• Energy Research

Future advanced turbine systems for electric power generation systems, based on coal-gasified fuels with CO2 capture and sequestration, are aimed for achieving higher cycle efficiency and near-zero emission. Most promising operating cycles being developed are hydrogen-fired cycle and oxy-fuel cycle. Both cycles will likely have turbine working fluids significantly different from that of conventional air-based gas turbines. In addition, the oxy-fuel cycle will have a turbine inlet temperature target at approximately 2030K (1760°C), significantly higher than the current level. This suggests that aerothermal control and cooling will play a critical role in realizing our nation’s future fossil power generation systems. This paper provides a computational analysis in comparing the internal cooling performance of a double-wall or skin-cooled airfoil to that of an equivalent serpentine-cooled airfoil. The present results reveal that the double-wall or skin cooled approach produces superior performance than the conventional serpentine designs. This is particularly effective for the oxy-fuel turbine with elevated turbine inlet temperatures. The effects of coolant-side internal heat transfer coefficient on the airfoil metal temperature in both hydrogen-fired and oxy-fuel turbines are evaluated. The contribution of thermal barrier coatings (TBC) toward overall thermal protection for turbine airfoil cooled under these two different cooling configurations is also assessed.

Abstract A Composite Thermoelectric Device (CTED) is comprised of thermoelectric (TE) elements made of TE semiconductor materials bonded to highly electrically and thermally conductive material in a segmented fashion. Thermoelectric performance of such devices using numerical methods with temperature-dependent thermoelectrical properties has been investigated. The CTED performance in terms of produced electrical current, Ohmic and Seebeck potentials, power output P 0 , heat input Q h , and conversion efficiency η is studied for various hot surface temperature T h , load resistance, semiconductor thickness d , and convection heat transfer coefficient h values. The aforementioned CTED performance characteristics are compared to those of a conventional TED with geometrical equivalence. For a given T h , a maximum P 0 is achieved at a load resistance value that is equal to the total internal resistance R i of the device; an optimum η , at an optimum load resistance R optmL which is typically higher than the R i . A CTED with d = 1 mm, the optimum η values are 24.8%, 26.2% and 29.9% higher than conventional TED values at T h = 350 K, 450 K and 550 K, respectively. At R optmL values, the difference in P 0 and Q h show significant and minor increases, respectively, in relation to differences in η with an increase in T h . The variation of the semiconductor thickness d has a substantial effect on the CTED characteristics and R optmL values; as d decreases, a continuous increase in P 0 and Q h and an optimum value of η are achieved. Intuitively, R optmL increases with an increase in d and it reaches a maximum value at the conventional TED limit. With d = 0.5 mm, T h = 450 K and h = 20 W m −2 K −1 at a corresponding R optmL value, P 0 and Q h exhibit an eight- and six-fold increase, respectively, and η is increased 22% compared to a conventional TED. The convective heat transfer coefficient has a pronounced effect on the CTED performance when it is greater than 100 W m −2 K −1 . From this study, the CTEDs show promise of extracting more heat in waste heat recovery applications when compared to conventional TEDs.