Open-cycle Ocean Thermal Energy Research Articles

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.

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.

Dual-use open cycle ocean thermal energy conversion (OC-OTEC) using multiple condensers for adjustable power generation and seawater desalination

Multiple condensers are used for dual-use open cycle ocean thermal energy conversion to generate both electric power and desalinated water with preset ratios. Fundamentals of heat and mass transfer phenomena are scrutinized to identify optimal operational conditions of various OC-OTEC plant scales. Important control parameters include warm water temperature and its intake rate, vacuum pressure in the evaporator, and cold water temperature. Intake rate of the cold deep seawater is estimated based on the operational mode and conditions. Performances of the multiple condenser OC-OTEC system, power generation and seawater desalination capacities are analytically evaluated in terms of steam flow fractions, temperature of intake seawater, and vacuum pressure. This research provides unprecedented levels of theoretical depth, mathematical details, and design criteria for highly optimized dual-use OC-OTEC operations. Future research directions for OC-OTEC technologies are also discussed in detail.

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.

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.

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).

Gas Desorption From Seawater in Open-Cycle Ocean Thermal Energy Conversion Barometric Upcomers

Gas desorption from warm and cold seawater under open-cycle ocean thermal energy conversion (OC-OTEC) conditions is addressed in this paper. The desorption process of dissolved O2, N2, and CO2 in the barometric upcomers of an OTEC plant is simulated mathematically. The model considers the growth of bubbles originating in the ocean and bubbles formed in the upcomers. Bubble growth is induced by gas mass transfer and water evaporation at the bubble-liquid interface, as well as by the decreasing hydrostatic pressure. Heterogeneous nucleation at pipe wall crevices and on suspended particles in the water stream is also modeled. Bubble coalescence due to turbulent shear and differential buoyancy is simulated. The results generated show the deaeration efficiency as a function of flow and geometric parameters. The calculations show that gas desorption in the barometric upcomers can be appreciable. Such desorption is enhanced by increasing the concentration of the incoming and/or the heterogeneously formed bubbles. Results of existing experiments are discussed and predictions are shown for the selected test conditions.

Thermo-Fluid Dynamic Design Study of Single and Double-Inflow Radial and Single-Stage Axial Steam Turbines for Open-Cycle Thermal Energy Conversion Net Power-Producing Experiment Facility in Hawaii

The results of the study of the optimum thermo-fluid dynamic design concept are presented for turbine units operating within the open-cycle ocean thermal energy conversion (OC-OTEC) systems. The concept is applied to the first OC-OTEC net power producing experiment (NPPE) facility to be installed at Hawaii’s Natural Energy Laboratory. Detailed efficiency and performance calculations were performed for the radial turbine design concept with single and double-inflow arrangements. To complete the study, the calculation results for a single-stage axial steam turbine design are also presented. In contrast to the axial flow design with a relatively low unit efficiency, higher efficiency was achieved for single-inflow turbines. Highest efficiency was calculated for a double-inflow radial design, which opens new perspectives for energy generation from OC-OTEC systems.

Optimum Design of a Low-Pressure, Double Inflow, Radial Steam Turbine for Open-Cycle Ocean Thermal Energy Conversion

This paper describes the thermo-fluid design concept of a low-pressure, radial inflow steam turbine for the Open-Cycle Ocean Thermal Energy Conversion (OTEC). For the optimum design consideration, the necessary theoretical tools are developed. The design concept permits the utilization of single-stage and multistage configurations for small and medium power ranges, respectively.

Seawater Test Results of Open-Cycle Ocean Thermal Energy Conversion [OC-OTEC] Components

Key components of open-cycle ocean thermal energy conversion (OC-OTEC) systems—the flash evaporator, mist eliminator, passive predeaerator, two surface condenser stages, and mo direct-contact condenser stages—have been tested using seawater. These components operate at lower steam pressures and higher inlet noncondensable gas concentrations than do conventional power plant heat exchangers. The rate of heat exchanged between the evaporator and the condenser is on the order of 1.25 MW (thermal), requiring a warm seawater flow of about 0.1 m3/s; the cold seawater flow is on the order of half the warm water flow. In addition to characterizing the performance of the various components, the system has produced potable water from condensation of the steam produced in the evaporator. The information obtained in these tests is being used to design a larger-scale experiment in which net power production is expected to be demonstrated for the first time using OC-OTEC technology.

Heat transfer of evaporating droplets in low pressure systems

Analytical study for evaporation from superheated water droplets in low pressure systems is presented for application to Open-Cycle Ocean Thermal Energy Conversion. The droplet size is considered to be approximately constant and sensible heat is the only source of energy for evaporation. Comparison between the experimental data and present prediction of the evaporator effectiveness are also provided.

Fundamental study of binary heat sources and multi-purpose open cycle ocean thermal energy conversion. Heat transfer performance of solar heater and fundamental performances of cycle.

This paper aims to propose and study a new open-cycle ocean thermal energy conversion featuring binary heat sources and multi-purpose uses. The circular pipe solar heater of warm sea water is cleand by sponge balls and the flow system adequate to this new cycle using high, medium and low pressure turbine stages is shown. Heat transfer performances of the solar heater are experimentally studied by use of halogen lamps simulating uniform flux. Based on the experimental results of the solar heater, the conditions for optimum thermal efficiency and specific output for various solar heater outlet temperatures are calculated. The results show that under the given sea water flow rate, the day-mean output is over four times more than the conventional one and the steam flow rate of the low pressure steam to produce fresh water is about five times than that of the conventional open cycle OTEC.

Ocean Thermal Energy Conversion Experiment Slated For Hawaii

An experiment to test conclusively the feasibility of open-cycle ocean thermal energy conversion (OTEC) to generate electricity has been designed by Solar Energy Research Institute, Golden, Colo. Following five years of gathering data and modeling systems, and with funding from the Department of Energy, SERI will conduct the experiment, as yet unscheduled, at the Natural Energy Laboratory of Hawaii. NELH has been involved in ancillary studies of OTEC for some time and already has in place some of the equipment needed for the experiment. The design calls for a 165-kW, open-cycle system. That size is considered the smallest that can be used satisfactorily to validate the basic design and also permit scaleup to projected commercial systems with generating capacities in the range of 5 to 15 MW. Interest in open-cycle OTEC systems was revived in 1978, when DOE funded analyses of the concept with an eye to cost-effectiveness. Work by several groups, including Westinghouse, the ...

Falling jet flash evaporators for open cycle ocean thermal energy conversion

Evaporation from falling superheated water jets for application to open cycle ocean thermal energy conversion is considered. Analyses are performed to show that the interfacial resistance is of no importance to evaporator design and that evaporation is liquid side controlled. The heat exchanger performance is presented in terms of its effectiveness and change of bulk temperature. Unbroken planar and round jets and broken jets which are assumed to be composed of spherical droplets are considered. The analysis is shown to provide a rational basis for correlating experimental data for broken and unbroken jets. Corresponding desorption rates of dissolved noncondensable gas from water jets are then predicted.

An Experimental Investigation of OC-OTEC Direct-Contact Condensation and Evaporation Processes

Heat transfer data are presented for direct-contact evaporator and condenser geometries suitable for Open-Cycle Ocean Thermal Energy Conversion (OC-OTEC) applications. Falling turbulent jets and films were tested at typical operating conditions. The flash evaporator performance was relatively constant over the range of conditions tested, with efficiencies as high as 95 percent due to the breakup of the jets (or films) into sprays. The condenser performance was only affected by the jet or film Reynolds number and the steam air content. Condenser heat transfer coefficients of the order of 27 kW/m2 °C were achieved with jets which were higher than those obtained with films. An empirical correlation was developed for the condenser data after it was shown that none of the existing correlations found in the literature could correlate all of the data trends observed.

Heat and Mass Transfer in Open-Cycle OTEC Systems

The temperature difference between surface and deep water in the oceans represents a vast resource of thermal energy. A promising method of harnessing this resource is the open-cycle ocean thermal energy conversion (OC-OTEC) system, which utilizes steam evaporated from the surface water to power the turbine. In this paper the state of the art of heat and mass transfer related to evaporation and condensation of steam at low pressures in OC-OTEC is summarized and relevant research issues are discussed.

Vapor/Droplet Coupling and the Mist Flow (OTEC) Cycle

The present experiments have demonstrated that water droplets (∼200-μm dia) can be lifted to substantial heights (∼50 m) by their own vapor produced in flashing over temperature differences typical of the tropical seas. The coupling between the vapor and droplets is found to be excellent. The efficiency for momentum coupling is over 90 percent, and that for energy coupling is shown to vary inversely with the slip ratio. For the conditions in the present experiments, it varies from 50 to 80 percent. The momentum transfer is correlated with an interaction parameter which is the product of the liquid fraction, the slip, and the amount of flashing. For the high vapor flow cases, the pressure difference across the lift column is found to be proportional to the interaction parameter. The relevance of the two-phase flow to a class of open-cycle ocean thermal energy conversion (OTEC) schemes is considered, and the implications of the observed strong vapor/droplet coupling to the feasibility of the mist-flow OTEC cycle are discussed.

Bubble Nucleation and Growth in Open-Cycle OTEC Subsystems

Bubble nucleation and growth in the evaporator, condenser, upcomers, and feedwater distribution systems of open-cycle ocean thermal energy conversion (OTEC) power plants are examined. The phenomenon that will probably have the most impact on system design is cavitation in the warm water feed near the entrance of the evaporator. The critical bubble size for cavitation is about 105 μm. Sources of bubbles in the warm water feed are those entering from the ocean, those nucleating on suspended particles, and those nucleating on the upcomer wall. Analyses of bubble growth induced by changes in hydrostatic pressure, mass transfer, and coalescence are presented. Using available information for bubble size distribution in seawater at California locations, it is shown that cavitation will probably have a significant impact on evaporator performance unless a debubbler is provided upstream of the evaporator entrance.