Low Heat Loss Research Articles

SiO2@MXene-based fabric featuring low heat loss for high-efficiency solar-driven freshwater production

Interfacial solar-driven steam generation is a promising approach for addressing the global freshwater crisis. However, the lower evaporation rate owing to high heat-loss is still a primary obstacle to commercialization. Given the high transmittance to solar radiation and superior isolation to thermal radiation of SiO2 aerogel, herein we design a hanging SiO2@MXene-based fabric (SMF) evaporator for high-efficiency solar-driven freshwater production. Under light irradiation, the internal MXene-based fabric with high solar absorption can rapidly convert solar radiation into heat-energy, while the outermost transparent SiO2 aerogel with excellent thermal insulation performance can efficiently prevent the internal heat-quantity from escaping outward. As such, the solar-to-thermal is well confined within the MXene-based fabric, endowing SMF with low heat loss towards surroundings. An evaporation rate as high as 1.87 kg•m−2•h−1 along with heat-loss as low as 5.9% is achieved under 1-sun, superior to other prior evaporators. Better yet, neither a decrease in evaporation rate nor fouling on SMF is observed even when treating high-salinity brine. This work provides new insights into reducing heat loss to enhance evaporation rate during solar desalination.

Simulation methodology and numerical comparison of boxer and opposed free piston linear generator configuration in 0D/1D environment

Free Piston Linear Generators (FPLGs) are thermal machines suitable for both stationary power generation unit (PU) and vehicle propulsion. They convert chemical energy from the fuel into electric energy to charge a battery using a linear electric generator. Due to the reduction of friction losses and the ability to work with different compression ratios, FPLGs are potentially characterized by high overall global efficiency with respect to a conventional alternative internal combustion engine. Like conventional internal combustion engines, FPLGs have several configurations: single piston, dual piston, opposed piston and boxer are the configurations considered in this work. Two configurations are compared in this paper from a functional and energetic perspective: opposed piston (Opposed) and Boxer. This is done with the aim to highlights pro and cons of these configurations. They are modelled using the 0D/1D modelling coupling GT-Power® and Simulink®. The comparison between the two configurations was done modelling the architecture with the same size and in two-strokes operating mode. The comparison of the two configurations highlights differences between them in terms of scavenging process, thermodynamic efficiency and global energy efficiency, putting in evidence what aspects need to be improved for each configuration and what are their advantageous features. Boxer configuration can provide less power due to lower volumetric efficiency compared to the Opposed configuration. Boxer configuration can provide 102.2 kW while Opposed can provide 110.0 kW at full load with a frequency of 20 Hz (equivalent to 1200 rpm). The Opposed configuration results in lower heat loss but higher energy lost at the exhaust. The final balance shows that Boxer configuration has a slightly higher brake efficiency (equal to 28.9 %) compared to the value found for Opposed (27.6 %).

Spatial Patterned Interfacial Solar Evaporators toward Recovering Heat Loss.

The novel freshwater production technology, such as interface solar-steam conversion (ISSC) technology, has advanced so rapidly recently, where its energy capture and conversion process was localized at the air-water interface so as to achieve high efficiency of energy utilization and transformation. However, when enlarging the evaporation surface and application scale, the inevitably increased heat loss and reduced conversion efficiency put it in a dilemma: should we exploit innovative steamer constructs for practical applications. In order to effectively mitigate heat loss from the evaporator to the surrounding environment, a series of spatial pattern evaporators (SPEs) are specifically designed in this article. By recovering the energy of radiation and convection heat loss, SPEs achieved low heat loss in an open evaporator through unequal height auxiliary heat exchange platforms. In an open environment, it achieves a maximum evaporation rate of 1.68 kg m-2 h-1, with approximately 52.41% of the heat loss being reabsorbed. This sophisticated pattern design provides a promising guideline for optimizing thermal management strategies and promoting practically scalable applications.

Dual-structured PTFE/PI-PI/PANI composite membranes for photothermal membrane distillation with excellent photothermal conversion and open pathways for water vapor transport

Photothermal membrane distillation (PMD) achieves localized temperature increase by absorbing and converting light energy to drive evaporation and phase change processes. It is expected to solve the challenges of low energy conversion efficiency and fouling of traditional membrane distillation (MD). A dual-structured photothermal composite membrane was constructed by oxidatively polymerizing photothermal functional materials on an electrospinning substrate membrane. The superhydrophobic Polytetrafluoroethylene/Polyimide (PTFE/PI) substrate membranes offer a spacious pore structure and excellent mechanical properties with low mass transfer resistance and low heat loss. The presence of the densely connected region of the Polyaniline (PANI) in the photothermal layer accelerates the local photothermal conversion. The porous structure formed by interconnecting fibers loaded with PANI is conducive to the repeated refraction of light beams and thus enhances the absorption of light energy. Under 1 kW·m−2 light intensity, the permeate flux of the PTFE/PI-PI/PANI membrane is stable at 1.44 ± 0.02 L·m−2·h−1 within 480 h PMD. With the outstanding salt rejection performance of 99.99 % and high photothermal conversion efficiency of 90.3 %, the photothermal membrane can be applied in areas where fresh water and fuel resources are scarce but solar energy resources are abundant.

Multi objective optimization for house roof using artificial neural network model

Roof models with low heat loss and low heating costs for buildings are crucial for reducing energy consumption and greenhouse gas emissions in cold regions. This study aimed to calculate the heat loss for various roof models with low heat loss, which are widely used in houses in Europe and other cold regions. We created 324 models for three types of roofs (light, medium, and heavy) with different materials and thicknesses using the HAP program. We then trained a neural network (ANN) to generate a mathematical function that can be used to calculate the optimal surface using a multi-objective genetic algorithm method. The resulting optimal roof design was simulated using the Ansys program on a January day. We compared the heat loss results for the optimal roof for HAP, ANN, and transient thermal analysis by Ansys, respectively, showing a close agreement among the methods.

Effect of inner channel geometry on solidification performance of phase change material (PCM) in double-pipe thermal energy storage

Advantages such as high energy storage density and low heat losses have made phase change materials (PCMs) preferable in energy storage. However, the very low thermal conductivity of PCMs is the most important obstacle to improving melting and solidification performance of PCMs. To improve the thermal conductivity of PCM, the combination of PCM with nanoparticles, fins, and metal foam is among the methods widely researched in the literature. The innovation of this study was to examine the effect of inner and outer channel configurations on the solidification rate in double-pipe energy storage with PCM. Thus, it was aimed to benefit from the same amount of stored energy more and at a lower cost. In this regard, solidification analyses were carried out for three different basic inner channel geometries. With the triangle-in-circle configuration, the solidification rate was increased by 5.5% to 13.2% compared to other configurations. It is anticipated that the data obtained in this study will be a guide for novel inner channel geometry designs that will increase solidification performance and heat transfer in double-pipe energy storage with PCM.

Feasibility Analysis on Compression-Assisted Adsorption Chiller Using Chlorides for Underground Cold Transportation

Thermal-driven refrigeration technologies, e.g., absorption- or adsorption-type, are gathering momentum since they can utilize low-grade heat from industrial, solar or geothermal energy. However, heat sources and end users are usually mismatched, which could lead to potential heat pollution and increased carbon emissions. Long-distance thermal energy transportation is good for district heating and cooling, which is of great significance if it can achieve a high energy-transportation density and low heat loss. In this paper, a compression-assisted chemisorption chiller driven by a low-temperature heat source for cold transportation is initially proposed, which aims to transport liquid ammonia with chemical potential and generate a cooling effect for end users. A feasibility analysis of the compression-assisted chemisorption chiller is preliminarily performed for 2 km cold transportation. The results show that the highest theoretical coefficient of performance and the energy efficiency of the compression-assisted adsorption chiller using a sodium bromide–ammonia working pair can reach 0.46 and 0.25, respectively, when the evaporation temperature is 20 °C. Among the three selected low-temperature salts, ammonium chloride–ammonia shows the best performance, which is up to about 40% higher than those of sodium bromide–ammonia and barium chloride–ammonia. It is demonstrated that compared with common absorption chillers, a compression-assisted adsorption system has a reasonable working efficiency to transport cold energy when the low- or ultralow-temperature heat source, e.g., lower than 60 °C, is required to be utilized.

Numerical sensitivity analysis of the energy performance of building envelope with dynamic conditions

The accurate energy performance assessment of building elements remains an active research topic. Studies have indicated that the thermal mass of building envelopes could influence heat losses in dynamic conditions and lead to significant difference in performance compared to steady-state conditions. This article aims to identify the relevance of using dynamic simulations instead of steady-state calculations for the energy performance assessment of building elements in Denmark. A numerical sensitivity analysis is performed on various parameters. The Monte-Carlo approach is used to perform numerous BSim simulations, which are then compared to standard steady-state calculations. The varied parameters include solar absorptance, ventilation in cavities, insulation material, insulation thickness, and orientation. Preliminary results indicate that dynamic conditions can significantly alter the wall heat losses, by up to 20% when compared to steady-state conditions. However, when aggregated, the differences average out to only 4% lower heat losses. When focusing on the thermal inertia of insulation materials, denser insulation materials only slightly delay the heat loss in the building element, but this effect is nullified over longer periods. The sensitivity analysis indicates that the most influential parameter is the solar absorptance, while the type of material is close in significance to the other parameters. The specific heat capacities and density of the insulation layer does not have a significant influence compared to its thermal conductivity. Thus, insulation materials with higher thermal mass do not seem to significantly improve the thermal performance of a building envelope.

A Comparative Study of Current Harmonics and Switching Frequency With Different Pulse Patterns in Duty-Cycle-Based Model Predictive Current Control

Model predictive current control with duty cycle optimization has been widely studied and applied in permanent magnet synchronous motor drives. However, the commonly adopted pulse arrangement of virtual voltage vectors may not be optimal, since no rigorous mathematical proof is given and too many other types of pulse arrangements exist. Firstly, six pulse arrangement combinations of virtual vectors are defined as six patterns in this paper. Then, the effects of six patterns on current distortion and switching frequency are investigated. Based on the analysis, the existing commonly adopted pattern is proved not to be the best. Instead, another optimal pattern with minimal current harmonics and the lowest switching frequency is selected to achieve better performance and lower heat loss. Finally, an experimental comparative study is implemented in this paper. The results have verified that the pulse pattern selected by the proposed analytical approach can effectively reduce the current harmonics and switching frequency, compared with the commonly adopted one.

Developing thermal prediction models for the elderly under temperature step changes

Daily transitions between indoor and outdoor environments involving temperature step changes have been extensively studied in young people, but little attention has been given to age-related differences and the development of thermal prediction models specifically for the elderly. To address this knowledge gap, a study was conducted in a climate chamber using warm-neutral-warm experimental sessions. Twenty-four elderly participants were selected from volunteers who met the health and body shape requirements. Thermal sensation vote (TSV) and mean skin temperature (MST) responses of the elderly were collected and analyzed. The results demonstrate that temperature step changes (3–9 °C) and neutral thermal experiences significantly influence the thermal responses of the elderly. Compared to the young, the elderly exhibit lower TSV, lower MST, lower skin heat loss, and different associations between TSV and skin heat loss. Leveraging these age-related differences, an MST prediction model was established for the elderly. The predicted MST can also serve as an input for the TSV prediction model established in this study. The TSV prediction model is based on skin heat loss and applicable to both transient and steady-state conditions. This study introduces a new approach for predicting the thermal responses of the elderly and enhances understanding of age-related differences in thermal responses.

Influence of N-Butanol-Diesel Fuel Blends on the Performance Characteristics of a Four Stroke, Compression Ignition Engine

Aims: This study investigates the impact of blending n-butanol with diesel fuel on the performance of a single-cylinder, 4-stroke diesel engine under varying engine loads.
 Study design: The performance assessment followed SAE J1349 on a Tec-Quipment TD110-115 4-stroke engine at 1500 rpm with varying loads and fuel samples.
 Place and Duration of Study: The study was conducted for 2 months the at Automotive Engineering Technology Workshop, Federal Polytechnic, Bauchi Nigeria.
 Methodology: The SAE J1349 test protocol was followed, using diesel fuel (D95) and different blended fuel samples (D90, D85, D80, D75, and D70). While diesel fuel generated higher brake power due to its higher calorific value, the D95 blend demonstrated comparable performance to diesel fuel across all parameters.
 Results: The brake power of the D95 blend initially decreased by 69.7% and then increased as the load increased to 2500g and 3000g, indicating improved combustion due to oxygenation. The D95 blend exhibited lower fuel consumption compared to diesel fuel, although blends with higher n-butanol percentages showed increased brake-specific fuel consumption due to lower calorific values, densities, and viscosities. Under maximum load, the D95 blend exhibited higher exhaust gas temperature and heat loss, reflecting the increased fuel quantity required for additional power. Lower heat losses at lower loads were attributed to factors such as lower calorific values, n-butanol's heat of vaporization, temperature differentials between the exhaust and ambient conditions, and engine size.
 Conclusion: The engine load has diverse effects on parameters across different fuel samples. D95 exhibits similar performance to diesel, yet discrepancies arise, especially with higher n-butanol content at lower loads.

Numerical Modelling and Performance Evaluation of Vacuum Membrane Distillation for Energy-Efficient Seawater Desalination: Towards Energy-Efficient Solutions

Vacuum membrane distillation (VMD) is a compelling technique for desalinating water because it exhibits superior pure water permeability at lower operating temperatures compared to other membrane distillation technologies. This leads to reduced energy consumption, lower heat loss via conduction across the membrane surface, and minimal heat transfer through conduction due to the low pressure on the permeate side. Detailed modelling of heat and mass transfer in VMD is essential for optimizing the process as it provides valuable insights that contribute to the advancement and successful implementation of seawater desalination using VMD technology. The aim of this study is to establish a comprehensive numerical model that describes the water vapor transfer across a hydrophobic micro-porous membrane in single-stage and multi-stage VMD processes for seawater desalination. The numerical predictions were compared to experimental data in addition to numerical computations based on an existing literature database, and good agreement has been found. The investigation also conducted a sensitivity analysis of process variables and membrane specifications on the VMD performance, as well as an assessment of the impact of temperature and concentration polarization. The obtained results showed that the permeation flux reached 18.42 kg/m2·h at 35 g/L feed concentration, 65 °C feed temperature, 50 L/h feed flow rate, and 3 kPa vacuum pressure. Moreover, the findings revealed that the feed temperature was the most significant factor, while the feed flow rate was the least important in determining the permeation flux. Additionally, the findings suggested that the effectiveness of the VMD process heavily relies on the composition and permeability of the support materials. Finally, the results confirmed that temperature polarization had a more significant effect on the reduction of the permeate flux than the concentration polarization.

Thermodynamic performance of CaCl2 absorption heat pump thermal energy storage system with triple storage tanks

Renewable energy source is playing an increasingly important role in district heating based on cogeneration units. Absorption thermal energy storage has the characteristics of high thermal energy storage density and low heat loss in long-term storage. In this paper, an absorption heat pump thermal energy storage system with CaCl2-water solution as the working fluid is proposed for solving the problem of insufficient wind power accommodations due to coal-fired cogeneration heat-power coupling. A steady-state thermodynamic model of the system is constructed in the Aspen Plus software. Under the considered design conditions, the volume of the tank required for one hour of operation of the system, thermal energy storage density, thermal energy storage ratio, and heating ratio are 464.61 m3, 29.81 kW h/m3, 46.50%, and 53.54%, respectively. The thermal energy storage density is 1.43 times and 1.25 times, and the tank volume is 0.7 times and 0.8 times, of those of a dual tank thermal energy storage system with H2O and CaCl2-water solution as the working fluids respectively. The effects of the system parameters on the thermal energy storage performance are simulated to obtain the optimal performance. At the optimum system performance, the tank volume required to operate the system for one hour, thermal energy storage density, heating ratio, and thermal energy storage ratio are found to be 439.56 m3, 37.39 kW h/m3, 44.83%, and 55.17%, respectively. Accordingly, the thermal energy storage performance of this system is also found to be superior to that of the traditional thermal energy storage system, which will be beneficial for reducing the volume of storage tanks in cogeneration units and increasing the thermal storage temperature.

Thermoregulation in the wild boar (Sus scrofa)

The wild boar (Sus scrofa) originates from warm islands but now inhabits large areas of the world, with Antarctica as the only continent not inhabited by this species. One might be tempted to think that its wide distribution results from increasing environmental temperatures. However, any effect of temperature is only indirect: Abundant availability of critical food resources can fully compensate the negative effects of cold winters on population growth. Here, we asked if temperature as a habitat factor is unimportant compared with other habitat indices, simply because wild boars are excellent thermoregulators. We found that the thermoneutral zone in summer was approximately 6–24 °C. In winter, the thermoneutral zone was lowered to 0–7 °C. The estimated increase in the heart rate and energy expenditure in the cold was less than 30% per 10 °C temperature decline. This relatively small increase of energy expenditure during cold exposure places the wild boar in the realm of arctic animals, such as the polar bear, whereas tropical mammals raise their energy expenditure several fold. The response of wild boars to high Ta was weak across all seasons. In the heat, wild boars avoid close contact to conspecifics and particularly use wallowing in mud or other wet substrates to cool and prevent hyperthermia. Wild boars also rely on daily cycles, especially of rhythms in subcutaneous temperature that enables them to cheaply build large core–shell gradients, which serve to lower heat loss. We argue it is predominantly this ability which allowed wild boars to inhabit most climatically diverse areas in the world.

Janus microbial membrane constructed by multiple biological self-assembly for photothermal conversion

Biological self-assembly as a kind of biological spontaneous behavior presents low energy consumption and high efficiency. In this work, biomass membrane is closely combined with carbon membrane derived from microorganism by biological self-assembly, forming Janus membrane (FAS-MC@M). The biomass-derived carbon membrane is endowed with superior photothermal conversion efficiency, also with the enhanced salt resistance via its hydrophobic nature and salt excretion domains. On the other hand, the biomass layer with good hydrophilicity and abundant capillary transport pipes ensure rapid water transfer to the Janus interface, conducing to stable heat exchange. The intrinsic micropores of the mycelium fiber can secure the reliable salt ion exchange between the tube water and bulk water during evaporation, which inhibit salt crystals formation as well. Meanwhile, biomass layer also reveals the relatively high thermal insulation properties, which can lower heat loss of the carbon membrane and improve the heat accumulation effect. For oily sewage, the biomass layer is also effective in protecting the photoheat exchange interface (carbon membrane) from oil pollution. In fact, the Janus evaporator in this work achieved 3.2 kg m-2h−1 evaporation rate under one solar intensity, showing desirable evaporation desalination potential.

Decoding the performance of module-scale interfacial desalination from a thermodynamic perspective

Passive solar-driven interfacial evaporation (PSDIE) is a highly efficient desalination technology that localizes heating at the evaporation surface, resulting in low heat loss and high evaporation rates. While many studies have focused on improving solar absorber performance, few have systematically analyzed the PSDIE thermodynamics. In this study, we propose a novel PSDIE distiller that effectively avoids salt accumulation. A module-scale model is developed that captures the mass and heat transfer characteristics of the distiller, and comprehensive thermodynamic analysis is performed. Our results show that the relative thickness between the evaporation and condensation layers, namely α, is a critical design parameter that optimizes process performance. An evaporation restricted region (α>5) and a condensation restricted region (α<5) can be identified. We also investigate the influence of hydrophobic membrane properties, finding that their effects depend on α. Our analysis demonstrates that inlet salinity of seawater feed has a detrimental effect on the water productivity of the distiller, while distiller with larger α tends to be more salt tolerable. Finally, we propose a design for a multistage distiller system and evaluate its water production performance, the results show that additional evaporation stages are beneficial to the distiller with smaller α.

Impermeable Graphene Skin Increases the Heating Efficiency and Stability of an MXene Heating Element.

A Joule heater made of emerging 2D nanosheets, i.e., MXene, has the advantage of low-voltage operation with stable heat generation owing to its highly conductive and uniformly layered structure. However, the self-heated MXene sheets easily get oxidized in warm and moist environments, which limits their intrinsic heating efficiencies. Herein, an ultrathin graphene skin is introduced as a surface-regulative coating on MXene to enhance its oxidative stability and Joule heating efficiency. The skin layer is deposited on MXene using a scalable solution-phased layer-by-layer assembly process without deteriorating the excellent electrical conductivity of the MXene. The graphene skin comprises narrow and hydrophobic channels, which results in ≈70 times higher water impermeability of the hybrid film of graphene and MXene (GMX) than that of the pristine MXene. A complementary electrochemical analysis confirms that the graphene skin facilitates longer-lasting protection than conventional polymer coatings owing to its tortuous pathways. In addition, the sp2 planar carbon surface with a low heat loss coefficient improves the heating efficiency of the GMX, indicating that this strategy is promising for developing adaptive heating materials with a tractable voltage range and high Joule heating efficiency.

Modification of perforated plate in fluidized-bed combustor chamber through computational fluid dynamics simulation

The combustion of solid fuels in a combustion chamber can be perfect if all the fuel included can burn out, and nothing remains. The combustion chamber requires sufficient air to burn all the fuel; therefore, meeting the air supply requires modifications with various considerations. The perforated plate modeled through computational fluid dynamics (CFD) was modified to investigate the excess air supply into the combustion chamber. The CFD simulation modeled in this study was used to validate the results of the experiments carried out with the previous fluidized-bed combustor technology. The simulation results show that the temperature increased for all fuels tested in geometry-2 compared with geometry-1. The best boiler efficiency from the simulation results is in geometry-1 for 45% oil palm midrib fuel with a mass flow rate of 7.78 kg/s. The maximum radiation was recorded from geometry-2 at a variation of 66.939 W/m2, with a heat loss level of 1.10 W/m2. Overall, the simulation was performed by modifying the fuel floor plate in geometry-2 to increase the temperature and radiation with lower heat loss.

Slug flow-enhanced vacuum membrane distillation for seawater desalination: Flux improvement and anti-fouling effect

Vacuum membrane distillation (VMD) is characterized by high permeate flux and low conductive heat loss among the membrane distillation technologies. However, the inescapable polarization effect severely deteriorates the distillation performance and service life. Thus, this study aimed at establishing a novel slug flow-enhanced VMD system to effectively address the polarization problem and investigating the influence of flow behaviors on MD process. First, a novel slug flow-enhanced VMD system was developed. The transmembrane permeate flux was intensified by 28.1 %. The performance enhancement was mainly ascribed to the increased turbulence and intensified shear stress. What’s more, further anti-fouling test demonstrated that the effect of slug flow on membrane fouling was divided into two stages. In Stage I, the permeate flux just dropped from 18.1 to 17.7 kg/(m2·h) and only 2.2 % reduction was observed in slug flow-enhanced VMD system, while it dropped rapidly from 15.2 to 12.9 kg/(m2·h) and a significant reduction of 15.2 % was observed in conventional VMD system. This research would be useful to membrane distillation for seawater desalination.

Thermal cycle performance of thermocline storage: numerical and experimental exergy analysis

The performance of a dual-media thermocline storage is studied in stationary periodic operation. Successive cycles are carried out until a stabilised behaviour is obtained. Numerical results are compared to experiments realised on a 107 kWh prototype combining thermal oil and alumina. Two performance indicators are studied: utilisation rate and exergy efficiency. The utilisation rate decreases as the thermocline thickens over the cycles, while the exergy efficiency increases over the cycles, mainly due to lower heat losses improving the energy efficiency. Experimental values stabilise around UR = 53% and ηex= 73% at the fifth cycle. A new indicator is introduced: the storage quality factor Ψs, which quantifies the exergy losses due to the process. Experiments and simulations show good agreement, with a slight underestimation of heat losses by the model. Exergetic performance of thermocline storage is thus validated experimentally on a prototype-scale setup in stationary periodic regime, with satisfying value: Ψs≈ 0.99. The influence of stop thresholds on performance is then examined numerically. The utilisation rate decreases with strict thresholds (κch=κd= 0.05) because it prevents the storage to be fully loaded and because the thermocline thickens over the cycles if not extracted from the tank. The exergy efficiency globally increases for hybrid threshold (strict during charge, soft during discharge), with a maximum of ηex= 96.5% for κch= 0.05, κd= 0.95. In that configuration, the time during which the storage is hot is minimised, decreasing heat losses. Besides, setting strict thresholds limits the variation of the outlet temperature and reduces exergy destruction.