Effect of heat extraction optimization with supercritical carbon dioxide on transcritical power cycle

Summary An experimental and thermodynamic analysis was conducted to explore the match in operating conditions for the heat extraction of supercritical CO 2 and the CO 2 transcritical organic Rankine cycle (CTORC). The results revealed that in the optimal conditions of the experiment, the difference between the pseudocritical temperature and the inlet temperature ( ΔT pc  − in ) was <10 K and T b /T pc (ratio of the bulk temperature to the pseudocritical temperature) was ≤1 (ideal scenario: T b /T pc = 1). Furthermore, the heat transfer and fluid flow of CO 2 as well as the CTORC system performance at the optimal T b /T pc could be simultaneously improved with respect to those at ΔT pc  −  in < 10 K. The peak values of system efficiency for the inlet temperature of the expander of 100°C and 150°C were 5.1% at 12.5 MPa and 8.0% at 17 MPa, with the corresponding T b /T pc being 1.24 ( T pc of 55.9°C) and 1.45 ( T pc of 70°C), respectively. Consequently, to simultaneously improve the heat transfer, fluid flow and system efficiency, T pc of the supercritical CO 2 in the CTORC should be sufficiently high to approach half the inlet temperature of the expander for obtaining an optimal T b /T pc at a low condensing temperature.