Thermal Energy Conversion Research Articles

Theoretical analysis on temperature-lifting cycle for ocean thermal energy conversion

A significant obstacle to the widespread large-scale utilization of ocean thermal energy is the low temperature of the heat source, which restricts the turbine inlet to saturated steam conditions and results in notably reduced turbine efficiency. In this paper, a novel temperature-lifting based Ocean Thermal Energy Conversion cycle (OTEC) is proposed. The cycle combines the second-type absorption heat pump and the Rankine power cycle to overheat the working fluid at the turbine inlet using both surface and deep seawater, thereby mitigating the adverse effects of low surface seawater temperatures on energy conversion. The thermodynamic model is developed for this proposed cycle, 22 wet working fluids are selected for the Rankine sub-cycle. A comparative analysis with the traditional Rankine cycles is conducted to evaluate the cycle performance. The results show that the overheated temperature at the turbine inlet reaches up to 44.7 °C relying on the ocean thermal energy. In contrast to the traditional Rankine cycle, the proposed cycle achieved an average thermal efficiency improvement of 40 % and a substantial increase in exergy efficiency, ranging from 17.99–57.58 % to 32.83–68.88 %. The economic analysis reveals that the optimal cost is reduced by 38 % compared to the Rankine cycle, leading to a lower levelized cost of energy, down from 0.18 $/kWh to 0.13 $/kWh. This novel cycle offers an economically efficient and easily implementable approach to enhance low-grade thermal energy conversion.

Parameter Optimization and Performance Research: Radial Inflow Turbine in Ocean Thermal Energy Conversion

Combining one-dimensional parameter optimization and three-dimensional modeling optimization, a 30 kW radial inflow turbine for ocean thermal energy conversion was designed. In this paper, the effects of blade tip clearance, blade number, twist angle, and wheel–diameter ratio on the radial inflow turbine were analyzed. The results show that the model prediction method based on 3D numerical simulation data can effectively complete secondary optimization of the radial turbine rotor. The prediction model can be used to directly obtain the optimal modeling parameter of the rotor. The tip clearance, blade number, twist angle, wheel–diameter ratio, and shaft efficiency were found to be 0.273 mm, 16, 43.378°, 0.241, and 88.467%, respectively. The optimized shaft efficiency of the turbine was found to be 2.239% higher than the one-dimensional design result, which is of great significance in reducing the system’s power generation costs and promoting the application of this approach in engineering power generation using ocean thermal energy.

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.

Preface

This Conference Proceedings volume contains the written versions of the contributions presented during the 78th Conference of the “Associazione Termotecnica Italiana” (ATI). The conference took place on September 14-15, 2023, at the Carpi “Polo Universitario” (Modena province, Italy), of the Department of Engineering “Enzo Ferrari” at the University of Modena and Reggio Emilia.ATI represents the oldest association in Italy that brings together universities, industries, and technical experts in the field of thermal engineering and energy conversion systems. The 2023 edition of the ATI Conference focused on energy transition problems and provided a platform for discussing recent developments in a wide range of themes. Technical sessions were organized on the following topics: clean, sustainable, and renewable energy production and storage systems internal combustion engines and sustainable mobility, hydrogen production, transport, storage, and utilization, efficient energy use and conversion in systems and processes, energy efficiency in buildings, energy storage systems, hydraulics, pneumatics, and drive systems, energy and indoor air quality (IAQ), measurement and monitoring in energy systems, refrigeration and heat pumps, smart energy systems, smart grids, and distributed power production, thermal control in electric and hybrid vehicles, turbomachines, carbon sequestration, and carbon-negative applications, climate change mitigation in the built environment, innovation in heat transfer problems, and policies to support the energy transition.The opening session of the conference included five keynote lectures by Prof. Marco Baratieri (Libera Università di Bolzano), Ing. Nicola Troiano (Gruppo HERA S.p.A, Conference financial supporter), Ing. Davide Coduri (KSB ITALIA S.p.A, Conference financial supporter), Ing. Bruno Grispino (CPL CONCORDIA Soc. Coop., Conference financial supporter), and Dott. Antonio Panvini (Comitato Termotecnico Italiano).The conference contributions were presented through approximately 80 oral presentations, distributed across 14 technical sessions, and around 30 posters displayed in a dedicated session. More than XXX delegates from the Italian academia as well as the local industrial sector attended the conference.List of Organizing Committee, Scientific Committee are available in this Pdf.

Performance analysis of a novel small-scale radial turbine with adjustable nozzle for ocean thermal energy conversion

Ocean thermal energy is acknowledged as one of the most promising ocean renewable energy sources in low latitude sea areas. In the ocean thermal energy conversion system, the turbine plays a significant role, and it is responsible for converting the working medium enthalpy into the shaft output power. The present study is focused on the performance analysis of a novel radial inflow turbine with an adjustable nozzle in the OTEC system in order to adapt to the changing operating conditions of the turbine, which vary with the change in seawater temperature. At the design point, the predicted overall isentropic efficiency is 86.5%, and the shaft output power is 15.3 kW, slightly higher than the expected 15 kW. Furthermore, a parametric study is performed, respectively, for the nozzle vane stagger angle and the nozzle-impeller radial clearance to explore the favorable geometric parameters for different conditions. The turbine’s overall efficiency increases slightly with deceasing nozzle-impeller radial clearance, and the variation of the nozzle vane stagger angle is much more influential on the turbine shaft power and overall efficiency. The optimum stagger angle point moves from 32° to 36° gradually with the increase in nozzle-impeller clearance. Finally, the feasibility of an adjustable nozzle for the turbine under off-design conditions was verified by combining the radial clearance and nozzle stagger angle.

Structural Design and Analysis of a 100 kW Radial Turbine for an Ocean Thermal Energy Conversion–Organic Rankine Cycle Power Plant

In this paper, a 100 kW radial inflow turbine is designed for an ocean thermal energy conversion (OTEC) power plant based on the organic Rankine cycle (ORC) with ammonia as the working fluid. Based on one-dimensional (1D) and three-dimensional computational fluid dynamics (3D-CFD) modeling, the mechanical structure design, static and modal analyses of the turbine and its components are carried out to investigate its mechanical performance. The results show the stress and strain distribution in the volute, stator and rotor, and their maximum values appear, respectively, at the inlet cutout, the tip of the stator outlet and the connection position between the rotor and the shaft. After optimization, all the stresses in the above components are below the allowable values. The frequencies from the first order to the sixth order of the rotor and whole turbine were obtained through modal analysis without prestress and under prestress. The maximum frequency of the rotor and whole turbine is 707.75 Hz and 40.22 Hz, both of which are far away from the resonance frequency range that can avoid resonance. Therefore, the structure of the designed turbine is safe, feasible and reliable so as to better guide actual production.

Advancements in Phase Change Materials: A Path to Sustainable Energy Solutions in China

With the rapid development of China, the country's demand for energy has significantly increased. According to the "World Energy Outlook 2020," China's annual average growth rate of primary energy consumption is 1.9%, and it is predicted that China's energy consumption will account for over 20% of global energy consumption by 2050. In the energy budget, 90% is allocated to thermal energy conversion, transmission, and storage. Facing an impending energy crisis, enhancing energy production while minimizing energy loss has become a matter of widespread concern. Phase Change Materials (PCMs), also called phase change energy storage materials, have garnered attention as a novel energy-efficient and environmentally friendly material. PCMs refer to substances that undergo a change in state and provide latent heat without changing their temperature. Phase change thermal storage materials utilize phase transitions during heat exchange processes to store energy by means of these phase changes. Compared to chemical energy storage materials, PCMs offer high thermal efficiency and energy storage density, maintain constant temperature, and are more easily controllable. They hold great potential as a promising material in the field of thermal energy storage and have been widely applied in areas such as solar energy, lithium batteries, building insulation, and medical applications. This article offers a comprehensive overview of the principles and classification of Phase Change Material (PCM), including organic PCM, inorganic PCM, and composite PCM. It discusses their applications in solar thermal energy storage, cooling of photovoltaic panels, building materials, and industrial waste heat recovery. The article also highlights China's research and development efforts in these areas, emphasizing the country's significant contributions to advancing PCM technology.

Role of R717 blends in ocean thermal energy conversion organic Rankine cycle

Ammonia (R717) is widely deemed to be the potential working fluid of the ocean thermal energy conversion (OTEC) system. To further improve the performance of OTEC Rankine cycle, R717-based binary non-azeotropic blends are employed as the working fluids, where R134a and R125 are selected as the additives. The thermodynamic model as well as the cost estimation model of OTEC system is developed to demonstrate the potential of power generation. The energy, net efficiency, and Levelized cost of electricity are evaluated and examined for OTEC system employing different blends, encompassing R717/R134a and R717/R125. The results indicate that the thermodynamic and economic indexes of R717-based blends are superior to pure fluids (i.e., R134a, R125, and R717). To maximize the net power, the mass fractions of R717 are 0.1 and 0.5 for the R717/R134a and R717/R125 blends, respectively, in a 30 kW-scale OTEC system, where the net power are higher than that of the pure R717 by 134% and 87%. In addition, R717-based blends perform better in energy cost than pure fluids, with 0–67% lower Levelized cost of electricity. Especially, a large-scale OTEC power plant is more economically profitable than a small one.

A Review on the Nanofluids-PCMs Integrated Solutions for Solar Thermal Heat Transfer Enhancement Purposes

The current review offers a critical survey on published studies concerning the simultaneous use of PCMs and nanofluids for solar thermal energy storage and conversion processes. Also, the main thermophysical properties of PCMs and nanofluids are discussed in detail. On one hand, the properties of these types of nanofluids are analyzed, as well as those of the general types of nanofluids, like the thermal conductivity and latent heat capacity. On the other hand, there are specific characteristics of PCMs like, for instance, the phase-change duration and the phase-change temperature. Moreover, the main improvement techniques in order for PCMs and nanofluids to be used in solar thermal applications are described in detail, including the inclusion of highly thermal conductive nanoparticles and other nanostructures in nano-enhanced PCMs and PCMs with extended surfaces, among others. Regarding those improvement techniques, it was found that, for instance, nanofluids can enhance the thermal conductivity of the base fluids by up to 100%. In addition, it was also reported that the simultaneous use of PCMs and nanofluids enhances the overall, thermal, and electrical efficiencies of solar thermal energy storage systems and photovoltaic-nano-enhanced PCM systems. Finally, the main limitations and guidelines are summarized for future research in the technological and research fields of nanofluids and PCMs.

Assembling all-wood-derived carbon/carbon dots-assisted phase change materials for high-efficiency thermal-energy harvesters

The collection and storage of renewable, sustainable and clean energy including wind, solar, and tidal energy has attracted considerable attention because of its promising potential to replace fossil energy sources. Advanced energy-storage materials are the core component for energy harvesters, affording the high-efficiency conversion of these new-style energy sources. Herein, originated from nature, a series of all-wood-derived carbon-assisted phase change materials (PCMs) were purposed by incorporating carbon dots-modified polyethylene glycol matrix into carbon skeletons via a vacuum-impregnation strategy. The resultant PCMs possessed desired anti-leakage capability and superior thermophysical behaviors. In particular, the optimum sample posed high latent heat (131.5 J/g) and well thermal stability, where the corresponding enthalpy still reserved 90 % over 100 heating/cooling cycles. More importantly, the as-fabricated thermal-energy harvester presented prominent capability to strorage and release multiple forms of thermal energy, as well as high-efficiency solar-energy utilization, corresponding to a photothermal conversion efficiency of 88 % in simulated sunlight irradiation, far exceeding some reported PCMs. Overall, with the introduction of wood-derived carbon dots and carbon skeletons, the assembled all-wood-derived carbon-assisted PCMs afforded trinity advantages on thermal performance, cycling stability, and energy conversion efficiency, which provide a promising potential for the practical application in thermal-energy harvesters.

Numerical analyses on the feasibility of TC type nozzles in ocean thermal energy Conversion turbines

ABSTRACT The turbine is a crucial component in harnessing ocean thermal energy (OTE), and the impact of the nozzle on turbine performance is significant. TC-profile nozzles have been proven to operate efficiently in air turbines under high-temperature and high-pressure conditions. However, their performance in ocean thermal energy conversion (OTEC) turbines using organic working fluids (low-temperature and low-pressure) still requires further research. Therefore, we focused on a practical 100 kW OTEC turbine equipped with different types of TC-profile nozzles. Three-dimensional numerical models were established, and the simulation results demonstrated that the turbine efficiency using TC-3A was generally higher than other turbines, reaching an optimal efficiency of 89.4%. The turbine can operate efficiently at deviations from the design point of + 10.27% or −31.39% in mass flow rate, ±3°C in inlet temperature, and + 20% or −11.43% in rotor speed during off-design conditions. The results revealed the adaptability of TC-3A nozzles to the OTEC environment and their excellent off-design performance. The study could provide valuable guidance and references for the application of TC-type nozzles in the field of OTEC turbines.

H-ZIF-Based Shape Memory Phase Change Materials for Thermal Management, Energy Conversion, and Electromagnetic Shielding

H-ZIF-Based Shape Memory Phase Change Materials for Thermal Management, Energy Conversion, and Electromagnetic Shielding

The Global Technical, Economic, and Feasible Potential of Renewable Electricity

Renewable electricity generation will need to be rapidly scaled to address climate change and other environmental challenges. Doing so effectively will require an understanding of resource availability. We review estimates for renewable electricity of the global technical potential, defined as the amount of electricity that could be produced with current technologies when accounting for geographical and technical limitations as well as conversion efficiencies; economic potential, which also includes cost; and feasible potential, which accounts for societal and environmental constraints. We consider utility-scale and rooftop solar photovoltaics, concentrated solar power, onshore and offshore wind, hydropower, geothermal electricity, and ocean (wave, tidal, ocean thermal energy conversion, and salinity gradient energy) technologies. We find that the reported technical potential for each energy resource ranges over several orders of magnitude across and often within technologies. Therefore, we also discuss the main factors explaining why authors find such different results. According to this review and on the basis of the most robust studies, we find that technical potentials for utility-scale solar photovoltaic, concentrated solar power, onshore wind, and offshore wind are above 100 PWh/year. Hydropower, geothermal electricity, and ocean thermal energy conversion have technical potentials above 10 PWh/year. Rooftop solar photovoltaic, wave, and tidal have technical potentials above 1 PWh/year. Salinity gradient has a technical potential above 0.1 PWh/year. The literature assessing the global economic potential of renewables, which considers the cost of each renewable resource, shows that the economic potential is higher than current and near-future electricity demand. Fewer studies have calculated the global feasible potential, which considers societal and environmental constraints. While these ranges are useful for assessing the magnitude of available energy sources, they may omit challenges for large-scale renewable portfolios.

Test and Analysis of the Heat Exchanger for Small Ocean Thermal Energy Power Generation Devices

The application of ocean thermal energy conversion is an effective method to extend underwater vehicles’ running times and operating ranges, and the solid–liquid phase transition of the phase change material (PCM) in the heat exchanger is a key process for underwater vehicles to collect ocean thermal energy. This study proposes a heat exchanger structure for a small-size thermal energy power generation device and establishes the heat transfer model for the heat exchanger. Simulations were conducted considering convective heat transfer, and the obtained results demonstrated the feasibility of the designed structure. A prototype of the heat exchanger was developed, and physical experiments were conducted to validate the performance of the prototype. The results show that the melting process of the heat exchanger can be completed within 6 to 12 h, the solidification process can be completed within 3 to 7 h, and the heat transfer time decreases with the increase in temperature difference, verifying the compatibility with the underwater vehicles’ working patterns. Moreover, the heat exchanger could theoretically extend their lifetime. The results can provide a reference for the structural design and optimization of the heat exchanger for small ocean thermal energy power generation devices in the future.

Optimized design and application performance analysis of heat recovery hybrid system for radioisotope thermophotovoltaic based on thermoelectric heat dissipation

Radioisotope thermophotovoltaic (RTPV) generators face a fundamental challenge, which is the significant output loss and energy wastage caused by the substantial thermal deposition resulting from the arrival of infrared radiation from the heat source at the energy conversion unit. This contradiction has gradually become a key limiting factor for its further development in recent years. In this work, a novel hybrid system based on the thermophotovoltaic-thermoelectric (TPV-TE) effect is proposed, in which the Bi2Te3-based I/W-type thermoelectric leg plays both heat dissipation and heat recovery for power supply. The heat recovery thermoelectric legs were prepared using a cold pressing mould and sintering method, and their Seebeck effect was used to convert the unwanted heat recover from InGaAs thermophotovoltaic cells into electrical power. The design preparation scheme of the heat recovery hybrid module and its potential application performance at 700–1000 K heat source temperature were investigated through experimental tests and simulations. The W-type TPV-TE heat recovery hybrid system exhibits good gain of 12.9% at 1000 K, and the output power can be increased by additional 21.6% in series mode. Simulations for environmental applications show better heat recovery in deep space than on Earth, with an output gain of 63.7% and an energy conversion efficiency of 11.23%. The heat recovery design enhances the competitiveness of the RTPV and provides new ideas for optimizing other heat recovery applications for thermal energy conversion systems.

Progress and Prospects for Research and Technology Development of Supercritical CO2 Thermal Conversion Systems for Power, Energy Storage, and Waste Heat Recovery

CO2 is an environmentally friendly heat transfer fluid and has many advantages in thermal energy and power systems due to its peculiar thermal transport and physical properties. Supercritical CO2 (S-CO2) thermal energy conversion systems are promising for innovative technology in domestic and industrial applications including heat pump, air-conditioning, power generation, renewable energy systems, energy storage, thermal management, waste heat recovery and others. Both S-CO2 and transcritical CO2 thermodynamic cycles have been extensively investigated in order to improve the efficiencies of thermal and power systems and achieve net zero carbon emissions. This paper focuses on the progress and prospects for current research and technology development of S-CO2 thermal energy conversion systems and their applications including power generation, energy storage and waste heat recovery. First, the CO2 thermal transport and physical properties and benefits using CO2 as a heat transfer fluid in thermal energy and power systems are discussed. Then, classification of CO2 thermodynamic systems is presented. Next, S-CO2 for power generation, energy storage and waste heat recovery systems are presented. Finally, research needs of subcritical and supercritical CO2 heat transfer, fluid flow and heat exchangers for the development of various thermal energy and power systems are discussed.

Reliability assessment of the ocean thermal energy conversion systems through Monte Carlo simulation considering outside temperature variation

AbstractThe ocean thermal energy conversion (OTEC) systems, as renewable energy-based power plants, have the potential to play a significant role in meeting future electricity demands due to the vast expanse of the world's oceans. These systems employ the temperature difference between surface ocean waters and deep ocean waters to drive a thermodynamic cycle and produce electricity. The temperature of deep ocean waters, approximately 1000 m below the surface, is approximately 4 °C, while surface ocean temperatures typically range between 20 and 30 °C. The generated power of OTEC systems is dependent on these temperature differences and may vary with changes in surface ocean temperatures. In this study, the main focus is to find the impact of temperature variation on the failure rates of OTEC system components and the generated power output of these plants. The findings indicate that as the demand for the power system increases, its reliability decreases. In order to improve the reliability of the power system, the integration of a new generation unit, such as the close cycle OTEC power plant under investigation, could be necessary. The findings also indicate the importance of considering temperature variation in the evaluation of the reliability of such types of power plants based on renewable energy.

A comprehensive review on solar to thermal energy conversion and storage using phase change materials

A comprehensive review on solar to thermal energy conversion and storage using phase change materials

Experimental investigation on the interaction characteristics of lead‑bismuth liquid metal and water

The energy conversion and utilization of liquid metals is becoming one of the effective ways to solve energy problems. The conversion and transfer of thermal energy from liquid metals to water are significant in advanced nuclear energy and metallurgy. It has been an urgent concern, especially in Generation IV of advanced nuclear energy systems‑lead-cooled fast reactors and the ultimate energy source for humanity-fusion engineering. In this paper, a large-scale liquid metal and water interaction test platform was developed to investigate the lead‑bismuth liquid metal and water interaction and the cover gas pressurization characteristics. Firstly, the evolution of phase behavior and water/vapor migration paths were inferred based on the transient temperature law. Secondly, the cover gas pressurization patterns above the molten pool were analyzed, and the test data showed three pressurization patterns, including two types of local vapor explosions. Additionally, this paper predicted the existence of another dangerous pressurization mode, i.e., overall vapor explosion. Finally, the cover gas pressurization rate model was established, and there is an appropriate matching between the model and test data with an average relative error of 12.6%. This paper provides an essential reference for profoundly understanding the energy conversion and thermal interactions between liquid metals and water.

Vibrational Responses of an Ultra-Large Cold-Water Pipe for Ocean Thermal Energy Conversion: A Numerical Approach

The transportation of seawater on a grand scale via an ultra-large cold-water pipe situated within the context of ocean thermal energy conversion (OTEC) floating installations inherently presents challenges associated with instability and potential malfunction in the face of demanding operational circumstances. This study endeavors to augment the stability and security of cold-water pipe (CWP) operations by scrutinizing their vibrational attributes across diverse boundary configurations. Initially, we invoke Euler–Bernoulli beam theory to forge the analytical framework and proffer a semi-analytical resolution by utilizing the generalized integral transform technique (GITT). Subsequently, we authenticate the convergence and precision of our proposed approach through comparative analysis with extant theories. Our findings underscore the conspicuous influence of boundary conditions on the convergence of transverse displacement. The influence of internal flow on the transverse displacement and the natural frequency manifests substantial variability under different boundary conditions. Significantly, an escalation in the internal flow velocity triggers a concomitant reduction in the natural frequency, ultimately culminating in instability once the critical velocity threshold is reached. Additionally, the reliance of the transverse displacement and the natural frequency on the clump weight at the bottom is markedly pronounced. Our discoveries propose that pipe stability can be ameliorated by adjusting the clump weight at the bottom. Furthermore, the novel insights obtained through our proposed approach can significantly aid in the early-stage design and analysis of CWP.