Lec 26: Fundamentals of concentrating collectors

Fundamentals of Solar Concentrating Collectors

Introduction to Concentrating Solar Power Technology

• Concentrating solar power (CSP) technology utilizes focused sunlight to increase the amount of incident energy on a surface compared to that on the concentrator itself.

• CSP employs mirrors or lenses to concentrate solar energy, converting it into high-temperature heat for various applications.

Working Mechanism of Concentrating Solar Collectors

• The system consists of a reflector and a receiver; sunlight strikes the reflector, which directs it onto an absorber where heat transfer fluid flows.

• A tracking device is essential for concentrating collectors to follow the sun's path, unlike flat plate collectors which are fixed in position.

Advantages and Limitations of CSP Technology

• Key advantages include better thermodynamic efficiency due to higher operating temperatures and reduced material requirements compared to flat plate collectors. High temperatures up to 350°C can be achieved.

• Limitations include reliance on direct beam radiation, necessitating clear skies for optimal performance, as diffuse radiation cannot be utilized effectively.

Conversion Process from Heat Energy to Electricity

Mechanism of Electricity Generation

• CSP systems generate electricity by first converting solar thermal energy into mechanical energy through heat engines using cycles like Rankine or Stirling cycles. This mechanical energy is then converted into electricity via generators.

• The conversion efficiency ranges from 9% to 18%, with efficiency decreasing as collection temperature increases while heat engine efficiency improves with higher working fluid temperatures.

Challenges in Thermal Energy Transportation

• Significant thermal energy loss occurs during transportation from collector systems to heat engines, requiring large areas for installation and effective storage solutions due to intermittent solar availability.

Parameters Characterizing Solar Concentrators

Key Parameters Defined

• Partial Area: The area through which solar radiation is incident on the concentrator.

• Absorber Area: Total area receiving concentrated radiation where useful energy can be extracted.

• Acceptance Angle: Defines how much deviation from normal allows rays to reach the receiver effectively.

These parameters influence overall system design and performance metrics such as optical efficiency and concentration ratio calculations.

Types and Configurations of Concentrating Collectors

Collector Configurations Overview

• Various configurations exist including tubular absorbers with reflectors designed for increased concentration ratios; parabolic trough systems focus sunlight along a linear axis while Fresnel reflectors utilize segmented mirrors for similar purposes.

Comparison Between Different Systems

• Point focus systems achieve high concentration ratios but require precise tracking mechanisms, whereas line focus systems may allow seasonal adjustments depending on their acceptance angles.

Classification of Concentrating Collectors

Modes of Classification

1. Aperture Type: Reflecting types using either mirrors or Fresnel lenses.

2. Image Formation: Non-imaging vs imaging types based on focusing capabilities.

3. Operating Temperatures: Low, medium, or high temperature classifications.

4. Tracking System: Single-axis vs double-axis tracking mechanisms are crucial for maximizing exposure throughout the day.

This classification helps in understanding different technologies available within CSP frameworks such as parabolic trough collectors and central receiver systems like solar towers and Fresnel collectors.

Applications of CSP Technologies

Commercial vs Domestic Applications

Commercial Scale:

• Power generation in grid-connected systems or hybrid setups; applications also extend towards hot water generation, air conditioning, desalination processes among others.

Domestic Level:

• Utilized primarily for hot water generation, cooking solutions like solar ovens/cookers, food drying techniques etc., showcasing versatility across scales.

This structured overview encapsulates key insights regarding concentrating solar power technology fundamentals while providing timestamps linked directly back into specific sections within the transcript ensuring ease-of-use when referencing detailed discussions presented therein .

Understanding Concentrating Collectors

Overview of the System Components

• The system includes a glass cover that neglects temperature drop, maintaining consistent conditions.

• Key dimensions include the aperture width (w), tube length (l), inner diameter (d_i), and outer diameter (d_o) of the absorber.

• The concentration ratio is defined as the effective area of the concentrator to the absorber tube area, calculated using specific dimensions.

Energy Balance on Absorber Plate

• An energy balance expression is derived for an elementary slice of the absorber tube, focusing on useful heat gain (q).

• Useful heat gain is influenced by beam radiation and losses from conduction and convection, with contributions from both direct solar exposure and reflected radiation.

• The equation incorporates factors like reflectivity and ambient temperature to quantify heat transfer dynamics.

Heat Gain Rate Calculation

• The heat gain rate can be expressed in terms of mass flow rate (m), specific heat capacity (c_p), and temperature difference between inlet and outlet.

• Collector efficiency can be calculated using combined equations that account for initial conditions at x = 0 where fluid temperature equals inlet temperature.

Instantaneous Efficiency Analysis

• Instantaneous collector efficiency relates useful heat gain to total incident solar radiation, including diffuse components when applicable.

• Beam radiation calculations are essential for determining efficiency; only beam radiations are considered in this analysis.

Heat Transfer Coefficient Calculations

• Understanding how heat transfer occurs from collector to absorber involves calculating overall heat transfer coefficients through various correlations.

• Nusselt number correlations help estimate convective heat transfer coefficients based on Reynolds numbers across different flow regimes.

Pressure Drop Considerations

• Pressure drop in concentrating collectors can be analyzed using established correlations that relate friction factor to Reynolds number.

• Knowing these parameters allows for accurate pressure drop calculations which are crucial for system performance evaluation.

Summary of Key Concepts Discussed

• Fundamental principles of concentrating collectors were covered, including classifications based on reflecting types and operational characteristics such as tracking methods.

• Energy balance equations were emphasized as critical tools for understanding useful heat gain in relation to environmental factors.