Technology

Power Generation By Solar Thermal Concentration

Solar thermal power plants produce electricity in much the same way as conventional power stations. The difference is that they obtain their energy input by concentrating solar radiation and converting it to high temperature steam or gas to drive a turbine or engine. Four main elements are required: a concentrator, a receiver, some form of heat transport media or storage, and power conversion. Many different types of systems are possible, including combinations with other renewable and non-renewable technologies, but the four most promising solar thermal technologies are:

  • Parabolic Trough Concentration System.
  • Central Receiver or Solar Tower System,
  • Parabolic Dish Concentration System.
  • Linear Fresnel Reflectors System.

Out of the above four methods, Parabolic Trough Concentration System is most frequently used for large solar power plants. Parabolic trough-shaped mirror reflectors are used to concentrate sunlight on to thermally efficient receiver tubes placed in the trough focal line. In these tubes a thermal transfer fluid is circulated, such as synthetic thermal oil. Heated to approximately 400°C by the concentrated sun’s rays, this oil is then pumped through a series of heat exchangers to produce superheated steam. The steam is converted to electrical energy in a conventional steam turbine generator, which can either be part of a conventional steam cycle or integrated into a combined steam and gas turbine cycle.

For complete utilization of solar energy, a combined solar power plant along with the water desalination plant would be most appropriate as waste energy in the steam condensation is utilized for water distillation.

Thermal Collector Unit

The metallic reflector material from ReflecTech has been identified. Its reflectivity and useful life cycle are claimed to be superior to glass mirrors. It is preferred to the glass mirrors as they are bulky and there is always a possibility of breakage. This could add to maintenance costs as well as loss of down time. Thermoplastic composite materials (FRP) have been selected to be used as support sheets for ReflecTech film, the suppliers for which have been identified.
Suppliers for receiver vacuum tube as well as the receiver tube design and its material are also short-listed.
Solar tracking system for the thermal collector also has been identified and the suppliers have been short-listed. The prices have to be called for. It is proposed to be either single axis or two axis tracking system. The later one will be preferred for its better efficiency.
The design and fabrication of the tracking mechanism could be done in-house and help from vendor workshops could be sought. Design, fabrication of components, working fluid conveying system and the assembly of the unit could be completed and trials carried out within three months after the receipt of the materials. It is proposed to make the Thermal Collector Unit of area 10m x 60m mainly for the economy of scale.

Pumping, Storage And Piping System For Thermic Fluid:

Different materials are available for the piping and storage of heat transfer fluid. Carbon alloy steel with high temperature resistance could be used for this purpose as it is cheaper in cost compared to stainless steel. However, alloy steels have better temperature resistance as well as longer life but are more expensive. Combination of materials could be used for optimum costing without compromising on the quality and for longer life.
Generally, the media used for this purpose are thermic fluid oils, molten salts, molten metals and solids like stones etc. Only thermic fluid oils remain in liquid state at ambient temperatures and are suitable for transfer of heat by pumping through heat exchangers at any temperature. The salts like nitrates of Sodium and Potassium and their combinations, solidify below 250ºC but this temperature could be lowered by addition of small quantities of other salts. Solids like stones; metals etc. can be used for storage only as cheaper alternatives. Thermic fluid oils are very expensive and, since large quantities are required in Solar Thermal Collection Power Plants, cheaper alternatives like combination of nitrates of Sodium and Potassium are used as heat transfer media. However, they must be used above 250ºC in order to keep them in fluid condition. In practice, all the above-mentioned media are used in combination to optimize the costs and operating expenses.  
It is proposed that molten salts may be used as medium for high temperature uses like superheating of steam. For steam generation, thermic fluid oils may be used as medium and for thermal storage; a combination of thermic fluid and stones could be used in order to optimize the costs.
The material used for construction of the storage tanks as well as pipelines for conveying the working fluids must have the qualities to withstand high temperatures. Special carbon steels are available which can withstand temperatures up to 500ºC. Alloy steels and some types of stainless steels can be used for higher temperature applications.
A good insulation is required for storage tanks, pipelines as well as heat exchangers in order to minimize the heat wastage and to improve the overall efficiency of the plant.

Steam Generating Plant

Superheated steam at pressures from 60 bar to 100 bar and superheat temperatures up to 450ºC is required for operation of turbine operated electrical generators. Special heat exchangers to generate steam of required quality are required to be provided for uninterrupted supply. The design of these heat exchangers can be made in house and the fabrication work could be outsourced to some reputed industry. The equipment has to be procured under third party inspection.
The steam pipelines have to be kept as short as possible and must be properly insulated to minimize the heat loss. The turbo-generators and steam condensing equipment have to be properly insulated.
The heat in the exhaust steam could be utilized to fresh water from seawater if available nearby or, from underground brackish water. Thus, not only the fresh water requirements for the power plant can be met from the water generated but also extra water could be supplied to the nearby cities.

Tidal Energy

The technology required to convert tidal range energy into electricity is very similar to the technology used in traditional hydroelectric power plants. It requires building up a reservoir of certain head that will be continuously filled by turbine pumps operated by movement of large mass of water during the rising and ebbing tide. The water in the reservoir can be used to operate hydro-generators.
A large number of hydro-turbine operated pumps can be installed in a turbine farm in coastal areas where there is considerable difference in low and high tide, in order to collect water in the reservoir continuously by turbine pumps using rising and ebbing tide. Coastal areas with deep canals can be selected for this purpose. In India, Gujarat coast is an ideal location for this purpose.

Marine underwater currents technology is an alternative method to harness tidal energy from sea. Meeting points of rivers into sea or, entry to certain bays along the coast can be selected where under water currents exist. Cambay on the Gujarat coast is ideal location to harness ocean energy by this method. A farm of large number of hydro-turbine can be installed to directly produce electricity. Alternatively, water turbine operated pumps could used to fill the reservoirs for hydro-generators.

In both the above alternatives, the main equipment is the hydro-turbines operated by the moving mass of water that could be used to generate electricity. The equipment is readily available all over the world. The equipment may be installed underwater and it can be designed to have minimum or no impact on sea creatures and the environment. The maintenance of the equipment will be cumbersome but the same can be designed to require minimum maintenance in the easier manner.

Ocean Current Energy

What is ocean current energy?

The massive oceanic surface currents of the world are untapped reservoirs of energy. Their total energy flux has been estimated at 280 trillion watt-hours. Because of their link to winds and surface heating processes, the ocean currents are considered as indirect sources of solar energy. If the total energy of a current was removed by conversion to electric power, these currents would cease to exist; but only a small portion of any ocean current's energy can be harnessed, owing to the current's size.

Why use current energy?

One of the primary advantages of this technology is the energy density. While solar and wind systems are well-suited for remote off grid locations, ocean energy is ideal for large-scale developments in the multiple gigawatt range. Seawater is 832 times as dense as air, providing 5 knots ocean current with more kinetic energy than a 350 km/h wind.

Already developed equipment is available in the market all over the world. The system has to be designed according to the available natural resources.

Waves Energy:

Sea waves are created due to high velocity winds along the surface of sea along the coast as well as in deep waters. The sea waves have large amounts of kinetic energy, which gets dissipated along the surface of seawater over long distances due to friction. This energy can be utilized to compress air using huge water body compression and the electricity can be produced by compressed air operated turbo-generators. The state-of-the-art equipment for this could be installed off shore to generate electricity.

Sea waves are created due to high velocity winds along the surface of sea along the coast as well as in deep waters. The sea waves have large amounts of kinetic energy, which gets dissipated along the surface of seawater over long distances due to friction. This energy can be utilized to compress air using huge water body compression and the electricity can be produced by compressed air operated turbo-generators. The state-of-the-art equipment for this could be installed off shore to generate electricity.
Alternatively, the kinetic energy of waves can be used to pump water using float operated reciprocating pumps into a reservoir from which, a hydro-generator can be operated to produce electricity.
The equipment for the compressed air method could be floating/fixed type with compressors as well as the electrical generators being placed on floating/fixed platforms or in enclosed vessels. This way the maintenance problems can be minimized.

Designs for both the above types of equipments developed in-house are available indigenously taking into consideration the harsh sea conditions prevailing at times for the tidal energy but for wave energy harnessing, the conditions will always be harsh which have to be taken into considerations while designing the equipment in order to have trouble free electricity generation.

Why Energy from Oceans?

While lagging behind wind and solar in commercial development, ocean wave power is a more promising resource than either:
  • Because waves originate from storms far out to sea and can travel long distances without significant energy loss, power produced from them is much steadier and more predictable, both day to day and season to season. This reduces project risk;
  • Wave energy contains roughly 1000 times the kinetic energy of wind, allowing much smaller and less conspicuous devices to produce the same amount of power in a fraction of the space;
  • Unlike wind and solar power, power from ocean waves continues to be produced around the clock, whereas wind velocity tends to die in the morning and at night, and solar is only available during the day in areas with relatively little cloud cover;
  • Wave power production is much smoother and more consistent than wind or solar, resulting in higher overall capacity factors;
  • Wave energy varies as the square of wave height, whereas wind power varies with the cube of air speed. Water being 850 times as dense as air, this results in much higher power production from waves averaged over time;
  • Estimating the potential resource is much easier than with wind, an important factor in attracting project lenders;
  • Because wave energy needs only 1/200 the land area of wind and requires no access roads, infrastructure costs are less;
  • Wave energy devices are quieter and much less visually obtrusive than wind devices, which typically run 40-60 meters in height and usually requiring remote siting with attendant high transmission costs. In contrast, 10 meter high wave energy devices can be integrated into breakwaters in busy port areas, producing power exactly where it is needed;
  • When constructed with materials developed for use on off-shore oil platforms, ocean wave power devices (which contain few moving parts) should cost less to maintain than those powered by wind.

Energy From Oceans:

Advantages of Energy from Oceans:

The oceans of the world cover about 71% of the earth’s surface, which is a lot of resource to generate energy from. Tides, currents and waves are predictable, renewable sources of energy. Collecting energy from them has a minimal impact on the environment because they do not emit greenhouse gases during generation. Because of the consistent nature of ocean energy, some researchers think ocean energy can be relied on to provide power when other energy sources can’t. For example, hydro energy generation is hampered by low rainfall and wind turbines function less effectively in calm weather. In addition, there are increasing shortages of non-renewable resources, such as coal and natural gas.

Disadvantages of Energy from Oceans:

Ocean energy uses waterways to generate electricity, as well as the open ocean and therefore building and using them can have effects on the natural eco-system and aquatic life. Ocean energy technology is currently expensive to install, but costs are expected to fall in the future.

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