Nickel-based alloys and nickel-containing stainless steels play a key role in a new type of renewable energy called solar thermal power or concentrated solar power (CSP).
The use of these materials has allowed the industry to overcome challenges in heat transfer and storage technology and, for projects with a design life of 40 years or more, can help reduce material degradation or replacement costs.
According to the International Energy Agency (IEO 2017), between 2015 and 2040, the consumption of renewable energy (including CSP) will grow at an annual rate of 2.3%. There are currently more than 40 solar thermal power plants worldwide, and more than 20 such plants are in the planning or construction stage. They tend to be located in areas with high solar irradiance, such as Spain, India, South Africa, China, Chile, Australia, the Middle East and North Africa (MENA) and the southern United States.
Demonstration CSP power plants were in operation as early as the 1980s. Since then, much progress has been made in energy harvesting and storage. The solar tower is a newly developed concentrating technology in which molten salt is used as the heat transfer fluid. The melting temperature of the nitrate mixture is generally 130°C (268°F) or above. They remain in liquid form in insulated storage tanks at temperatures of 288°C (550°F). The liquid salt is then pumped through a pipe into a solar receiver and heated by concentrated radiation to temperatures as high as 566°C (1,050°F). The high temperature liquid is then fed into a high temperature storage tank. The high temperature storage tank is also insulated and can store thermal energy for a long time.
Thermal Electricity Storage (TES) is used to compensate for changes in demand and environmental conditions. The molten salt is pumped into a steam generator as needed to generate steam to drive conventional turbines and generators. TES has unique advantages over other large-scale renewable energy sources, and in some cases does not even require backup power generation fuel. In the summer of 2013, a system of molten salt towers in Spain produced continuous electricity 24 hours a day for 36 days - the first time ever.
Crescent Dunes is one such power station in operation. Located in Tonopah, Nevada, in the desert north of Las Vegas, it has a net generation capacity of 110 megawatts and a TES time of 10 hours. This means that the power station can generate electricity at full capacity for up to 10 hours during peak demand periods. More than 10,000 heliostats reflect sunlight and concentrate the energy at a receiver on top of a 200-meter (640-foot) tower. Each heliostat consists of small mirrors with a total area of 115.7 square meters (1,245 square feet). The total light collecting area exceeds 1.2 million square meters (12 million square feet). Since its commercial launch in 2015, Crescent Dunes has generated more than 173 gigawatts of electricity, enough to meet the peak electricity demand of an estimated 75,000 homes. Solar towers and molten salt CSPs form the design basis for new planned projects in South Africa, Australia and Nevada. The 100 MW Redstone project in South Africa will meet the peak electricity demand of about 200,000 households with a TES time of 12 hours. A massive new project in Nevada has 10 solar towers with a generating capacity of 2 gigawatts (2,000 megawatts).
The use of stainless steel and nickel alloys has made high temperature systems possible, where handling molten salts was previously a challenge. Designers and engineers started making receiver tubes from nickel-based alloys such as UNS N06617, N06625 and N06230 because they needed to maintain high-temperature strength for extended periods of time -- a property known as creep resistance. These alloys are stable at operating temperatures mainly because of their high nickel content and their resistance to oxidation.
Type 347H stainless steel (S34709) is used for high temperature storage tanks.
As of 2016, the total installed capacity of CSPs worldwide reached 4,815 MW. In 2017, Spain had 2,300 MW of installed capacity in operation, almost half of the world's total. The United States has an installed capacity of 1,740 MW and has two of the largest projects in the world: the Avenpa Solar Power Station (392 MW) and the Mojave Solar Project (354 MW).
Crescent Dunes, located in the desert north of Las Vegas, Nevada, has a net power generation capacity of 110 megawatts and a TES time of 10 hours. This means that the power station can generate electricity at full capacity for up to 10 hours during peak demand periods.
Solar tower concentrating technology
1. Sunlight is concentrated through a large field of heliostats and reflected to a receiver at the top of a 195-meter (640-foot) tower; 2. Liquid salt from the cold salt tank is pumped into the receiver and heated to 566°C (1050°F); 3. Heated salt in the receiver is stored in a hot salt tank; 4. The hot salt is pumped into the hot salt tank through the steam generator to form steam, which drives the steam turbine to generate electricity; 5. The cold salt at 288°C (550°F) flows back to the cold salt tank; 6. The condensed steam from the steam turbine is recycled repeatedly.
