Solar thermal power generation is a technology that converts sunlight into heat energy using specialized collectors, which then drives a thermal cycle to produce electricity. It plays a vital role in the broader field of solar energy utilization. Since the 1980s, countries such as the United States, Europe, and Australia have developed various demonstration systems, contributing significantly to the advancement of this technology. Globally, there are three main types of solar thermal power systems: parabolic trough systems, tower systems, and dish systems.
**Classification of Solar Thermal Power Generation Systems**
1. **Trough Line Focusing System**
This system uses parabolic trough mirrors to concentrate sunlight onto a central tube, heating the fluid inside. The heated fluid is then used to generate steam in a heat exchanger, which powers a conventional turbine to produce electricity.
2. **Tower System**
In this system, a large number of heliostats (sun-tracking mirrors) focus sunlight onto a receiver located at the top of a central tower. This creates extremely high temperatures, which can be used for power generation or heat storage.
3. **Dish System**
The dish system employs a parabolic mirror that focuses sunlight onto a receiver at its focal point. This setup often uses a Stirling engine to convert the heat into mechanical energy, which is then transformed into electricity.
**Comparison of Three System Performance**
Among these systems, only the trough line focusing system has been commercialized so far. The other two are still in the demonstration phase but show great potential for future commercial use. All three systems can operate using only solar energy or in combination with other fuels, offering flexibility in different conditions.
To dive deeper, let's explore the **trough solar thermal power generation system**, one of the most widely used technologies today.
**Trough Solar Thermal Power Generation System**
The trough system, also known as the parabolic trough system, consists of long rows of curved mirrors that focus sunlight onto a pipe running along the focal line. This pipe contains a heat transfer fluid, typically oil or water, which is heated to around 350–390°C. The heated fluid is then used to generate high-pressure steam, which drives a turbine connected to an electric generator.
**Working Principle of Trough Solar Thermal Power Generation System**
A parabolic trough mirror, which curves in only one direction, focuses sunlight onto a central tube positioned along its focal line. The heat transfer medium inside the tube is heated to high temperatures, and this heat is transferred to water through a heat exchanger, producing superheated steam. This steam then powers a conventional steam turbine, generating electricity.
**Technologies Used in Trough Systems**
1. **Medium Temperature Technology**
This method uses heat transfer oil or molten salt as the working fluid, operating at temperatures below 400°C. The heat is exchanged to produce steam for power generation.
2. **High-Temperature Technology**
Here, a mixture of nitrates is used as the heat carrier, allowing operation at temperatures up to 550°C. This enhances efficiency and reduces the need for additional fuel.
3. **DSG Direct Steam Generation**
Instead of using oil, water is directly heated in the collector tubes to generate steam, eliminating the need for a separate heat exchanger and improving system efficiency.
Depending on the technology used, the system can be designed as either a single-loop or dual-loop system.
**Components of the Trough Solar Thermal Power System**
The system includes several key components:
1. **Concentrating Collector System**
This is the core part, consisting of mirrors, receivers, and tracking devices. Receivers can be either vacuum tubes or chambers, and tracking methods include north-south, east-west, or polar-axis alignment.
2. **Heat Transfer System**
Includes preheaters, steam generators, superheaters, and reheaters. When using oil, a dual-loop system is typically employed, while direct water use simplifies the process.
3. **Power Generation System**
Similar to traditional power plants, it includes turbines and generators, with additional equipment to manage fluid switching between the receiver and auxiliary systems.
4. **Heat Storage and Exchange System**
Storing thermal energy allows the system to operate during cloudy periods or at night. Common storage methods include sensible, latent, and chemical heat storage.
5. **Auxiliary Energy System**
Used during low solar availability, such as at night or in rainy weather, to maintain system performance.
**Heat Transfer and Exchange System**
The heat transfer fluid, often oil or water, is heated by the collectors and transported via long pipelines. To reduce heat loss, insulation is applied, and pumps are used to circulate the fluid efficiently. The heat is then transferred to water, producing steam that powers the turbine. Heat exchangers come in various designs, including plate and tube types.
To ensure stable operation, the system includes heat storage tanks—both high-temperature and low-temperature—allowing continuous power generation even when sunlight is limited. For fluids that may freeze, an auxiliary heater is essential to prevent damage.
**Heat Storage System**
Due to the intermittent nature of solar energy, a heat storage system is crucial. There are four temperature categories for storage: low, medium, high, and ultra-high. Corresponding materials include hydrated salts, oils, molten salts, and refractory balls.
Storage methods include:
1. **Sensible Heat Storage**
Uses materials like water, oil, or sand. It’s cost-effective but has lower energy density.
2. **Latent Heat Storage**
Utilizes phase-change materials that store more energy per unit volume, making it ideal for compact systems.
3. **Chemical Heat Storage**
Offers high storage capacity and can store reactants separately, making it suitable for long-term applications.
Each method has its advantages and challenges, and the choice depends on the specific application and system design.
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