Chapter 6SOLAR POWER TOWER 6.1 Abstract:The largest power towers ever built are the 30 MW (Solar One and Solar Two plants). The next plants could be assumed to be scaled-up to between 30 and 400 MW. As non-polluting energy sources become more preferable, molten-salt power towers will have features which include high value as the thermal energy storage gives the plant the benefit of being dispatch able.A power tower plant can deliver energy during peak load times when it is more valuable which explains why the value of power is worth more.Unfortunately the negatives of this technology are the high water usage as well as large land usage.6.2 History of Solar Power TowerSolar technology has been around since centuries ago. Its history started from the 7th Century B.C. to today. Utilization of solar energy started out by concentrating the sun’s heat with glass and mirrors to light fires. Now, we have far more improved and developed technologies everything from solar-powered buildings to solar powered vehicles. 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 possible. During the operation of Solar One, research began on the more advanced molten-salt power tower design described. This development culminated in the Solar Two project. Solar Two, which is currently going through its startup phase, will generate (in addition to electric power) information on the design, performance, operation and maintenance of molten-salt power towers.6.3 The Processing: Solar Power Tower Solar power towers generate electric power from sunlight byFocusing concentrated solar radiation on a tower-mounted heat exchanger (receiver).1) It uses an array of flat, moveable mirrors (called heliostats) toFocus the sun’s rays on the collector tower.2) The high energy of concentrated sunlight is transferred to a substance which stores the heat.3) Liquid sodium is one of the latest heat transfer material that has been successfully demonstrated.4) Sodium is used as it is a metal with a high heat capacity, which is capable of allowing that energy to be stored and drawn off throughout the evening.5) Stored energy can be used to boil water for use in steam turbines.6.4 Solar One – The First Generation of Power Tower PlantSolar One was the world’s largest power tower plant, which operated from 1982 to 1988. The Solar One thermal storage system operates by storing heat which is generated from steam by 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 increases the power generation capability of the plant at night and provided heat for generating low-grade steam for keeping parts of the plant warm during off-hours and for morning startup.The main cons for solar one were that it was complex to operate 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 transience and lack of effective thermal storage.6.5 Solar Two – The Next Generation Solar Power TowerThe solar two project was an expansion of the solar one project with added improvements such as increasing the efficiency and reducing the risks technically and economically. These improvements were made by adding nitrate salt receiver, salt storage system, salt steam generator, and a new master control system to the existing Solar One heliostat field, receiver tower, turbine-generator, and balance-of-plant. The risks were reduced as the nitrate salt receiver’s technical characteristics were authenticated along with the technology system. In addition more heliostats were added to the collector field which in turn increased the reflective area making the morning start time shorter and giving more energy to the thermal storage system. 6.6 Design and Construction of the Solar Two Power TowerThe Solar Two power tower is composed of a series of panels, each made of thin walled, stainless steel tubes. Molten salt flows through the stainless steel tubes in a sinuous path. The panels form a shell around the piping, structural supports, and control equipment. For high temperature resistance and high absorbance of the incident light a black paint which is robust, 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 also reducing the heat losses due to convection and radiation.To rapidly change the temperature in the receiver without being damaged, the construction has been designed to include non-contact measurement devices laser welding, tube nozzle header connections as well as a tube clip design which makes the tube expansion and contraction easier. The Salt MixtureThe salt storage medium is primarily made up of a mixture of 60 percent sodium nitrate and 40 percent potassium nitrate. The melting point for this salt is 220ºC and it is maintained in a molten state of 290ºC in the cold storage tank. It then travels through the receiver where it is heated to 565ºC and then on to a hot tank for storage.Hot salt is then pumped to a steam generating system when power is required from the plant. The Rankin-cycle turbine is generated by the superheated steam which is produced from the hot salts. After coming from the steam generator, the salt is returned to the cold tank where it is stored and eventually reheated in the receiver.6.7 Advantages of Using Molten SaltMolten 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 high-pressure and high-temperature steam turbines, and it is non-flammable and nontoxic. In addition, molten salt is used in the chemical and metals industries as a heat-transport fluid.6.8 Molten-Salt Environment and Metal corrosion In order for us to prevent corrosion of metal the pipes, valves and vessels for hot salt were designed and constructed from stainless steel as it has a high resistance to corrosion in the molten salt environment. Chapter 7Conclusion The sun’s radiation withholds an immense amount of energy that can be utilized in many ways for a number of applications. The capturing of solar energy into electricity is the most crucial and substantial way of exploiting the suns energy. As there is a growing exponential demand for safer and more environmentally friendly sources of energy. Throughout the years scientists, engineers and experts have been designing and creating devises that meet our needs and convert solar energy into useful form. The undeniable growth and development of the production of solar plants and devices have noticeably contributed in popularizing the significance of utilizing the sun’s energy, as these devices have become more and more convenient and safer to use for domestic purposes. Thus, expanding the market and diminishing the dependency on fossil fuels. Solar power tower has stood out amongst the crowd as it proved to be a hugely efficient and easy to operate the plant. Furthermore, it is less costly than most of the major solar plants. 7.2 Recommendations for Future WorkIn the constant unstoppable development of solar energy there are also things that we shouldn’t look past. As there will always be ways we can further improve. These improvements should include:Increasing the power-conversion efficiency 2. Reducing the amount of material needed per cell. Thinner, more flexible films and substrates could reduce cell weight and cost for photovoltaic cells.3. Reducing the complexity and cost of manufacturing of solar plants.