A self-condensing CO2 power system for widely adaptive underwater conditions

In the underwater environment, a nuclear-powered CO₂-based transcritical recuperative power cycle can effectively utilize the low temperature of seawater to achieve high-efficiency. To address the challenge of non-condensable working fluids in the epipelagic zone, a self-condensing subloop offers an effective solution. This study introduces a configuration for a self-condensing CO₂-based transcritical recuperative power cycle, establishes a thermodynamic model, investigates the negative impacts of the self-condensing subloop, and analyzes its operational strategies at various underwater depths. Results indicate, when CO₂ can condense in the cooler, the subloop consumes between 17.4 % and 36.9 % of generated power, which decreases as seawater temperatures rise at a cooler pressure of 8.5 MPa. Since the high heat capacity of the heat source, increasing turbine inlet temperature and pressure significantly improves system efficiency. Activation of the self-condensing subloop enhances power output with higher storage tank temperatures. Furthermore, when seawater temperatures exceed 23.4 °C, a linear functional relationship between seawater temperature and optimal cooler pressure is specifically proposed, which effectively optimizes system power output. The study recommends activating the self-condensing subloop when CO2 at cooler outlet exceed 28 °C, broadening applicable temperature range of transcritical power cycle systems in the ocean. Methods in this research include first-principle modeling and optimization.