The solar power tower (also known 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, movable mirrors (called heliostats) to focus the sun’s rays upon a collector tower (the target). Concentrated solar thermal is seen as one viable solution for renewable, pollution free energy production with currently available technology.
Early designs used these focused rays to heat water, and used the resulting steam to power a turbine. Newer designs using liquid sodium have been demonstrated, and systems using molten salts (40% potassium nitrate, 60% sodium nitrate) as the working fluids are now in operation. These working fluids have high heat capacity, which can be used to store the energy before using it to boil water to drive turbines. These designs allow power to be generated when the sun is not shining.
The central receiver solar concentrator design concentrates sunlight to a receiver mounted on a tower with the use of mirrors. The concentrated sunlight causes the solar central power tower to generate electricity. The receiver is also called a heat exchanger. This system uses hundreds of sun-reflecting mirrors.
The large size of the receivers makes them useful in utility scale applications. The concentrator works by pumping liquid salt at 290 degrees C from a cold storage tank. The salt pumps through the receiver. The receiver heats the salt to 565 degrees C. The salt leaves the receiver and goes to a hot tank for storage. Pumping hot salt through the steam-generating system produces the steam needed for the Rankine-cycle turbine system. As a result, power generates. After receiving enough power, the salt goes back to the cold storage tank.
Solar CRT uses proprietary software to control thousands of tracking mirrors, known as heliostats, to directly concentrate sunlight onto a boiler filled with water that sits atop a tower. When the sunlight hits the boiler, the water inside is heated and creates high temperature steam. Once produced, the steam is used either in a conventional turbine to produce electricity or in industrial process applications. In order to conserve precious water, the steam is air-cooled and piped back into the system in a closed-loop process.
Tracking mirrors, known as heliostats, are highly engineered and designed for accuracy, durability and longevity with minimal maintenance. The heliostat consists of two flat glass mirrors (supported by a lightweight steel support structure) that are mounted on a single pylon equipped with a computer-controlled drive system. This control system enables the heliostats to track the sun in two-directions, maximizing the collection of the sunlight while accurately aiming at the solar receiver. A 130 MW plant may utilize up to 60,000 heliostats, depending on land area and shape, and site-specific considerations. The low-impact design of the heliostat allows solar plant sites to accommodate a slope of up to 5%, avoid areas of sensitive habitat and eliminate the need for the concrete pads used with other solar thermal technologies, reducing the system’s environmental impact.
Solar Field Optimization Software and Control System
A proprietary solar field optimization software is used during the system design phase to determine the optimal position of each heliostat to maximize output and meet the customer’s power production profile. The technology also provides considerable design flexibility, allowing projects to be built on sites with irregular topographies and shape. Using actual site conditions and custom-built meteorological datasets, the software produces precise GPS-ready mappings ready for download to solar field installation crews.
Proprietary heliostat control software system, the Solar Field Integrated Control System (SFINCS), controls the heliostats arrayed in the solar field to track the sun and aim the sunlight onto the receiver. SFINCS performs a number of functions including:
Solar energy management, to focus the ideal amount of solar energy on the receiver at various times of the day to maximize electricity production while ensuring that the solar receiver’s flux and temperature limits are not exceeded.Solar field control, to provide aiming points on the solar receiver surface for each individual heliostat, as well as facilitating start-up and shutdown. Heliostat tracking maintenance, to calibrate the heliostats based on three-dimensional laser scanning and other photogrammetric methods.
At the core of the SFINCS are proprietary algorithms that perform real-time optimization of the distribution of energy across our solar receiver using real-time, heliostat-aiming and closed-loop feedback systems. In addition, SFINCS can automatically configure the heliostats to protect them from inclement weather.
Solar Receiver (Boiler)
The solar receiver is a standard utility-scale industrial boiler designed to be heated from the outside using concentrated solar radiation reflected onto the boiler by the heliostats. The boiler is designed to withstand the rigors of the daily cycling required in a solar power plant over the course of its lifetime, and is treated with a proprietary solar-absorptive coating to ensure that maximum solar energy is absorbed in the steam.
In electricity generation applications, the high-temperature, pressurized steam generated in the solar receiver is piped to a conventional steam turbine generator. The electricity generated is then delivered to the transmission grid for consumption.
In a solar-to-steam application, such as thermal enhanced oil recovery, the process is similar to generating electricity. However, for solar-to-steam applications, saturated steam is piped from the receiver to a heat exchanger to generate the process steam.