Recuperators for micro gas turbines: A review

Micro gas turbines are a promising technology for distributed power generation because of their compact size, low emissions, low maintenance, low noise, high reliability and multi-fuel capability. Recuperators preheat compressed air by recovering heat from exhaust gas of turbines, thus reducing fuel consumption and improving the system efficiency, typically from 16-20% to ∼30%. A recuperator with high effectiveness and low pressure loss is mandatory for a good performance. This work aims to provide a comprehensive understanding about recuperators, covering fundamental principles (types, material selection and manufacturing), operating characteristics (heat transfer and pressure loss), optimization methods, as well as research hotspots and suggestions. It is revealed that primary-surface recuperator is prior to plate-fin and tubular ones. Ceramic recuperators outperform metallic recuperators in terms of high-temperature mechanical and corrosion properties, being expected to facilitate the overall efficiency approaching 40%. Heat transfer and pressure drop characteristics are crucial for designing a desired recuperator, and more experimental and simulation studies are necessary to obtain accurate empirical correlations for optimizing configurations of heat transfer surfaces with high ratios of Nusselt number to friction factor. Optimization methods are summarized and discussed, considering complicated relationships among pressure loss, heat transfer effectiveness, compactness and cost, and it is noted that multi-objective optimization methods are worthy of attention. Moreover, 3D printing and printed circuit heat exchanger technologies deserve more research on manufacturing of recuperators. Generally, a metallic cost-effective primary-surface recuperator with high effectiveness and low pressure drop is a currently optimal option for a micro gas turbine of an efficiency of ∼30%, while a ceramic recuperator is suggested for a high efficiency micro gas turbine (e.g. 40%).