SOLAR POWER TOWER | Working ,HELIOSTAT ,RANKINE CYCLE

SOLAR POWER TOWER

1. INTRODUCTION

            The solar power tower (also knows as 'Central Tower' power plants or 'Heliostat' power plants or power towers) is a type of solar furnace using a tower to receive the focused sunlight. It uses an array of flat, moveable mirrors (called heliostats) to focus the sun's rays upon a collector tower (the target). The high energy at this point of concentrated sunlight is transferred to a substance that can store the heat for later use. The more recent heat transfer material that has been successfully demonstrated is liquid sodium. Sodium is a metal with a high heat capacity, allowing that energy to be stored and drawn off throughout the evening. That energy can, in turn, be used to boil water for use in steam turbines. Water had originally been used as a heat transfer medium in earlier power tower versions (where the resultant steam was used to power a turbine). This system did not allow for power generation during the evening.

1.1 OVERVIEW

              To date, the largest power towers ever built are the 10 MW Solar One and Solar Two plants. Assuming success of the Solar Two project, the next plants could be scaled-up to between 30 and 100 MW in size for utility grid connected applications in the Southwestern United States and/or international power markets. New peaking and intermediate power sources are needed today in many areas of the developing world. India, Egypt, and South Africa are locations that appear to be ideally suited for power tower development. As the technology matures, plants with up to a 400 MW rating appear feasible. As non-polluting energy sources become more favored, molten-salt power towers will have a high value because the thermal energy storage allows the plant to be dispatch able. Consequently, the value of power is worth more because a power tower plant can deliver energy during peak load times when it is more valuable. Energy storage also allows power tower plants to be designed and built with a range of annual capacity factors (20 to 65%). Combining high capacity factors and the fact that energy storage will allow power to be brought onto the grid in a controlled manner (i.e., by reducing electrical transients thus increasing the stability of the overall utility grid); total market penetration should be much higher than an intermittent solar technology without storage. One possible concern with the technology is the relatively high amount of land and water usage. This may become an important issue from a practical and environmental viewpoint since these plants are typically deployed within desert areas that often lack water and have fragile landscapes. Water usage at power towers is comparable to other Rankine cycle power technologies of similar size and annual performance. Land usage, although significant, is typically much less than that required for hydro [3] and is generally less than that required for fossil (e.g., oil, coal, natural gas), when the mining and exploration of land are include

1.2 SYSTEM DESCRIPTION                                 

   Solar power towers generate electric power from sunlight by focusing concentrated solar radiation on a tower-mounted heat exchanger (receiver). The system uses hundreds to thousands of sun-tracking mirrors called heliostats to reflect the incident sunlight onto the receiver. These plants are best suited for utility-scale applications in the 30 to 400 MWe range. In a molten-salt solar power tower, liquid salt at 290ºC  is pumped from a ‘cold’ storage tank through the receiver where it is heated to 565ºC  and then on to a ‘hot’ tank for storage. When power is needed from the plant, hot salt is pumped to a steam generating system that produces superheated steam for a conventional Rankin 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. Figure 1 is a schematic diagram of the primary flow paths in a molten-salt solar power plant. Determining the optimum storage size to meet power-dispatch requirements is an important part of the system design process. Storage tanks can be designed with sufficient capacity to power a turbine at full output for up to 13 hours.

The heliostat field that surrounds the tower is laid out to optimize the annual performance of the plant. The field and the receiver are also sized depending on the needs of the utility. In a typical installation, solar energy collection occurs at a rate that exceeds the maximum required to provide steam to the turbine. Consequently, the thermal storage system can be charged at the same time that the plant is producing power at full capacity. The ratio of the thermal power provided by the collector system (the heliostat field and receiver) to the peak thermal power required by the turbine generator is called the solar multiple.

  

2. RANKINE CYCLE

There are four processes in the Rankine cycle, each changing the state of the working fluid. These states are identified by number in the diagram above.

• Process 4-1: First, the working fluid is pumped (ideally isentropic ally) from low to high pressure by a pump. Pumping requires a power input (for example mechanical or electrical).
• Process 1-2: The high pressure liquid enters a boiler where it is heated at constant pressure by an external heat source to become a superheated vapor. Common heat sources for power plant systems are coal, natural gas, or nuclear power.
• Process 2-3: The superheated vapor expands through a turbine to generate power output. Ideally, this expansion is isentropic. This decreases the temperature and pressure of the vapor.
• Process 3-4: The vapor then enters a condenser where it is cooled to become a saturated liquid. This liquid then re-enters the pump and the cycle repeats.   

  Rankine cycles describe the operation of steam heat engines commonly found in power generation plants. In such vapor power plants, power is generated by alternately vaporizing and condensing a working fluid (in many cases water, although refrigerants such as ammonia may also be used).

The working fluid in a Rankine cycle follows a closed loop and is re-used constantly. Water vapor seen billowing from power plants is evaporating cooling water, not working fluid.

                                         

3.HELIOSTAT

It comes from Helios, the Greek word for sun and stat, as in stationary.

             A Heliostatis a device that tracks the movement of the sun. It is typically used to orient a mirror, throughout the day, to reflect sunlight in a consistent direction. When coupled together in sufficient quantities, the reflected sunlight from the heliostats can generate an enormous amount of heat if all are oriented towards the same target. It was originally developed as an instrument for use in surveying, allowing the accurate observation of a known point from a distance. Heliostats have been used for sunlight-powered interior lighting, solar observatories, and solar power generation. Mirrors and reflective surfaces used in solar power that do not track the sun are not heliostats.

             The simplest heliostat devices use a clockwork mechanism to turn the mirror in synchronization with the rotation of the Earth. More complex devices need to compensate for the changing elevation of the Sun throughout a Solar year. Even more advanced heliostats track the sun directly by sensing its position throughout the day.

        The heliostat reflects the sunlight onto the transmission system. This is typically a set of mirrors that direct the reflected sunlight into the building or, alternatively, a light tube. Fiber optic cabling has also been used as a transfer mechanism. Various forms of commercial products have been designed for the point of termination (the "light bulb").

4. WORKING of solar tower

            Solar power towers consist of a large field of sun-tracking mirrors, called heliostats, which focus solar energy on a receiver atop a centrally located tower. The enormous amount of energy, coming out of the sun rays, concentrated at one point (the tower in the middle), produces temperatures of approx. 550°C to 1500°C. The gained thermal energy can be used for heating water or molten salt, which saves the energy for later use.Heatened water gets to steam, which is used to move the turbine-generator. This way thermal energy is converted into electricity.

            As already mentioned there are two main fluids which are used for the heat transfer, water and molten salt. Water for example is the oldest and simplest way for heat transfer. But the difference is that the method in which molten salt is used, allows storing the heat for the terms when the sun is behind clouds or even at night. Molten salt - better: the heat of it - can be used until the next dawn when the sun will be back to heat the cooled down salt again.

            The molten salt consists of 60% sodium nitrate a 40% potassium nitrate (saltpeter). The salt melts at about 700°C and is liquid at approx. 1000°C; it will be kept in an insulated storage tank until the time, when it will be needed for heating up the water in the steam generator. His way of energy storage has an efficiency of approx. 99%, i.e. due to the imperfect insulation 1% of the stored energy gets lost.

5. SOLAR ONE

    Solar One, which operated from 1982 to 1988, was the world’s largest power tower plant. It proved that large-scale power production with power towers was feasible. In that plant, water was converted to steam in the receiver and used directly to power a conventional Rankine-cycle steam turbine. The heliostat field consisted of 1818 heliostats of 39.3 m reflective area each. The project met most of its technical objectives by demonstrating

                       (1) The feasibility of 2 generating power with a power tower

                       (2) the ability to generate 10 MW for eight hours a day at summer solstice and  four hours a day near winter solstice.

              During its final year of operation, Solar One’s availability during hours of sunshine was 96% and its annual efficiency was about 7%. The Solar One thermal storage system stored heat from solar-produced steam in a tank filled with rocks and sand using oil as the heat-transfer fluid. The system extended the plant’s power-generation capability 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 storage system was complex and thermodynamically inefficient. While Solar One successfully demonstrated power tower technology, it also revealed the disadvantages of a water/steam system, such as the intermittent operation of the turbine due to cloud transience and lack of effective thermal storage. During the operation of Solar One, research began on the more advanced molten-salt power tower design described previously. This development culminated in the Solar Two project.

                                         

6. SOLAR TWO

            The goals of the redesigned plant, called Solar Two, are to validate nitrate salt technology, to reduce the technical and Economic risk of power towers, and to stimulate the commercialization of power tower technology. Solar Two has produced 10 MW of electricity with enough thermal storage to continue to operate the turbine at full capacity for three hours after the sun has set. Long-term reliability is next to being proven. The conversion of Solar One to Solar Two required a new molten-salt heat transfer system including the receiver, thermal storage, piping, and a steam generator and a new control system. The Solar One heliostat field, the tower, and the turbine/generator required only minimal modifications. The Solar Two receiver comprises 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. The external surfaces of the tubes are coated with a black Pyromark paint that is robust, resistant to high temperatures and thermal cycling, and absorbs 95% of the incident sunlight. 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, which includes laser-welding, sophisticated tube-nozzle-header 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 Solar Two is designed with a minimum number of casketed flanges and most instrument transducers, valves, and fittings are welded in place.

            The energy storage system for Solar Two consists of two 875,000 liter storage tanks which were fabricated on-site by Pitt-Des Moines. The tanks are externally insulated and constructed of stainless steel and carbon steel for the hot and cold tanks, respectively. Thermal capacity of the system is 110 MWh. A natural convection cooling system is used t in the foundation of each tank to minimize overheating and excessive dehydration of the underlying soil. All pipes, valves, and vessels for hot salt were constructed from stainless steel because of its corrosion resistance in the molten-salt environment. The cold-salt system is made from mild carbon steel. The steam generator system (SGS) heat exchangers, which were constructed by ABB Lummus, consist of a shell-and-tube super heater, a kettle boiler, and a shell-and-tube preheater. Stainless steel cantilever pumps transport salt from the hot-tank-pump sump through the SGS to the cold tank. Salt in the cold tank is pumped with multi-stage centrifugal pumps up the tower to the receiver. Solar Two is expected to begin routine daily power production in late 1997. Initial data collected at the plant show that the molten-salt receiver and thermal storage tanks should perform as predicted during.

7. LAND, WATER, AND CRITICAL MATERIALS REQUIREMENTS

  The land and water use values provided in Table 4 apply to the solar portion of the power plant. Land use in 1997 is taken from Solar Two design documents. Land use for years 2000 and beyond is based on systems studies. The proper way to express land use for systems with storage is ha/MWh/yr. Expressing land use in units of ha/MW is meaningless to a solar plant with energy storage because the effect of plant capacity factor is lost. Water use measured at the SEGS VI and VII [20] trough plants form the basis of these estimates. Wet cooling towers are assumed. Water usage at Solar Two should be somewhat higher than at SEGS VI and VII due to a lower power block efficiency at Solar Two (33% gross). However, starting in the year 2000, water usage in a commercial power tower plant, with a high efficiency power block (42% gross), should be about 20% less than SEGS VI and VII. If adequate water is not available at the power plant site, a dry condenser-cooling system could possibly be used. Dry cooling can reduce water needs by as much as 90%. However, if dry cooling is employed, cost and performance penalties are expected to raise level zed-energy costs by at least 10%.

                                

8. SOLAR POWER APPLICATIONS

 Several kinds of very practical solar energy systems are in use today. The two most common are passive solar heated homes (or small buildings), and small stand-alone photovoltaic (solar electric) systems. These two applications of solar energy have proven themselves popular over a decade of use. They also illustrate the two basic methods of harnessing solar energy: solar thermal systems, and solar electric systems. The solar thermal systems convert the radiant energy of the sun into heat, and then use that heat energy as desired. The solar electric systems convert the radiant energy of the sun directly into electrical energy, which can then be used as most electrical energy is used today

REFERENCES

1) www.wikepedia.org

2) www.solar.org

SEMINAR TOPIC FROM :: www.edufive.com/seminartopics.html

3) Solar power engineering By B.S Magal

4) Solar energy fundamentals and applications By H P Garg and J Prakash

5) Renewable energy sources and their environmental impact By S A Abbasi                                                    and Naseema Abbasi