct and operate the plants

Installing a system:

WATEEN is committed to professional system designs and installations that comply with all applicable codes. We are proud to have a association of nationally certified installers working for us. WATEEN can also provide technical assistance for "do it yourself" installations performed by a sufficiently skilled homeowner.