A comprehensive review of micro-scale expanders for carbon dioxide related power and refrigeration cycles

Abstract

• Developments of micro-scale expanders in carbon dioxide cycles are summarized. • Comparison between turbines and volumetric expanders are analysed. • Relations between flow rate, pressure ratio and rotational speed are highlighted. • Challenges and future development directions for expanders outlined. In recent decades, the carbon dioxide cycles, including supercritical carbon dioxide cycle, transcritical carbon dioxide Rankine cycle and refrigeration cycle, have been proven effective due to the high efficiency and compact structure, and received increasing interests. The performance of the expander in the power cycles, particularly in micro-scale applications, is one of the essential components that determine the cycle performance and still remains a significant challenge. This paper presents a critical overview of micro-scale (<200 kW) expanders for carbon dioxide cycles. Five types of expanders, including axial and radial turbines (dynamic expanders), scroll, piston and vane expanders (volumetric expanders), are evaluated from experimental and numerical aspects. Expansion mechanisms, rotational speed, isentropic efficiency, pressure ratio and mass flow rate are considered. The review suggests that the volumetric expanders stand certain advantages in the micro-scale carbon dioxide cycles due to the more suitable rotational speed and potentially less leakage. For all expanders, internal and external leakages remain a vital challenge that greatly reduce their performance and efficiency, which urgently needs to be addressed.