CFD – Aided Design Methodology for PCHE-Type Recuperators in Supercritical CO2 Recompression Power Cycles

The supercritical carbon dioxide (sCO₂) cycle has emerged as a promising power cycle for various types of power conversion systems, based on its high thermal efficiency, (approaching 60%), small-size components and compact footprint. Applications of sCO₂ power cycle for different heat sources have been investigated. They include nuclear energy, solar energy, fossil fuel, fuel cell and industrial waste heat.

Since the recompression Brayton cycle with sCO₂ is based on high capacity regenerators processing a large amount of heat (for example, the HTR’s duty is typically about 150% that of the main heater), the effectiveness of these regenerators is critical to attaining high overall cycle efficiency figures. This is proved by basic availability analysis of this cycle.

Effectiveness values up to 99% are currently set as a target for these heat exchangers, which can be achieved by use of the Printed Circuit Heat Exchanger (PCHE) technology. Thus, the design process for these heat exchangers is demanding, especially if one takes into account the peculiarities of variation of CO₂ density and thermal properties (especially c_p which augments by orders of magnitude) near the critical temperature.

A hybrid design methodology for the high-pressure and low-pressure recuperator (HTR and LTR) is formulated and presented in this paper, which employs 3D CFD conjugate heat transfer computation of the performance of a small two-channel module of the PCHE type heat exchanger. Specific design alternatives for channel dimensions, roughness and zig-zag angles are compared and discussed.

The main results of the CFD computation are the heat transfer coefficient and Colburn j-factor for the module, as well as the Fanning friction factor for each typical channel (hot and cold side of the module). The results from the small module computation are employed in a routine segmental method for the performance computation of the full channel length. Thus, the effectiveness and pressure drop characteristics for the full heat exchanger are computed with high accuracy.

Application of the proposed methodology is carried out for the HTR and LTR computation for a recompression sCO₂ Brayton cycle of a 600 MW_th size power plant with turbine entry temperature of 850 K under study. Both regenerators are studied according to the PCHE design approach. Typical hot side inlet temperature to the HTR is 720 K and the respective hot side inlet temperature for the LTR is 430 K. The complexities induced by CO₂ thermal properties variation, especially for the LTR, are presented and discussed.

Finally, overall design and geometry layout for the full heat exchangers, both HTR and LTR, along with their collectors’ layout are proposed, based on the current state of PCHE technology.