Intermittent Operating Conditions Research Articles

Impact of variable hydrostatic pressure and intermittent operation on filtration performance and biofouling layer in gravity-driven membrane system for practical decentralized water supply

The gravity-driven membrane (GDM) systems offer a promising solution for decentralized drinking water supply due to its low maintenance and energy consumption. However, research on the practical application of GDM under variable hydrostatic pressure (variable load) and intermittent operation conditions, particularly in response to high-turbidity water, remains limited. This study investigates the long-term performance and characteristics of the biofouling layer of GDM under ultra-low variable hydrostatic pressure (20–60 mbar) and intermittent operation (8–12 h) conditions in practical decentralized water supply. The results indicate that the membrane flux (1.9–6.0 L∙m−2∙h−1, LMH) remained relatively stable despite variations in gravity-driven pressure (ΔP). Long-term operation of GDM can stably remove turbidity and potential pathogens from raw water. The total protein/polysaccharide ratios of extracellular polymeric substances (EPS) in the biofouling layer were below 0.50, reducing contaminant adhesion on the membrane surface. The 0.2–0.4 μm transparent exopolymer particles (TEP) fraction might be crucial for biofouling layer formation. The key species, belong to Proteobacteria and Patescibacteria, significantly alleviated membrane fouling by degrading polysaccharide and proteins in biofouling layer. Comammox Nitrospira were significantly enriched, contributing to efficient nitrification. GDM with variable load maintained a stable flux of 2.2–5.2 LMH under high-turbidity water conditions (300–1200 NTU), and simple forward flushing combined with sludge discharge can quickly restore ΔP. Overall, the variable load GDM system effectively manages membrane fouling and maintains stable filtration performance through the combined effects of variable load and intermittent operation, ensuring long-term and low-maintenance operation for decentralized water supply.

Understanding Mechanisms and Reversibility of Anode Degradation in PEM Water Electrolysis

Water electrolysis is an important route toward sustainable industrial production of hydrogen, as well as mitigation of the CO2 emissions associated with current hydrogen production from natural gas. The industrial deployment of clean electrolytic hydrogen will depend on long-term electrolyzer durability under the intermittent operating conditions associated with coupling with renewable sources of electricity. Thus, understanding cell degradation mechanisms is required to develop mitigation strategies and minimize performance degradation over extended periods of operation.Here, we investigate the effect of temperature and intermittent conditions on proton exchange membrane (PEM) water electrolyzer performance over a variety of timescales. Performance losses consisting of both reversible losses and irreversible degradation accelerate quickly during holds at high current densities and low temperatures, reaching high cell voltages within several hours. However, the cell can be returned to near beginning-of-test performance by interrupting these current holds with a variety of intermittent states of operation, such as interruption of the applied current or operation at higher temperatures. Reversible losses can harm the overall cell efficiency without being captured in short-term polarization curves, and may also contribute to stressors causing long-term degradation. The impact of different shut-down and idle states on the catalyst layers and cell degradation will also be discussed. The presented insights could contribute to the development of accelerated stress test protocols as well as performance recovery strategies for long-term operation of PEM water electrolyzers.

Unraveling the impact of reverse currents on electrode stability in anion exchange membrane water electrolysis

Anion Exchange Membrane Water Electrolysis (AEMWE) stands out as one of the promising ways of producing green hydrogen. However, significant strides in performance and durability are necessary for commercial competitiveness. Shunt currents and reverse currents are common problems associated with electrolyzers using conductive liquid electrolytes during start/stop conditions and can enhance electrode degradation. This study incorporates a dual Pt-wire reference electrode within the flow cell consisting of a NiFe-layered double hydroxide anode and different cathode materials to decouple individual electrode kinetics under steady state and intermittent operating conditions. The performance of bimetallic cathode catalysts like PtRu/C and NiMo/C was assessed in comparison with traditional Pt/C catalysts in the context of the hydrogen evolution reaction. The initial observed catalyst activity displayed an evident trend in the order of PtRu/C > Pt/C > NiMo/C. When subjected to reverse currents, all three systems showed degradation in performance. The use of reference electrodes illustrated that all cathode coatings degraded as a result of the reverse currents while the anode remained relatively stable. The degradation followed the trend of NiMo/C > PtRu/C > Pt/C. This work thus shows that reverse currents are a real issue for AEMWE and demonstrates the importance of investigating electrodes under intermittent conditions.

Effect of Different Control Strategies on the Heat Transfer Mechanism of Helical Energy Piles

In this paper, numerical simulations of a special energy pile, which constitutes a spiral-injected pipe and one straight discharge pile for Geothermal Heat Pump Systems (SGHEs-P(parallel)), were conducted by Fluent software. The effects of the spiral pitches on the heat transfer rate based on the G-function method and peripheral soil temperature of the pile were investigated under continuous and intermittent operation strategies. The impact of spiral tube sizing on the surface heat transfer coefficients was studied. The results indicated that SGHEs-P may be preferred for office buildings under intermittent operation conditions. For a short period, the temperature profiles and heat transfer efficiency of SGHEs-P were mainly influenced by the fluid type, length of the spiral tube, and spiral pitch. The smaller the spiral pitch, the more uniform the temperature distribution, and the better the heat transfer effect, but the heat transfer per unit depth of pile decreased. The average temperature variation curve of the soil around the energy pile with different spiral pitches was simulated and obtained over time. Meanwhile, the impact of spiral radius, spiral pitch, and spiral tube radius on the convective heat transfer coefficient was also presented. Through data fitting, the formulas for the correction coefficients of spiral radius, spiral pitch, and spiral tube radius on convective heat transfer coefficient were obtained, respectively.

Influence of different types of pipes on the heat exchange performance of an earth-air heat exchanger

An earth-air heat exchanger (EAHE) is a green technology suitable for regulating the thermal environment of buildings; however it requires a large space and high construction costs. In this study, five types of pipes, such as aluminum, stainless steel, polyvinyl chloride, corrugated, and perforated corrugated pipes were selected as the heat exchange pipes to study the effects of pipe material, roughness of pipe, and bottom perforation on the performance of the EAHE under short-time, intermittent, and continuous operation conditions, and to identify the most suitable EAHE pipe. The results showed that in the case of thinner wall thickness, the pipe material had a negligible effect on EAHE performance, with a preference for cheaper pipes. Compared to smooth pipes, corrugated and perforated corrugated pipes significantly improved the heat transfer performance and reduced the pipe length by 28.33–33.33% and 22.67–29.33%, respectively. Although perforations in the bottom of the pipe could increase the heat exchange between the air inside the pipe and the lower soil layer, accelerate the recovery of the soil temperature, and increase the cooling time, it significantly reduced the heat exchange efficiency. Thus, the corrugated pipe with the best overall performance indicated that a pipe with rough surfaces was the most suitable pipe type for EAHE. This can significantly lower the amount of space and money needed for EAHE construction, making it suitable for a variety of building types and contributing to the solution to the issue of rising global temperatures brought on by the use of conventional energy in the construction sector.

Ammonia synthesis over cesium-promoted mesoporous-carbon-supported ruthenium catalysts: Impact of graphitization degree of the carbon support

Carbon-supported ruthenium catalysts facilitate electrically-assisted Haber–Bosch ammonia synthesis. However, the relationship between carbon supports and catalytic performance remains ambiguous. We developed ordered mesoporous carbon plates (MCPs) with varying graphitization degrees as Cs-promoted Ru catalyst supports, examining correlations between ammonia synthesis rate and key structural parameters, included graphitization degree, Ru nanoparticle size, and Cs/Ru ratio. High-graphitization-degree carbon supports resisted methanation and facilitated formation of reductive activation enabled dynamic Cs0 species as electronic promotor, induced by spillover hydrogen from the Ru surface to CsOH. Density functional theory calculations further revealed that CsOH alleviated hydrogen poisoning. Notably, the catalyst supported on MCP-1100—which exhibited the highest graphitization degree among the supports and superior stability—with 10 wt% 2.3-nm-sized Ru nanoparticles and Cs/Ru = 2.5 achieved high ambient-pressure ammonia synthesis rates (7.9–43 mmolNH3·g−1·h−1) below 410 °C. Furthermore, it functioned under intermittent operating conditions, potentially integrating renewable-electricity-based electrolytic hydrogen production.

A comprehensive evaluation of water spray system on transparent roof towards optimal operation schedule

Indoor overheating induced by transparent roofs sharply increases the building energy consumption. Water spray offers a potential solution but requiring effective strategy to balance cooling effects and cost-effectiveness. Aiming at a spray system installed on transparent roof (63.2 × 24.6 m) of a mixed-use building in Guangzhou, China, this study provides a comprehensive evaluation of multiple strategies including continuous and intermittent operation with different spray/interval time. In sunny weather, continuous spray reduces the exterior, interior surface temperature, and long-wave radiation by 17.5 °C, 9.6 °C, and 66.3 W/m2, respectively. Besides, the indoor air temperature, globe temperature, and mean radiant temperature are also lowered by 4.4, 5.2, and 5.7 °C, respectively. Comparatively, the intermittent spray produces worse cooling effect, the reduction of exterior, interior surface temperature, and long-wave radiation are 9.4–15.9 °C, 4.1–7.8 °C, and 24.1–47.7 W/m2, respectively. The intermittent operating conditions, sorted according to the exterior surface temperature reduction, are ranked as 30/10 > 20/10 > 10/10 > 10/50 > 10/20 min, with the cooling effect not decreasing linearly with interval time, but increasing monotonically with spray time. The air conditioning electricity saved by continuous spray is higher than that by intermittent spray, but the water pump electricity and water consumptions are higher as well. Technical-economy evaluation indicates that water consumption is primary cost factor in spray systems and intermittent spray with 10/50 min is the optimal scenario. This study provides insights into the economical and efficient operation of transparent roof spray system, and lays a foundation for its smart control.

A comprehensive study on transient thermal behaviors and performances of the modular pipe-embedded energy wall system under intermittent operation conditions

The active thermal insulation technology based on utilizing low-grade thermal energy makes the opaque envelopes gradually be treated as multifunctional components with structural and energy properties. In this study, the dynamic thermal behaviors of modular pipe-embedded energy (MPE) walls integrated with pipe installation cavity and backfilling material are numerically studied, and then the impacts of key variables on MPE walls are explored. Results show that the thermal behaviors of MPE walls can be significantly improved under all intermittent charging modes, while the maximum extra heat loss caused by applying filler cavity and backfilling material is only 2.4%. When the injection time is kept constant, MPE walls operated in multi-pulse modes, especially those with higher heat injection frequency (e.g., 3On3Off mode), can achieve a better performance enhancement and operating energy reduction effect. Besides, a reasonable increase in λf-value or a:b-value can effectively alleviate the heat accumulation around water pipes. In addition, the energy consumption of heat charging systems can be significantly reduced when performance indicators can be slightly sacrificed. Results highlight the effectiveness of adding pipe cavity and backfilling materials in enhancing the performance of MPE walls, which can provide theoretical guidance and data support for such walls in the future.

Multi-objective optimal scheduling of laminar cooling water supply system for hot rolling mills driven by digital twin for energy-saving

As a result of the complex structure of laminar cooling water supply systems, both facets of strict process requirements and various intermittent running conditions could cause challenges of energy-saving and equipment upkeep. Given the energy wastage issue of the laminar cooling system of hot rolling, this paper develops an optimal scheduling system according to digital twin using Online Sequential Extreme Learning Machine (OS-ELM) and multi-objective evolutionary optimization using Improved Sparrow Search Algorithm (ISSA). The optimal scheduling system according to digital twins can accurately predict the water consumption trend of the water supply system and optimize the scheduling instructions and operation scheme through dynamic information interaction and mapping of process constraints, intermittent operating conditions, rolling rhythm, measured data, etc. between physical space and virtual space. Experimental results reveal that the proposed method can lessen power usage by 13.60% and water consumption by 10.54% regarding the premise of ensuring what is needed of cooling procedures. In addition, the water pump can maintain high effectiveness during operation to guarantee the security and stability of laminar cooling water supply systems.

Numerical assessment of integrated thermal management systems in electrified powertrains

Temperatures in internal combustion engines (ICE) impact fuel consumption and pollutant emissions, especially under transient operating conditions. In hybrid powertrains, where the reciprocating internal combustion engine has intermittent operating conditions, a optimum control of these temperatures is critical. In this work, a detailed methodology for studying integrated thermal management systems for hybrid propulsion system was presented. Both experimental measurements and 0D/1D models were implemented and validated for the different components of the hybrid vehicle powertrain. The novelty of this work consists in the extensive experimental measurements involved for the development of the different models, specially the ICE, in order to study the integration of the different thermal flows of a hybrid powertrain. Furthermore, the simulation methodology used in this work integrates different modelling tools and takes advantage of their strengths when compared to using a single modelling tool. Two different thermal management systems have been evaluated under different Real Driving Emission (RDE) cycles at two different temperatures (at 20 °C and -20 °C). Results have shown that during the ICE warming up, the integrated thermal management system improved energy consumption by 1.74% and 3% for warm and cold conditions, respectively. This was because, the integrated TMS allows to avoid the temperature drop of the ICE when the propulsive system is in pure electric mode.

A Finite Element Model for Investigating Unsteady-State Temperature Distribution and Thermomechanical Behavior of Underground Energy Piles

The underground energy geostructure represented by the energy pile is one of the key paths for the cooperative development of underground space and geothermal energy. Because of its advantages of low cost, high efficiency and no extra occupation of underground space, it has become a feasible alternative to the borehole heat exchanger. The change in the temperature field of the energy pile and its surrounding ground not only affects the geological environment but also influences the thermomechanical performance and the durability of the structure. However, the temporal and spatial unsteady-state temperature distribution of piles and surrounding rock under typical intermittent and unbalanced thermal load conditions is still unclear. In this paper, a finite element model was applied to analyze the unsteady-state temperature distribution, and the thermomechanical behavior of the energy pile group was developed and verified. The temperature field distribution of pile and surrounding rock under typical intermittent working and unbalanced thermal load conditions were determined. Moreover, the thermomechanical behavior characteristics of the energy pile group were investigated. Finally, the influences of pile layout on the thermomechanical behavior of the energy pile group were identified by designing six different scenarios. The results indicate that under typical intermittent operation conditions, the temperature of the energy pile and surrounding ground near the heat exchange pipe varies periodically. For areas with unbalanced cooling and heating loads, long-term operation of energy piles leads to thermal accumulation, and the maximum temperature of energy piles occurs in the first daily cycle. In summer/winter working conditions, the increase/decrease in pile temperature induces axial compression/tensile stress. When the pile group is partially used as the energy pile, the non-energy pile acts as the “anchor pile”, and it generates the added tensile stress.

A super-growth carbon nanotubes-supported, Cs-promoted Ru catalyst for 0.1–8 MPaG ammonia synthesis

For the decarbonization of ammonia industry, a super-growth carbon nanotube (SGCNT)-supported, Cs-promoted Ru catalyst (Cs-Ru) was developed for mild ammonia synthesis (≤400 °C and ≤8 MPaG), particularly under intermittent operation conditions. This catalyst with well-dispersed Cs-promoted Ru particles was superior to previously reported analogs in ammonia synthesis concerning activity and stability. It produced ≈ 15 vol% of ammonia (rate ≈ 35 mmol g−1 h−1) at 380–400 °C and 5 MPaG (an H2/N2 molar ratio of 3 and a GHSV of 7000 h−1) that was close to thermodynamic equilibrium. The active components of Cs-promoted Ru particles confined in the nanospace of SGCNTs facilitated the adsorption of hydrogen and nitrogen during the ammonia synthesis. Hence, it exhibited high resistance to hydrogen inhibition in H2-rich ammonia synthesis conditions, particularly under pressurization, and facilitated nitrogen dissociation, resulting in high activity and stability in 0.1–8 MPaG ammonia synthesis at ≤400 °C.

Development of Stable Mixed Microbiota for High Yield Power to Methane Conversion

The performance of a mixed microbial community was tested in lab-scale power-to-methane reactors at 55 °C. The main aim was to uncover the responses of the community to starvation and stoichiometric H2/CO2 supply as the sole substrate. Fed-batch reactors were inoculated with the fermentation effluent of a thermophilic biogas plant. Various volumes of pure H2/CO2 gas mixtures were injected into the headspace daily and the process parameters were followed. Gas volumes and composition were measured by gas-chromatography, the headspace was replaced with N2 prior to the daily H2/CO2 injection. Total DNA samples, collected at the beginning and end (day 71), were analyzed by metagenome sequencing. Low levels of H2 triggered immediate CH4 evolution utilizing CO2/HCO3− dissolved in the fermentation effluent. Biomethanation continued when H2/CO2 was supplied. On the contrary, biomethane formation was inhibited at higher initial H2 doses and concomitant acetate formation indicated homoacetogenesis. Biomethane production started upon daily delivery of stoichiometric H2/CO2. The fed-batch operational mode allowed high H2 injection and consumption rates albeit intermittent operation conditions. Methane was enriched up to 95% CH4 content and the H2 consumption rate attained a remarkable 1000 mL·L−1·d−1. The microbial community spontaneously selected the genus Methanothermobacter in the enriched cultures.

Study on Heat Transfer Model and Applicability of Energy Piles in Nanning Basin under Subtropical Climate

In order to meet the requirements of peak carbon dioxide emissions, energy pile systems have been widely used in more and more regions because of their low-carbon, energy-saving, and emission-reducing characteristics. To explore the applicability of the energy pile project in Nanning, which has typical subtropical climate characteristics with warm winters and hot summers, some research work has been carried out in this paper. Firstly, through indoor thermophysical property tests, the geotechnical thermophysical parameters of typical composite strata in this area are obtained. Then a transient heat transfer model of the energy pile is established, which can fully reflect the heat transfer characteristics of the pile and the soil. On this basis, the superposition principle and stepped heat flow theory are used to analyze the temperature recovery characteristics of typical strata in Nanning area under different intermittent operation conditions. The results show that: the rock-soil mass of the typical stratum in Nanning area has good heat transfer characteristics, which is conducive to the promotion of energy pile engineering. The heat transfer model proposed in this paper has higher accuracy than the classical linear heat source model, which can reflect the material characteristics of energy pile system and avoid overestimating or underestimating the temperature field of energy pile system. The operation of energy pile system in a subtropical climate will produce an unbalanced heat load, and corresponding measures need to be taken to achieve the balance of heat load. The research results can provide a reference for energy pile engineering design in different climate regions.

Failure cause analysis of an expansion bellow of integrated turbine unit

The bellow is a flexible component which compensates an expansion allowance imposed by axial strain during high temperature service. The bellow failure during turbine operation of boiler unit has been systematically investigated. The metallurgy of bellow was AISI 316L stainless steel. The comprehensive laboratory investigation includes visual inspection, chemical analysis, mechanical tests, metallographic and fractographic studies. Visual inspection in the field revealed multiple cracks in the bellow and failed debris were collected for laboratory investigation. The mechanical tests of the expansion bellow confirmed to material property specification. The microstructure revealed austenitic structure and fracture surface revealed striation marks. The striation marks attributed to fatigue failure. Finite element analysis revealed that maximum stress induced in the trough region and stress could be higher as compared to ASME VIII Div 1 design equations. It can be concluded that actual operating pressure range could resulted in sufficient stress cycles to initiate fatigue cracks from surface of the bellow. The large number of stress cycles owing to actual varied pressure range attributed to fatigue failure. It is recommended to reduce many intermittent operating conditions by good operational practice and installation of strain gauges at critical locations of the bellow. It is further proposed to increase the existing ply thickness and install reinforced expansion bellow & sandwich/layered plies for prevention of fatigue failure.

Enhanced exergy analysis of a full-scale brackish water reverse osmosis desalination plant

Brackish water desalination by reverse osmosis membranes is energy-driven process. With the purpose of improving the energy performance of such unit a full-scale BWRO desalination plant located in Canary Island (Spain) has been working under intermittent operating conditions for 14 years was assessed with conventional and enhanced (exogenous and endogenous/avoidable and unavoidable exergy destructions) exergy analysis. The exergies across the major components of the plant are calculated. The conventional exergy analysis reveals that, largest irreversibilities were identified in the RO system (membrane modules), the high pressure pump and the feed pump which amounts to about 64.28%, 40.84% and 38.48% respectively. Results of advanced exergy analyses shows that 70.61%, 92.94% and 7.83% of the total exergy destruction in the high pressure pump, feed pump and the RO system respectively are avoidable. Moreover, the highest endogenous avoidable exergy destruction rate was determined owing to the feed and high pressure pumps. Using these findings, instructions were proposed to ensure optimal effectiveness of the RO system and to improve the exergy destruction in the pumps currently in use.

Long-term intermittent operation of a full-scale BWRO desalination plant

The use of renewable energy sources (RES) to power reverse osmosis (RO) desalination plants is a promising option for remote communities that face water scarcity. One drawback, however, is that the plants are generally obliged to operate intermittently due to the nature of the RES, which can reduce RO system performance in terms of production, salt rejection and energy consumption. This work analyses the long-term performance of a full-scale brackish water reverse osmosis (BWRO) desalination plant that has been working under intermittent operating conditions for 14 years (~9 h d−1). The plant has two stages, a 2:1 arrangement and five spiral wound membrane elements per pressure vessel (Filmtec™BW30-400). The feed water is pumped from a groundwater well. The feed flow and water flux recovery remained practically constant (416 m3 d−1 and around 65% respectively) over the 14-year period. The plant uses antiscalant dosing (5 mg L−1) and two cartridge filters in series (25 and 5 μm) as a pre-treatment. Rinses are carried out before shut-downs. Feedwater conductivity, permeate conductivity, feed pressure, pressure drop, permeate flow, and feed flow operating data were collected. The results show a decrease of about 50% in the average water permeability coefficient and an increase in specific energy consumption from 1.82 to 2.21 kWh m−3. The results provide useful information about the behavior of BWRO desalination systems under intermittent operating conditions and should be taken into account when designing RES-powered BWRO desalination plants.

X-ray absorption spectroscopy of Ba- and Cs-promoted Ru/mesoporous carbon catalysts for long-term ammonia synthesis under intermittent operation conditions

The active sites of Ba- and Cs-promoted Ru/mesoporous carbons were examined by advanced techniques, especially X-ray absorption spectroscopy, and correlated to their activities in long-term ammonia synthesis under intermittent operation conditions.

Study on the effects of air conditioners under intermittent operation conditions on residential demand response

This paper studies the applicability of the intermittent operation of air-conditioners in residential building demand response (DR). Firstly, the thermal inertial model of the building is established based on the dynamic heat transfer theory and verified by the experimental platform; then, based on the validated thermal inertial model of the building, the power demand change of intermittent operation of a residential air conditioner is simulated; because intermittent operation of an air-conditioner cannot achieve a smooth reduction of power demand, this article studies a group of residential building further and evaluates the DR effect of multiple air-conditioners interacting with each other. The study found that by use of the thermal inertia of the building, intermittent operation of air conditioners can maintain the indoor temperature within a certain range while achieving temporary reduction in electricity demand; through the cooperation between multiple air conditioners, the long-term stable reduction can be achieved under the premise of guaranteeing the temperature of the air-conditioned rooms.

Intermittent operating characteristics of an ecological soil system with two-stage water distribution for wastewater treatment

Ecological soil systems (ESSs) are usually used to remove nitrogen from wastewater. Due to the poor denitrification performance of traditional ecological soil systems (ESSs), this study proposes a two-stage water distribution system to improve the nitrogen removal. The effects of different distribution ratios on the system treatment effect were studied in an intermittent operation mode. After determining the optimal distribution ratio and intermittent operation conditions, the dynamics of system inflow, outflow, and nitrogen removal were monitored. Theoretical analysis of the denitrification mechanism was carried out. The results showed that the optimum water distribution ratio was 2: 1, and a mean total nitrogen removal rate of 60.42% was achieved, which is 23.09% greater than that is typically achieved by the single-section ecological system. Under optimum distribution ratio conditions, the system also demonstrated effective removal of chemical oxygen demand (COD), total phosphorus (TP) and ammonia nitrogen (NH4+-N), allowing the effluent to satisfy China's urban sewage treatment plant level B emission standards.