查看更多>>摘要：This study proposes a comprehensive evaluation of the actual greenhouse effect related to the operation of ORC systems in the kW scale. The method is derived from the TEWI (total equivalent warming impact) concept for refrigeration systems, since it includes both direct and indirect contributions to the greenhouse gas emission related to the ORC system. A comparison between traditional HFC-134a (R134a) and some of its low-GWP replacements has been performed, accounting for the effect of the operating fluid leakage during system operation, but also for the indirect contribution associated to the lower performance that can be achieved using more sustainable working fluids, such as hydrofluoroolefins (HFO). Alternative fluids that have been tested are two pure compounds (R1234yf and R1234ze(E)), and four mixtures (R134a-R123434; R-134a-R1234ze(E); R515A; R430A). A semi-empirical lumped-parameters model has been employed for simulating the behavior of the ORC system. For the model validation, the experimental data collected on a reference 2-kW ORC test bench with R134a have been used. The model was then applied to investigate the performance of the system working with alternative fluids. The results show that the indirect emissions associated to HFOs may lead to higher values of total equivalent CO2 emissions, with respect to the employment of R134a as working fluid. The main factors affecting the environmental evaluation, such as emission factors, fluid leak rate and R134a concentration in the mixture, can be decisive and are discussed in this work.
查看更多>>摘要：The focus of this study is to evaluate the effects of porous media inside the gas flow channel of Proton Exchange Membrane Fuel Cells (PEMFC) on four different output parameters of voltage, power density, pressure drop, and Nusselt number (Nu) considering the impacts of its thickness, viscous resistance, and current density. Although it is proved that the new design will improve the convective heat transfer, there have not been studies to evaluate the effects of this porous layer on the electrochemical performance and heat transfer simultaneously. The results showed that viscous resistance has by far the highest impact on the power densities in high current densities while thicker inserted porous layer improves the performance. Results also demonstrate that a parameter is needed to consider all these output parameters at the same time, hence the Evaluation Criterion of Proton Exchange Membrane (ECPEM) is defined using artificial neural network (ANN) modeling. Single-objective optimization of the ECPEM is developed using the ANN models to produce 250,000 data. The optimum value of ECPEM was obtained 78.88 in the thickness of 500 mu m and the viscous resistance of 2,111,000 (1/m(2)) while the current density is equal to 0.65 (A/cm(2)).
查看更多>>摘要：R134a will be phased out soon due to its high GWP. Replacing R134a with a low Global Warming Potential (GWP) and non- or slightly flammable fluids in the systems in stock should maintain the same level of system performance. This study focuses on the drop-in alternatives for R134a regarding a Mobile Air-Conditioning system. A simulation model coupled with a microchannel heat exchanger model is developed and validated for calculating the system Coefficient of Performance (COP) and capacity of the Mobile Air-Conditioning system. A comprehensive method is developed to evaluate the alternative candidates combining the performance simulation and the flammability estimated of mixtures. The evaluation includes the system performance, environmental impact, and flammability of alternative mixtures. 15 pairs of binary mixtures from six pure fluids (R134a, R1234yf, R1234ze(E), R32, R227ea, and R152a) with 99 compositions are evaluated as R134a drop-in replacements, and the results are discussed. There is no perfect mixture found that is nonflammable, has low GWP, and has higher system efficiency and capacity than R134a in this study. Using R152a as a drop-in replacement of R134a can increase the system efficiency and capacity, but R152a is flammable. By adding R134a or R227ea to R152a can reduce the flammability of the mixture. R152a/R134a, R152a/R1234yf, and R32/R152a have higher COP and capacity than R134a, but they are not safe or environmental friendly enough. The mixtures for replacing R134a as drop-in alternatives with low Global Warming Potential (less than 150 or 700), nonflammable or slightly flammable are listed and discussed. The method developed in this study can be extended to evaluate alternatives for replacing other working fluids in any thermal application that will be restricted.
查看更多>>摘要：Upgrading the energy density of lithium-ion batteries is restricted by the thermal management technology of battery packs. In order to improve the battery energy density, this paper recommends an F2-type liquid cooling system with an M mode arrangement of cooling plates, which can fully adapt to 1C battery charge-discharge conditions. We provide a specific thermal management design for lithium-ion batteries for electric vehicles and energy storage power stations. In addition, the influence of the type of liquid cooling system, discharge rate, inlet temperature and flow rate are investigated, along with the effect of cooling plate arrangement on the temperature uniformity, maximum temperature, cooling efficiency factor and comprehensive heat transfer performance of cooling systems. The experimental results showed that the F2-type liquid cooling system has more advantages in cooling efficiency and comprehensive heat transfer performance than other liquid cooling systems. The best arrangement mode is M and the optimal inlet temperature is approximately 18.75 degrees C The upper limits of cooling water rate of flow at different charging and discharging rates are also determined. Cooling water rates of flow should be no less than 6 and 12 L/h when batteries are discharged at the rates of 1 and 2C, respectively.
查看更多>>摘要：The aim of this paper is to thoroughly analyse the influence of condensation models on the modelling of condensation phenomena in transonic flows of moist air. The reason for the study was the fact that different condensation models are used by researchers to obtain satisfactory results of numerical modelling. The condensation models tested herein differ in the nucleation rate formula and the droplets growth equation. Four most often used condensation models were selected for detailed investigations. The results obtained from each model were compared with experiments for the internal flow through a nozzle and the external flow around an airfoil. The main focus was on the location of the onset of the nucleation process. Moreover, the droplet growth intensity was compared and discussed. The nozzle flow CFD calculations were performed using the ANSYS Fluent commercial tool. Finally, the condensation model which is the most suitable for the moist air transonic flow was recommended.
查看更多>>摘要：The stability of superheated steam temperature (SST) is severely challenged by the adjustment of thermal power plants under a wide-load range. Accurate and efficient prediction of SST plays an important role in the control of superheat system. To this end, an SST prediction model based on a multi-mode integrated method is proposed in this paper. Firstly, conservation of energy, as an equality constraint, is introduced into the loss function of the data-driven model based on Long Short-Term Memory (LSTM) architecture. Subsequently, the physical relationship between SST and the spray water flow, as an inequality constraint, is introduced into the above loss function. Finally, an individual hybrid model for each operating mode is developed and integrated with multimode switching strategies based on attention mechanism. Operating data with a wide-load range is sampled from a supervisory information system (SIS) to validate the superiority of the proposed method. The comparison results demonstrated that the predictive effect of the hybrid model with physics-based loss function is not only generalizable but also scientifically consistent with the dynamic characteristics of the step response. Furthermore, the results of the multi-mode integrated model prove the effectiveness of the mode switching strategy based on the attention mechanism.
查看更多>>摘要：Research on nanofluids has increased markedly in the last two decades. Initial attention has focused on conventional or mono nanofluids, dispersions of one type of solid nano-sized particles in a base fluid. Despite various challenges such as dispersion stability or increased pumping power, nanofluids have become improved working fluids for various energy applications. Among them, convective heat transfer has been the main research topic since the very beginning. Hybrid nanofluids, dispersions of two or more different nanoadditives in mixture or composite form, have received attention more recently. Research on hybrid nanofluids aims to further enhance the individual benefits of each single dispersion through potential synergistic effects between nanomaterials. Multiple experimental studies have been conducted independently analysing the convective heat transfer performance of mono or hybrid nanofluids for single-phase and two-phase convective heat transfer applications. However, there are still no general conclusions about which nanofluids, mono or hybrid, present better prospects. This review summarizes the experimental studies that jointly analyse both hybrid and mono nanofluids for these applications and the results are classified according to the heat transfer device used. Based on this criterion, three large groups of devices were noticed for single-phase convective heat transfer (tubular heat exchangers, plate heat exchangers and minichannel heat exchangers/heat sinks), while one group was identified for twophase convective heat transfer (heat pipes). The main outcomes of these studies are summarized and critically analysed to draw general conclusions from an application point of view.
查看更多>>摘要：The offshore floating storage and regasification unit (FSRU) is developing rapidly as a new type of storage and gasification system for liquefied natural gas (LNG), and the heat transfer and system stability of offshore LNG vaporizers are greatly affected by ocean rolling. To reveal the regular pattern and mechanism of supercritical methane heat transfer performance under ocean motion, this research numerically studied the flow and heat transfer characteristics of supercritical methane in a 1 x 1 x 100 mm mini channel under the additional inertial force by large radius rolling. Based on the non-inertial coordinate system, the normal inertial acceleration, radial inertial acceleration and Coriolis inertial acceleration were considered. Moreover, Nusselt number, degree of fluctuation and friction factor were investigated. The ranges of Reynolds number and Nusselt numbers are 15,000 to 45,000 and 35 to 130 respectively. The results show that the Nusselt number and friction factor violently fluctuate under rolling motion, and the fluctuation period is similar to the rolling period. Besides, the fluctuation degree of Nusselt number is greater than that of friction factor, and the largest fluctuation degree of Nusselt number is 27.9%. The effect of the fluctuation increases with the decrease of the rolling period and the increase of the rolling amplitude. Hysteresis exists in the coupling of rolling, flow fluctuation and temperature fluctuation. In addition, the degree of fluctuation is reduced with the increase of fluid temperature and mass flux. For the effect of rolling, the additional force in the mainstream direction plays a leading role. Besides, the influence of radial force is greater than that of normal force for horizontal flow condition.
查看更多>>摘要：Dynamic hydrothermal management is the critical issue for improving the performance of Proton exchange membrane fuel cell (PEMFC). In this study, a multi-input and multi-output (MIMO) fuzzy control method is proposed for the hydrothermal management of PEMFC in real time. For PEMFC, stack temperature and relative humidity at cathode are chosen as the control objectives, and the cooling water flow rate and inlet gas humidity at cathode are set as the control variables. To establish the transient synergy control of temperature and humidity, a fuzzy logic controller is constructed. The input parameters of controller include the errors of temperature, humidity and their derivatives. The dynamic performances of temperature and humidity transient synergy control under the disturbance of load current and operating parameters are analyzed. The results show that the temperature and humidity transient synergy control based on MIMO fuzzy method presents faster response speed and better control performance than traditional Proportion Integration Differentiation (PID) control. The output voltage of the proposed control method is 3.14% and 4.21% higher than that of traditional PID controllers. Furthermore, the power density of THTSc is 9.37% higher than that of PID controller.
Garcia-Afonso, OscarDelgado-Torres, Agustin M.Gonzalez-Diaz, Benjamin
15页
查看更多>>摘要：Waste Heat Recovery with Organic Rankine Cycle (ORC) in piston engine-based power plants placed in isolated systems is addressed in this paper. The power plant subjected to study is located in La Palma, Canary Islands, considered a representative example of small-to-medium isolated power system where the diesel engine is the solely or the main conventional technology for generation. In those systems, the power plant load control to cover the grid demand results in a noticeable variability of the available heat. This challenge is addressed in this paper through a performance and economic evaluation of an intermediate thermal energy storage system. A techno-economic analysis of the proposed power plant is performed taking as input the annual exhaust thermal energy available. The ORC-Diesel unit interaction is studied by means of an in-house developed plant load management model. An annual power plant fuel economy gain and pollutant emission reduction between 5% and 7% is estimated. In addition, the extra power and spinning reserve provided by the ORC allows the power plant to reduce its operational cost through the reduction of the number of diesel units starts.
NSTL
Elsevier