Ocean Energy Technology Program

Performance analysis of ocean thermal energy conversion system integrated with waste heat recovery from offshore oil and gas platform

South Korea is currently in the preparatory stage of commercializing an ocean thermal energy conversion (OTEC) system, as the demonstration of a 1 MW scale OTEC system has been accomplished. However, the commercialization of OTEC requires the establishment of a control system for various environmental changes. Therefore, pre-emptive identification of the system’s risk factors and the process of analyzing the impact of the system, building control items, and optimizing control are necessary. This study aims to establish and analyze an optimized control system for MW-scale OTEC risk factors, such as the shutdown of seawater or refrigerant pumps. The selected OTEC system was designed for 1070 kW class facilities, with a 36.6% portion of total electricity usage by the seawater pump and refrigerant pump. As a result, an on/off control system was adopted in order to eliminate the risk factors. By adjusting this option, dry operation of the refrigerant pump, water hammering, and liquid inflow into the turbine were successfully prevented. To be more specific, the initial system was to be shut down due to a sharp decrease in power at the point where the deep seawater flow rate was 538 kg/s (35.7% of max flow rate) and the surface seawater flow rate was 715 kg/s (38.4% of max flow rate). This situation was improved by adopting parallel operation of seawater pumps and on/off control, thereby leading to a more stabilized operation by maintaining a flow rate of over 1864 kg/s for surface seawater and 1507 kg/s for deep seawater. Moreover, it was confirmed that the flow rate of the pump was reduced by 1.89 kg/s per second in the process of pump shutdown during a single operation mode of the refrigerant pump. Parallel operation made it possible to maintain 60.2% of the output by increasing the power of the second pump’s flow rate in the event of the first pump shutting down. The final seawater temperature differential power generation model derived from this study consists of two refrigerant pumps and two surface seawater and deep seawater pumps in order to prevent system shutdown caused by a single pump failure. The final design was reflected in the final delivery to Kiribati, which is located near the equator.

Ocean thermal energy conversion system uses from water in surface of the ocean as high temperature source and water in the depth of the ocean as low temperature source. Three types of ocean thermal energy conversion systems including close cycle, open cycle and hybrid systems are available. In a close cycle system, working fluid through a thermodynamic cycle based on the Rankine cycle can rotate the turbine and generate electricity. Due to the variation in the ocean surface temperature, the output power of the ocean thermal energy conversion system is not fixed and controllable and so this uncertainty nature results in the numerous states in the generated power of this plant. Thus, in integrating ocean thermal energy conversion systems to the power system, many aspects of power system such as reliability may be affected and so new approaches must be developed for investigation these effects. In this regard, in this paper for the first time, the reliability of power system containing ocean thermal energy conversion system is evaluated and the valuable indices such as loss of load expectation, expected energy not supplied and peak load carrying capability that can be used for generation expansion planning of power system, are calculated.

This research focuses on enhancing the efficiency of Bi2Te3-based thermoelectric generators (TEGs) in ocean thermal energy conversion (OTEC) systems through innovative heat exchanger designs. Our comparative study uses computer simulations to evaluate three types of heat exchangers: cavity, plate-fins, and longitudinal vortex generators (LVGs). We analyze their impact on thermoelectric conversion performance, considering the thermal energy transfer from warm surface seawater to TEGs. The results demonstrate that heat exchangers with plate-fins and LVGs significantly outperform the cavity heat exchanger regarding thermal energy transfer efficiency. Specifically, plate-fins increase TEG output power by approximately 22.92% and enhance thermoelectric conversion efficiency by 38.20%. Similarly, LVGs lead to a 13.02% increase in output power and a 16.83% improvement in conversion efficiency. These advancements are contingent upon specific conditions such as seawater flow rates, fin heights, LVG tilt angles, and locations. The study underscores the importance of optimizing heat exchanger designs in OTEC systems, balancing enhanced heat transfer against the required pump power. Our findings contribute to a broader understanding of materials science in sustainable energy technologies.

Multi-objective optimization of an open-cycle, ocean thermal energy conversion system with desalinization

The organic Rankine cycle is one of the key methods to utilize ocean thermal energy, which is abundant and continuous renewable energy. In this paper, a non-dominated sorting genetic algorithm-II (NSGAII) based approach is utilized to optimize an organic Rankine cycle-based ocean thermal energy conversion system. The objective is to maximize the degree of thermal energy utilization and the maximum system power output in. The decision variables considered for optimization include evaporation temperature, condensation temperature, degree of superheating, and working fluid mass flow. The objective functions are defined as thermal efficiency and net power output. The optimization process for the ocean thermal energy conversion Rankine cycle is performed using NSGA-II. The results indicate an inverse relationship between thermal efficiency and power output, meaning that achieving the highest thermal efficiency does not necessarily result in the highest net power output. The utilization of NSGA-II enables the identification of multiple optimal solutions that lie on the Pareto front, representing the best trade-off between thermal efficiency and power output. By considering the investment cost constraint, the most suitable solution for achieving the maximum net power output can be determined.

The study explores the feasibility of utilizing Ocean Thermal Energy Conversion (OTEC) systems collocated with deepwater oil and gas facilities to power monitoring activities for plugged and permanently abandoned (PA) offshore oil and gas wells in the Gulf of Mexico (GOM). As offshore oil and gas operations expand, the need for sustainable monitoring solutions for PA wells, which may release hydrocarbons, may become critical. Current regulations do not mandate long-term ongoing monitoring post-PA, presenting an opportunity for innovative approaches. This research evaluates the potential of OTEC, a renewable marine energy technology, to provide continuous, dispatchable power for such monitoring systems. The study models an OTEC system collocated with an oil and gas platform, assessing its capability to power long-range autonomous underwater vehicles (AUVs) shuttled to the relevant locations for periodic surveys of PA wells. The findings suggest that while OTEC can technically support PA well monitoring, logistical challenges such as and economic viability remain. The study underscores the need for further research and investment in OTEC technology to enhance its economic competitiveness and integration with offshore operations. This approach not only aids in addressing potential PA well hydrocarbon release concerns but also offers a model for other offshore monitoring applications, including subsea geologic carbon dioxide (CO2) storage projects.

Thermodynamic, economic, exergoeconomic analysis of an integrated ocean thermal energy conversion system

This chapter mainly introduces the development and prospect of turbines utilized in ocean thermal energy conversion (OTEC), including brief introduction, aerodynamic design, mechanical and electric control system, problems, and prospect of the turbine in OTEC. At the beginning, the first section mainly introduces compositions and types of turbine in OTEC systems, different working fluids in the turbine, status of OTEC turbine currently in the world, and so on. After that, the aerodynamic design of turbine has the greatest impact on the efficiency of the turbine and is the most important process during the turbine development. Therefore, the second section focuses on turbine aerodynamic design. Furthermore, the third section mainly introduces the mechanical system and the electric control system of the turbine. Finally, the problems in the turbine currently are listed, and the prospects for the development of the turbine in OTEC are discussed in the fourth section.

This chapter presents ocean wave energy, tidal energy, ocean current energy, ocean thermal energy, and geothermal energy techno‐economic summaries including information on resource characteristics, conversion technologies, power electronics approaches as applicable, and grid interface issues. It also presents autonomous, non‐grid‐connected ocean wave energy applications, including adapting utility‐scale wave energy devices for small‐scale autonomous and sensor applications. Ocean thermal energy conversion systems convert the energy available in the temperature gradient of the warm surface‐water layers and the deep, cold ocean depths of approximately 800‐1000m. Marine renewable energy can also be harnessed from the tides, from tidal currents, and non‐tidal ocean currents. Geothermal energy conversion is an abundant, renewable resource consisting of the natural heat generated and stored in rock and fluids in the Earth's crust that can be used for electricity generation and for heating and cooling purposes.

The limited petroleum resources available for mankind have led to the exploration and development of oil fields in the offshore deep-sea regions, especially in light of the rising oil prices. Different studies and projects on the ocean thermal energy conversion (OTEC) technology, which operates the power generation facilities using the temperature difference between the deep-sea water and the sea surface water, are currently being undertaken. In this study, various methods of applying the OTEC technology to a floating production storage and offloading (FPSO) facility installed in a deep-sea area were simulated and analyzed. In the closed cycle, a 1595.5 kW net power generation capacity was obtained. In the open cycle, a 863.5 kW generating capacity and 54.4 m3/h fresh water were obtained. In the hybrid cycle, a 643 kW generating capacity and 55.1 m3/h fresh water were obtained. In addition, the cooling water of the heating, ventilation, and air-conditioning systems (HVACs) obtained using deep-sea water cooling was also investigated. Ultimately, a 100 MW OTEC power plant was designed, and the required seawater amount and operating fluid capacity were confirmed. Considering the similarity of OTEC power generation facilities and FPSOs, the conversion of an FPSO in the deep-sea region to an OTEC power generation facility after the end of the life of the FPSO was proposed. It will be an alternative way of addressing two important issues: the need for petroleum resource development considering the limited petroleum resources that are currently available, and the need to ensure the sustainable development of countries.

In the ocean thermal energy conversion (OTEC) system, electrical power is generated by exploiting the temperature difference between cold deep seawater and warm surface water. This study introduces an innovative method that uses a solar collector to increase the temperature of the water entering the evaporator and a wind turbine to increase the useful power, thereby increasing the energy efficiency and power output of the cycle. The research findings indicate that the proposed cycle achieves an average net production power of 36.63 megawatts, with energy productivity and exergy at 9.70% and 29.12%, respectively. Additionally, freshwater production has increased by 170.26% compared to the base cycle. A sensitivity analysis of various parameters was also conducted on the proposed cycle, which demonstrated a reduction of 89.69 tons in carbon dioxide emissions. Finally, to optimize the designed system, the response level multiobjective optimization method has been used to find the best set of objective functions and decision variables. The four objective functions of this optimization included exergy efficiency, total system energy, production work, and system cost rate.

Ocean energy is one of the renewable energy sources. Ocean thermal energy is the most abundant in marine energy, so it has a good development prospect. At present, the main way to use ocean energy is power generation. However, it has disadvantages: low efficiency and high energy consumption. In view of the above shortcomings, this study puts forward a new type of ocean energy comprehensive utilisation system of water–electricity cogeneration. The system is based on the ocean thermal energy conversion: wave energy power generation and solar pond are integrated in the system. The basic principle and working process were analysed, and the energy consumption and the product of the system were calculated. As shown in the calculation results, it is effective to reduce energy consumption and improve the system efficiency. The research of this study has valuable reference for the development of ocean energy in the future.

This paper presents a review of the research on closed thermodynamic cycles of ocean thermal energy conversion (OTEC) system, including a description of thermodynamic cycles with either pure or mixture working fluids, and describes the effects of various working fluids on cycle efficiency. For cycles with pure working fluids, the efficiency changes due to change in the evaporation and condensation temperature caused by heat resource differences. For cycles with mixture working fluids, the efficiency may be improved by a number of techniques, such as heat recovery of ammonia-depleted solution and the intermediate extraction regeneration. Furthermore, the effect of the ejector on performance of the cycle is also reviewed. Finally, the techniques used to improve efficiency are discussed and summarized. In general, the thermodynamic efficiency can be improved by adopting suitable working fluids and measures which could increase the utilization rate of ocean thermal energy. The related methods need to be compared and analyzed under the same working conditions to determine which is the most effective.

In the present study, we investigated a multiple energy generation system based on the use of solar, wind and ocean thermal energy for the study area with high renewable energy potential. In this study, all existing capacities in coastal areas (wind, sun, ocean temperature difference) are used to design a renewable system. The present system consists of subsystems including organic Rankin cycle (ORC), wind turbine, thermoelectric, heat exchanger and flat panel solar collector. EES thermodynamic software has been used to model the system and obtain thermodynamic results. The system designed for the city of Bushehr, which has good ocean thermal energy, wind energy and solar radiation potential, has been studied. The most important effective and practical parameters in the proposed system are collector area, solar radiation, and wind turbine speed. The results of the investigation of the system in Bushehr City showed that the production power of the system is 4,840,913.8 and 1,138,957.02 kWh compared to the climate changes. Also, the amount of freshwater produced by the system is 103,495.99 and 429,080.51 m3/h compared to the climate changes. Finally, in order to optimize the designed system, the response level multi-objective optimization method has been used to find the best set of objective functions and decision variables. The two objective functions of this optimization included the exergy efficiency of the whole system and the system cost rate. The most optimal value of exergy efficiency was 13.25% and the cost rate was 72.17 $/hour.