Lec 27: Concentrating collector technologies and working principle

Solar Concentrating Technologies for Power Generation

Overview of Solar Concentrating Technologies

• The discussion focuses on various solar concentrating technologies used for power generation, including parabolic troughs, central receiver systems, and Fresnel collectors.

Parabolic Trough Technology

• Parabolic trough systems utilize reflectors to concentrate sunlight onto a receiver tube, achieving fluid temperatures around 400°C. This technology employs linear focusing systems with parabolic mirrors.

• It is noted for its maturity and commercial availability, suitable for hybridization with other technologies but requires flat land for optimal performance.

• Key components include mirrors, receiver tubes, metal support structures, and tracking systems that ensure maximum solar radiation capture.

• Applications range from generating thermal energy in industrial processes to electricity generation; however, the heat transfer fluid can degrade over time at high temperatures.

• The system's design allows it to effectively concentrate sunlight onto a U-shaped receiver tube where heat transfer fluids flow.

Design Parameters of Parabolic Trough Systems

• Aperture areas typically range from 1 to 60 square meters with concentration ratios between 10 and 80.

• Absorber tube diameters vary from 2.5 to 5 cm and are often made of steel or copper coated with heat-resistant black paint to enhance absorptivity.

Tracking Mechanisms

• Parabolic trough systems may employ single-axis or dual-axis tracking mechanisms to maximize solar radiation absorption throughout the day.

Energy Generation Process

• The heated fluid can be used directly in turbines for electricity generation; water is commonly used as a heat transfer fluid due to its high temperature capabilities.

Stirling Engines and Their Applications

Stirling Engine Technology

• Stirling engines operate at higher temperatures (up to 800°C), utilizing point focus systems that concentrate sunlight onto a single focal point.

• Each unit can generate approximately 25 kW of power with an efficiency rate around 30%.

System Configuration

• These systems require dual-axis tracking for optimal performance and feature mirrored reflectors similar in appearance to satellite dishes.

Central Receiver Systems

Central Receiver System Overview

• Central receiver systems consist of a tower topped by a receiver surrounded by heliostats (mirrors). They achieve very high operational temperatures (up to 800°C).

Heat Storage Capabilities

• These systems have significant heat storage capabilities allowing them to store excess energy generated during peak sunlight hours for later use.

Energy Balance in Central Receiver Systems

Energy Gain Calculations

• Useful heat gain calculations involve assessing the energy received by the absorber minus losses due to inefficiencies within the system.

Concentration Ratio Definition

• The concentration ratio is defined as the area covered by mirrors divided by the area of the absorber. This ratio is crucial for understanding system efficiency.

Practical Considerations in Solar Power Generation

Installation Insights

• A megawatt-level power tower installation showcases how heliostats direct solar radiation towards a central tower where it is converted into usable energy through heating processes.

This structured approach provides clarity on each technology discussed while linking back directly to specific timestamps in the transcript for further exploration or review.

Overview of Solar Thermal Power Generation

Heat Transfer and Electricity Generation

• The process begins with heating a receiver to high temperatures, where heat is collected and transferred through a heat exchanger using water as the heat transfer fluid.

• Steam generated from heated water expands in a turbine, leading to electricity generation via a generator. Low-temperature water is then cooled in a condenser.

Energy Storage Solutions

• If electricity generation is insufficient or excess heat is produced, energy storage systems are necessary for later use.

• This technology is suitable for large-scale applications, typically ranging from 30 to 400 megawatts.

Heliostats: Key Components of Solar Power Systems

Structure and Functionality

• Heliostats consist of multiple mirrors; each unit can have about 12 modules that reflect sunlight towards the receiver.

• The Redstone Solar One facility features 1818 heliostats with an 80-meter tower capable of producing 10 megawatts of electrical energy.

Reflectivity Concerns

• Heliostat reflectivity starts at approximately 0.903 but can decrease to around 0.82 due to dirt accumulation, which poses operational challenges.

Receiver Types in Central Power Systems

External vs. Cavity Receivers

• There are two main types of receivers: external and cavity types, with cavity receivers offering higher optical efficiency by allowing solar flux entry through smaller openings.

• Various plants worldwide utilize these technologies, including installations in Spain, Italy, Japan, France, and the USA.

Fresnel Collectors: An Alternative Technology

Design and Operation

• Fresnel collectors use curved mirrors to focus sunlight onto a linear receiver that can achieve temperatures up to 500 degrees Celsius.

• These systems employ thermal oil or molten salt as heat transfer fluids and possess storage capabilities for energy management.

Stationary Concentrating Collectors

Mechanism and Efficiency

• Stationary collectors utilize compound parabolic reflectors that direct solar energy without requiring tracking devices due to their wide acceptance angle.

• This class includes various designs such as flat plate collectors with parabolic boosting reflectors suitable for high-temperature applications.

Comparison of Solar Technologies

Categories of Applications

• Different solar technologies cater to varying temperature applications: low (flat plate collectors), moderate (parabolic trough), and high (central power tower).

Performance Metrics

• Parabolic trough systems range from 30 to 320 megawatts while power towers vary between 10 to 200 megawatts; operating temperatures also differ significantly across technologies.

Future Prospects in Solar Power Technologies

Summary Insights

• The lecture concludes by emphasizing that parabolic trough systems are currently the most mature technology available for large grid-connected projects.

Potential Developments

• Power tower systems promise efficient thermal storage solutions with high capacity factors for future solar-only power plants.