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CO2 Working Fluid

Using CO₂ as the working fluid to generate power from low-grade heat presents unique opportunities, particularly due to the properties of CO₂ at different temperature and pressure ranges. Let's evaluate three key cycles—Organic Rankine Cycle (ORC), Supercritical CO₂ Rankine Cycle (sCO₂), and the Standard Rankine Cycle (SRC)—to determine which one offers the highest efficiency when working with low-grade heat sources.

1. Standard Rankine Cycle (SRC) with CO₂:

• Working Principle: The standard Rankine cycle typically uses a phase change between liquid and vapor. CO₂ in this cycle would follow a similar process, where heat is added to CO₂ in liquid form to convert it to vapor, which then drives a turbine. The vapor is then condensed back to liquid.

• Efficiency with Low-Grade Heat: CO₂ has a relatively low critical temperature (around 31°C), making it challenging to achieve a liquid-vapor phase change at low-grade heat sources (below 150°C). For low-grade heat applications, maintaining the CO₂ in a liquid phase at ambient conditions may not be possible, limiting the effectiveness of this cycle.

• Conclusion: The Standard Rankine Cycle using CO₂ is generally not well-suited for low-grade heat because CO₂ tends to stay in the gas phase at temperatures where other refrigerants would condense. This leads to reduced efficiency in extracting work from the phase change.

2. Organic Rankine Cycle (ORC) with CO₂:

• Working Principle: The Organic Rankine Cycle is specifically designed to operate with low-boiling-point fluids (like CO₂ or hydrocarbons) at lower temperatures. In the ORC, CO₂ could act similarly to organic fluids, absorbing heat at a low temperature and expanding in a turbine before being condensed.

• Efficiency: CO₂’s critical temperature of 31°C limits its ability to undergo a liquid-vapor phase transition in low-grade heat applications. Unlike organic fluids, CO₂ doesn't efficiently undergo phase change below or near ambient temperatures, making its use in an ORC cycle less efficient compared to specialized organic fluids like R245fa or n-pentane.

• Conclusion: The ORC with CO₂ has some potential, but it is not the most efficient when compared to using fluids specifically designed for low-grade heat applications. ORCs generally achieve higher efficiencies when using organic fluids rather than CO₂ for low-grade heat sources.

3. Supercritical CO₂ (sCO₂) Rankine Cycle:

• Working Principle: In the supercritical CO₂ (sCO₂) cycle, CO₂ operates at conditions above its critical point (31°C and 73.8 bar), where it does not undergo a distinct phase change between liquid and gas. Instead, the CO₂ exists in a supercritical state, which allows for efficient heat absorption and expansion at relatively low temperatures.

• Efficiency for Low-Grade Heat: The sCO₂ cycle is known for its high efficiency in converting low• to mid-grade heat sources (50-150°C) into mechanical work, particularly because supercritical CO₂ has excellent thermodynamic properties in this range. Its high density in the supercritical state allows for compact heat exchangers and high cycle efficiency due to lower compressor work.

• Advantages:

• Compact system design: Due to the supercritical properties of CO₂, heat exchangers and turbomachinery can be made smaller while maintaining high efficiency.

• Higher thermal efficiency: The sCO₂ cycle can reach thermal efficiencies between 30-40% for low• and medium-grade heat sources, significantly higher than conventional ORCs or SRCs operating with CO₂.

• Wider temperature range: Supercritical CO₂ can extract energy more efficiently across a wider temperature range than conventional cycles.

• Conclusion: The sCO₂ cycle is generally regarded as the best cycle for converting low-grade heat to power using CO₂ as the working fluid. Its ability to operate in a supercritical state, avoiding the inefficiencies associated with phase change, makes it highly efficient for such applications.

4. Other Cycles: Kalina Cycle:

• Working Principle: The Kalina Cycle uses a mixture of fluids (typically ammonia and water) as the working fluid. The mixture allows for a continuous boiling process, which can be tuned to specific temperature ranges for better heat transfer.

• Efficiency with CO₂: The Kalina Cycle typically operates with an ammonia-water mix, and adapting it to work with CO₂ would be difficult. However, its efficiency is generally higher than standard Rankine cycles due to better heat recovery. Using a different mixed working fluid (not CO₂) could be more efficient, but CO₂ alone would not benefit much from this cycle.

• Conclusion: The Kalina Cycle is not typically used with CO₂, and it is unlikely to be efficient for low-grade heat applications compared to sCO₂.

Overall Assessment:

• Supercritical CO₂ (sCO₂) Rankine Cycle is the most efficient cycle for utilizing CO₂ as the working fluid in low-grade heat applications. It leverages the supercritical properties of CO₂ to improve heat transfer, reduce system size, and increase efficiency, making it ideal for converting low-temperature heat sources into usable power.

• Organic Rankine Cycle (ORC) with CO₂ is less efficient than an ORC using organic fluids because CO₂'s critical temperature is too low for effective phase change at ambient conditions.

• The Standard Rankine Cycle (SRC) is not well-suited for CO₂ in low-grade heat because of the limited ability to achieve phase change and extract maximum work from low-temperature sources.

Conclusion:

The supercritical CO₂ Rankine cycle (sCO₂) is the best option for achieving the highest efficiency when using CO₂ as the working fluid to convert low-grade heat into power. This cycle provides a significant efficiency advantage by operating CO₂ in its supercritical state, making it ideal for low• to medium-grade heat recovery. Other cycles, such as the standard Rankine cycle or Organic Rankine cycle with CO₂, are not as efficient in this application.

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