Maximum Allowable Temperature Research Articles

New hybrid system of PV/T, solar collectors, PEM electrolyzer, and HDH for hydrogen and freshwater production: Seasonal performance investigation

A new hybrid energy system of PV/T, flat plate collector, PEM electrolyzer, and humidification-dehumidification desalination is introduced here. A comprehensive mathematical model for the system seasonal performance is constructed and solved numerically using MATLAB software. A year-round comparison between the traditional PV/T performance and its concentrated counterpart indicates that the latter performs better compared to power, hydrogen, and freshwater production and system overall efficiency. The maximum monthly achieved PV/T efficiency, electrolyzer efficiency, desalination system Gain Output Ratio, and overall system efficiency are 52 %, 62 %, 4.8 %, and 44 %, respectively at a concentration ratio of 4 and maximum allowable PV temperature. The system produces 82,200 kWh of energy annually and provides 77,700 kWh and 1500 kWh to the electrolyzer and the electric heater, respectively. Moreover, the system produces an annual freshwater production of about 52,700 kg, out of which approximately 10,000 kg are consumed in the electrolysis process, and an annual hydrogen production of approximately 1120 kg.

Mechanical Design Guidelines to Inhibit Fracture in Perovskite Solar Cells

Perovskite (PVSK) solar cells offer significant benefits over conventional silicon cells including low‐cost solution processibility, minimal materials usage related to strong photon absorption in thin‐film cell architectures, and a tunable bandgap. However, PVSK films are mechanically fragile, and fracture of PVSK layers and adjacent interfaces are a significant concern during fabrication, encapsulation, and operation. Herein, a thin‐film mechanics fracture analysis tailored for p–i–n and n–i–p PVSK solar cells on both soda lime glass and polyimide substrates fabricated with three PVSK crystallization methods is presented. The role of thermal processing of each cell layer is explored to determine the maximum allowable temperature below which fracture is inhibited. In the analysis, the mechanics basis for processing and materials selection guidelines for preventing fracture in PVSK solar cells is provided.

Retrofitting HVAC systems for enhanced energy performance and resilience in university data center room

Data centers have been extensively implemented globally to satisfy the increasing need for IT services. Small data centers with insufficient cooling require much power to keep IT equipment running reliably. As a result, retrofitting has become an efficient method to increase energy efficiency and save operating costs. This study examines the impact of retrofitting on energy efficiency in a data center room at a university. The data center under consideration underwent a comprehensive retrofitting process. Implementing a closed in-row type data center increases cooling capacity, resulting in more efficient cooling and improved temperature control. Measurement data were collected before and after retrofitting, including power usage effectiveness (PUE), energy consumption, temperature, and velocity distribution. Computational fluid dynamics (CFD) simulations were also conducted to determine the characteristics of the data center's airflow distribution and temperature profile. Numerical simulations carried out for two operational situations have indicated that a slight modification has the potential to greatly enhance the efficiency of the cooling procedure. Numerical simulations have revealed that developing hotspots in the racks has resulted in air temperatures exceeding the allowable maximum temperature, which can be improved after retrofitting. The PUE of the data center before/after retrofitting was 2.17 and 1.52, respectively. Free cooling technology is also implemented, making the data center more efficient and reducing PUE to 1.32. The results showed a significant improvement in energy efficiency, with a PUE reduction of 0.85, which reveals that energy efficiency has been improved by 48.71 %. The retrofitting results in significant cost savings and environmental benefits.

Comparison of Maximum Heat Transfer Rate of Thin Vapor Chambers with Different Wicks under Multiple Heat Sources and Sinks

In this paper, we present a new analytical model to investigate the maximum heat transfer rate of a thin vapor chamber (TVC) with multiple heat sources and sinks. The model can specifically consider different heat flux conditions for each heat source. Both capillary limitations and allowable maximum temperature constraints were employed to determine the maximum heat transfer rate. The liquid and vapor pressure distributions within the TVC were analytically derived using the Brinkman-extended Darcy equation and the Hagen–Poiseuille equation, respectively. Additionally, the theoretical wall temperature distribution was calculated based on the 3D energy equation, considering different heat flux conditions for multiple heat sources with a weighting factor. Our results demonstrate that the heat flux conditions applied to the heat sources significantly impact the internal flow pattern of the TVC. These changes in flow patterns influence the pressure distributions of the liquid and vapor, thereby affecting the maximum heat transfer rate. Furthermore, the effects of wick parameters on the maximum heat transfer rate under various heat flux conditions were examined.

Analysis and verification of electrodynamic force, thermal stress and current sharing for CRAFT converter structure design

In the design realm of fusion power supplies, structural components play a pivotal role in ensuring the safety of fusion devices. To verify the reliability of the converter structure design at the Comprehensive Research Facility for Fusion Technology (CRAFT), meticulous analysis of the converter's dynamic impact is carefully performed based on the worst fault current (400 kA), firstly. Subsequently, the thermal stress analysis based on the maximum allowable steady-state temperature is finished, and the equivalent thermal stress, thermal deformation, maximum shear stress of a single bridge arm and the whole converter are studied. Furthermore, a simple research method involving the current-sharing characteristics of a bridge arm with multi-thyristor parallel connection is proposed using a combination of Simplorer with Q3D in ANSYS. The results show that the current-sharing characteristics are excellent. Finally, the structural design has been meticulously tailored to meet the established requirements.

Assessment of the cold storage capacity used in bulk handling of perishable agricultural produce in Ugandan cities

Investment in the food cold chain has been reportedly identified as a solution to extensive postharvest losses worldwide. However, cold chain investment in the Ugandan food value chain is largely unknown. This study located, characterised, and determined the efficacy of bulk cold storage facilities used in handling perishable agricultural food produce in the 16 city districts of Uganda. Only 9/16 city districts had bulk cold storage facilities, translating to a cold storage capacity of 4.5 m3 per 1,000 inhabitants. The private sector owned the highest proportion by volume (90.3%) of these facilities with the highest volume used for handling multiple perishable products (11,590 m3). Generally, temperature varied significantly from the respective set points in the facilities. Variations were observed for between 1/3 to 3/4 of the logged period above maximum allowable storage temperatures for the different products, showing efficacy lapses. These results indicate investment and management gaps and opportunities in this indelible sector in food value chain by both government and private sector players.

Performance analysis of a novel air source ammonia/water cascade heat pump for heating buildings in subarctic climate

Performance of available air-source heat pumps (ASHP) for buildings is extremely influenced by the outdoor air temperature (OAT). When OAT falls below –20 °C, especially in subarctic climate, most commercially available ASHPs lose their defined functionality. To address this issue, a novel cascade heat pump (CHP) is proposed that utilizes two natural (with zero global warming index) refrigerants: ammonia, aka R717, as the low stage (LS) refrigerant, and water, aka R718, as the high stage (HS) working fluid. The proposed system is equipped with two rotary vane compressors for LS and HS. To desuperheat the compressor discharge temperatures and increase the HS heating capacity, liquid refrigerant is injected into the chamber of both LS and HS compressors. The wet compression is thermodynamically modeled and validated against experimental data available in the literature. Optimum design parameters such as the maximum allowable temperatures during compression and the minimum vapor quality after injection are obtained where the COP is maximized. Total exergy destruction, number/location of injectors and the injected amounts are also investigated. The proposed CHP can deliver 6.35 kW heating load at 50 °C with a COP of 1.985 when OAT is –40 °C. From a broader perspective, the proposed CHP not only proves its efficacy in extremely cold subarctic climates but also emerges as a pivotal player in addressing the global warming issue. This is accomplished through a dual approach to decarbonization—firstly, by electrifying heating processes and secondly, by substituting phased-out refrigerants with environmentally friendly natural alternatives.

Optimization of MTI structure based on material characteristics under extreme aerodynamic heating environment

Thermal insulation structure is a key factor to protect a high-speed aircraft to work safely and complete various tasks. The multilayer thermal insulation (MTI) structure has attracted much attention because of its light weight and good thermal insulation effect. The performance of an MTI structure is not only related to its physical sizes, but also closely related to the material optimization and temperature influences. A transient nonlinear thermal transfer model of the MTI structure is established, and a numerical solution strategy of the nonlinear thermal transfer model is proposed. Considering material selection, introducing the temperature influence of materials, and taking the geometric dimensions and equivalent mechanical properties of the MTI structures as constraints, an optimization frame of the MTI structures is established. Then two intelligent methods are adopted to solve the problem. Using the aerodynamic heating environment of X-43 flight trajectory, the results of a typical high-speed aircraft show that the proposed optimization framework can reduce the structural mass density from 13.95 kg‧m−2 to 9.10 kg‧m−2 by about 34.8 %. The temperature effect analysis shows that the temperature variation has an important influence on the optimization results. If the temperature effect on material properties is not considered, the optimized design scheme may not meet the temperature design constraints (The maximum temperature of the inner surface is 410 K, far exceeding the maximum allowable temperature of 320 K) which further leads to structural failures. Finally, the reliability and rationality of the optimization scheme are verified by a finite element model (FEM). The equivalent modulus of the optimized MTI structure is 146 GPa and the mass area density of the MTI structure is 7.7 kg‧m−2. The optimization design method of an MTI structure proposed in the current paper provides a technical support and lays a foundation for the overall design of a high-speed aircraft.

Integrating Wire Arc Additive Manufacturing into Selective Paste Intrusion for Reinforced Concrete Elements: Effect of Temperature on the Mechanical Performance

AbstractThis research aims to integrate the Wire Arc Additive Manufacturing (WAAM) process into the Selective Paste Intrusion (SPI) process to enable the production of reinforced concrete elements with complex geometries. However, the high temperatures generated during the steel melting process in WAAM can locally lead to a full stop of hydration which consequently is linked to a decrease of the mechanical properties of the concrete in the bonding zone around the reinforcing bar. This study aims to determine the maximum allowable temperature at which concrete retains sufficient hardening conditions which allows strength development when combining SPI with WAAM. In practical application, this temperature limit can directly be affected by e.g. the protruding length of the WAAM‐reinforcing bar above the SPI particle bed or external cooling strategies. As continuation of previous research, this study establishes a link between actual and targeted temperatures in the steel bar to allow the combination of WAAM and SPI processes. Temperature limits without compromising mechanical performance are investigated, and the need for and extent of additional cooling measures are evaluated. The results show that temperatures of the concrete in the fresh state can reach up to 70°C without weakening its mechanical performance.

On the design of a hydrogen micro-rectangular combustor for portable thermoelectric generators

Micro-combustion based thermoelectric generators have been proposed to power micro-electromechanical units. The present study aimed to optimize the thermal design of micro-combustor to best suit the requirement of thermoelectric generators. An equivalent heat transfer coefficient was used to consider the arranged thermoelectric generators and its effect was elucidated. Increasing the coefficient enhances combustor wall heat loss, reducing flame stability and uniformity index of temperature. It is thus impossible to constrain combustor outer wall temperature below the maximum allowable temperature of commercial thermoelectric generators (623K) simply through increasing equivalent heat transfer coefficient. To address this issue, three simple strategies of combustor channel modification, adding copper layer and using flue gas heat recirculation were developed. A notable improvement in uniformity index of temperature over 6.5% is achieved for the latter two strategies. Nevertheless, none of these three strategies can ensure outer wall temperature below 623K. Motivated by the different mechanisms to improve temperature uniformity and reduce wall temperature for the two simple strategies of adding copper layer and using flue gas heat recirculation, a hybrid strategy combining them was proposed. The hybrid strategy can maintain flame stability, improve uniformity index of temperature approach to 100% and control combustor outer wall temperature below 623K, making it a qualified strategy for designing high-performance micro-combustors.

Features of hydrodynamic calculation of highly loaded plain bearings of heavy ship’s gears

Despite the high level of modern vessel engineering development, there are a number of unresolved design and technological problems, the most urgent of which is the issue of unification of the main components and assemblies of the vessel. At the same time, an important and unresolved task is to create a unified image of the ship’s reverse gear for the needs of the domestic civilian fleet and navy. The object of study is highly loaded bimetallic plain bearings, which are elements of the “heavy” marine reverse gears of a new generation. As part of the work, a hydrodynamic calculation that refines the parameters of the plain bearings of a highly loaded marine transmission at the stage of a prototype production is performed. The applicability of the State Standard ISO 7902 methodology to the products under consideration is assessed, as well as the approximation of the performance characteristics of bearings using interpolation of two variables function is performed. The rationale for the coverage angle and the location of the pockets for supplying lubrication of bearings taking into account the thermal and viscous properties of the oil is presented. Calculations of the parameters of the critical thickness of the lubricating layer, the maximum allowable specific load and the maximum allowable temperature of bearings are performed. Based on the results of the calculations, an additional assessment for the compliance of bearings with the requirements of State Standard ISO 7902 in terms of bearing capacity and stability parameters is carried out. The comparison of the obtained results with the data of the preliminary calculation performed at the stage of the technical project is given. It is revealed that when calculating dynamically loaded plain bearings, it is necessary to take into account a number of additional factors, namely the magnitude of the relative eccentricity and relative clearance. The risks of bearings failure in the process of dynamic loading are determined. The result of the work is the correction of the geometric parameters of the plain bearings of the marine reverse gear unit in order to increase its reliability and the overhaul interval.

Designs of ultra-HPHT Electrical Component Packages for Downhole and Geothermal Wellbore Logging Tool Integrations

Conventional dielectric sealing materials (PEEK, glass and glass-ceramic) used for sealing electrical feedthroughs, connectors, interconnectors and bulkheads in electronic packages can provide reliable downhole wellbore long-term logging tool service up to 150C or short-term logging service for greater than/equal 200C, due primarily to low glass transition temperature of the used sealing materials. This paper will demonstrate that the maximum allowable operating pressure and temperature could be from 20KSI to 100KSI and from 200C to 400C with Borosilicate and Bismuth-Boron-Silica (XTS) sealing glasses. Borosilicate glass sealed Inconel-Kovar/Kovar electronic housing could maintain reliability up to the maximum pressures of ~35KSI at 200C or 25KSI at 400C. Similarly, a Bismuth-Boron-Silica XTS glass sealed stainless steel electronic package housing could have a maximum operating pressure of ~100KSI at 150C or ~50KSI at 300C temperature. It has been found that high glass transition temperatures of (440-560)C for Borosilicate glass and (44010)C for Bismuth-Boron-Silicate (XTS) glass are key for making highly reliable ultra-HPHT electronic component packages. It is shown that the tetrahedral diamond-like microstructures of these sealing glasses are keys to ensure water-repelling properties to maintain sufficient electrical insulation resistance regardless of their use in water or moisture-rich wellbores. Moreover, compressive pin stress is found to be additional key for mitigate frequently observed glass seal cracks during logging tool field services.

Контроль качества светодиодных COB матриц по пороговому току

A description of a method for measuring the threshold current, the minimum current at which optical emission is occurs, of chips that are part of COB (chip on board) LEDs is given. The method consists in registering with a digital camera the average values of the emission intensities of the chips at three small values of the electric current and determining the threshold currents by solving a system of equations relating the measured intensities to a function approximating the P-I characteristic in the range of low currents. The measurement method was tested on commercially available COB LEDs consisting of ten series-connected chips. It was determined that the values of the threshold current of the chips differ: for the investigated samples, the coefficient of variation ranged from 2.6% to 39.2%. As a result of the tests of COB LEDs under the action of a current of 140 mA at a maximum allowable ambient temperature of 110 °C for 11 days, it was found that changes in the average values of the threshold currents during tests correlate with changes in the current-voltage characteristics of the matrices measured in the voltage range from 17 to 24 V at which leakage current dominates: the threshold current increases for those matrices in which leakage current increases. The correlation coefficient between the relative changes in the leakage current and the average values of the threshold currents is 0.94. It is shown that the correlation coefficient between the magnitude of the decline in the luminous flux of the matrices during testing and the change in the average value of the threshold current is 0.96. The established strong correlations show the possibility of diagnosing the quality by the threshold current of individual chips in the composition of COB LEDs.

Two-stage stochastic unit commitment for renewable energy integrated power systems considering dynamic capacity-increase technologies of transmission lines

Dynamic capacity-increase technologies (DCITs), as a solution to reduce the stress of transmission networks, avoid long-term constructions and update the maximum transmitted capacities based on real-time environment factors and the maximum allowable operation temperatures (MAOTs), which provide flexibility in optimizing the operation modes for power systems. This paper proposes a detailed two-stage stochastic unit commitment model considering the physical processes of heat transfer as well as the uncertainties of renewable energy. The dynamic thermal rating (DTR) is calculated based on the heat balance of transmission lines in the steady state, including radiated and convective heat loss and gain from the solar heat and power flows. To simulate the uncertainties from renewable energy outputs, this paper applies the Latin hypercube sampling method to generate output scenarios in stochastic optimization. Finally, the proposed model is verified on a modified IEEE-24-bus test system. The effectiveness of DCITs is illustrated through the case study.

A Case Study on PV-Aided Net Zero-Energy Building: the Daycare in IKCU

At the core of our growing societies, energy supply stands as one of the major concerns today, and it will be an inevitable challenge for our near future. As the nations are looking to find solutions for the transition from fossil fuels – depleting at a high rate – to alternative energy sources, solar energy through PV cells is getting attention as an affordable and easily implemented option especially for power supply in commercial and residential buildings. This work consists in analyzing the possibility to cover the entire energy needs of a building via PV solar cells for the case of a constructed daycare. In this case study, HVAC energy requirement has been calculated by the TS825 standard. The standard specifies a method for calculating the net heating/ventilation energy need and provides the rules for calculating the maximum allowable temperature in buildings. First, dimensions of the investigated building are taken and characteristics affecting the thermal insulation are assessed. Then, other energy needs, mainly lighting and electrical devices, are computed in the analysis as internal electricity needs. The scope of this work extends to the assessment of indoor air quality for occupants of building, which is an important aspect in our case study where the occupants are children. ASHRAE standards 62.1 is utilized for this purpose. The standard specifies minimum ventilation rates and other measures intended to provide acceptable indoor air quality to human occupants and that minimizes adverse health effects. The results are obtained for monthly varying solar exposition in the specified area where the building is located to provide supply for the determined energy demand via solar energy. Finally, monocrystalline PV panel system has been proposed with proper orientation and adequate power potential. Based on the obtained results, as well as the economical aspect, inferences and suggestions are made for improvements.

Increasing the efficiency and environmental safety of ship power plants cooling systems with jet heat removal devices

There are numerous cases when during the ship operation it is advisable to exclude for some time the intake of sea water used to cool the ship’s power plant. The issues of the possibility of such operation of widely used open double-circuit cooling systems are considered. To this end, the systems can be switched to the sea water tanks available on board. Work according to this scheme is limited by the time of heating the available water to the maximum allowable temperature. The issues of increasing this time by rising the heat removal through the ship’s plating are considered. To do this, the heated water is not simply discharged into the tank, but is jet supplied through the pipeline system directly to the inner surface of the ship’s plating. Conducted visual and thermal engineering studies have shown that the best results are achieved with the use of a jet supply, supplemented by a screen one. As a result, a channel for hot water movement is formed between the screen and the ship’s plating. Water radially spreads outward from the center of the screen, forming a fan jet. At the same time, the maximum flow velocity around the ship’s plating and the temperature difference between hot and outboard water are ensured. The advantages of this method for supplying cooled water are confirmed by the thermal engineering studies. It is shown that in this case the heat transfer increases by 25…30 %, compared with the case of a simple jet water supply. As a result of processing numerous experimental data, generalizing similarity equations have been obtained. They allow you to calculate the heat transfer for various water flow regimes. The use of these devices allows you to increase the time of systems operation in a closed loop without receiving outside water. These devices are quite simple in design and can be used both at the stage of building a new ship, and when modernizing existing ships. An example of such devices introduction on self-propelled floating cranes is given.

Conventional and advanced exergy and exergoeconomic analysis of a biomass gasification based SOFC/GT cogeneration system

In this paper, a small scale biomass gasification based solid oxide fuel cell/gas turbine (SOFC/GT) combined heat and power (CHP) plant is investigated by means of both conventional and advanced exergy and exergoeconomic analysis. A one-dimensional model of an internal reforming planner SOFC is employed to account for the temperature gradient within the fuel cell solid structure, which is maintained at the maximum allowable temperature gradient (150 K) under different operating conditions. Two main parameters of the gasification process, namely, air-to-steam ratio and modified equivalence ratio, are investigated, and the key parameters of the cycle exergy and exergoeconomic study are analyzed. Moreover, a multi-objective optimization procedure is applied to determine the unavoidable gasifier conditions required for the advanced exergy analysis of the system. The results of the conventional exergy and exergoeconomic analysis reveal that the highest rate of exergy destruction occurs in the gasifier, followed by the afterburner (AB) with 41.87% and 21.98%, respectively. Also, the lowest exergoeconomic factor is related to AB by 5.34%, followed by heat recovery steam generator (HRSG), gasifier, air compressor, and SOFC, which implies that the priority is to improve these components to reduce the exergy destruction cost rate. The results obtained from the advanced exergy and exergoeconomic analysis indicate that the most of the total exergy destruction rate is unavoidably in the CHP plant. The AB shows the least improvement potential in terms of reduction of the exergy destruction by almost 2% avoidable part, followed by Heat Exchanger 3 (H.X.3), gasifier, and SOFC duo to their lowest avoidable exergy destruction parts of almost 5%, 10% and 13%f respectively. Furthermore, the unavoidable part of the investment cost rate for all the components of the cogeneration plant is larger than the avoidable part, which means that it is difficult to reduce the investment cost rate of the system components. Meanwhile, the endogenous/exogenous analysis shows that the exergy destruction is completely endogenous for all components of the integrated plant, except for HRSG, GT, and HX1. Compressors and turbines have the highest potential to reduce endogenous exergy destruction. This is due to their higher avoidable endogenous exergy destruction. Reducing the investment cost rate seems difficult, as the main investment cost rate was found to be an unavoidable endogenous part for all system components. Finally, some results obtained from the advanced analysis approach are the opposite to those of the conventional method. This fact emphasizes that the results of conventional exergy analysis alone are insufficient and unreliable. For example, based on the advanced analysis perspective, the gas turbine and H.X.2 by 8.9% and 8.46% modified exergoeconomic factor, respectively, should be considered for reducing investment cost rate, while the conventional method gives opposite results.

A Review of Advanced Cooling Strategies for Battery Thermal Management Systems in Electric Vehicles

Electric vehicles (EVs) offer a potential solution to face the global energy crisis and climate change issues in the transportation sector. Currently, lithium-ion (Li-ion) batteries have gained popularity as a source of energy in EVs, owing to several benefits including higher power density. To compete with internal combustion (IC) engine vehicles, the capacity of Li-ion batteries is continuously increasing to improve the efficiency and reliability of EVs. The performance characteristics and safe operations of Li-ion batteries depend on their operating temperature which demands the effective thermal management of Li-ion batteries. The commercially employed cooling strategies have several obstructions to enable the desired thermal management of high-power density batteries with allowable maximum temperature and symmetrical temperature distribution. The efforts are striving in the direction of searching for advanced cooling strategies which could eliminate the limitations of current cooling strategies and be employed in next-generation battery thermal management systems. The present review summarizes numerous research studies that explore advanced cooling strategies for battery thermal management in EVs. Research studies on phase change material cooling and direct liquid cooling for battery thermal management are comprehensively reviewed over the time period of 2018–2023. This review discusses the various experimental and numerical works executed to date on battery thermal management based on the aforementioned cooling strategies. Considering the practical feasibility and drawbacks of phase change material cooling, the focus of the present review is tilted toward the explanation of current research works on direct liquid cooling as an emerging battery thermal management technique. Direct liquid cooling has the potential to achieve the desired battery performance under normal as well as extreme operating conditions. However, extensive research still needs to be executed to commercialize direct liquid cooling as an advanced battery thermal management technique in EVs. The present review would be referred to as one that gives concrete direction in the search for a suitable advanced cooling strategy for battery thermal management in the next generation of EVs.

SORPTION AND THERMODYNAMIC PROPERTIES OF AROMATIC PLANTS AS DRYING OBJECTS

The high demand for aromatic herbs in the biotechnological, cosmetic, pharmaceutical and food industries led to increased requirements for the technological processes of processing aromatic raw materials to preserve biologically active substances and essential oils. Peppermint (Mentha piperita L.) is a well-known medicinal essential oil crop that is widely distributed throughout Ukraine. It is widely used due to its beneficial properties and therapeutic effect on the human body.
 The purpose of the article is studies of sorption and thermodynamic properties of aromatic plants to determine technological parameters of drying and storage processes, which ensure minimum specific energy consumption and maximum preservation of essential oils.
 The results of study on sorption properties of peppermint leaves are presented. Adsorption moisture isotherms of peppermint leaves were determined at three different temperatures 30, 40 and 50 ºC, using the standard gravimetric static method. The net isosteric heat of adsorption was calculated from moisture isotherms, using the Clausius-Clapeyron equation.
 The specific heat consumption for drying peppermint leaves at temperatures of 40 and 50 ºС was obtained experimentally, using the differential microcalorimeter of evaporation. The effect of volatile substances of peppermint essential oil on the heat of vaporization was revealed.
 The choice of drying temperature of aromatic plants is primarily determined by the thermal sensitivity of essential oils. The study of the effect of drying temperature on the removal of volatile aromatic compounds from peppermint leaves showed that the maximum allowable temperature of the material during dehydration should not exceed the melting point of menthol at 42,5 ºС.

Experimental validation and numerical assessment of a temperature-controlled thermoelectric generator concept aimed at maximizing performance under highly variable thermal load driving cycles

Although the internal combustion engine (ICE) dominance in mobility is being challenged, ICE R&D enables a non-disruptive transition towards sustainable transportation through hybridization and carbon neutral fuels. The efficiency improvement of ICE-based mobility through Waste Heat Recovery (WHR) is thus a priority, especially in the area of hybrid, plugin hybrid and long-haul transportation, where the recovery potential is high, and ICEs will still be used for a long time. Thermoelectric (TE) Generators (TEGs) have been assessed as a possible low maintenance WHR solution for long. However, viable market solutions are still not a reality. While material costs are steadily being overcome with novel affordable TE materials, there is still the issue of low average conversion efficiency under realistic driving conditions. This is caused by the fact that the efficiency of TEGs is further deprecated by the big challenge of thermally optimizing the TEGs under highly variable conditions of exhaust flow rate and exhaust temperature during real-world driving. This thermal optimization mainly means that the hot side temperature should be as close as possible to the maximum allowable temperature of the modules, but not higher.The authors have been exploring exhaust heat exchanger (HX) concepts that allow achieving this thermal optimization under highly variable thermal loads by using phase change. The strategy used incorporates the spreading of excess heat along the HX by variable conductance heat pipes (VCHPs) embedded in it, thus enabling to maximize the energy conversion without thermal dilution under low thermal loads or overheating risk under high thermal loads. This was done by pre-regulating the pressure of the non-condensable gas (NCG) present inside the VCHPs so that these devices would start performing the spreading of excess heat from over-heated to under-heated regions of the HX at a precise phase change temperature, optimized to the optimal thermal level of the thermoelectric generators.The merit of the aforementioned strategy was confirmed through modelling and illustrated in previous work. The scientific novelty of present study relatively to previous work by the authors or other researchers is that it finally provides a full experimental validation of this concept with two different prototypes attached to a light duty engine. The experimental proof-of-concept results were successfully compared against predictions. Finally, theoretical performance analysis of the impact of the application on a light duty vehicle in terms of fuel economy and GHG emissions during the WLTC class 3 b was assessed with the help of driving cycle and engine simulations performed with AVL Suite software.The unique heat spreading and temperature control features of the concept were confirmed experimentally. In addition, predictions made by the model were far beyond the current state of the art for light duty automotive TEG generators. Namely, maximum output powers exceeding 2 kW and 1.2 kW were predicted during the WLTC cycle when using commercial bismuth telluride and the silicide-Tetrahedrite modules explored by the authors, respectively. Average fuel/CO2 savings of 4% were predicted for the WLTC cycle, with these savings raising to 12% during the highway portion of the cycle. This unparalleled performance is explained by the possibility of achieving both an optimized hot face temperature and a high heat exchanger effectiveness even under highly variable thermal loads.