How Does a Solar Thermal Power Plant Work?
The operation of solar thermal power plants is based on obtaining heat from solar radiation and transferring it to a heat carrier medium, generally water.
A set of solar collectors reflects solar radiation to one point to heat a fluid. In this way, the liquid reaches temperatures of 300ºC to 1000ºC, which will serve to generate steam using a heat exchanger.
Afterward, we can use the steam generated to drive steam turbines converting thermal energy into mechanical energy. Finally, the turbines are connected to an electrical generator to produce electricity.
The higher the temperature, the higher the performance of the solar thermal power plant.
Concentrating Solar Thermal Systems
The capture and concentration of solar rays are done using mirrors with an automatic orientation that point to a central tower where the fluid is heated. In addition, some solar thermal power plants use smaller parabolic geometry mechanisms such as parabolic trough solar collectors.
The set of the reflecting surface and its orientation device is called a heliostat.
Thermodynamic Cycles Used
There are several thermodynamic flows and cycles used in the experimental setups.
Wide other varieties have also been made, such as the Stirling engine. However, the most used cycles are those that combine solar thermal energy with natural gas.
Types of Solar Thermal Power Plants
There are different solar power generation technologies for solar thermal power plants. We have different types of solar thermal power plants depending on the technology used. These are the main ones:
The Solar Thermal Plant with Parabolic Troughs
This technology uses concave mirrors mounted in a channel to direct the sun's rays into the fluid.
Mirrors concentrate the sun's rays received on a pipe through which a heat transfer fluid circulates. This fluid acquires all the solar energy and is transported to an exchanger. There it will transfer the thermal energy for its transformation. This process will be for the generation of electrical power, or it will be stored for later use.
Both processes are part of this type of solar thermal power plant.
Concave mirrors are called parabolic trough collectors. These are mounted on a structure capable of rotating them. This system's objective is to follow the solar path throughout the maximum possible solar hours at the location of the solar power plant.
Currently, the parabolic trough solar thermal power plant is the most widespread type worldwide.
Heliostats with a Central Solar Power Tower Receiver
It requires a large area where heliostats are placed and distributed on the plant's surface.
Heliostats are large surface mirrors mounted on a structure. They can rotate in two directions. The objective is to capture the sun's rays following the sun's path and reflecting the sun's rays at a specific point. This reflection goes to a central tower of the installation.
Due to this phenomenon, we can concentrate at a high temperature at the same point.
This technology presents a great difficulty. Each heliostat has a different location concerning the focus of the tower.
Linear Fresnel Reflectors
Solar thermal power plants with linear Fresnel reflectors mount flat mirrors on a structure. In such a way, the mirror's rotation in an axis is allowed to follow the solar path and focus the capture on the conduit that contains the heat transfer fluid.
This type of solar thermal power plant is under development, and better results are expected. It is based on the experiences of parabolic trough plants.
What Is the Efficiency of a Thermoelectric Power Plant?
The efficiency of a concentrating solar power system will depend on the following factors:
The technology we choose to convert solar energy into electrical energy.
The operating temperature of the heat sink.
There are some thermal losses in the system.
The presence or absence of other system losses
In addition to conversion efficiency, the optical system concentrating sunlight also adds additional losses.
The maximum conversion efficiency for "power tower" type systems, operating at temperatures of 250-565 degrees Celsius, is 23-35%. With a combined cycle turbine, the efficiency is higher.
The efficiency of dish Stirling systems, which operate at temperatures of 550-750 degrees Celsius (277 to 477 degrees Kelvin), offer an efficiency of around 30%.
Due to the variation in the incidence of the sun during the day, the average conversion efficiency achieved is not equal to these maximum efficiencies.
Annual net efficiencies are around 7-20% for solar power tower systems and 12-25% for Stirling dish systems.
What Environmental Effects Do Thermal Solar Power Plants Cause?
Generating electrical energy through a solar thermal power plant has a series of environmental effects, particularly in:
The high use of water.
The large occupied surfaces.
The use of hazardous materials.
It has some damaging effects on birds.
Water is generally used to cool and clean mirrors. Cleaning agents (hydrochloric acid, sulfuric acid, nitric acid, hydrogen fluoride, acetone, and others) are also used for cleaning reflective surfaces.
Some projects are looking at various approaches to reduce water use and cleaning agents, including barriers, non-stick coatings on mirrors, water mist systems, and others.
Effects on Wildlife and Birds
Insects can be attracted to bright light caused by the concentration of solar radiation. The result is that insect-eating birds get close and can be burned if they fly close to the point where the light is focused.
This problem also affects the birds of prey that hunt the birds.