Supercritical CO2 power cycles for waste heat recovery: A systematic comparison between traditional and novel layouts with dual expansion

The recovery of waste heat from thermal engines or industrial processes is prerequisite to achieve high efficiencies in the utilization of primary energy resources. When the waste heat is available at high temperatures, the most obvious solution for power production is the multi-pressure steam Rankine cycle, where the staging of the evaporation process is required to reach a good thermal match in the heat transfer process. Different options have been proposed in the search of simpler plant layout and more compact equipment, which include the use of water-ammonia mixtures, high critical temperature organic fluids or the operation at supercritical pressures. The last option is receiving an increasing attention when carbon dioxide is used as working fluid. Supercritical CO2 power cycles have been deeply investigated for nuclear and concentrating solar power, whereas their potential in waste heat recovery (WHR) applications is still largely unexplored. While the plant design for the former applications appears already standardized on only few traditional layouts showing a high cycle thermal efficiency like the recompression cycle, there is much more dynamism in the WHR field where an effective heat extraction from the open loop heat carrier asks for a dedicated plant design. Even though there is still a lack of consensus on the best CO2 power cycle design for WHR, a few layouts are recurrent in the recent literature since their first proposal at the beginning of this decade. This work investigates the potential of two of these novel layouts, namely the single and dual flow split with dual expansion, in the recovery of waste heat in a wide temperature range between 400 and 800 degrees C in comparison to the more traditional single recuperated and recompression layouts. This is evaluated not only in terms of cycle thermal efficiency, but also considering their capability in extracting heat from the heat carrier in a wide domain of the decision variables. The results of the thermodynamic optimization for a 1 MW system show that the total heat recovery efficiency, which is the ratio between net power output and heat available from the heat carrier, for the most advanced layout reaches 17.8%-28.5%, which is 5.8-9.5%-points higher than the traditional layouts, and raises with the heat source temperature. The multi-objective optimization shows that the marked increase in performance is obtained at the expenses of only a limited increase of 5.0-6.2% in specific investment cost compared to the traditional cycles. Thus, the development of s-CO2 power cycle layouts specifically developed for WHR appear mandatory especially for the upper temperature range.