Start-stop Operation Research Articles

Post mortem Study of Catalyst Degradations Occurring in High-Temperature Proton Exchange Membrane Fuel Cells Upon Start-Stop Operation

High-temperature proton exchange membrane fuel cells (HT-PEMFCs) could replace fossil fuel-based technologies for applications which cannot involve bulky/heavy cooling systems, such as aeronautics. However, severe materials degradations upon operation prevent performance retention for acceptable lifetimes. While others have already reported degradations in HT-PEMFC, post mortem characterizations of used HT-PEMFC membrane electrode assemblies (MEAs) remain scarce. Herein, HT-PEMFC performance degradation is studied by applying a startup/shutdown protocol to a short-stack operated at 160 °C; one MEA is characterized using complementary physicochemical/electrochemical techniques to identify/understand the degradation mechanisms and their origin. This start/stop operation mode (co-flow gas reactants) leads to substantial degradation inhomogeneity. For the anode, migration, coalescence, and detachment of Pt nanoparticles are witnessed induced by high-surface-area carbon support functionalization and corrosion. The anode electrochemical surface area (ECSA) remains constant at the inlet and increases significantly at the outlet, following inhomogeneous degradation of the cathode catalyst: the Ptz+ ions formed at high potential/oxidizing conditions concentrate towards the outlet, where they redeposit locally or at the anode, after diffusion/migration across the PBI membrane. Hence, the cathode ECSA decreases significantly at the inlet. Furthermore, intense Ni-leaching from the initial PtNi alloy catalyst is reported as a result of O2 mass-transport and phosphoric acid dilution inhomogeneity.

Study on the dynamic characteristics of the head cover-bolts-stay ring of a variable-speed pump-turbine

Abstract The design and operation of variable-speed pump-turbine units is more difficult than that of conventional units due to their adjustable speed, frequent start stop, and forward and reverse operation. Due to the development of variable-speed pump-turbines towards variable-speed, high head, and large capacity, the vibration instability caused by frequent switching between turbine and pump operating conditions is more prominent. The study on the dynamic characteristics of the head cover-bolts-stay ring system is an important guarantee for the long-term safe and stable operation of the units. This article uses the finite element method to conduct modal analysis on the head cover-bolts-stay ring system of a variable-speed pump-turbine. The natural frequency and mode shape are analyzed, and resonance analysis is carried out in combination with the dynamic and static interference characteristics of the variable-speed unit. The research results show that the dynamic and static interference frequency of the unit during the variable-speed process may be close to the natural frequency of the head cover-bolts-stay ring system, which needs to be paid attention to during operation. The results and findings in this article provide theory and practical basis for the research and design of high head, large capacity, and variable-speed pump-turbine units.

Simulated Start-Stop and the Impact of Catalyst Layer Redox on Degradation and Performance Loss in Low-Temperature Electrolysis

Stress tests are developed that focus on anode catalyst layer degradation in proton exchange membrane electrolysis due to simulated start-stop operation. Ex situ testing indicates that repeated redox cycling accelerates catalyst dissolution, due to near-surface reduction and the higher dissolution kinetics of metals when cycling to high potentials. Similar results occur in situ, where a large decrease in cell kinetics (>70%) is found along with iridium migrating from the anode catalyst layer into the membrane. Additional processes are observed, however, including changes in iridium oxidation, the formation of thinner and denser catalyst layers, and platinum migration from the transport layer. Increased interfacial weakening is also found, adding to both ohmic and kinetic loss by adding contact resistances and isolating portions of the catalyst layer. Repeated shutoffs of the water flow further accelerate performance loss and increase the frequency of tearing and delamination at interfaces and within catalyst layers. These tests were applied to several commercial catalysts, where higher loss rates were observed for catalysts that contained ruthenium or high metal content. These results demonstrate the need to understand how operational stops occur, to identify how loss mechanisms are accelerated, and to develop strategies to limit performance loss.

Effect of pump start-stop operation in drilling of offshore Extreme-HPHT directional wells upon bottomhole transient fluctuating pressure

The engineering roadblocks and challenges — extreme-high temperature + extreme-high pressure + extremely narrow pressure window — confronted by the extreme high temperature-high pressure (extreme-HPHT) well in the Yingqiong Basin in the South China Sea have put forward much more stringent requirements for accurate prediction of the pressure peak and change rule of directional well transient flow during pump start-circulation-pump stop. This paper has given sufficient consideration to the synthetic effects of the directional well type, changes in drilling fluid temperature-pressure physical properties (density and rheology), and transient friction, which are used in conjunction with the drilling fluid temperature-pressure physical property, continuity, momentum and energy equations to build a mathematical model of directional well pump start-stop transient flow coupled with drilling fluid temperature-pressure physical properties-unsteady friction in transient flow. Calculations show that the peak fluctuating pressure and the peak steady circulating pressure at the bottomhole of the coupling model is smaller than the result of uncoupled consideration. The coupling consideration of the influence of temperature and pressure and transient friction cannot be ignored; otherwise it will increase the risk of pump starting and stopping operations. In the meantime, the established transient flow model significant for the optimal design of pump start-stop index, pump displacement, drilling fluid density and well inclination angle can be used to realize the accurate prediction of the magnitude, fluctuation period, and change law of wellbore pressure during the whole pump start-stop process of the Extreme-HPHT well.

Analysis of Equipment Management Methods for Pumped Storage Power Stations Under the “Dual-Carbon” Goals

Pumped-storage, as the most mature technology, economically optimal, and most suitable for largescale development, plays a crucial role in promoting the consumption of clean energy and supporting the construction of new energy systems. Pumped-storage power stations involve various types of equipment such as hydraulic and electrical devices. The frequent start-stop operation in the context of new energy system construction will pose significant challenges to the equipment. This paper proposes a reliability-centered health management method for pumped-storage equipment. It implements differentiated and hierarchical management strategies for various types of equipment and establishes differentiated equipment maintenance strategies based on this.The project takes the manhole door of a power station governor pressure oil tank with nitrile rubber sealing ring as an example to calculate the service life in actual working conditions. Through calculation, it is considered that when reliability is 99%, the service life can be extended for more than 5 years, and if the reliability is 100%, the service life can be extended for more than 1 year.

Highly Durable Membrane–Electrode-Assemblies Using High M w and Ether-Free Polyfluorene-Based Electrolytes for Anion Exchange Membrane Water Electrolyzer

Water electrolysis technology, which produces hydrogen through electrolysis of water using electricity derived from renewable energy, is attracting attention as a way to realize a carbon-neutral society. Among water electrolysis systems, anion exchange membrane water electrolysis (AEMWE) can achieve high energy conversion efficiency and quick response to fluctuating power supplies by using thin polymer electrolyte membranes, and operates in an alkaline environment, allowing the use of inexpensive non-precious metals for bipolar plates and electrodes. However, conventional AEMs and ionomers exhibit low durability in high-temperature and alkaline environments, which is a critical issue in realizing high-performance and highly durable membrane–electrode assemblies (MEAs) for AEMWE.Our group developed high-molecular-weight (M w) and ether-free, polyfluorene-based anion conducting polyelectrolyte (PFT-Cx-TMA, Cx represent the number of carbon atoms in the alkyl side chain) as shown in Fig. 1a.1 These polyelectrolytes show high OH− conductivity over 100 mS cm− 1, high chemical durability (stable in 8 M NaOH at 80 °C for 168 h), and high tensile strength (25–42 MPa, comparable to a Nafion membrane). This study applied PFT-Cx-TMA to AEMs and ionomers in MEAs to evaluate the durability against various AEMWE operations (constant current operation and start-stop operation in different voltage ranges), demonstrating MEAs with both high performance and high durability.Fig. 1b shows the constant current durability of the MEA with PFT-C8-TMA as AEM and ionomer.2 This test was performed at the constant current density of 0.2 A cm− 2 and 80 °C using a 1 M KOH aqueous solution fed to the anode. Here, to investigate the effects of AEMs and ionomers on MEA durability, conventional precious metal catalysts (anode: IrO2, cathode: Pt/C or PtRu/C) were used in the MEA. The MEA with PFT-C8-TMA exhibited a current density of 1.0 A cm− 2 at 1.79 V and there was almost no voltage change even after the constant current operation for over 100 h. It is noteworthy that the MEAs using PFT-C8-TMA and PFT-C10-TMA demonstrated high performance and high durability not only in the alkaline-fed operation but also in the pure-water-fed operation.Next, this study investigated the start-stop durability of MEAs using voltage cycles simulating the start-stop operation of water electrolyzer. Here, square-wave voltage cycling of holding at 1.8 V and 0.1 V for 1 min each was used for the start-stop test at 80 °C with a 1 M KOH solution fed to the anode. As a comparison, the MEA was prepared using a commercial AEM (Sustainion X37-50, 50 μm in thickness) and ionomer (Sustaionion XB-7). As shown in Fig. 1c, the MEA using a PFT-C10-TMA AEM (56 μm in thickness) and ionomer was highly durable in the start-stop test compared to the MEA using commercial Sustaionion. During the test, the elution of black solid particles from the catalyst layer and the increase in charge transfer resistance (Rct) were observed in the MEA with Sustaionion, whereas no elution of catalyst materials and no change in Rct were confirmed in the MEA with PFT-C10-TMA. These results suggest that nonlinear mechanical stress via rapid bubble formation and shutdown during start-stop cycles caused the leaching of catalysts, resulting in the performance loss of the MEA with Sustainion. The commercial ionomer (Sustainion XB-7) exhibits a large dimensional change (156%) compared to the PFT-C10-TMA membrane (34%) at 80 °C in a 1 M KOH aqueous solution. Catalyst particles covered with a highly swollen ionomer are easily detached from a catalyst layer by the nonlinear mechanical stress. On the other hand, the PFT-C10-TMA ionomer has a high molecular weight (M w = 240000 g mol−1), low swelling degree, and well-entangled polymer chains, hence preventing leaching of the catalyst particles.In summary, this study demonstrates that MEAs using high M w and ether-free polyfluorene-based electrolytes with both high chemical and mechanical durability have excellent durability for AEMWE operations including constant and dynamic (start-stop voltage cycling) operations. Acknowledgement This presentation is based on results obtained from a project, JPNP14021, commissioned by the New Energy and Industrial Technology Development Organization (NEDO). References (1) S. Miyanishi and T. Yamaguchi, Polym. Chem., 2020, 11, 3812–3820.(2) R. Soni, S. Miyanishi, H. Kuroki, and T. Yamaguchi, ACS Appl. Energy Mater., 2021, 4, 1053–1058. Figure 1

Start–Stop Cyclic Durability Analysis of Membrane–Electrode Assemblies Using Polyflourene-Based Electrolytes for an Anion-Exchange Membrane Water Electrolyzer

Highly durable anion-exchange membranes and ionomers are essential for developing cost-effective green hydrogen generation. The anion-exchange membrane and ionomer must withstand chemical attacks by OH– ions as well as nonlinear mechanical stress caused by irregular bubble formation when the electrolyzer is coupled with intermittent power sources. Herein, we examined the durability of a trimethylammonium-modified poly(fluorene-alt-tetrafluorophenylene) (PFT-C10-TMA)-based membrane and ionomer under dynamic operation to evaluate its competency to couple with intermittent power sources. The durability of membrane–electrode assembly (MEA) prepared using these polymer membrane and ionomer was evaluated by performing 1000 start–stop voltage cycles in 1 M KOH solution at 80 °C while varying the applied cell voltage. The electrochemical performances of the MEA and electrodes were simultaneously evaluated using an electrolyzer with reference electrodes, and the chemical structure of the polymer was examined before and after the durability test using NMR techniques. The MEA based on a commercial Sustainion XB-7 ionomer and X37-50 RT anion-exchange membranes showed the catalyst leaching during start–stop cycles, resulting in reduced cell performance. On the other hand, the MEAs with the PFT-C10-TMA membrane and ionomer demonstrated promising high chemical and mechanical durability under dynamic start–stop operation, encouraging the development of electrolyzers that can function with intermittent renewable power sources.

Control strategy for actual constraints during the start–stop process of a supercritical CO2 Brayton cycle

Supercritical carbon dioxide (sCO2) Brayton cycles have been actively researched for potential use in a wide range of energy-conversion applications. The start–stop process, as an important transition stage, has a significant impact on the stability and security of system operation. Previous studies have ignored practical constraints which are important for the safe operation. This study developed a dynamic model of the recompression Brayton cycle and validated its components carefully. The effect of the operating variables on different constraints is analysed, and corresponding measures to practical constraints are proposed. A complete start–stop scheme considering multiple constraints is developed, and a simulation verification is performed. The results indicate that the rotation speeds of compressors should be changed simultaneously and the rate of temperature change must be constantly controlled for successful start–stop operation. Compared to the start-up process, overloading is more likely to occur during the shutdown process. Timely lowering of the turbomachinery rotation speed after the system load drops can effectively prevent overloading. There are various practical constraints and strong coupling relationships between the countermeasures and constraints. The coordination between the heat source and rotation speeds of the two compressors is the key factor that affects each constraint condition.

An improved memetic algorithm to solve the energy-efficient distributed flexible job shop scheduling problem with transportation and start-stop constraints.

In the current global cooperative production environment, modern industries are confronted with intricate production plans, demanding the adoption of contemporary production scheduling strategies. Within this context, distributed manufacturing has emerged as a prominent trend. Manufacturing enterprises, especially those engaged in activities like automotive mold production and welding, are facing a significant challenge in managing a significant amount of small-scale tasks characterized by short processing times. In this situation, it becomes imperative to consider the transportation time of jobs between machines. This paper simultaneously considers the transportation time of jobs between machines and the start-stop operation of the machines, which is the first time to our knowledge. An improved memetic algorithm (IMA) is proposed to solve the multi-objective distributed flexible job shop scheduling problem (MODFJSP) with the goal of minimizing maximum completion time and energy consumption. Then, a new multi-start simulated annealing algorithm is proposed and integrated into the IMA to improve the exploration ability and diversity of the algorithm. Furthermore, a new multiple-initialization rule is designed to enhance the quality of the initial population. Additionally, four improved variable neighborhood search strategies and two energy-saving strategies are designed to enhance the search ability and reduce energy consumption. To verify the effectiveness of the IMA, we conducted extensive testing and comprehensive evaluation on 20 instances. The results indicate that, when faced with the MODFJSP, the IMA can achieve better solutions in almost all instances, which is of great significance for the improvement of production scheduling in intelligent manufacturing.

Can heat pump heat storage system perform better for space heating in China’s primary schools? A field test and simulation analysis

The space heating system has a significant influence on the energy consumption and comfortable indoor environment for primary schools in China’s cold area. This study investigated the operational performance of heat pump systems in two primary schools. Results showed that the primary school buildings had high heating load at turn-on period every morning, which then quickly decreased when the students came to classroom. With this wide-range variation of heating load and unreasonable control strategy, the mismatch between heat source and heating demand happened very easily among the whole heating season. It then led to typical issues and poor energy efficiency of heat pump systems, including excessive space heating after school, low load ratio and frequent start-stop operation of heat pump systems, small water temperature difference and excessive water flow rate of water distribution system, etc. In allusion to those core issues, the heat pump heat storage system (HPHS) was raised for space heating in primary schools to decouple the heat supply side and the heat demand side. Then this study simulated and analyzed the operational performance of HPHS. Results showed that with reasonable control strategy, the excessive heat supply after school was effectively avoided to save almost 51% of the total heat consumption. Besides, the heat pumps could operate stably with almost full heating load for heat storage, thus reaching better energy performance with annual average COP of 5.13. Also the water distribution systems in the ground side, heat storage side and user side could reach better energy performance with high energy efficiency and reasonable water temperature difference among the heating season. Therefore, the HPHSs with reasonable control strategy could effectively solve those typical issues and are more suitable for primary schools.

Operation performance of an ultralow-temperature cascade refrigeration freezer with environmentally friendly refrigerants R290-R170.

In the present study, the operation performance of an ultralow-temperature cascade refrigeration freezer is experimentally researched. The natural refrigerants R290-R170 are adopted as high-temperature and low-temperature fluids. The experimental test is conducted in a type laboratory with a dry bulb temperature of 32.0°C and awet bulb temperature of 26.5°C. Different state monitors are set to display the system operation performance, and several temperature monitors are arranged to study the pull-down performance and temperature variations in the freezer. Based on the established experimental rig, three freezing temperatures, including - 40°C, - 80°C, and - 86°C, are measured and compared. The results show that it takes about 240min for the freezer to be pulled down to - 80°C. During the pull-down period, different monitors all experience rapid temperature drop, and the power consumption reduces from 1461.4W to 997.5W. Once the target temperature is achieved, the freezer comes into periodic start-stop operation. With the set temperature ranging from -40°C to -86°C, the inlet temperature of two compressors gradually decreases, while the discharge temperature has an increase trend. The cooling effect of the pre-cooled condenser reduces with the freezing temperature, while the long connection pipe has opposite variation profile. Moreover, it is observed that for different freezing temperatures, most of the space in the freezer can be cooled down to the target temperature. It is confirmed that the present ultralow-temperature freezer can be used for the storage and transportation of COVID-19 vaccines. However, it is also found that the cascade refrigeration system is not suitable for high freezing temperature, due to high power consumption and extensive start-stop switch of refrigeration system.

Correlation analysis and prediction of PEM fuel cell voltage during start-stop operation based on real-world driving data

The accurate prediction of fuel cell voltage is critical for fuel cell health management, which directly affects the performance and cost of fuel cell vehicles. This paper proposes analysis and prediction approaches for the start-stop voltage of fuel cell based on data from a proton exchange membrane fuel cell (PEMFC) bus operated under real traffic conditions in Chengdu, China. First, three correlative analysis methods are used to identify the correlation between the PEMFC voltage and other variables during start-stop operation. Then, the original sequence of PEMFC voltage at start-stop is processed using the grey model, residual grey model, logarithmic-power function grey model, and cotangent function grey model. Four grey prediction models are combined and optimized based on a backpropagation neural network (BPNN). Finally, an organic grey BPNN model (OGNNM) is established to predict the PEMFC voltage during start-stop operation and analyze its accuracy. Results indicate that the PEMFC voltage changes sharply during start-stop, which is closely connected to the current, hydrogen gas tank pressure, and PEMFC stack temperature. In addition, the proposed OGNNM has higher prediction precision than the other data-driven models that provides a basis for the control of fuel cell hybrid systems.

Potential of Membrane Alkaline Water Electrolysis in Connection with Renewable Power Sources

Hydrogen is an efficient energy carrier with numerous applications in various areas as industry, energetics, and transport. Its potential depends also on the origin of the energy used to produce the hydrogen with respect to its environmental impact. Where the standard production of hydrogen from fossil fuels (methane steam reforming, etc.) doesn’t bring any benefit to decarbonisation of society. The most ecological approach involves water electrolysis using ‘green’ electricity, such as renewable power sources. Such hydrogen thus stores energy which can be used later. Hydrogen, used in the transport sector, can minimize its environmental impact together with preserving the driving range and decrease the recharge/refill time in comparison with a pure battery-powered vehicle. For transportation the hydrogen filling stations network is required. Local production of hydrogen is one of proposed scenarios. The combination of electrolyser and renewable power source is the most viable local source of hydrogen. It is important to know the possible amount of hydrogen produced with respect to local environmental and economic conditions.Hydrogen production by water electrolysis is an extensively studied topic. Among the three most prominent types, which are the alkaline water electrolysis (AWE), proton-exchange membrane (PEM) electrolysis and high-temperature solid-oxide electrolysis, AWE is the technology which is widely used in the industry for the longest time. In the recent development, AWE is being modified by incorporation of anion-selective membranes (ASMs) to replace the diaphragm used as the cell separator. In comparison with the diaphragm, ASMs perform acceptably in environment with lower temperatures and lower concentrations of the liquid electrolyte, thus, allowing for very flexible operation similarly to the PEM electrolysers. On the other hand, ASMs are not yet in a development level where they could outperform the diaphragm and PEM in long-term stability.Renewable sources of energy, predominantly photovoltaic (PV) plants and wind turbines, operate with non-stable output of electricity. Considering their proposed connection to the water electrolysis, flexibility of such electrolyser is of the essence for maximizing hydrogen production.The aim of this work is to consider a connection of a PV plant with an AWE. Power output data from a real PV plant are taken as a source of electricity for a model AWE. The input data for the electrolyser were taken from a laboratory AWE. The AWE data were measured using a single-cell electrolyser using Zirfon Perl® cell separator with nickel-foam electrodes. Operation including ion-selective membranes was also taken into consideration. Data from literature were used to set possible operation range and other electrolyser parameters. Small-scale operation was then upscaled to match dimensions of a real AWE operation.Using the before mentioned data, a hydrogen production model was made. The model takes the power output of the PV plant in time and decides whether to use the power for preheating of the electrolyser or for electrolytic hydrogen production. Temperature of the electrolyser is influenced by the preheating, thermal-energy loss of the electrolytic reactions, or cooling to maintain optimal conditions.The advantage of the created model is its variability for both energy output of the power plant or other instable power source and the properties of the electrolyser. It can be used to predict hydrogen production in time with respect to the electrolyser and PV power plant size. The difference between standard AWE and AWE with ion exchange membrane is mainly shown during start-up time where membrane based electrolyser shows better efficiency. Frequency of start-stop operation modes thus influences the choice of suitable electrolyser type.Another output is to optimize design of an electrolyser to fit the scale of an existing plant from economical point of view. This knowledge is an important input into the plan which is set to introduce hydrogen-powered transport options where fossil-fuel powered vehicles is often the only option, such as unelectrified low-traffic railroad networks.Acknowledgment:This project is financed by the Technology Agency of the Czech Republic under grant TO01000324, in the frame of the KAPPA programme, with funding from EEA Grants and Norway Grants.

The importance of target product engineering for long-term operation of CO2 zero-gap electrolysers

Zero-gap electrolysers lead the race for industrial implementation in the electrosynthesis of carbon-based products from waste CO2. Due to their compact structure, these reactors have lower ohmic losses and attain higher energy efficiencies than the alternatives. However, most research conducted on these electrochemical cells dismisses key product-specific considerations. In this work, we optimised a zero-gap system and tuned it for the production of both formate and carbon monoxide. We assessed the industrial applicability of alternative metal-nitrogen-doped carbon catalysts and benchmarked them with state-of-the-art metallic nanoparticles. We achieved energy efficiencies as high as 50.4% for the production of HCOO- and 58.0% for the production of CO (86.9 mA cm−2). Finally, a rapid degradation study was conducted for start-stop operation, which revealed the current challenges of this novel family of catalysts. This study constitutes an essential step in the direction of upscaled zero-gap CO2 electrolysis with metal-nitrogen-doped carbon catalysts.

Hollow-Structure Pt-Ni Nanoparticle Electrocatalysts for Oxygen Reduction Reaction

An electrocatalyst with high oxygen reduction reaction (ORR) activity and high stability during start–stop operation is necessary. In this paper, hollow-structure Pt-Ni electrocatalysts are investigated as ORR catalysts. After synthesis via sacrificial SiO2 template method, the electrocatalyst exhibits much higher specific activity (1.88 mA/cm2) than a commercial Pt/C catalyst. The mass activity (0.49 A/mg) is 7 times higher than the commercial Pt/C catalyst. The kinetics of the ORR is evaluated using Tafel and K-L plots. It also exhibits a higher durability than commercial Pt/C catalyst during accelerated durability test (ADT). Moreover, the electrocatalyst shows good resistance against accelerated durability test for start–stop, the specific activity and mass activity drops 34.6% and 40.8%, respectively, far better than the commercial catalyst.

Characterisation of a controllable loop thermosyphon for precise temperature management

Controllable loop thermosyphon in frequently start-stop operation offers an efficient heat transfer regulation. This low-cost device has a great protentional in precise temperature management of refrigerators, industrial buildings, and cutting-edge equipment. In this study, a test platform is built to investigate the thermodynamic characteristics of controllable loop thermosyphon and its effectiveness on precise temperature control of a fresh-keeping box. The comprehensive behaviours in continuous and start-stop operations are explored using R407C, R410A, R404A and R134a as working fluids. The filling ratios for above refrigerants are optimised, and the influence of thermal boundaries are studied. Given a suitable filling ratio of R407C, the average fresh-keeping temperature is reduced from 25 °C to 5 °C in 90 min and to 0 °C in 180 min during continuous operation. In start-stop operation, a minimum fresh-keeping temperature fluctuation of ± 0.3 °C is achieved at various fresh-keeping temperatures. The maximum and average heat transfer rates in the running period are 74.0–94.5 W and 48.3–80.2 W, respectively. Results indicate that the controllable loop thermosyphon with R407C provides efficient heat transfer and thermal control, and thus perform well in precise temperature management.

Numerical prediction of the frictional losses in sliding bearings during start-stop operation

With the increased use of automotive engine start-stop systems, the numerical prediction and reduction of frictional losses in sliding bearings during starting and stopping procedures has become an important issue. In engineering practice, numerical simulations of sliding bearings in automotive engines are performed with statistical asperity contact models with empirical values for the necessary surface parameters. The aim of this study is to elucidate the applicability of these approaches for the prediction of friction in sliding bearings subjected to start-stop operation. For this purpose, the friction performance of sliding bearings was investigated in experiments on a test rig and in transient mixed elasto-hydrodynamic simulations in a multi-body simulation environment (mixed-EHL/MBS). In mixed-EHL/MBS, the extended Reynold’s equation with flow factors according to Patir and Cheng has been combined on the one hand with the statistical asperity contact model according to Greenwood and Tripp and on the other hand with the deterministic asperity contact model according to Herbst. The detailed comparison of simulation and experimental results clarifies that the application of statistical asperity contact models with empirical values of the necessary inputs leads to large deviations between experiment and simulation. The actual distribution and position of surface roughness, as used in deterministic contact modelling, is necessary for a reliable prediction of the frictional losses in sliding bearings during start-stop operation.

A case study on particulate emissions from a gasoline plug-in hybrid electric vehicle during engine warm-up, taking into account start–stop operation

Concerning the discussions about emissions caused by individual mobility, it is foreseeable that future vehicle concepts will increasingly be based on hybrid powertrains. These systems lead to more complex operating scenarios, which have a significant influence on the resulting emissions of the engine. This work shows a case study and the results in the operation and emission behavior of a plug-in hybrid electric vehicle with a direct injection gasoline engine when operated in an internationally recognized driving cycle. The vehicle’s exhaust aftertreatment system consists of a three-way catalytic converter; a particulate filter is not installed. The emissions are analyzed with a focus on particulate number emissions (from soot), especially during the warm-up phase and the frequent start–stop events (in total, there are 12 internal combustion engine operating phases), which are typical for hybrid vehicles. The results show that approximately 50% of the emitted particulates have a smaller size, 23 nm (a very high number of particulates with a mean size of 10 to 15 nm are present), which are currently not regulated, but are expected to have a high risk of adverse health effects.

State Evolution of Dry Gas Seal during Repeated Start–Stop Operation Using Acoustic Emission Method

Seal failure is a common phenomenon that occurs when using a dry gas seal. Many seal failures occur during the start-up process of the seal. In this study, the state evolution of a dry gas seal during repeated start–stop operations was investigated using an acoustic emission (AE) method and a modified empirical mode decomposition using masking signals (MS-EMD). It was evident that the liftoff speed and root mean square (RMS) of the full-speed period changed in stages. According to the change in these parameters, the state evolution of the dry gas seal during the experiment occurred in four stages: the running-in stage, normal working stage, transition stage, and abnormal stage. During the normal working stage, the RMS curve of the late liftoff period oscillated, but its wave trough was flattened due to misalignment of the seal face. After entering the transition stage, the period flattened at the wave trough disappeared and was replaced by a period of slow decline, which is attributed to the existence of abrasive particles between the seal faces. The unsteady accumulation of abrasive particles caused the liftoff speed to fluctuate significantly. By monitoring the liftoff speed of the seal and the RMS of the full-speed period, seal operators may assess the state of the seal and take intervention measures. This has great engineering value for the safe operation and life management of the seal.

Durability Loss Mechanisms and Sources in Proton Exchange Membrane-Based Electrolysis

In commercial electrolysis today, the cost of electricity input drives the cost of hydrogen production. As electrochemical water splitting is coupled directly with low-cost power sources, however, capital cost at lower capacity (renewable input) will be a significant factor in hydrogen cost [1,2]. While a variety of components affect electrolysis capital cost, reducing catalyst loading (several milligrams per square centimeter today) will be necessary to approach hydrogen production cost targets. Efforts have begun to address electrolyzer durability at low catalyst loading and variable load/input profiles, to: establish single-cell durability baselines for catalyst development projects; evaluate how the type and magnitude of stressor influences single-cell performance over time; and assess the ability of potential control to limit in-situ durability losses [3]. Although commercial electrolyzers are durable today, thick catalyst layers appear to mask catalyst corrosion and delay loss observations. Moreover, intermittent operation associated with renewable load profiles significantly accelerate loss compared to constant load. Performance loss has been tied to decreasing kinetics, where both catalyst layer thinning and changes to the electrode structure were observed. Cell diagnostics has been used to evaluate resistances in the catalyst layer, and changes at the catalyst layer interface may contribute to the observed loss. A variety of factors were examined as the sources of catalyst loss, including the expected dissolution rate of test profiles, the frequency of load cycling, and the ramp rate to peak load [4,5]. Testing has expanded to expected operation profiles (wind, solar) to correlate accelerated stress tests to anticipated use and device lifetime. Recently, studies have expanded into other catalyst loss mechanisms, including start-stop operation and hydrogen crossover. Lower potentials in the anode catalyst layer reduce the catalyst surface and lead to particle agglomeration and increased dissolution rates. These factors have been included to assess various stressors resulting in catalyst loss and shorter device lifetime. [1] H2 at Scale: Deeply Decarbonizing our Energy System. Presented at Annual Merit Review, U.S. Department of Energy; Washington, DC, June 6−10, 2016. https://www.hydrogen.energy.gov/pdfs/review16/2016_amr_h2_at_scale.pdf.[2] Denholm, P.; O’Connell, M.; Brinkman, G.; Jorgenson, J. Overgeneration from Solar Energy in California: A Field Guide to the Duck Chart; Vol. NREL/TP-6A20-65023; National Renewable Energy Laboratory: Golden, CO, 2015. Available at the following: http://www.nrel.gov/docs/fy16osti/65023.pdf.[3] S. M. Alia, H2@Scale: Experimental Characterization of Durability of Advanced ElectrolyzerConcepts in Dynamic Loading, 2018. https://www.hydrogen.energy.gov/pdfs/review18/tv146_alia_2018_p.pdf. [4] Cherevko, S.; Geiger, S.; Kasian, O.; Kulyk, N.; Grote, J. P.; Savan, A.; Shrestha, B. R.; Merzlikin, S.; Breitbach, B.; Ludwig, A.; Mayrhofer, K. J. J. Cat. Today 2016, 262, 170.[5] S. M. Alia and G. C. Anderson, J. Electrochem. Soc., 2019, 166, F282-F294.