To effectively use solar power, solar energy incident on a large area has to be concentrated to get high temperatures. The heat then has to transfer to a working media like steam to produce power. How is this done ? Read on…
The first part is the concentration of the solar power. Current developments are in three methods described below. The second part is storing excess energy for use when the sun is down.
Parabolic Trough.
Using parabolic troughs to concentrate solar radiation is most common in the plants worldwide. Parabolic troughs concentrate the solar radiation on to tubes carrying thermal oil located at the focal line. Hundreds of long parabolic troughs spread over vast areas concentrate the incident solar radiation onto the tubes to produce a continuous flow of hot thermal oil. The Parabolic shape has the advantage of focussing all the incident rays to a focal point irrespective of the direction of the incident ray. Electronic tracking of the mirrors to follow the sun optimizes the incident solar rays throughout the day. This system heats the oil to around 400 °C. The hot oil transfers the heat to the water in a heat exchanger to produce superheated steam.
The steam cycle is almost the same as the normal Rankine cycle in a combined cycle power plants with pumps , de-aerators and extraction heaters. Because of the lower temperature of the steam, the steam cycle efficiency is low.
Solar Tower
Another method under development to concentrate the solar radiation is the solar tower. Hundreds of parabolic mirrors or heliostats reflect the sunlight to a tower. The concentrated heat converts water to steam. Temperatures in the range of 565 °C are achievable by this system. The steam rotates the steam turbines using the conventional Rankine cycle. Higher steam cycle efficiencies are possible in this method due to the higher temperature. The high cost of heliostats , almost 40 % of the power plant cost is one, which makes this system very costly.
Parabolic Dish
Yet another method is the dish system . In this method concentrated solar radiation from parabolic dish incidents on to a coil or tube carrying Helium, Hydrogen or air . A Stirling engine uses these hot gases to rotate a generator to produce power. This is good for stand alone small power generation and has better conversion efficiencies.
Thermal storage
The second part of a Concentrated Solar Thermal power plant is the storage of thermal energy. This can really make the solar power plants as replacements to base load power plants giving a continuous supply of uninterrupted power at least for six seven hours.
Molten salt tanks or other media stores the excess solar heat for recovery when the sun is down. Sodium and Potassium nitrate mixture are the salts that are in use in pilot plants.
The high cost of the mirrors or reflecting surfaces are the ones that make the Concentrated Solar Thermal Systems very costly. Developments in making mirrors from low cost materials should offset the cost in future. In addition, there is the requirement of enormous quantity of water to keep the mirrors clean.
As the technology develops, the unit cost is expected to come down drastically.