Operating Temperature Levels Research Articles

Development of a Semi-Empirical Model for Estimating the Efficiency of Thermodynamic Power Cycles

Power plants constitute the main sources of electricity production, and the calculation of their efficiency is a critical factor that is needed in energy studies. The efficiency improvement of power plants through the optimization of the cycle is a critical means of reducing fuel consumption and leading to more sustainable designs. The goal of the present work is the development of semi-empirical models for estimating the thermodynamic efficiency of power cycles. The developed model uses only the lower and the high operating temperature levels, which makes it flexible and easily applicable. The final expression is found by using the literature data for different power cycles, named as: organic Rankine cycles, water-steam Rankine cycles, gas turbines, combined cycles and Stirling engines. According to the results, the real operation of the different cases was found to be a bit lower compared to the respective endoreversible cycle. Specifically, the present global model indicates that the thermodynamic efficiency is a function of the temperature ratio (low cycle temperature to high cycle temperature). The suggested equation can be exploited as a quick and accurate tool for calculating the thermodynamic efficiency of power plants by using the operating temperature levels. Moreover, separate equations are provided for all of the examined thermodynamic cycles.

A detailed investigation of an evacuated flat plate solar collector

The evacuated flat plate collector is a pioneering solar device for operation at medium temperatures (<200 °C) with high efficiency and without the need for tracking. The goal of this investigation lies in the detailed parametric analysis of an efficient evacuated flat plate collector and its daily performance for the city of Athens (Greece). The parametric analysis presents the instantaneous thermal and exergy performance of the collector for different inlet temperatures, solar irradiation levels, ambient temperatures, specific mass flow rates and the number of riser tubes. The daily analysis presents results for the thermal and exergy performance of the collector for different daily profiles of solar irradiation and ambient temperatures using real weather data. Also, the main innovation of this study lies in the development of a correlation of the daily useful output by taking into account the operating temperature level, the daily irradiation level and the mean ambient temperature. According to the yearly analysis, the optimum exergy efficiency is found at 13.33% for operation at 125 °C and the respective thermal efficiency is found at 47.24%. For this case, the levelized cost of energy and exergy were calculated at 0.0274 €/kWh and 0.1043 €/kWh respectively.

Embrittlement of WCLL blanket and its fracture mechanical assessment

In the European fusion programme, the Water Cooled Lithium Lead breeding blanket (WCLL BB) uses EUROFER as a structural material cooled with water at temperatures between 295 °C–328 °C and a pressure of 155 bar. The WCLL BB will be significantly irradiated (>2 dpa), while some parts will not receive significant heat loads, e.g. the sidewalls or the back-supporting structures. The irradiation, together with the irradiation temperature of EUROFER below 350 °C, produces a shift of the ductile-to-brittle-transition temperature (DBTT) to levels above room temperature at neutron doses, causing material damage as low as 2–3 dpa. Even though the DBTT does not reach the operating temperature level, brittle/non-ductile fracture is a concern during in-vessel maintenance when the BB temperature is below the DBTT. Two loading scenarios were identified as severe in this respect: (i) re-pressurization of the WCLL BB cooling loop after in-vessel maintenance, and (ii) dead weight loads during lifting of the BB segment. The embrittlement of the WCLL BB was investigated by quantifying the local DBTT shift in its parts based on current knowledge of the embrittlement behaviour of EUROFER under neutron irradiation. Therefore, a suitable, not overly conservative procedure was derived considering dpa damage and transmuted helium effects. The results demonstrate the ability to identify the 3D spread of the severely embrittled zones in the structure whose impact on the structural integrity was assessed considering the risk of brittle/non-ductile fracture. Thereby, the fracture mechanics approach established in nuclear codes was applied assuming its applicability to EUROFER. The embrittled zones in the first wall (FW) and its sidewalls pass the criteria when assessing the relatively low stresses resulting from the coolant pressure. The assessment was then continued considering stresses appearing in the FW during maintenance, in particular, when lifting the BB segment and transporting it out of the vacuum vessel. In this context, the maximum tolerable flaw sizes were determined in a parameter study considering designs of the FW with different cooling channel wall thicknesses.

Designing and Commissioning an Experimental Setup to Evaluate AC Losses in Superconductors Under Transverse Rotating Fields

The development of low AC loss MgB <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> conductors and the increasing interest in a liquid hydrogen-based economy have reignited research into high power density fully superconducting (SC) electrical machines. In these machines, the armature winding experiences rotating fields that generate AC losses, making it essential to estimate these losses during machine design. While analytical and finite element analysis (FEA) models are available in the literature for estimating AC losses, these models for multi-filament MgB <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> superconductors have yet to be experimentally validated for machine operating regions. This paper presents a high-precision AC-loss test setup to measure AC losses in SC MgB <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> windings under rotating fields at the air-gap field, frequency, and operating temperature levels relevant to electrical machine applications. The experimentally measured AC losses are then compared with analytical and FEA models. Results demonstrate good agreement between measurements and predictions. The paper discusses the experimental setup, sample preparation, calibration, measurement method, and results. The study provides a significant contribution to the development of high power density fully SC electrical machines.

Fuzzy Approach for Reliability Modeling of Lead-Free Solder Joints in Elevated Temperature Environmental Conditions

The mechanical and fatigue properties of microelectronic interconnection materials are important issues in critical applications in the life of electronic assemblies. Due to the growth in the applications of electronic components in new technological products used in tough conditions, evaluating the reliability of solder alloys has become crucial to the prediction of product life. SAC (Sn-Ag-Cu) solder alloys are common lead-free alloys used to form solder joints. In the current study, the reliability of individual SAC305 solder joints in actual setting conditions with an organic solderability preservative (OSP) surface finish was examined using an accelerated shear fatigue test at different load levels (16 MPa, 20 MPa, and 24 MPa). Four operating temperature levels (−10, 25, 60, and 100 °C) were also used. Seven samples were utilized at each experimental condition. An orthogonal array (L12) was employed for this experiment. The fatigue behavior of the SAC305 solder joints in actual setting conditions was described by a two-parameter Weibull distribution. The characteristic life and the shape parameter were extrapolated from the Weibull distribution for each experimental condition. Characteristic life was employed to represent the fatigue resistance of the solder joints. For each sample, the inelastic work per cycle and plastic deformation were determined. The results indicated a notable increase in the inelastic work per cycle and the plastic strain as a result of increasing either the stress load or the working temperature. Combinations of stress amplitude, working temperature, inelastic work, and plastic deformation were applied as inputs to the fuzzy system for computing a comprehensive output measure (COM-Value) using the Mamdani method. A negative relationship was observed between the solder life and the four fuzzy inputs. To fuzzify the inputs of the fuzzy system, two membership functions, “MFs”, were formed for each input, and five MFs were set for the output. The center-of-gravity (COG) theorem was utilized as a defuzzification method in the fuzzy inference system. The characteristic life of the solder joints was predicted by the COM-Value, which was used as an independent variable. Finally, The COM-Value was used to generate a general predicted reliability model for SAC305 solder joints with an acceptable model adequacy index.

A review of exploratory studies at RSC Energia into high-temperature voltage conversion systems for an electrically-propelled transportation spacecraft

The paper presents studies conducted at RSC Energia that looked into the feasibility of developing a high-temperature high-power voltage converter of a low-voltage space power supply system for an electrically-propelled space transportation spacecraft based on classical voltage conversion systems using electroplasma gates (key elements) and high-temperature transformers. It provides electrical schematics fro voltage conversion systems. It presents results of experimental studies on lab prototypes of electroplasma gates with electrical power of tens and hundreds kilowatts, which have demonstrated that in principle it is possible to use them as a basis for development of high-temperature radiation-proof voltage converters with mass fraction of ~0.01 kg/A for such power sources as nuclear power-supply systems with sub-megawatt and megawatt thermionic converter reactor. It provides a rationale for selecting the voltage conversion frequency in systems without forced cooling. It provides an experimental proof that in principle it is possible to implement high-temperature transformers with 600°С operating temperature level, and gives recommendations on selection of materials. Key words: space power system, electrically-propelled transportation spacecraft, voltage converter, electroplasma gate, high-temperature transformer, conversion frequency.

A review of exploratory studies at RSC Energia into high-temperature voltage conversion systems for an electrically-propelled transportation spacecraft

The paper presents studies conducted at RSC Energia that looked into the feasibility of developing a high-temperature high-power voltage converter of a low-voltage space power supply system for an electrically-propelled space transportation spacecraft based on classical voltage conversion systems using electroplasma gates (key elements) and high-temperature transformers. It provides electrical schematics fro voltage conversion systems. It presents results of experimental studies on lab prototypes of electroplasma gates with electrical power of tens and hundreds kilowatts, which have demonstrated that in principle it is possible to use them as a basis for development of high-temperature radiation-proof voltage converters with mass fraction of ~0.01 kg/A for such power sources as nuclear power-supply systems with sub-megawatt and megawatt thermionic converter reactor. It provides a rationale for selecting the voltage conversion frequency in systems without forced cooling. It provides an experimental proof that in principle it is possible to implement high-temperature transformers with 600°С operating temperature level, and gives recommendations on selection of materials. Key words: Space power system, electrically-propelled transportation spacecraft, voltage converter, electroplasma gate, high-temperature transformer, conversion frequency.

Highly dispersed Co nanoparticles embedded in a carbon matrix as a robust and efficient Fischer–Tropsch synthesis catalyst under harsh conditions

Series of catalysts with Co nanoparticles embedded in matrix of porous carbon is directly synthesized via a unique melting approach. No obvious deactivation is observed at high operating temperature and high CO conversion levels.

Flue Gas Emissions and Performance Evaluation of Small-scale Solid Waste Incinerators at Njokerio and Ng’ondu in Njoro, Kenya

Solid waste management is challenging and incineration technique is more preferred to other methods in reduction of mass and volume, removal of odour and energy recovery in both industrial and residential environments. The challenges facing residents at Njokerio, Ng’ondu and Green Valley estates in Njoro, Kenya included poorly designed open-wastes collection systems, exceeding incinerator loading rates and inappropriate operating temperatures. It also include inadequate design specifications, poorly mixed solid wastes with high moisture contents resulting to high emissions of noxious heavy dense smoke. The aim of this study was to evaluate factors influencing flue gas emissions and performance of small-scale incinerators. Data collected were statistically analysed to determine trends, means, F-values and Least Significant Different (LSD) at . Wastes incineration at varying moisture contents (MC) from 15 to 75% produced mean emission values for carbon monoxide (CO), carbon dioxide (CO2), and hydrocarbon (HC) ranging between 5 and 11 ppm, 5 and 14%, and from 508 to 1168 ppm, respectively. Varying the incinerator loading rates from 15 to 75 kg/h yielded means CO ranging between 5 and 12 ppm, CO2 from 5 to 14%, and HC between 252 and 1096 ppm. Waste incineration at varying operating temperature levels from 180 to 900°C contributed to mean emissions for CO, CO2 and HC ranging from 14 to 5 ppm, 15 to 6% and 1253 to 316 ppm, respectively. The Egerton University dispensary incinerator had the best incineration performance compared to the rest. High moisture contents, overloaded incinerators and low operating temperature levels contributed to high emission levels of flue gases leading to dark and dense smoke which resulted into incomplete wastes combustion indicating poor incineration performance. Wastes incineration at low loading rates, low moisture contents and high operating temperatures produced white and fine bottom ash, low levels of carbon, implying complete wastes combustion.

Energy, Environmental, and Economic Analyses of Geothermal Polygeneration System Using Dynamic Simulations

This paper presents a thermodynamic, economic, and environmental analysis of a renewable polygeneration system connected to a district heating and cooling network. The system, fed by geothermal energy, provides thermal energy for heating and cooling, and domestic hot water for a residential district located in the metropolitan city of Naples (South of Italy). The produced electricity is partly used for auxiliaries of the thermal district and partly sold to the power grid. A calibration control strategy was implemented by considering manufacturer data matching the appropriate operating temperature levels in each component. The cooling and thermal demands of the connected users were calculated using suitable building dynamic simulation models. An energy network dedicated to heating and cooling loads was designed and simulated by considering the variable ground temperature throughout the year, as well as the accurate heat transfer coefficients and pressure losses of the network pipes. The results were based on a 1-year dynamic simulation and were analyzed on a daily, monthly, and yearly basis. The performance was evaluated by means of the main economic and environmental aspects. Two parametric analyses were performed by varying geothermal well depth, to consider the uncertainty in the geofluid temperature as a function of the depth, and by varying the time of operation of the district heating and cooling network. Additionally, the economic analysis was performed by considering two different scenarios with and without feed-in tariffs. Based on the assumptions made, the system is economically feasible only if feed-in tariffs are considered: the minimum Simple Pay Back period is 7.00 years, corresponding to a Discounted Pay Back period of 8.84 years, and the maximum Net Present Value is 6.11 M€, corresponding to a Profit Index of 77.9% and a maximum Internal Rate of Return of 13.0%. The system allows avoiding exploitation of 27.2 GWh of primary energy yearly, corresponding to 5.49∙103 tons of CO2 avoided emissions. The increase of the time of the operation increases the economic profitability.

A Compact Thermally Driven Cooling System Based on Metal Hydrides

Independent of the actual power train, efficiency and a high driving range in any weather conditions are two key requirements for future vehicles. Especially during summertime, thermally driven air conditioning systems can contribute to this goal as they can turn the exhaust heat of internal combustion engines, fuel cells or of any additional fuel-based heating system into a cooling effect. Amongst these, metal hydride cooling systems (MHCSs) promise very high specific power densities due to the short reaction times as well as high reaction enthalpies. Additionally, the working fluid hydrogen has a very low global warming potential. In this study, the experimental results of a compact and modular MHCS with a specific cooling power of up to 585 W kg MH − 1 referred to one cold generating MH are presented, while reactor and MH weight in total is less than 30 kg and require a volume &lt; 20 dm3. The system is driven by an auxiliary fuel heating system and its performance is evaluated for different operating conditions, e.g., temperature levels and half-cycle times. Additionally, a novel operation optimization of time-shifted valve switching to increase the cooling power is implemented and investigated in detail.

A Superconducting Permeameter for Characterizing Soft Magnetic Materials at High Fields

A superconducting permeameter is proposed to characterize the magnetic properties of high-energy superconducting magnet yokes at their operating temperature and saturation level. The main problem of superconducting coils, an undesired quench, was faced by specific protection simulations, which has led to a self-protected system. The superconducting permeameter was used to perform the magnetic characterization of ARMCO Pure Iron, the material for the new High-Luminosity Large Hadron Collider (HL-LHC) superconducting magnet yokes, which was performed at the cryogenic temperature of 4.2 K and a saturation level of nearly 3 T. Two case studies based on the new HL-LHC superconducting quadrupole and dipole magnets highlight the impact of the magnetic properties of the yoke on the performance of the superconducting magnets, showing that the common assumption that heavily saturated steels with similar chemical composition behave precisely the same way has been proven wrong.

Optical and thermal analysis of different cavity receiver designs for solar dish concentrators

Abstract Cavity receivers are the most usual design in solar dish concentrators in order to achieve high thermal performance. The objective of this work is the investigation of five different cavity receivers under different operating temperature levels and the selection of the most appropriate designs. More specifically, the examined cavities have the following shapes: cylindrical, rectangular, spherical, conical and cylindrical-conical. All the cavities are optimized in order to determine the best design which maximizes the thermal efficiency of the solar collector. The optimization variables are different for every design and they regard the cavity length, the cone angle and the distance from the concentrator base. According to the results, the best design is the novel one with cylindrical-conical shape, while the conical and the spherical are the next choices. The worst design is rectangular, while the cylindrical is the fourth design in the performance sequence. For operation at 300 °C, the cylindrical-conical design is found to have 67.95% thermal efficiency, 35.73% exergy efficiency while the optical efficiency is 85.42%. The analysis is conducted with a developed model in SolidWorks Flow Simulation which is validated with literature experimental data.

Geyser boiling in a thermosyphon with nanofluids and surfactant solution

Experiments with a two-phase closed thermosyphon are carried out with the aim of investigating the influence of different working fluids on geysering. The length of the analysed device is 1.800 mm and its inner diameter is 20 mm. It is filled with various working fluids: from pure deionised-water and deionised-water plus sodium dodecyl sulphate (surfactant) as reference fluids to two different gold nanofluids, a silica nanofluid, and a nanofluid with nanohorn nanoparticles. Thermal performance of the thermosyphon is investigated with respect to time averaged parameters, such as amount of heat transferred, thermal resistance, and temperature distribution in the working fluid sump, but also with respect to geysering frequency and time dependent internal pressure. The inlet temperature of heating water is controlled between 35 °C and 85 °C and of the cooling medium between 15 °C and 25 °C.The most important conclusion is that geysering is strongly affected not only by nanoparticles, but also by chemical stabilisers like sodium dodecyl sulphate or polyvinylpyrrolidone. It is shown experimentally that nanofluids consisting of base fluid, chemical stabiliser, and nanoparticles in some cases diminish or even completely suppress geysering. Effects related to the nanoparticles alter the solid-vapour interface (a nanoporous layer on the evaporator wall) and the vapour-liquid interface (bubble surface). The influence of nanoparticles is restricted to the evaporator section. Replacing water with one of the other studied solutions/suspensions shifts the working point of the thermosyphon, changing the amount of transferred heat (increase even up to 80% for low evaporator temperature) and operating temperature levels.

Describing the effect of Ag/Au modification on operating temperature and gas sensing properties of thick film SnO2 gas sensors by gas diffusion theory

Abstract The effect of Ag and Au additives on the optimum operating temperature and response level of thick film SnO2 gas sensors is investigated and compared with the results of pure SnO2 ones. Fabricated samples were examined at various atmospheric conditions and at different operating temperatures (130–400 °C). It is shown that compared with pure SnO2 samples, in Ag-SnO2 samples 50 °C reduction of operating temperature and 7 times increase of response level is achieved. The achieved modifications are 70 °C reductions in operating temperature and 4 times increase of response level for Au-SnO2 samples. In addition to reporting the improving properties of SnO2 samples by adding Ag and Au, the validity of the gas diffusion model was also verified in these metal added samples for the first time. Gas diffusion theory is a comprehensive theory which its validity in resistive gas sensors has been verified by many researchers. The parameters of gas diffusion model were extracted for all fabricated sensors to achieve better understanding of their sensing properties.

Daily performance of a solar dish collector

Solar energy exploitation is one of the most promising techniques for achieving the sustainability in the energy domain. The objective of this work is to investigate the daily performance of a solar dish collector under different operating temperature levels. A solar dish collector with 10.28 m2 aperture and a spiral coil absorber is investigated. The analysis is performed with a developed numerical model in engineering equation solver which has been validated with experimental results. The analysis proved that the daily thermal efficiency of the collector is ranged from 67.36% to 54.65% for inlet temperatures from 50?C to 350?C, respectively. On the other hand, the exergy efficiency presents an increasing rate of the inlet tem?perature and it is found to be ranged from 8.77% up to 31.07% for the respective temperatures. The daily exergy production of the collector can reach up to 26 kWh with a respective thermal production of 50 kWh for inlet temperature equal to 350?C. The results of this work can be exploited for the suitable evaluation of the solar dish collector on a daily basis.

Experimental study of thermal performance of the backfill material around underground power cable under steady and cyclic thermal loading

Abstract An experimental program is conducted to investigate the response of a dry soil heated by a cylindrical heater. The motivation for the work comes from its application to underground power cables which are employed for low, medium and in future for the high voltage electricity transportation and distribution. Due to the Joule heating, the power cables generate heat that has to be dissipated through the surrounding soil to prevent melting of the insulation and keep the cable below the safe operating temperature levels. Heat generation also causes the migration of moisture from the vicinity of the cable and allowing the formation of a dry out zone. The dry out zone due to loss of moisture becomes less thermal conductive and starts to heat up the power cable even faster. To prevent the damage in the cable the Ampacity or the current carrying capacity of the cable is reduced. Therefore, in this study, we modified the backfill material around the cable based on Fuller modification to achieve maximum density in the dry state (critical state). A large-scale experimental device is set up and tests were performed to observe the change in the thermal behavior of the backfill. A 5 days heating and cooling cycles of 12 hours are performed on the sand and modified backfill material and later allowed to relax for two days. The heat relaxation time for different heating conditions has been studied to quantify the nature of the backfill material for different loading scenarios.

Assessment of linear solar concentrating technologies for Greek climate

The objective of this work is to compare and to evaluate the performance parabolic trough solar collector (PTC) and the linear Fresnel solar reflector (LFR) for the climate conditions of Athens, Greece. These collectors are examined under the same operating conditions and they are evaluated energetically, exergetically and financially. The analysis is conducted for all the year period using twelve typical days, one for every month. Performance curves for the solar systems are taken from the literature in order to estimate the collector performance for different operating temperature levels, solar irradiation level and solar angles. According to the final results, the mean yearly incident angle modifier for the PTC is about 83.73%, while for the LFR is 58.66%, at the examined climate conditions. It has been found that the mean yearly thermal efficiency at 350 °C is 52.25% for the PTC and 35.39% for the LFR, while the mean yearly electrical efficiency of the equivalent power production system is 16.11% with the PTC and 10.92% with the LFR. The simple payback period for process heat production was found close to 3–4 years, while for power production close to 8–9 years. Moreover, it is proved that the specific cost of the LFR has to be lower than 185 €/m2 in order to be a more attractive choice financially compared to the PTC.

Enhancing the performance of a linear Fresnel reflector using nanofluids and internal finned absorber

The objective of this paper is to investigate thermal efficiency enhancement methods in a linear Fresnel reflector (LFR) with evacuated tube receiver. The primary reflectors of the collector are flat mirrors of 27 m2 total net aperture, while the secondary reflector has a parabolic shape. The working fluid is Syltherm 800, and the analysis is performed for temperatures up to 650 K. The use of nanofluids and internal fins is the investigated thermal enhancement methods in the receiver of the LFR. The examined nanofluid is Syltherm/CuO for concentrations 2, 4 and 6%, while the examined internal fins are 8 longitudinal fins which are symmetrically located in the absorber. The LFR is examined using nanofluids and pure thermal oil in smooth or finned absorber. According to the final results, the maximum thermal efficiency enhancement is up to 1% and it is greater for higher operating temperature levels. The use of internal fins is better enhancement method compared to the use of nanofluids, while the combination of these two techniques leads to the highest possible performance. For the inlet temperature of 600 K with 200 L min−1 flow rate, the thermal efficiency enhancement with 4% nanofluid and finned absorber is found 0.82%, while it is found 0.61 and 0.28% with finned absorber with pure oil and 4% nanofluid with smooth absorber, respectively.

Thermal analysis of parabolic trough collector operating with mono and hybrid nanofluids

In this paper, the use of mono and hybrid nanofluids in parabolic trough collectors is investigated. The LS-2 PTC module is studied for operation with Syltherm 800 as the base fluid, while the examined nanofluids are the following: 3% Al2O3/Oil, 3% TiO2/Oil and 1.5% Al2O3-1.5% TiO2/Oil. As it is obvious, in all the cases the total volumetric nanoparticle concertation has the same value in order to perform a suitable comparison. The analysis is conducted for turbulent flow conditions with a flow rate equal to 150l/min, a value which leads to sufficient thermal performance results. The solar collector is investigated for inlet temperatures from 300K to 650K in order to cover a great range of possible operating temperature levels. According to the final results, the thermal efficiency enhancement for the hybrid nanofluid reaches up to 1.8%, while it is up to 0.7% with for mono nanofluids. This higher thermal efficiency enhancement is based on the great Nusselt number enhancement for the hybrid nanofluid case which is about 2.2 higher than the respective for operation with pure oil. The analysis is conducted with a developed thermal model in EES (Engineering Equation Solver) which is validated with literature results.