Development study of automatic control system for the demonstration plant of ocean thermal energy conversion (OTEC) using sequence controller

Based on the success of the demonstration experiment of MW-class Ocean Thermal Energy Conversion (OTEC) in East Sea, Korea, OTEC technology is expected to spread in the future. In order for this to happen, technology improvement factors, such as minimizing human error and reducing operating costs through automatic start and stop for stable operation, are to be applied. In this paper, the operation performance of a 20 kW-class pilot model for the automated start and stop of the recently demonstrated MW-class OTEC served as the basis for the scenario. The scenario derived from the operation experiment of the 20 kW-class pilot plant established the starting conditions of the surface water and deep water seawater inflow conditions with maximum flow conditions of 1,864 kg/s and 1,507 kg/s, respectively, at the initial start-up of the MW-class OTEC. The refrigerant circulation pump produces 86.4 kW initial power at 700 RPM (Max 1,150) when the turbine bypass valve is closed by 50%. The maximum flow condition of 104.0 kg/s through the sequential increase in refrigerant flow and decrease in the opening of the turbine bypass valve generated an output of 886.9 kW. In the stop condition, the RPM of the refrigerant pump is reduced to 800, and the bypass valve is fully closed. Moreover, the power generation output is reduced to 154.3 kW. In accordance with the normal stop condition of 200 kW or less, the bypass valve, which is fully opened, and the valve at the turbine inlet, which is completely opened and closed, block the inflow into the turbine. On the other hand, the opening degree of the bypass valve, which is fully opened, reduces the turbine output to 0 kW in the emergency stop condition. Then, the turbine inlet valve, which is completely opened and closed, blocks the working fluid inflow. As the bypass valve opens abruptly, the flow rate of the working fluid increases up to 328.0 kg/s due to the decrease in turbine resistance. In doing so, the flow rate decreases, and the working fluid amount in the liquid separator decreases. The construction of an automated system for the empirical model in the future will use the OTEC start and stop simulation results derived through this study.