Techno-economic efficiencies and environmental criteria of Ocean Thermal Energy Conversion closed Rankine cycle using different working fluids

Abstract

Ocean Thermal Energy Conversion (OTEC) is a foundation for an appealing renewable energy technology owing to its vast and inexhaustible resources of energy, stability, and sustainable output. Development of OTEC power plant is to exploit the energy accumulated in between the top layer of warm surface seawater (heat source), and the cold layer of deep seawater (heat sink). It operates based on Rankine cycle to produce electricity between the source and the sink at the smallest temperature difference of approximately 20 K. OTEC power plant commonly utilized ammonia as working fluid. Nevertheless, ammonia poses potential lethal health risks and hazardous fluid. Hence, the effect of the working fluid types and the subsequent operation conditions may be critical and therefore become the subject of this study. In addition to OTEC power plant’s thermodynamic efficiencies study, this research also explores the economic efficiencies in term of capital cost per net power output ($/kW) and environmental criteria of different working fluids including that of ammonia, ammonia-water mixture (0.9), propane, and refrigerants (R22, R32, R134a, R143a, and R410a). The results showed that ammonia-water mixture gave the excellent performance with regard to the characteristics of heat transfer with the best thermodynamic efficiency of 4.04% compared to pure ammonia with 3.21%, propane with 3.09%, followed by refrigerants from 3.03% to 3.13%. Capital cost of using propane was economically efficient with 15730 $/kW compared to ammonia-water mixture at 16201$/kW, refrigerants from 16990 $/kW to 21400 $/kW, and pure ammonia being the costliest at 21700 $/kW. Despite being lower in its thermodynamic efficiency, propane gave the lowest capital cost and had the lowest toxicity in contrast to all other working fluids. Therefore, propane has the potential to be used as a clean and safe working fluid that would further enhance the OTEC technology.

System Parameters Sensitivity Analysis of Ocean Thermal Energy Conversion

“…In contrast, the cycle employing R134a demonstrates the lowest net power output. These results are consistent with the research conducted by Samsuri et al [25] to find the most ideal working fluid for OTEC systems, including ammonia and R134a. Based on this research, ammonia working fluid produced the highest net power output among other pure working fluids, including R134a.…”

“…From the evaluation of the standard deviation at each depth of cold seawater for the three cycles, the average standard deviation at the Bungkulan site is only 0.0269, while the standard deviation at each depth of cold seawater at the Celukan Bawang location is only 0.0270 The efficiency trend in each cycle is inversely proportional to the net power trend. The cycle efficiency results are also inversely proportional to research conducted by Samsuri et al [25], in contrast to the similarity in net power output. This difference in results occurs because Samsuri et al [25] used the same seawater mass flow rate between each working fluid in their research, whereas in this study, the mass flow rate between each working fluid is differentiated according to the mass flow rate required to achieve the heat exchanger performance.…”

“…The cycle efficiency results are also inversely proportional to research conducted by Samsuri et al [25], in contrast to the similarity in net power output. This difference in results occurs because Samsuri et al [25] used the same seawater mass flow rate between each working fluid in their research, whereas in this study, the mass flow rate between each working fluid is differentiated according to the mass flow rate required to achieve the heat exchanger performance. Thus, the required pump power is different for each working fluid, with the increase in pump power affecting the efficiency of the OTEC cycle negatively.…”

“…On the other hand, most research on the potential of OTEC power generation, especially in terms of net power calculation methods, has failed to include the energy required to operate the seawater pump. This is the case in the study by Samsuri et al [25], which compared the efficiency of several working fluids but only calculated the efficiency of the working fluid pump. Similarly, research on the Rankine cycle OTEC performance by Thirugnana et al [23] and Lee et al [26] did not include seawater pumping.…”