Dynamics of an Organic Rankine Cycle for OTEC

Abstract

The global energy demand is growing, while climate change is demanding a sustainable way of generating this energy. Ocean Thermal Energy Conversion (OTEC) can be a part of the solution for this problem. OTEC generates electricity by using the temperature difference between the surface water of the ocean and the water at 1000 meters depth as a driving force. As this temperature difference is present all year round, there is no need for energy storage, which is the case for wind- and solar energy. An OTEC system utilizes an Organic Rankine Cycle (ORC), which uses ammonia as a working fluid. A lot of research is conducted on this cycle, all assuming steady-state. However, as the system is constantly changing its state, either by a changing temperature difference or by variations in the operating conditions of components, more research is required to investigate the impact of these changes. Therefore, a dynamic model has been developed. In order to cope with these changes and to ensure the optimal power output at all times, a control strategy is developed and implemented on the model. As the system has been developed with the use of a control system as a boundary condition, a model is adapted which prioritises computational time over accuracy, but, according to literature, is still accurate enough for the small transients. This model has been implemented for the OTEC cycle and improved from normal dynamic models by including pressure drops and storage tanks. Allseas, in cooperation with the TU Delft, has built an experimental set-up of an OTEC cycle. Experiments conducted on this set-up were used to compare the model with reality, both steady-state and dynamically. It has been proven that the steady-state values matched the experiments within 1%, and the  dynamics matched the experiments almost perfectly. As a next step, the system has been scaled to match the desired 3MW output. With this scaled model, realistic scenarios were simulated to check the response on larger transients. The scenarios consists of a start-up and shutdown of the system, a changing inlet temperature, pumps being shut off and a number of heat exchangers that are decoupled from the system, for example when a number of heat exchangers need maintenance.  The outcome proved that, from a dynamic perspective, the influence of a seawater temperature change was negligible. As the temperature changes per second are very small, the net output scales linearly with the temperature difference. A start-up and shutdown of the system was successfully simulated, which is the largest possible transient in the system. The influence of pumps that are stopped and heat exchangers being turned off for maintenance have been simulated, which showed just a slight decrease in net output. It also showed that the system, when running at the nominal conditions of the pump and turbine curve, was not running at an optimum. Therefore, a control strategy was developed by conducting a sensitivity analysis and, after using an optimisation to find the optimum point, the resulting control strategy was implemented. This new strategy resulted in an increase of 15% in the net output, compared to the nominal conditions. This proves that an OTEC cycle could greatly benefit from using a dynamic model to predict its dynamic performance and the implementation of a control system.