scholarly journals Finite-Time Thermodynamic Model for Evaluating Heat Engines in Ocean Thermal Energy Conversion

Entropy ◽  
2020 ◽  
Vol 22 (2) ◽  
pp. 211 ◽  
Author(s):  
Takeshi Yasunaga ◽  
Yasuyuki Ikegami
Keyword(s):  
Energy Conversion ◽  
Thermal Energy ◽  
Temperature Difference ◽  
Thermal Efficiency ◽  
Atmospheric Condition ◽  
Ocean Thermal Energy Conversion ◽  
Dead State ◽  
Heat Engines ◽  
Thermal Energy Conversion ◽  
Ocean Thermal Energy

Ocean thermal energy conversion (OTEC) converts the thermal energy stored in the ocean temperature difference between warm surface seawater and cold deep seawater into electricity. The necessary temperature difference to drive OTEC heat engines is only 15–25 K, which will theoretically be of low thermal efficiency. Research has been conducted to propose unique systems that can increase the thermal efficiency. This thermal efficiency is generally applied for the system performance metric, and researchers have focused on using the higher available temperature difference of heat engines to improve this efficiency without considering the finite flow rate and sensible heat of seawater. In this study, our model shows a new concept of thermodynamics for OTEC. The first step is to define the transferable thermal energy in the OTEC as the equilibrium state and the dead state instead of the atmospheric condition. Second, the model shows the available maximum work, the new concept of exergy, by minimizing the entropy generation while considering external heat loss. The maximum thermal energy and exergy allow the normalization of the first and second laws of thermal efficiencies. These evaluation methods can be applied to optimized OTEC systems and their effectiveness is confirmed.

scholarly journals Dynamic Simulation of System Performance Change by PID Automatic Control of Ocean Thermal Energy Conversion

2020 ◽  
Vol 8 (1) ◽  
pp. 59 ◽  
Author(s):  
Lim Seungtaek ◽  
Lee Hoseang ◽  
Kim Hyeonju
Keyword(s):  
Control System ◽  
Energy Conversion ◽  
Thermal Energy ◽  
Temperature Difference ◽  
Pid Control ◽  
Seawater Temperature ◽  
Ocean Thermal Energy Conversion ◽  
Control Value ◽  
Thermal Energy Conversion ◽  
Ocean Thermal Energy

Near infinite seawater thermal energy, which is considered as an alternative to energy shortage, is expected to be available to 98 countries around the world. Currently, a demonstration plant is being built using closed MW class ocean thermal energy conversion (OTEC). In order to stabilize the operation of the OTEC, automation through a PID control is required. To construct the control system, the control logic is constructed, the algorithm is selected, and each control value is derived. In this paper, we established an optimal control system of a closed OTEC, which is to be demonstrated in Kiribati through simulation, to compare the operating characteristics and to build a system that maintains a superheat of 1 °C or more according to seawater temperature changes. The conditions applied to the simulation were the surface seawater temperature of 31 °C and the deep seawater temperature of 5.5 °C, and the changes of turbine output, flow rate, required power, and evaporation pressure of the refrigerant pump were compared as the temperature difference gradually decreased. As a result of comparing the RPM control according to the selected PID control value, it was confirmed that an error rate of 0.01% was shown in the temperature difference condition of 21.5 °C. In addition, the average superheat degree decreased as the temperature difference decreased, and after about 6000 s and a temperature decrease to 24 °C or less, the average superheat degree was maintained while maintaining the superheat degree of 1.7 °C on average.

scholarly journals The extraction of power and fresh water from the ocean off the coast of KZN utilising ocean thermal energy conversion (OTEC) techniques

2021 ◽  
Author(s):  
◽  
Makhosonke Gumede
Keyword(s):  
Energy Conversion ◽  
Fresh Water ◽  
Thermal Energy ◽  
Temperature Difference ◽  
Power Output ◽  
Warm Water ◽  
Ocean Thermal Energy Conversion ◽  
Thermal Energy Conversion ◽  
Kwazulu Natal ◽  
Ocean Thermal Energy

Ocean thermal energy conversion (OTEC) is an electric power generation system which uses the temperature difference between warm water at the surface (26 oC) and cold water from the depths (5 oC) of the ocean. Generating electricity is not the only function of OTEC as it can also produce significant amounts of fresh water. This can be very important, for example on islands and in some regions, such as Port Edward, where fresh water is limited. This thesis sets out to harness this fluidic energy, thus generating significant amounts of useful electric power for insertion into the national grid, as well as fresh water in Port Edward on the KwaZulu-Natal (KZN), South Coast. The site of Port Edward is naturally suited to the establishment of alternate energy collection sources such as OTEC; the geographical location of this region is additionally suited to the development of Open Cycle - Ocean Thermal Energy Conversion (OC- OTEC). Port Edward lies just beneath the tropic of cancer and on the shore of the Indian Ocean thus two important elements needed for OTEC namely constant sunlight and large coastal areas can easily be found in this region. More importantly, the steep drop in water depth down to 3000 meters makes this an ideal research site for ocean thermal energy conversion in KwaZulu-Natal (KZN). If the proposed theories are correct, this can possibly be used for base generated energy capacity and fresh water. The results are presented with reference to the temperature difference between the sea surface and the sea bottom because it is an important parameter in choosing an actual plant site and system design of OC-OTEC. This research is mainly laboratory based concentrating on design, calculations, modelling and simulation of OC-OTEC. The thermodynamic fluid calculations were undertaken with a view to design the main mechanical components of an OC-OTEC system, i.e. flash evaporator, condenser and steam turbine. SOLID EDGE software was utilized to design OC-OTEC plant and ASPEN PLUS V8.6 software was used to simulate and model the experiment. An OC-OTEC demonstration plant was designed and constructed in an Electrical Power Laboratory at Durban University of Technology (DUT). The experimental study was carried out on the demonstration plant with consideration given to water temperature, mass flow rate of fluid, and pressure. The measurements were taken before and after each component. The selection of a good process modelling and simulation tool was of extreme importance for the success of this work. Throughout the measurements, we found that the thermal efficiency (%) and the power output increased with increasing temperature difference Δt = tw - tc. The power output was produced when the total temperature difference was sufficient to allow heat transfer within the evaporator and provide a pressure drop across the turbine. There was more heat transfer (steam produced) in the flash evaporator at a constant flow rate because the warm water continuously supplied heat energy to the evaporator without losing much energy through the process, therefore continuous feed to the turbine improved constant power output. The thermal efficiencies were increased with increasing pressure across the turbine. The increase of pressure drops across the steam turbine caused the output power to increase. The larger flow rates of the warm water lead to higher amounts fresh water produced from the condenser. The final step in this process was the design of the main components of a practical plant to be used as a pilot plant at a selected location on the KwaZulu-Natal South coast. This will address the problem of lack of water in the region.

Performance Simulation of Solar-Ocean Thermal Energy Conversion

2018 ◽  
Author(s):  
Yue Juan ◽  
Li Dashu ◽  
Li Zhichuan ◽  
Xiao Gang ◽  
Zhang Li ◽  
...  
Keyword(s):  
Energy Conversion ◽  
Thermal Energy ◽  
Power Output ◽  
Thermal Efficiency ◽  
Performance Simulation ◽  
Comprehensive Utilization ◽  
Ocean Thermal Energy Conversion ◽  
Thermal Energy Conversion ◽  
Ocean Thermal Energy ◽  
Net Power Output

Compared to the restriction of intermittency of solar power generation, ocean thermal energy conversion (OTEC) is not only 24/7 base-load, but also comprehensive utilization of fresh water production, air-conditioning, mariculture etc. However, limited temperature difference between warm surface seawater and the cold deep seawater is a crucial factor that restricts the thermal efficiency of OTEC. But today, with the appliance of solar collector in OTEC net power output and the net thermal efficiency have been significantly improved. In this study theoretical analysis and performance simulation of 1MW solar-ocean thermal energy conversion (SOTEC) in South China Sea area is conducted. Net power output and net thermal efficiency of SOTEC with solar-boosted temperature of 20K and OTEC under the condition of weather conditions in South China Sea are compared and analyzed. The results show that the net power output and net thermal efficiency of SOTEC have been significantly improved by combining the solar collector. This study is practical for autonomous supply of islands and coastal areas, and instructive for the comprehensive utilization of renewable energy.

Thermodynamic Analysis of Ocean Thermal Energy Conversion System With Different Working Fluids

2017 ◽  
Author(s):  
Dashu Li ◽  
Li Zhang ◽  
Xili Duan ◽  
Xiaosuai Tian
Keyword(s):  
Energy Conversion ◽  
Thermal Energy ◽  
Thermal Efficiency ◽  
Sea Water ◽  
Working Fluid ◽  
Ocean Temperature ◽  
Water Pump ◽  
Ocean Thermal Energy Conversion ◽  
Thermal Energy Conversion ◽  
Ocean Thermal Energy

A thermodynamic model is developed for ocean thermal energy conversion (OTEC) systems. Considering the narrow temperature range in the evaporator, different refrigerants including R717, R134a and R600 were analyzed and compared under sub-critical state with practical ocean thermal conditions. The results show that larger ocean temperature differences will lead to higher evaporation pressures, and less pumping power requirements for all pumps, i.e., warm sea water pump, cold sea water pump and pumps for the working fluid. The thermal efficiency of different systems and the net power output were found to be closely related to ocean temperature difference, with a positive linear relationship. It was also found that R717 provides the highest thermal efficiency with the least pump power requirement. This working fluid could potentially be used for OTEC system development. This study provides useful insights to the design and equipment selection of OTEC systems.

scholarly journals Performance Evaluation Concept for Ocean Thermal Energy Conversion Toward Standardization and Intelligent Design

Energies ◽  
2021 ◽  
Vol 14 (8) ◽  
pp. 2336
Author(s):  
Takeshi Yasunaga ◽  
Kevin Fontaine ◽  
Yasuyuki Ikegami
Keyword(s):  
Performance Evaluation ◽  
Energy Conversion ◽  
Thermal Energy ◽  
Thermal Efficiency ◽  
Equilibrium Condition ◽  
Evaluation Methods ◽  
Exergy Efficiency ◽  
Ocean Thermal Energy Conversion ◽  
Thermal Energy Conversion ◽  
Ocean Thermal Energy

Ocean thermal energy conversion (OTEC) uses a very simple process to convert the thermal energy stored mainly in tropical oceans into electricity. In designs, operations, and evaluations, we need to consider the unique characteristics of OTEC to achieve the best performance or lower the electricity cost of projects. The concept and design constraints of OTEC power generation differ from those of conventional thermal power plants due to the utilization of a low temperature difference. This research theoretically recognizes the unique characteristics of the energy conversion system and summarizes the appropriate performance evaluation methods for OTEC based on finite-time thermodynamics and the equilibrium condition of the heat source. In addition, it presents the concept of normalization of thermal efficiency for OTEC and exergy efficiency based on the available thermal energy in the ocean defined as the transferable thermal energy from the ocean and the equilibrium condition as the dead state for exergy. The differences between conventional thermal efficiency and the effectiveness of the evaluation methods are visualized using the various reference design data, and it is ascertained that there is no clear relation between the conventional thermal efficiency and exergy efficiency, whereas the normalized thermal efficiency is definitely proportional to the exergy efficiency. Moreover, the exergy efficiency shows the effectiveness of the staging Rankine, Kalina, and Uehara cycles. Therefore, the normalized thermal efficiency and the exergy efficiency are important to analyze the heat and mass balance as well as improvement of the system.

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