5.1 Rankine cycle and its variations for CSP

The Rankine cycle is the backbone of steam power plants, including those using concentrated solar power. It converts thermal energy into mechanical work through a series of processes involving steam generation, expansion, condensation, and pressurization.

Various modifications to the basic Rankine cycle can boost efficiency. These include superheating, reheating, and regeneration. Advanced configurations like supercritical cycles push the boundaries of performance, aiming to maximize energy conversion in CSP systems.

Rankine Cycle Components

Core Elements of the Rankine Cycle

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• Rankine cycle functions as the fundamental thermodynamic cycle for steam power plants
• Steam turbine converts high-pressure steam into mechanical energy
• Condenser transforms exhaust steam back into liquid water
• Feed pump increases the pressure of the condensed water
• Boiler/steam generator heats water to produce high-pressure steam

Steam Generation and Expansion Process

• Boiler utilizes heat from concentrated solar power to generate steam
• Steam enters the turbine at high pressure and temperature
• Turbine blades rotate as steam expands, driving an electrical generator
• Multiple turbine stages extract energy from steam at different pressures
• Exhaust steam exits the turbine at low pressure and temperature

Condensation and Pressurization Stages

• Condenser uses cooling water or air to remove heat from exhaust steam
• Condensation process occurs at constant pressure and temperature
• Feed pump increases the pressure of condensed water to boiler pressure
• Pressurized water reenters the boiler to complete the cycle
• Closed-loop system continuously recycles working fluid (water/steam)

Rankine Cycle Variations

Enhanced Efficiency Techniques

• Superheating raises steam temperature above saturation point
• Reheat cycle reheats partially expanded steam between turbine stages
• Regenerative cycle uses turbine extraction steam to preheat feedwater
• Subcritical Rankine cycle operates below critical point of water (374°C, 22.1 MPa)
• Supercritical Rankine cycle operates above critical point, improving efficiency

Superheating and Reheating Processes

• Superheating reduces moisture content in turbine, minimizing blade erosion
• Reheat cycle typically involves two-stage turbine configuration
• First reheat stage expands steam to intermediate pressure
• Reheater increases steam temperature before entering second turbine stage
• Multiple reheat stages can further improve cycle efficiency (diminishing returns)

Advanced Cycle Configurations

• Regenerative cycle extracts steam from turbine at various stages
• Extracted steam heats feedwater in series of feedwater heaters
• Open feedwater heaters mix extracted steam directly with feedwater
• Closed feedwater heaters use heat exchangers to transfer energy
• Supercritical cycles achieve higher thermal efficiencies (up to 45%)

Rankine Cycle Performance

Thermal Efficiency Factors

• Thermal efficiency measures ratio of net work output to heat input
• Carnot efficiency sets theoretical maximum for any heat engine
• Actual Rankine cycle efficiency lower due to irreversibilities
• Higher boiler temperatures and lower condenser temperatures improve efficiency
• Typical subcritical Rankine cycle efficiencies range from 30-42%

Efficiency Improvement Strategies

• Increasing steam temperature and pressure raises overall efficiency
• Decreasing condenser pressure lowers the heat rejection temperature
• Superheating reduces moisture content, improving turbine performance
• Reheating allows higher average temperature of heat addition
• Regeneration increases the average temperature of heat addition

Performance Metrics and Calculations

• Thermal efficiency calculated using enthalpy values at cycle points
• Work output determined by enthalpy change across turbine
• Heat input calculated from enthalpy change in boiler
• Pump work typically small compared to turbine work (often neglected in quick calculations)
• Cycle analysis involves applying first law of thermodynamics to each component