The global climate agreement signed in Paris in 2015 sets the goal of limiting global warming, which is essential of saving the planet for future generations. The energy transition from a fossil-fuel living to a sustainable society depends on the technological development of using the renewable energy sources on earth. Wind and solar are common energy sources, but more sustainable energy sources need to be used to achieve this fossil-free living. Ocean Thermal Energy Conversion (OTEC) uses the temperature gradient of the ocean for the generation of electricity to accelerate the energy transition. The OTEC cycle contains plate heat exchangers (PHEs) which are made from titanium for high heat transfer performance and to cope with corrosive fluids, like ammonia and seawater. However, titanium is expensive and replacement is needed to reduce costs and environmental impact [114]. Polymers, on the other hand, are cheaper and are corrosion-resistant, but heat transfer is poor. Heat transfer and pressure drop indicate the performance of PHEs and increasing heat transfer and lowering pressure drop will increase the efficiency of the entire system, thereby lowering the overall cost of energy produced. The performance of composite polymer plates in heat transfer applications is still unknown. The thermal conductivity is lower compared to metal plates, which has a significant effect on heat transfer [37]. In this thesis, a small 100WOTEC plant, constructed at the TU Delft, is used as experimental setup. The test section of the setup is the condenser of the OTEC cycle, which is a gasketed plate heat exchanger (GPHE). In the first part of this thesis, two types of composite polymers are investigated to determine the design requirements of polymer plates in PHEs. Sealing of the GPHE seems to be an issue due to the low ductility and compression strength of the composite polymers. The plates are not able to withstand the compressive force needed for sealing. Three types of seals are tested in this research: conventional rubber gasket, epoxy and rubber orings. Unfortunately, none of the seals achieved sealing. A new plate and gasket design has been proposed to be able to prevent plate failure and achieve sealing for the testing of composite polymer plates in a GPHE for future research. The second part of this research focuses on the validation of heat transfer and pressure drop correlations in a 2-channel stainless steel GPHE, with ammonia (NH3) as working fluid, developed by Tao [96]. The correlations are based on 20-80 kg/m2s mass flux range, which is the mass flow per flow passage area. The experimental research is conducted with two adjustments compared to the research by Tao: ammonia-water (NH3/H2O) as working fluid and 4-channels. Experiments are conducted at mass fluxes of 15, 20 and 30 kg/m2s with changing vapour qualities. The data is analysed and heat transfer coefficients (HTCs) and pressure drops are calculated using a data analysis model. HTCs and pressure drop results are compared with the correlations developed by Tao for validation. HTC results are low compared to the expected values based on previous work. This is likely due to the presence of fouling on the stainless steel plates, the presence of a non-condensable gas in the system and a maldistribution of the flow at the inlet of the GPHE, which causes an uneven distribution of liquid and vapour across the channels. As a consequence, the applicability of the heat transfer correlation by Tao for multi-channel NH3/H2O mixture at low mass fluxes is inconclusive. The pressure drop results seem to be less effected by the factors negatively influencing the HTCs. The pressure drop correlation by Tao underpredicts the obtained pressure drop results, which is possibly due to the change of working fluid: NH3/H2O mixture. However, the applicability of the frictional pressure drop correlation by Tao is inconclusive due to the maldistribution of the flow at the inlet of the GPHE. This concludes that the knowledge of the distribution of vapour and liquid across the channels in the GPHE is important during performance testing of a GPHE.