5.3 Supercritical CO2 cycles for CSP

Supercritical CO2 cycles are revolutionizing Concentrated Solar Power systems. By using CO2 above its critical point, these cycles achieve higher efficiencies and more compact designs than traditional steam-based systems. This innovative approach promises to make solar power more competitive and sustainable.

The key advantages of supercritical CO2 cycles include reduced compressor work, improved heat transfer, and higher overall efficiency. Various cycle configurations like recompression and partial cooling further optimize performance, while specialized components handle the unique challenges of working with supercritical CO2.

Supercritical CO2 Cycles

Properties and Advantages of Supercritical CO2

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• Supercritical CO2 exists as a fluid above its critical point (73.8 bar and 31.1°C)
• Exhibits properties of both liquid and gas simultaneously
• High density similar to a liquid allows for compact turbomachinery design
• Low viscosity comparable to a gas enables efficient fluid transport
• Excellent heat transfer properties enhance overall cycle efficiency
• Chemically stable and non-toxic makes it safe for use in power cycles
• Abundant and inexpensive reduces operational costs

Brayton Cycle with Supercritical CO2

• Closed-loop Brayton cycle utilizes sCO2 as the working fluid
• Operates at high pressures (typically above 200 bar) and temperatures (up to 700°C)
• Consists of four main components: compressor, heater, turbine, and cooler
• Compressor pressurizes sCO2 to operating pressure
• Heater adds thermal energy from external heat source (solar, nuclear, fossil fuels)
• Turbine expands high-pressure, high-temperature sCO2 to generate power
• Cooler reduces sCO2 temperature before re-entering the compressor
• Achieves higher thermal-to-electric efficiency compared to traditional steam Rankine cycles
• Compact design reduces overall plant footprint and capital costs

Compressor Work Reduction and Efficiency Gains

• Critical point of CO2 occurs at 73.8 bar and 31.1°C
• Operating near the critical point significantly reduces compressor work
• Rapid density change near critical point minimizes compression energy requirements
• Compressor work reduction leads to increased overall cycle efficiency
• Thermal-to-electric efficiency can exceed 50% in advanced sCO2 cycle configurations
• Higher efficiencies result in reduced fuel consumption and lower environmental impact
• Improved efficiency translates to lower levelized cost of electricity (LCOE)

sCO2 Cycle Configurations

Recompression Cycle

• Advanced sCO2 cycle configuration designed to improve overall efficiency
• Splits the flow after the main compressor into two streams
• One stream goes through a low-temperature recuperator (LTR)
• Second stream bypasses the LTR and enters a recompression compressor
• Recompression compressor increases pressure of the bypassed stream
• Both streams recombine before entering the high-temperature recuperator (HTR)
• Reduces irreversibilities in the cycle by better matching heat capacities in recuperators
• Achieves higher thermal efficiency compared to simple recuperated cycle
• Typically used for high-temperature applications (nuclear, solar thermal)
• Can achieve thermal efficiencies up to 50% or higher depending on operating conditions

Partial Cooling Cycle

• Modified version of the recompression cycle with additional cooling stage
• Incorporates a precooler before the main compressor
• Precooler reduces the temperature of the working fluid below the critical point
• Main compressor operates with liquid-like CO2, further reducing compression work
• Includes a split-flow arrangement similar to the recompression cycle
• Offers improved efficiency at lower turbine inlet temperatures (400-550°C)
• Well-suited for waste heat recovery applications and lower-temperature heat sources
• Can achieve higher efficiencies than recompression cycle at moderate temperatures
• Provides flexibility in optimizing cycle performance for various heat source temperatures

sCO2 System Components

Turbomachinery Design and Challenges

• Compact turbomachinery design due to high density of sCO2
• Turbine and compressor sizes significantly smaller than in steam cycles
• High operating pressures (200-300 bar) require robust mechanical design
• Materials selection crucial for withstanding high temperatures and pressures
• Turbine inlet temperatures can range from 400°C to 700°C or higher
• Bearing systems must handle high rotational speeds (typically 30,000-100,000 rpm)
• Sealing systems critical to prevent CO2 leakage and maintain cycle efficiency
• Turbine blade design optimized for sCO2 properties (Mach number, Reynolds number)
• Compressor design accounts for rapid density changes near critical point
• Advanced manufacturing techniques (3D printing, precision machining) enable complex geometries

Heat Exchangers for Supercritical CO2 Systems

• Specialized heat exchangers required to handle high pressures and temperatures
• Printed circuit heat exchangers (PCHEs) commonly used for their compact design
• PCHEs feature chemically etched flow channels in stacked metal plates
• Diffusion bonding joins PCHE plates, creating a strong, leak-tight assembly
• Shell and tube heat exchangers utilized for certain applications (precoolers, coolers)
• Recuperators play crucial role in cycle efficiency, often using PCHE design
• High-temperature recuperator (HTR) operates at turbine outlet conditions
• Low-temperature recuperator (LTR) used in recompression and partial cooling cycles
• Material selection considers corrosion resistance, thermal expansion, and creep strength
• Common materials include stainless steels, nickel alloys, and advanced ceramics
• Thermal stress management critical due to large temperature gradients in heat exchangers