Ocean Thermal Energy Research Articles

STUDY OF POTENTIAL INSTALLATION OF OCEAN THERMAL ENERGY CONVERSION (OTEC) IN THE NORTH WATERS OF LEMBATA, NTT

Ocean thermal energy conversion is an attempt to convert potential energy in the variances heat content of seawater into other energy by utilizing the temperature change between the sea surface and deep sea at least 20°C. The Lembata waters is near to the equator, sea surface temperature tends to be warm and stable. This research was conducted to estimate the potential energy generated from a closed cycle OTEC system in North Lembata Waters. This study used temperature data from Global Ocean Physics Reanalysis from Copernicus Marine Environment Monitoring Service (CMEMS) for 9 years (2012-2020) in 6 stations. Validation was performed using the primary CTD Lembata OTEC Team of the Marine Geological Institute (MGI). Temperature data validation results on the MSE (Mean Square Error), RMSE (Root Mean Square Error), and MAPE (Mean Absolute Percentage Error) methods are considered to represent field temperature conditions. The variability value shows the station point in the North Lembata Waters has a temperature with slight differences. The vertical temperature change (ΔT) shows between 20.98°C to 23.44°C. Potential electric power resulting from the OTEC system using the technical estimation formula. The average net power generated from those temperature gradients ranges from 5.65 MW-7.56 MW, respectively. The Lembata waters have temperature conditions suitable for OTEC installations. Station C-4 has a power potential of 6.84 MW with a depth of 763 m and the distance of 1.86 km from the coastline. Station C-4 in the Omesuri sub-district is the best point for OTEC installation in North Lembata Waters.

Theoretical analysis on extraction-ejection combined power and refrigeration cycle for ocean thermal energy conversion

Theoretical analysis on extraction-ejection combined power and refrigeration cycle for ocean thermal energy conversion

Thermodynamic analysis and optimization of a combined cooling and power system using ocean thermal energy and solar energy

Ocean thermal energy and solar energy have great potential for development in energy conversion systems. In this study, a new type of combined cooling and power (CCP) system using solar energy and ocean thermal energy is proposed. The effects of evaporating temperature and condensing temperature of the organic Rankine cycle, evaporating temperature of refrigeration cycle, split ratio, mass flow, the proportion of solar energy to total heat absorption on the performance of CCP system are investigated. The results indicate that the increase of evaporating temperature resulting in a rise in Primary Energy Efficiency Ratio (PER) and exergy efficiency, while the increase of condensing temperature leads to the decrease of PER and exergy efficiency. R1234ze(E), R1234yf, R134a, R245fa, and R32 are screened under the optimal operating conditions, and R32 is the most suitable working fluid with the highest PER of 9.99% and the exergy efficiency of 11.94%. The exergy loss is concentrated in the heat transfer process, among which the exergy loss of the condenser is the highest of 168.29 kW, accounting for 46.68% of the total exergy loss. The increase in the proportion of solar energy absorption to the total heat absorption will reduce the PER and exergy efficiency.

Experimental study of the absorption refrigeration using ocean thermal energy and its under-lying prospects

The construction of a maritime cold chain network with islands as node is an important avenue for the economic development of the island, in which efficient refrigeration technology is essential to the development of the marine industry. Motivated by the substantial resources of tropical ocean, an absorption-compression refrigeration device is constructed and its technical feasibility is firstly demonstrated by experiment, achieving a refrigerating capacity of 6.8 kW and 10.2 kW at the evaporation temperature of −24.5 °C and −18 °C, respectively. The pressure rise by the compressor has been reduced by 185 kPa when compared to traditional vapor-compression refrigeration. The energy-saving potential of different refrigeration methods is also summarized, with the proposed refrigeration achieving the highest level of energy-efficient ratio, EER = 6.57. Especially, the proposed refrigeration can be cascaded with power generation, desalination, chilled-soil agriculture, and aquaculture to realize the gradient utilization of ocean thermal energy and hence improve the overall economy. This work opens up a new path for promoting and developing the use of ocean thermal energy for blue ocean economy in tropical oceans with bathymetry depths of 300 m or higher.

Opportunities and challenges of ocean thermal energy conversion technology

OPINION article Front. Energy Res., 18 May 2023Sec. Process and Energy Systems Engineering Volume 11 - 2023 | https://doi.org/10.3389/fenrg.2023.1207062

Review of Resources from the Perspective of Wave, Tidal, and Ocean Thermal Energy Conversion

The conversion of Ocean Renewable Energy (ORE) sources to electricity could meet increasing energy demand and diversify the energy supply in Malaysia. Possessing a long coastline overlooking the South China Sea (SCS) and the Malacca strait has encouraged the Malaysian government to promote ORE and assess available resources to reduce its dependence on fossil fuels. However, most of the previous attempts to assess the potential of Malaysian ORE resources have focused primarily on theoretical resource assessment, which in practice may not reflect the viability and suitability of the resource. Other technical and practical issues must be accounted for as well. This paper presents a brief description of ORE conversion technologies and reviews the existing studies on regional ORE resources in Malaysia with an emphasis dedicated to wave energy, tidal energy, and Ocean Thermal Energy Conversion (OTEC). It also highlights the essential technical and practical constraints limiting or excluding the utilization of ORE sources. While some ORE resources, particularly the OTEC, appear to be theoretically promising for exploitation in Malaysia, this review has shown a lack of precise resource mapping linked to socio-economic and environmental constraints.

Numerical and experimental study of the ocean thermal energy capture process utilizing metal foam–phase-change material (PCM) composites

Ocean thermal energy (OTE) is an ideal in-situ retrievable energy source for underwater profiling vehicles to maintain their long-term mission capacity. A potential method to capture OTE is to utilize the volume change in metal foam–phase-change material (PCM) composites. In this study, a numerical model for the phase change and energy conversion processes was established. The solidification and melting process were observed experimentally to validate the proposed model. Based on the numerical model, a parametric study was conducted to demonstrate the influence of the back pressure, metal foam, ambient temperature and chamber diameter on the phase change process. The results indicate that (i) increasing the back pressure reduces the temperature difference between the solid phase area and the environment, thus increasing the melting time; (ii) adding copper foam with low porosity in the phase-change chamber can effectively improve the net power density of an ocean thermal engine; (iii) the net power density is significantly affected by the hot end temperature; (iv) reducing the inner diameter of the phase-change chamber can reduce the phase transition time. However, to maximize the net power density, an appropriate inner diameter and back pressure must be selected; and (v) after optimization, the net power density is increased by 151.7 %, which reflects the value of the proposed numerical model in engineering applications.

Investigation of a distillation desalination system driven by solar and ocean thermal energy

Ocean thermal energy is a stable and abundant marine renewable energy that can be adopted for sustainable desalination in remote islands. However, its relatively low available temperature difference restricts its application. Therefore, to manipulate ocean thermal energy while overcoming its unique small thermocline temperature gradient, a dual-energy-driven distillation desalination system driven by solar and ocean thermal energy is put forward. The merits of this system include resolving the nightly failure of solar desalination systems and higher productivity. To quantify the system potential, a numerical model is developed and thermodynamic analysis is conducted. A comparison between dual-energy-driven system and single-energy-driven systems is conducted. The results indicate that the yield can be significantly improved by simultaneously using solar and ocean thermal energy. A single ocean thermal energy-driven desalination can achieve the minimum freshwater productivity of 0.07 kg·h−1, while coupled solar thermal energy can improve freshwater yield to 1.21 kg·h−1, which is 30.1 % higher than that of a single solar desalination. The system power consumption is only 1.77 kW·h/ton. Furthermore, based on the actual meteorological data in Haikou, China, an annual freshwater productivity of 3524 kg (for a 1 m2 absorber plate area) can be obtained, corresponding to a gain output ratio of 0.44.

Thermodynamic process control of ocean thermal energy conversion

The small temperature difference in ocean thermal energy conversion (OTEC) causes the high sensitivity of thermodynamic process to perturbations such as temperature and power load, which requires an efficient control strategy of thermodynamic process for OTEC. To this aim, an integral dynamic model is established to study the dynamic response of thermodynamic process in an OTEC system. To ensure the system efficiency and operating safety of power cycle, a multi-objective active-disturbance rejection control (ADRC) strategy is proposed and compared with corresponding single-objective ADRC and PI controller. Multi-objective ADRC includes a load-following control loop and two auxiliary control loops (evaporator control loop and condenser control loop). The results indicate that the multi-objective ADRC is most efficient to track the change of power load, in which the overshoot and integration absolute error (IAE) of load-following are reduced by 50% and 16% when compared with the single-objective ADRC. In addition, the multi-objective control strategy can maintain the stability of superheat degree and condensation pressure, and hence improves the system efficiency by 8.64% for the output power of 35 kW. Especially, the feedback-feedforward controller and feedback controller are attractive candidates to maintain the stable superheat degree and condensation pressure.

Assessment of the stiffened panel performance in the OTEC seawater tank design: Parametric study and sensitivity analysis

Ocean thermal energy conversion (OTEC) is a process of generating electricity by exploiting the temperature difference between warm surface seawater and cold deep seawater. Due to the high static and dynamic pressures that are caused by seawater circulation, the stiffened panel that constitutes a seawater tank may undergo a reduction in ultimate strength. The current paper investigates the design of stiffening systems for OTEC seawater tanks by examining the effects of stiffening parameters such as stiffener sizes and span-over-bay ratio for the applied combined loadings of lateral and transverse pressure by fluid motion and axial compression due to global bending moment. The ultimate strength calculation was conducted by using the non-linear finite element method via the commercial software known as ABAQUS. The stress and deformation distribution due to pressure loads was computed in the first step and then brought to the second step, in which the axial compression was applied. The effects of pressure on the ultimate strength of the stiffener were investigated for representative stiffened panels, and the significance of the stiffener parameters was assessed by using the sensitivity analysis method. As a result, the ultimate strength was reduced by approximately 1.5% for the span-over-bay ratio of 3 and by 7% for the span-over-bay ratio of 6.

Performance evaluation of Ocean Thermal Energy Conversion(OTEC) stabilization based on risk analysis

Performance evaluation of Ocean Thermal Energy Conversion(OTEC) stabilization based on risk analysis

Performance Analysis of a 10 MW Ocean Thermal Energy Conversion Plant Using Rankine Cycle in Malaysia

Ocean thermal energy conversion (OTEC) is a solution for environmental and climate change issues in the tropics. The OTEC potential in Malaysia using ocean conditions and bathymetry data has been previously studied and demonstrated. Following this, it is vital to perform a basic performance analysis of a 10 MW Rankine Cycle OTEC plant using the Malaysian ocean conditions. In this paper, the results of heat and mass balance will be reported for a 10 MW Rankine cycle OTEC plant which uses heat exchangers of plate-type and anhydrous ammonia as its working fluid. The value of a minimum objective function (γ) is derived by total heat surface area (AT) divided by the net power (PN). γ decreases when the inlet temperature difference (inlet temperature of warm seawater (TWSWI)—inlet temperature of cold seawater (TCSWI)) increases. PN is clarified to be approximately 70–80% of the PG (gross power) using Malaysian ocean conditions.

A performance study of R717 and R22 as the working fluid for OTEC plant

The purpose of this paper is to compare the thermal efficiency of an ocean thermal energy conversion (OTEC) cycle using R717 and R22 working fluids. An OTEC system typically operates at the temperature difference between the average warm surface seawater of 30 °C and the cold deep seawater of 8 °C. The temperature difference will then power a turbine to produce electricity. R22 and R717 are two potential working fluids to be used in the OTEC cycle to absorb heat due to their saturation temperature. However, R22 is associated with higher global warming potential and the global warming potential (GWP) of 1,760, compared to R717 with zero GWP. The study shows that the thermal efficiency of R22 is 0.211% higher than R717. Moreover, the thermal power required by the water circuit to cool/heat the OTEC circuit for R717 is 13% higher than R22. However, the required working fluid flow rate of the R717 system is 81.5% lower than R22, making the former significantly smaller in size.

Heat transfer mechanism of cold-water pipe in ocean thermal energy conversion system

Ocean thermal energy is clean and renewable. In recent years, many scholars have focused on improving the generation efficiency of the ocean thermal energy conversion system. To the best of our knowledge, the majority of the scholars focus on equipment improvement and working fluid optimization, and few study how to reduce the outlet temperature of cold-water pipes. This paper establishes a numerical model, which is solved by the finite volume method, to analyze the temperature distribution and heat transfer characteristics of cold-water pipes during pumping. The influence of cold-water pipe materials, wall thicknesses, flow rates, and insulation layer thicknesses on outlet temperature is explored. In addition, this paper qualitatively gives performance optimization recommendations based on technical and economic indicators. This study will be useful for the OTEC community.

Marine Energy: Comparison of Tidal and Wave Energy

Marine energy is one of the forms of renewable energy that is one of the most widely available sources of energy; the oceans cover around 71% of the earth's surface, which means that there can be huge potential in energy. The majority of the technologies that are used to harvest marine energy are still in their developmental stages. Marine energy includes tidal energy, wave energy, osmotic energy, ocean thermal energy, and ocean current energy. Researchers all across the world, as well as people developing industries, are paying attention to the potentially lucrative field of marine energy. Since more than half of the US population lives within 50 miles of the coast, marine energy resources are spread out and in a good place to power cities. These resources are prospective contributors to a reliable, steady sustainable energy infrastructure because they are also quite predictable. This article provides an overview of the current state of the art in marine energy and explores tidal and wave energy in detail including the benefits and limitations of both.

A transient model for clean electricity generation using Solar energy and ocean thermal energy conversion (OTEC) - case study: Karkheh dam - southwest Iran

Given the limitations of fossil fuels, humans must develop alternate energy sources. Solar energy and ocean thermal energy are known as a safe, secure, promising, and clean instrument for this purpose due to their enormous potential. A hybrid system of solar energy and ocean thermal energy with a thermoelectric generator is examined in this study to generate clean electricity and utilize the water temperature difference of the Karkheh dam. The system under consideration is made up of flat panel solar collector subsystems and a Rankin organic cycle. To model the analyzed system and acquire system analysis results, thermodynamic software of EES engineering equations solving was employed. The collector area, solar radiation intensity, ambient temperature, and input temperature to the turbine have all been studied as parameters affecting system outputs. The system's economic research revealed that the solar unit, evaporator, and ORC turbine have the highest cost rates among the system components. Furthermore, the system's exergy study revealed that the solar unit and evaporator suffered the highest exergy destruction. For one year, a case study was conducted on the water temperature difference at Karkheh Dam, and the findings of system performance were assessed. The results demonstrated that the best system performance is obtained under summer weather conditions in Andimeshk. The results showed that the proposed system's total output power in relation to the difference in water temperature of Karkheh Dam is 1,192,607/9 kW per year, and that this system can meet the energy needs of 111 households in Andimeshk throughout the year. According to the results, the system described in this study is suitable for the required applications.

Performance Evaluation of Control Compatibility for an OTEC Pump Shutdown Condition

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.

Objective Functions and Performance Optimization of Ocean Thermal Energy Conversion (OTEC) with CO2-Based Binary Zeotropic Mixture Power Cycle

Ocean thermal energy is a huge renewable and clean energy. For different applications of the Ocean Thermal Energy Conversion (OTEC) system, new objective functions are proposed and optimal CO2-based binary zeotropic mixtures for each objective function are discussed. Propane, butane, isobutane, and pentane were selected as the secondary component of the mixture working fluid. Based on each objective function, some parameters of the CO2-based binary zeotropic mixture power cycle were studied and the optimal working fluids were obtained. The results indicated that the thermal efficiency showed an upwards trend as the evaporating temperature increased from 20 to 23 °C and decreased as the molar fraction of the secondary component of the working fluid increased from 0.01 to 0.05, but the specific volumetric flow showed the opposite trend. The specific net power increased as the evaporating temperature increased, and the net power output had a greater effect on the variations in the specific net power than the molar fraction of the secondary component. When taking thermal efficiency or specific volumetric flow as the objective functions, CO2/propane was the suitable working fluid under the considered conditions; in the case that specific net power was taken as the objective function, CO2/butane (0.97/0.03 or 0.96/0.04 molar ratio) or CO2/isobutane (0.97/0.03, 0.96/0.04, or 0.95/0.05 molar ratio) were suitable under the considered evaporating temperature.

Non-linear assessment of cold water pipe (CWP) on the ocean thermal energy conversion (OTEC) installation under bending load

Non-linear assessment of cold water pipe (CWP) on the ocean thermal energy conversion (OTEC) installation under bending load

Three-point bending assessment of cold water pipe (CWP) sandwich material for ocean thermal energy conversion (OTEC)

Three-point bending assessment of cold water pipe (CWP) sandwich material for ocean thermal energy conversion (OTEC)