Analysis of external influences on an OTEC cycle

Abstract

Ocean Thermal Energy Conversion (OTEC) is a promising renewable energy technology that has vast potential in the tropics. The technology uses the temperature difference between different ocean layers to run a power cycle. The effects of temperature variations for a fixed design of the thermodynamic cycle is one of the questions that has to be answered for this technology. These effects are here analysed for three different working fluids, pure ammonia, ammonia - water mixture and R32 - R134a mixture. An off design model is implemented in MATLAB to analyse the effects. Experiments were performed with pure ammonia in an OTEC experimental set up to validate the model. The heat exchangers in the thermodynamic cycle are one of the most important components since they are large and costly in comparison to other components because of the small temperature difference in the system. Based on the experiments performed, suitable heat transfer correlations were selected. It is still a question if these correlations are suitable for other working fluids especially mixtures. This has to be confirmed with further experiments. The results of the model suggest that in all cases the net power output and the thermal efficiency are proportional to the temperature difference of the ocean layers. Fluctuations in the warm and cold seawater temperature seem to have similar effect on the outcome. Fluctuation of one degree for a 25 MW plant will result in approximately 10 % fluctuation in the net power output. This is considerable and has to be taken into account in economical evaluations for each suitable location. To minimize negative effects the mass flow of the working fluid can be varied however that will only improve the performance to a small degree.