Analysis of a carbon dioxide transcritic (1)

A detailed analysis of a carbon dioxide transcritical power cycle using an industrial low-grade stream of process gases as its heat source is presented. The methodology is divided in four steps: energy analysis, exergy analysis, finite size thermodynamics, and calculation of the heat exchangers’ surface. The results have been calculated for fixed temperature and mass flow rate of the heat source, fixed maximum and minimum temperatures in the cycle, and a fixed sink temperature by varying the high pressure of the cycle and its net power output. The main results show the existence of an optimum high pressure for each of the four steps; in the first two steps, the optimum pressure maximizes the thermal or exergetic efficiency while in the last two steps it minimizes the product UA or the heat exchangers’ surface. These high pressures are very similar for the energy and exergy analyses. The last two steps also have nearly identical optimizing high pressures that are significantly lower than the ones for the first two steps. In addition, the results show that the augmentation of the net power output produced from the limited energy source has no influence on the results of the energy analysis, decreases the exergetic efficiency, and increases the heat exchangers’ surface. Changing the net power output has no significant impact on the high pressures optimizing each of the four steps.