The increasing demand in renewable energy motivates the search for new renewable energy sources. The ocean is a vast energy source of both thermal and kinetic energy, however methods to harness this energy haven’t reached maturity yet. Ocean Thermal Energy Conversion (OTEC), a method to harness the thermal energy of the ocean, could change this. In order to get to this stage of maturity, experimental and numerical research on the OTEC power cycle was done by Goudriaan [12] and Kuikhoven [23]. However, the numerical model developed showed inaccuracy for the condenser and lacked physical insight into the processes of heat transfer. The condenser and consequently, the cold water pipe, are a significant part of the investment costs of an OTEC plant. Therefore, the goal in this research was to increase the accuracy of the performance calculations of the condenser and to gain more knowledge on the physical phenomena in the condenser heat exchanger. Two different methods were tested in order to improve the condenser model. In the first part a heat transfer correlation is fitted to the experimental data. A literature study was done on two phase heat transfer correlations to investigate what phenomena occur in two phase condensation flow. Relevant dimensionless quantities were chosen accordingly. The quantities used are the conventional Reynolds and Prandtl number. The mass transfer that occurs through absorption was represented by the Schmidt number. The resulting correlation fit was tested on accuracy using the experimental data obtained by Goudriaan [12] and Kuikhoven [23]. To be able to compare the results of this research with their research, the in-depth validation was done for the same working fluid mass flow of 0.010 kg/s that was used by Goudriaan [12] and Kuikhoven [23]. The Schmidt number showed a positive effect on the accuracy. The correlation was extrapolated for other working fluid mass flows as well. It was found that the correlation fit is a useful tool for the working fluid mass flows that have a large amount of experimental data available (the cases 0.007, 0.010 and 0.013 kg/s) but isn’t as accurate for working fluid mass flow with a small amount of experimental data available (0.005 kg/s). In the second part of the report a detailed condenser model was developed using the method of Fernández- Seara et al. [10]. This model was used to provide more physical insight into the two phase mixture behaviour in a condensation process. The model is also capable of predicting the required pressure on the working fluid side to provide full condensation. Literature research was done on different aspects of the condensation process in a plate heat exchanger. Film thickness correlations were tested. Different heat transfer correlations developed for plate heat exchangers were investigated. Flow pattern theory and flow pattern maps were used to predict the flow behaviour. The method of Fernández-Seara et al. [10] was used to predict mass transfer through the vapor-liquid interface in the working fluid. A pressure iteration loop was added to predict the condenser pressure on the working fluid side. The initial results were slightly off compared to the experimental measurements. The reason for the inaccuracy is expected to be the neglection of surface tension effects on the wettability of the working fluid. Dry surface voids are expected to occur in the liquid film with decreasing concentration. A correction factor based on surface tension was added to include this effect. The 0.010 kg/s case was validated in detail and showed decent accuracy. At 0.007 kg/s working fluid mass flow deviations were slightly more significant but the trends of the characteristic variables were similar to the experimental data. The mass transfer iteration loop showed inaccuracies and instabilities at the working fluid mass flows of 0.005 and 0.013 kg/s. The model showed that for the range tested the condenser performs better with increasing concentration of ammonia. The non-linear behaviour of the mass diffusivity decreases the mass transfer coefficients at decreasing concentrations. The surface tension correction factor also adds to the decrease in performance, however, if the mass flux of the working fluid would be higher, this correction factor might not be valid because the dry surface voids might not occur any more. The overall heat transfer coefficient increases towards the outlet of the condenser.

Ocean Thermal Energy Conversion (OTEC) uses the temperature difference between the surface and deep layers to generate electricity. The temperatures in the ocean hardly vary between day and night or between the seasons, which ensures a very constant energy production with OTEC. In order for OTEC to compete with the current forms of energy production, optimization is necessary such that costs are reduced to make OTEC power generation economically feasible. In cooperation with the Delft University of Technology, Bluerise B.V. constructed a small scale OTEC-plant to test the performance of the OTEC-cycle and optimize its outputs. The 100 W OTEC demo consists of an organic Rankine cycle using pure ammonia as the working fluid that is downward condensed in a gasketed plate heat exchanger (GPHE). Plate heat exchangers (PHEs) are known to have a wide range of applications due to their superior performance in relation to favourable heat transfer coefficients (HTCs), compactness, design flexibility and thermal effectiveness. However, the two-phase behaviour inside the PHEs is yet not fully understood, resulting in over- or underestimating by several published heat transfer and pressure drop correlations, which are limited by the range of conditions they cover. Tao et al. (2018) conclude that better predictions of flow patterns and dominant physical phenomena in PHEs will improve the calculation of heat transfer and pressure drop. Therefore, the goal in this research is to identify the flow patterns that occur in the GPHE and gain more knowledge on the dominant physical phenomena that are apparent in the downward condensing ammonia. In the future, this can be used to increase the accuracy of performance calculations of the condenser. Flow visualization experiments are performed on the GPHE to identify the flow patterns and how they relate to the performance of the GPHE. A visualization section is designed in the OTEC demo that allows for flow visualization. The visualization section involves the GPHE, a transparent visualization plate, illumination by a LED-strip and a high speed camera. The first part of this thesis involves material research on glass and several polymers to produce the visualization plate. The setup requires a plate that is chemically compatible with ammonia and has a mechanical performance that is fit for the operational pressures inside the GPHE. Chemical tests prove that Polystyrene is chemically well resistant to ammonia for four days. The results of a finite element analysis predict that the mechanical performance of PMMA is promising but glass is critical for the GPHE. To ensure a safe setup, a material combination of PS/PMMA is selected. A visualization plate is manufactured from a corrugated PS layer on top of a thick PMMA base plate. The PS layer acts as a chemical resistant barrier between ammonia and the PMMA base, while PMMA ensures the mechanical performance. However, after 7 days of performing the visualization experiments, the plate showed limited durability with respect to ammonia, as cracks in the material emerged and the plate lost its transparency. The second part of the research in this thesis involves two types of experiments. The first type involves flow visualization experiments on a single-channel configuration. For all conditions, only film flow and partial film flow are observed. A low mass flux shows partial film flow with a smooth film, and a high mass flux corresponds to film flow with rough film characteristics. Partial film flow includes dry-out areas on the plate, and it is concluded that these surface area voids increase with vapor quality and volumetric void fraction. Mass flux contributes to a pressure drop increase and therefore film flow corresponds to a higher pressure drop in the PHE with respect to partial film flow. The HTCs increase with mass flux but mostly with vapor quality. However, there seems to be an optimum for the HTCs with increasing vapor quality when partial film flow is observed. For partial film flow, the surface area voids increase with vapor quality and for large voids the slope of the HTCs decay. It is concluded that surface area voids have a negative influence on the HTCs. Proposed flow patterns maps by previous studies are not in accordance with the observed flow patterns inside the GPHE for the current experimental conditions. For this reason, a flow pattern map for downward condensing ammonia is proposed for this experimental configuration and conditions. The second type of experiments involves multi-channel experiments without visualization to predict the flow patterns for lower mass fluxes. The slope of the HTCs followed the same trend with increasing vapor quality for the observed partial film flow in the visualization experiments. This indicates that for this multi-channel configuration partial film flow occurs in the GPHE.

In the search for new forms of renewable energy production, an interesting option is to make use of the energy stored in the ocean in tropical areas. The sun heats up the surface water in these areas to a point it is feasible to generate electricity, using the temperature difference between the hot surface water and the cold ocean water at around 1 [km] depth. This form of renewable energy production is called Ocean Thermal Energy Conversion (OTEC). As the water temperature in these areas is quite constant, the electricity generation is quite constant as well, which is a big advantage as no inefficient energy storage or unsustainable base load generation is needed. To make OTEC power generation more economically attractive, design optimization and technology up-scaling need to be done, lowering the costs of the components and making sure the components work as efficiently as possible. A crucial component is the condenser. In the condenser, heat is transferred from the working fluid in the cycle (most commonly ammonia or ammonia-water) to the cold water. The working fluid is condensed, which can then be pumped around. The process of pumping the water up from 1 [km] depth is a costly operation (Kirkenier, 2014) and the condenser itself is a significant part of the cost of an OTEC-plant. It is relevant to gain more insight in the transport phenomena in the condenser by executing experiments, so the cost can be reduced. The experimental set-up is a small scale OTEC cycle prototype (OTEC-demo), located in the Process & Energy laboratory at the Delft University of technology. For this research, a gasketed plate heat exchanger is added to the set-up, which can be used as a condenser. On the gasketed plate heat exchanger it is possible to apply some modifications and to change the number of plates, as the heat exchanger can be disassembled. Nine miniature temperature sensors are placed along the flow direction of one of the heat transfer plates for local temperatures measurements. For future research on the flow patterns in the condenser, a transparent corrugated plate was designed and fabricated for visualization purposes. First, the plate was designed with 3D CAD software. Then, the plate was constructed by milling, at the central workshop of the Delft University of Technology. A single phase convective heat transfer coefficient correlation and a pressure drop correlation for the cold water side in the condenser were proposed, by executing experiments with water on both sides of the heat transfer plates in the gasketed plate heat exchanger. In order to research the influence of the vapour quality and the mass flux on the convective heat transfer coefficient on the working fluid side, ammonia-water mixture condensation experiments were executed. During the experiments, the measured cold water temperature profiles were linear for all the set mass flows. This means the convective heat transfer coefficient did not change much along the flow direction of the plate, which can be explained by the small change in vapour quality during the ammonia-water mixture experiments. The convective heat transfer coefficient on the working fluid side increases for increasing mass flux and also increases for increasing vapour quality. The results of these experiments were compared to the theory. A numerical condenser model, which can predict the measured data, was developed. The numerical condenser model is a modification of the numerical condenser model by Goudriaan (2017) and Kuikhoven (2017). By comparing the results of the experiments to the results from the model, the assumptions made in the model can be validated. The model can predict the cold water inlet temperature and the working fluid outlet temperature of the experimental data within an accuracy of 4%. However, this only applies to the measured range of experiments. More experiments are required to propose correlations that are valid in a higher range of mass flows and vapour qualities. Furthermore, pure ammonia experiments were executed. The results were compared to the ammonia-water mixture experiments. The convective and overall heat transfer coefficients decreased for increasing mass flux during the pure ammonia experiments, while the convective and overall heat transfer coefficients increased for increasing mass flux during the ammonia-water mixture experiments. More experiments are needed to investigate this phenomenon. The pressure drop of the pure ammonia flow is slightly higher than the pressure drop of the ammonia-water mixture, due to the lower density of pure ammonia.

In the quest for renewable energy for tropical islands, Ocean Thermal Energy Conversion (OTEC) has revealed itself as one of the most promising technologies. OTEC stands or falls with the performance of the condenser. Plate Heat Exchangers (PHEs) have proven their selves as effective heat transfer equipment for low enthalpy cycles such as OTEC. Nowadays, the design of PHEs mostly relies on models based on experimental data. This is deemed time-consuming and non-generic. Enormous successes in the application of Computational Fluid Dynamics to single-phase flows have raised the interest to use CFD as a predictive and generic tool for the design of PHEs. Moreover, CFD yields a picture of the complete flow field, enabling the designer to better understand the fundamental flow structures that are relevant for the heat transfer and pressure drop performance of the PHE. First, a comprehensive review of literature on condensation models is presented. Thereafter, the practicality of the phase change models in terms of real computation time needed is investigated. It is concluded that the stability issues associated with the vapor-liquid interface poses the biggest problem. That is, for the simple Nusselt condensation problem, an extremely long computation time is needed for reasonable accuracy. Finally, the ability of the phase change models to cope with wave evolution, flow circulation and flow separation is investigated. The multiphase model is found to properly predict the flow field. However, the heat and mass transfer predicted by the condensation models is estimated poorly. In the end, until the stability issues of the current phase change models are solved, or a significant improvement in computational performance is made, the application of CFD to the design of condensation PHEs is questionable. Yet, using CFD only to find trends in heat transfer and pressure drop in PHEs might prove to be fruitful.