Open-cycle Ocean Thermal Energy Conversion Research Articles

Data-driven model based adaptive feedback-feed forward control schemes for open cycle - OTEC process

Open Cycle - Ocean Thermal Energy Conversion (OC-OTEC) is one of the most important renewable energy sources that generate electricity and fresh water from seawater utilizing the temperature gradient between the warm surface seawater and the cold deep seawater. This paper aims to develop non-linear data-driven model-based adaptive Feedback Control (FBC) schemes for the OC-OTEC process to track the output power and a dynamic Feed-Forward Control (FFC) scheme to reject the effects of temperature disturbance on power caused by climate variations in OC-OTEC. The experiments are conducted on a laboratory-scale OC-OTEC experimental setup at the National Institute of Ocean Technology, Chennai. Firstly, linear data-driven models are developed using system identification techniques. Based on the developed models, gain scheduling-based adaptive control schemes are developed: Proportional Integral (PI) control and Model Predictive Control (MPC). Secondly, the closed-loop performances of the developed FBC schemes are analysed under servo and regulatory operations. Furthermore, it is observed from the sensitivity analysis results that the temperature disturbance highly influences output over the manipulated variable. Hence, a dynamic FFC scheme is implemented to reject known disturbance of 1 °C variation in temperature gradient from 18 °C to 19 °C due to Sea Surface Temperature (SST) changes in temperature gradient. The results show that the proposed MPC-based FBC-FF scheme effectively enhances the tracking and disturbance rejection performance compared to the PI-based FBC-FF control scheme with a minimum Integral Square Error (ISE) of 3.055 and Control Effort (CE) of 7.505. This experimental data-based, data-driven modelling and adaptive control research provides a new direction for automating OC-OTEC. Further, these studies will help to develop a rugged and automated upscaling OC-OTEC plant control system with proposed feasible control schemes.

Optimization of distribution networks for water and energy in isolated regions: A multi-objective approach incorporating ocean thermal energy conversion technologies

The limited resources and increased vulnerability to climate change and environmental degradation in isolated regions such as islands and archipelagos make the development of sustainable technologies for water and energy provision a crucial factor in addressing resource scarcity and mitigating anthropogenic impact. This work presents a multi-objective optimization analysis incorporating an Open Cycle Ocean Thermal Energy Conversion (OC-OTEC) plant in the water and energy distribution networks of an isolated region. The optimization considers three interrelated objectives: economic viability measured by annual profit, environmental sustainability reflected in reduced CO2 emissions and aquifer water usage, and social impact through job creation. The optimization results demonstrate the positive impact of integrating the OC-OTEC plant into the Tahiti distribution network, achieving an optimal configuration that balances economic, environmental, and social considerations. Finally, the proposed model is applied to Tahiti of French Polynesia as a case study.

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

Developing technologies that can supply water and energy in a sustainable manner is essential in addressing resource scarcity of small communities. The main motivation of this article is to find, through the analysis of different study cases, the optimal design of an Open-Cycle, Ocean Thermal Energy Conversion (OC-OTEC) system. Specifically, we propose optimization formulations to determine the operation policy and sizing of the system while considering four conflicting objectives: minimizing the total annual cost, minimizing CO2 emissions, and maximizing the generation potential of the products derived from the operation of the OC-OTEC (water and energy). The formulation captures seasonal effects and restrictions on water and energy demands determined by the end-user as well as diverse technical, operational, and design restrictions. The proposed model is applied to a study area in the Pacific region of Mexico.

Evaluation of hybrid ocean thermal energy conversion system plantwide performance

Abstract Ocean Thermal Energy Conversion (OTEC) is a renewable energy source in which energy is produced by converting the heat stored in the sea or the ocean thermal energy into valuable work, based on the temperature difference between the warm surface seawater and the cold deep seawater. One of the OTEC system requirements is to have a seawater temperature difference at a minimum of 20 °C within a depth of 1000 m below sea level. Recognizing the importance of optimum sea water temperature, several studies have been conducted to optimize the OTEC system. However, none of these studies was attempted under a hybrid ocean thermal energy conversion (H-OTEC) setup. A H-OTEC system is a combination of closed-cycle and open-cycle OTEC system. The objective of this study is to evaluate the performance of the H-OTEC process system based on the impact of seawater temperature variation by simulating H-OTEC process system. Aspen HYSYS was used as a chemical process simulation platform for conducting this study. After the model was completed, verification test was conducted before the simulated data was recorded. The data for the pump work input and the turbine work output were acquired to determine the net power output and system efficiency. The net power output, Carnot efficiency, and thermal efficiency were recorded approximately 1.39 kW, 5.7%, and 1.45%. The data for net power output and the efficiencies of the system was recorded for every 1 °C of increment in surface seawater temperature. The results showed that the net power output increased slightly by 0.5kW, with efficiency difference for both Carnot cycle and actual cycle, recorded to be less than 3% and 0.1% respectively.

Indirect air CO2 capture and refinement based on OTEC seawater outgassing

SummaryIn this paper, we propose a low-cost method to capture and purify CO2 from ocean water indirectly from the atmosphere. Atmospheric CO2 dissolves in seawater following Henry's law. In open-cycle ocean thermal energy conversion (OC-OTEC), being a heat engine, large quantities of water are used to generate electricity from temperature differences in the ocean. CO2 and other gases dissolved in seawater are extracted from seawater by a vacuum compressor, being essential for heat transfer in OC-OTEC. Non-condensable gases containing CO2 from OC-OTEC are currently considered a waste gas stream. Based on literature, we calculate cold water non-condensables containing up to 14% CO2. These non-condensables can be refined further to 80–90% purity with a water absorption process, inspired by those as used in the biogas industry. Levelized CO2 capture and purification costs were estimated to be 15–35 euro per ton, as only refinement costs are allocated of a waste gas stream of OC-OTEC.

Ocean Thermal Energy Conversion—Flexible Enabling Technology for Variable Renewable Energy Integration in the Caribbean

Many Caribbean island nations have historically been heavily dependent on imported fossil fuels for both power and transportation, while at the same time being at an enhanced risk from the impacts of climate change, although their emissions represent a very tiny fraction of the global total responsible for climate change. Small island developing states (SIDSs) are among the leaders in advocating for the ambitious 1.5 °C Paris Agreement target and the transition to 100% sustainable, renewable energy systems. In this work, three central results are presented. First, through GIS mapping of all Caribbean islands, the potential for near-coastal deep-water as a resource for ocean thermal energy conversion (OTEC) is shown, and these results are coupled with an estimate of the countries for which OTEC would be most advantageous due to a lack of other dispatchable renewable power options. Secondly, hourly data have been utilized to explicitly show the trade-offs between battery storage needs and dispatchable renewable sources such as OTEC in 100% renewable electricity systems, both in technological and economic terms. Finally, the utility of near-shore, open-cycle OTEC with accompanying desalination is shown to enable a higher penetration of renewable energy and lead to lower system levelized costs than those of a conventional fossil fuel system.

Simulation Data of Regional Economic Analysis of OTEC for Applicable Area

To mitigate the power shortage problem in the South Pacific island nations and the equatorial area, we studied the development characteristics of ocean thermal energy conversion (OTEC) using abundant clean ocean energy. Through the simulation of open- and closed-cycle OTEC, the generation amount and its economic efficiency were compared, and the application characteristics of the power generation cycle according to the seawater temperature distribution were compared by applying various seawater temperature conditions. According to the characteristics of seawater heat sources in the region, the power generation output was about 883.2 kW in Samoa, and the average power generation by region was about 650.5 kW for the open-cycle OTEC model. Regional revenue up to approximately $8,487,000 was generated in Kiribati, driven by the higher electricity tariff of $0.327/kWh and high water costs of $5.86/ton. With the spread of 50 MW commercial plants, Kiribati had a high net present value of $1,930,402,000, and its internal rate of return was more than 37.0%. This paper is presents a method of securing economic feasibility of OTEC according to various heat source conditions and economic conditions in the region, while it also analyzes the capacity and type of the power plant.

Ocean thermal energy conversion and open ocean mariculture: The prospect of Mainland-Taiwan collaborative research and development

Ocean thermal energy conversion (OTEC) is an energy generation technology that uses cold deep ocean water (DOW) and warm surface water to produce electricity. Active development of OTEC was started in the 1970s with Hawaii as a major research and development center. In the following several decades, small pilot-scale closed-cycle and open-cycle OTEC plants were successfully designed, constructed, and tested. An open-cycle OTEC plant produces both electricity and freshwater. Besides its low temperature, DOW is also nutrient-rich and free from pathogenic bacteria. DOW-enhanced open ocean mariculture can significantly increase the world fish catch and induce an air to water transfer of greenhouse gas CO2. Therefore, an integrated development of DOW as a natural resource is the center of a “blue revolution” that has the potential to solve four of the most urgent world problems: energy, freshwater, food, and global warming. This paper will review technological development of OTEC and open ocean mariculture, followed by a discussion of the prospect of Mainland-Taiwan research collaboration on the technological development and application of DOW resources.

Open‐cycle OTEC systems with freshwater product: Effects of noncondensable gases on performance of condenser

AbstractAn open‐cycle ocean thermal energy conversion (OC‐OTEC) system is one of the energy conversion methods used to generate electricity from ocean thermal energy. For the OC‐OTEC system, steam evaporated from the surface seawater due to flash evaporation drives the turbine. At that time, dissolved gas such as air is introduced into the low‐pressure system (OC‐OTEC system) as the noncondensable gas, which degrades the performance of condensation heat transfer.In this paper, a small‐scale OC‐OTEC experimental unit experimentally investigates the effect of noncondensable gas on the heat transfer performance in a condenser. The experimental results are discussed in comparison with theoretical estimation by the Sparrow–in method. It is shown that the condensation is occupied by heat and mass transfer near a condensation surface and that the condensation efficiency is affected by exhaust quantity of noncondensable gas at a relatively high concentration ratio of condensable gas. © 2005 Wiley Periodicals, Inc. Electr Eng Jpn, 154(1): 29–35, 2006; Published online in Wiley InterScience (www.interscience.wiley.com). DOI 10.1002/eej.20179

Open Cycle OTEC System with Fresh Water Product-Effects of Noncondensable Gases on Performance of a Condenser-

An open-cycle ocean thermal energy conversion (OC-OTEC) system is one of energy conversion methods to generate electricity from ocean thermal energy. For OC-OTEC system, steam evaporated from the surface seawater due to flash evaporation drives the turbine. At that time, dissolved gas such as air is introduced into the low-pressure system (OC-OTEC system) as the non-condensable gas, which degrades the performance of condensation heat transfer. In this paper, a small scale OC-OTEC experimental unit experimentally investigates the effect of non-condensable gas on the heat transfer performance in a condenser. The experimental results are discussed in comparison with theoretical estimation by Sparrow-Lin method. It is shown that the condensation is occupied by heat and mass transfer near a condensation surface and that the condensation efficiency is affected by exhaust quantity of non-condensable gas at relative high concentration ratio of condensable gas.

A financing strategy for small OTEC plants

The present paper proposes a financing scheme that would spread in time the capital cost of, and the technological risk associated with, small land-based Ocean Thermal Energy Conversion (OTEC) plants. It is based on the cost effectiveness of some OTEC by-products. Three separate phases are envisioned: the first one would consist in supplying air-conditioning (A/C) needs with deep cold seawater pumped through a pipe designed for larger OTEC flow rate requirements; the second one, in building a desalination unit based on some Open-Cycle OTEC hardware, with externally supplied power. The last phase represents the OTEC power plant itself. The capital expenditure would be shared relatively evenly by all phases. The philosophy of this financing strategy is that each phase has an acceptable cumulative payback period, even if the following phases cannot be implemented.

Synchronous power output fluctuations for an experimental open-cycle OTEC plant

A 210 kW experimental open-cycle ocean thermal energy conversion plant was completed in Hawaii in 1993, and equipped with a synchronous generator to test its connection to the local power grid. During shakedown tests, large power output fluctuations were observed. Linear mathematical models of the system were developed and numerical simulations reproduced observations well, for a given line frequency input, confirming in particular the resonant nature of a massive turbine rigidly connected to a small generator. The frequency-domain algorithm was extended to analyze the effect of inserting a fluid coupler between the turbine and the generator to eliminate large power output fluctuations. The actual installation of a fluid coupler in early 1994 allowed a validation of the model predictions.

Energy-efficient schemes for seawater desalination

The present paper proposes a financing scheme that would spread in time the capital cost of, and the technological risk associated with, small land-based Ocean Thermal Energy Conversion (OTEC) plants. It is based on the cost effectiveness of some OTEC by-products. Three separate phases are envisioned: the first one would consist in supplying air-conditioning (A/C) needs with deep cold seawater pumped through a pipe designed for larger OTEC flow rate requirements; the second one, in building a desalination unit based on some Open-Cycle OTEC hardware, with externally supplied power. The last phase represents the OTEC power plant itself. The capital expenditure would be shared relatively evenly by all phases. The philosophy of this financing strategy is that each phase has an acceptable cumulative payback period, even if the following phases cannot be implemented.

Performance characteristics of barometric-type open-cycle OTEC system

Open Cycle-Ocean Thermal Energy Conversion (OC-OTEC) system has a merit to use a heat exchanger of direct contact type without a heat transfer tube. Therefore, it is expected that the capital cost of OC-OTEC is reduced by use of this simply designed heat exchanger. However, non-condensable gas is released in the system, because in a direct contact evaporator, the steam driving a turbine is generated by surface sea water including air, and existing air causes a reduction of system performance. From the above point of view, we carried out an experimental study on the operating characteristics and the effect of structure of a heat exchanger and the existence of non-condensable gas on the performance of a direct contact heat exchanger in an experimental apparatus of barometric type OC-OTEC. As a result, the performance of the direct contact evaporator can be satisfied. However, we cannot get satisfactory results in a direct contact condenser. Therefore, we conducted further experiments through the improvement of the structure of the condenser and the control of the non-condensable gas. Finally, we concluded that the major factor affecting the system performance is the non-condensable gas, but its effect can be ignored when its concentration is below 8%. © 1997 Scripta Technica, Inc. Heat Trans Jpn Res 25(4): 226–237, 1996

Performance characteristics of barometric‐type open‐cycle OTEC system

Open Cycle-Ocean Thermal Energy Conversion (OC-OTEC) system has a merit to use a heat exchanger of direct contact type without a heat transfer tube. Therefore, it is expected that the capital cost of OC-OTEC is reduced by use of this simply designed heat exchanger. However, non-condensable gas is released in the system, because in a direct contact evaporator, the steam driving a turbine is generated by surface sea water including air, and existing air causes a reduction of system performance. From the above point of view, we carried out an experimental study on the operating characteristics and the effect of structure of a heat exchanger and the existence of non-condensable gas on the performance of a direct contact heat exchanger in an experimental apparatus of barometric type OC-OTEC. As a result, the performance of the direct contact evaporator can be satisfied. However, we cannot get satisfactory results in a direct contact condenser. Therefore, we conducted further experiments through the improvement of the structure of the condenser and the control of the non-condensable gas. Finally, we concluded that the major factor affecting the system performance is the non-condensable gas, but its effect can be ignored when its concentration is below 8%. © 1997 Scripta Technica, Inc. Heat Trans Jpn Res 25(4): 226–237, 1996

バロメトリック型オープンサイクル海洋温度差発電システムの動作特性

Open Cycle-Ocean Thermal Energy Conversion (OC-OTEC) system has a merit to use a heat exchanger of direct contact type without a heat transfer tube. Therefore, it is expected that the capital cost of OC-OTEC is reduced by use of this simply designed heat exchanger. However, non-condensable gas is released in the system, because in a direct contact evaporator, the steam driving a turbine is generated by surface sea water including air, and existing air causes a reduction of system performance. From the above point of view, we carried out an experimental study on the operating characteristics and the effect of structure of a heat exchanger and the existence of non-condensable gas on the performance of a direct contact heat exchanger in an experimental apparatus of barometric type OC-OTEC. As a result, the performance of the direct contact evaporator can be satisfied. However, we cannot get satisfactory results in a direct contact condenser. Therefore, we conducted further experiments through the improvement of the structure of the condenser and the control of the non-condensable gas. Finally, we concluded that the major factor affecting the system performance is the non-condensable gas, but its effect can be ignored when its concentration is below 8%. © 1997 Scripta Technica, Inc. Heat Trans Jpn Res 25(4): 226–237, 1996

オープンおよびクローズドサイクル海洋温度差発電システムの出力特性(隔壁式凝縮器の場合)

オープンおよびクローズドサイクル海洋温度差発電システムの出力特性(隔壁式凝縮器の場合)

An assessment of ocean thermal energy conversion as an advanced electric generation methodology

The history of ocean thermal energy conversion (OTEC), a process that employs the temperature difference between surface and deep ocean water to alternately evaporate and condense a working fluid, is reviewed. In the open-cycle OTEC configuration, the working fluid is seawater. In the closed-cycle configuration, a working fluid such as propane is used. OTEC is assessed for its practical merits for electric power generation. Because rather large amounts of seawater and working fluid are required, the energy requirements for pumping them may be greater than the energy recovered from the OTEC engine itself. The concept of net power production is discussed. The components of a typical OTEC plant are described with emphasis on the evaporator heat exchanger. Operation of an OTEC electric generating station is discussed, including transient operation. Recent experiments and efforts at the National Energy Laboratory-Hawaii (NELH) are summarized. Remarks are made on bottlenecks and the future of OTEC as an advanced electric generation methodology. >

Experiments on Oxygen Desorption From Surface Warm Seawater Under Open-Cycle Ocean Thermal Energy Conversion

This paper presents the results of scoping deaeration experiments conducted with warm surface seawater under open-cycle ocean thermal energy conversion (OC-OTEC) conditions. Concentrations of dissolved oxygen in seawater at three locations (in the supply water, water leaving the predeaerator, and discharge water from an evaporator) were measured and used to estimate oxygen desorption levels. The results suggest that 7 percent to 60 percent of the dissolved oxygen in the supply water was desorbed from seawater in the predeaerator for pressures ranging from 35 to 9 kPa. Bubble injection in the upcomer increased the oxygen desorption rate by 20 percent to 60 percent. The data also indicated that at typical OC-OTEC evaporator pressures, when flash evaporation in the evaporator occurred, 75 percent to 95 percent of the dissolved oxygen was desorbed overall from the warm seawater. The results were used to find the impact of a single-stage predeaeration scheme on the power to remove noncondensable gases in an OC-OTEC plant.

Direct Contact Condensation in the Presence of Noncondensables in OC-OTEC Condensers

Analyses are presented for the performance of direct contact jet and spray condensers in the presence of noncondensables. A solution procedure is developed which obtains analytically the liquid-side transient temperature profile, including the thermal entrance effect. The approach avoids iteration at the liquid-vapor interface, which usually is required to determine the interface temperature, by locally linearizing the energy and mass transfer relationship at the gas-liquid interface. Comparisons are carried out with some available data and correlations and show good agreement. The analyses are shown to provide a rational basis for correlating the experimental data. Unlike previous correlations, which are basically curve fits to the applicable data, the present analysis shows clearly the effects of noncondensables, flow, and geometric parameters on the condenser performance. Results are presented for conditions typical of open-cycle ocean thermal energy conversion (OC-OTEC).