Abstract
This study investigates the feasibility of a Turbo-recuperator Heat Exchanger (T-rHEX) as a novel compact regeneration concept for the ORC waste heat recovery system resembling the ORCHID facility. The motivation rises from the need to increase cycle efficiency and power density of the system by recuperating thermal energy discharged through the turbine diffuser, thereby reducing the required size of the primary heat exchangers. The T-rHEX concept is such that the coolant channels are wrapped around the exterior of the diffuser wetted surface so that the diffuser operates simultaneously converting kinetic energy to pressure and as a heat exchanger.
To analyze the fluid dynamic and thermal performance characteristics of the T-rHEX, an in-house low fidelity quasi-1D model (T-rHEX) is developed. Using the existing ORCHID turbine diffuser as a recuperator increases the thermal efficiency of a baseline high-temperature ORC system in simple configuration from 12.11% to 12.53%. To further improve ORC efficiency, the geometry of the diffuser is optimized in order to maximize the aero-thermal performance of regenerative diffuser using the low fidelity model. The resulting optimized T-rHEX provides an efficiency of 13.15%, improving heat exchanger effectiveness and compactness while maintaining an acceptable level of diffuser pressure recovery.
To verify the results, a high fidelity (3D) CHT model is constructed to capture 3D effects such as flow separation and solid–fluid interaction. The comparison between low and high fidelity model results shows that the low fidelity model is comparatively accurate, capturing trends well in terms of fluid dynamics and global heat transfer behavior when the flow remains attached. In the case of strong non-axisymmetric flow effects are present (such as separation), the accuracy of the low fidelity model breaks down. Overall, numerical results demonstrate that the T-rHEX concept improves cycle efficiency compared to the original cycle without recuperation. The addition of a compact recuperator such as the T-rHEX not only reduces thermal duty of the primary heat exchanger components of the cycle but also can increases diffuser pressure recovery due to cooling effect.
