Solar 1

PRESENTED BY 

SHASHANK BHATNAGAR & VANCHHIT KHARE B.TECH(E.C.) – II SEM

Introduction Solar power tower converts, sunshine into clean electricity. The

technology uses many large, sun-tracking mirrors commonly referred to as heliostats to focus sunlight on a receiver at the top of a tower. A heat transfer fluid heated in the receiver is used to generate steam, which, in turn, is used in a conventional turbine-generator to produce electricity. Early power towers such as the Solar One plant uses steam as the heat transfer fluid. Current power towers such as Solar Two use molten nitrate salt. Nitrate salt is used because of its superior heat transfer and energy storage capabilities.

A heat transfer fluid heated in the receiver is used to generate steam, which, in turn, is used in a conventional turbine-generator to produce electricity. Early power towers such as the Solar One plant uses steam as the heat transfer fluid. Current power towers such as Solar Two use molten nitrate salt. Nitrate salt is

Solar One - The First Generation of Power Tower Plant Solar One was the world’s largest power tower plant, which operated from 1982 to 1988. The Solar One thermal storage system works by storing heat in the form of steam generated using solar energy in a tank which is filled with rocks and sand and using oil as the heat-transfer fluid. The Solar One thermal storage system extended the power generation capability of the plant into the night and provided heat for generating low-grade steam for keeping parts of the plant warm during off-hours and for morning startup. Unfortunately, the Solar One thermal storage system was complex and thermodynamically inefficient. Solar One also showed the disadvantages of a water/steam system, such as the intermittent operation of the turbine due to cloud transcience and lack of effective thermal storage.

Solar Two The Next Generation Solar Power Tower The conversion of Solar One to Solar Two required a new moltensalt heat transfer system and a new control system. This includes the receiver, thermal storage, piping, and a steam generator. The Solar One heliostat field, the tower, and the turbine or generator required only minimal modifications

Advantages of Using Molten Salt A variety of fluids were tested to transport the sun's heat,

including water, air, oil, and sodium, before molten salt was selected as best. Molten salt is used in solar power tower systems because it is liquid at atmosphere pressure, it provides an efficient, low-cost medium in which to store thermal energy, its operating temperatures are compatible with todays highpressure and high-temperature steam turbines.  It is non-flammable and nontoxic. In addition, molten salt is used in the chemical and metals industries as a heat-transport fluid, so experience with molten-salt systems exists for non-solar applications.

Design and Construction of the Solar Two Power Tower  The Solar Two power tower is composed of a series of panels,

each made of 32 thin-walled, stainless steel tubes, through which the molten salt flows in a serpentine path. The panels form a cylindrical shell surrounding piping, structural supports, and control equipment.  A black Pyromark™ paint which is robust, resistant to high temperatures and thermal cycling and absorbs 95% of the incident sunlight is used to coat the external surfaces of the tubes. The receiver design has been optimized to absorb a maximum amount of solar energy while reducing the heat losses due to convection and radiation.  The design including laser-welding, sophisticated tube-nozzleheader connections, a tube clip design that facilitates tube expansion and contraction, and non-contact flux measurement devices, allows the receiver to rapidly change temperature without being damaged.

The Salt Mixture A mixture of 60 percent sodium nitrate and 40 percent

potassium nitrate is employed as the salt storage medium. This salt melts at 220ºC and is maintained in a molten state of 290ºC in the ‘cold’ storage tank. It is then traveled through the receiver where it is heated to 565ºC and then on to a ‘hot’ tank for storage. Hot salt is pumped to a steam generating system when power is needed from the plant. These hot salts produce superheated steam for a conventional Rankine-cycle turbine generator system. From the steam generator, the salt is returned to the cold tank where it is stored and eventually reheated in the receiver.

Metal Corrosion in the Molten-Salt Environment

What are the Benefits of Solar Power Towers Like all solar technologies, solar power towers

are fueled by sunshine and do not release greenhouse gases. Solar power towers are unique among solar electric technologies in their ability to efficiently store solar energy and dispatch electricity to the grid when needed, even at night or during cloudy weather.

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. Figure 1.10a shows the 10 MWe Solar One central receiver generating plant at Daggett, California with its adjoining steam power plant. 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.

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. Figure 1.10c shows one parabolic trough from the Kramer Junction, California field shown in Figure 1.1. 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. Figure 1.10b shows six 9kWe parabolic dish concentrators with Stirling engines attached to the receiver at the focus.

Some cases ->> We have some data to show that how much energy is granted by the sun to various countries in various month’s

As an example of the importance of the material variation of insolation over a full, clear day in March at Daggett, California, a meteorological measurement site close to the Kramer Junction solar power plant described previously. The outer curve, representing the greatest rate of incident energy, shows the energy coming directly from the sun (beam normal insolation) and falling on a square meter of surface area which is pointed toward the sun. The peak rate of incident solar energy occurs around 12:00 noon and is 1,030 Watts per square meter. Over the full day, 10.6 kilowatt-hours of energy has fallen on every square meter of surface area as represented by the area under this curve. Over the entire day, 6.7 kilowatt-hours of solar energy fall on every square meter of horizontal surface, of which 0.7 kilowatt-hours comes from all directions other than directly from the sun. . These data files based on long-term actual observations, form the time-dependent database of the computerized performance computations contained within this book and, indeed, much of the solar literature.

An example of a complete set of beam normal insolation data for a given location is shown in Figure. Here we see hourly insolation data, summarized over a day, for each month of a year. With this type of data for a specific site, it is possible to predict accurately the output of a solar energy conversion system, whether it is a low temperature thermal system, a high temperature thermal system or a photovoltaic system. Figure Time and date description of the global, horizontal insolation solar resource for Cairo Egypt. In addition to estimating the amount of energy coming from the sun, the solar designer must also be able to predict the position of the sun. The sun's position must be known to predict the amount of energy falling on tilted surfaces, and to determine the direction toward which a tracking mechanism must point a collector. Chapter 3 discusses the computation of the position of the sun with respect to any given point on the face of the earth. Using only four parameters (latitude, longitude, date and local time), equations are derived to determine the location of the sun in the sky. A characteristic fundamental to the capture of solar energy is that the amount of energy incident on a collector is reduced by a fraction equal to the cosine of the angle between the collector surface and the sun's rays. may be used to predict the fraction of incoming solar energy that falls on the collector. These include situations where the collector is fixed or is tracked about a single axis, no matter what the orientation.

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