Ocean Thermal Energy Conversion (OTEC) is a technique to generate electricity by using the thermal gradient available in the ocean to drive a heat engine. In order for OTEC to become successful it is important to reduce the levelized cost of electricity. The development of an effective control system can aid in this endeavour, by making sure that the plant is always operated at maximum capacity. Theoretically, the optimal control inputs can be determined offline by using models. However, due to modelling assumptions, this method can result in a suboptimal operation of the plant. Besides, performance of the plant varies over time as a result wear and tear, fouling, and fluctuating ocean water temperatures. Comparable obstacles are encountered in maximizing the electricity production in wind turbines. A promising solution in this field is Extremum Seeking Control (ESC). ESC is a model-free control strategy used for the real-time optimization of an unknown objective function. In this study, ESC has been implemented on a room-sized OTEC test-setup, called the OTEC-demo. Single parameter as well as multiparameter ESC algorithms are tested, and are shown to be successful in automatically finding the set of inputs for which the power output of the plant is maximized.