Thermodynamic study of supercritical CO₂ Brayton cycle using an isothermal compressor

A power cycle using supercritical CO₂ instead of steam as a working fluid has been considered a strong candidate for reaching higher efficiency limits. To enhance the theoretical limits further, a novel component called the ‘isothermal compressor’, known as an ideal component to minimize the compression work, is applied to the supercritical CO₂ cycle technology. This futuristic idea can improve its current efficiency limits and provides more options for cycle layout design.

Article  |  Spring 2018

In the current global warming dialogue, CO₂ is mostly perceived as a pollutant and, thus, our enemy. Recently, the supercritical CO₂ cycle technology, a next-generation power conversion system that uses the same CO₂ in its supercritical state to generate electricity, has been increasing research interest.

Supercritical CO₂, also used as a solvent for extraction processes, possesses favorable properties near the critical point that can be primarily utilized to reduce the work required for the compression process. Because CO₂ is known to have low compressibility near the critical point, it becomes easier to raise its pressure with less energy input.

In order to enhance the theoretical efficiency limits of the supercritical CO₂ cycle (s-CO₂ cycle) technology, an idea of using a novel component called the isothermal compressor in the system has been investigated by a research team led by Professor Jeong Ik Lee. The isothermal compressor has been known theoretically to minimize the compression work because it compresses the working fluid at a constant temperature while removing the heat. By using this technology, the performance of the s-CO₂ cycle can be further improved, especially since the compression process would take place closer to the critical point. Hence, the advantage of using an isothermal compressor becomes amplified for the s-CO₂ cycle utilizing the nature of CO₂ properties.

However, the thermodynamic framework for analysis has remained ambiguous for the isothermal compressor since it requires information about how to define its real work. Theoretical research focuses on how this ambiguity can be resolved by assuming the isothermal compression is a combination of infinitesimally divided steps of compression processes at constant entropy and cooling processes at constant pressure. With this new definition, the performance changes for the s-CO₂ replacing the conventional compressor with an isothermal compressor can be analyzed. Overall, the research demonstrates how this idea can bring about further improvements in the efficiency limits of the energy system.

Original paper: http://dx.doi.org/10.1016/j.apenergy.2017.08.081