Actual Temperature Difference Research Articles

A study of the temperature distribution in the OTEC cold water pipe using a heat and mass transfer approach

Abstract The difference between sea water temperature at a depth of around 1000 m and sea water temperature at sea level is generally used as a parameter in the design of Ocean Thermal Energy Conversion (OTEC). In practice, electricity generation is determined by the difference between the temperature of the cold seawater coming out of the Cold Water Pipe (CWP) and the temperature of the seawater at the surface. The temperature of cold sea water increases due to heat transfer experienced by cold sea water flowing through the CWP, which comes into contact with surrounding sea water which has a higher temperature. This in turn provides a lower actual temperature difference, and therefore reduces the design power capacity. However, many previous studies did not consider these lower temperature differences. This may be acceptable for cases with practically small heat transfer such as CWP with low thermal conductivity combined with good insulation used in 1000 m CWP vertical floating systems. Unfortunately, this may not be the case for many of OTEC’s proposed alternative sites, which are located on land systems that require CWP lengths of five km or more. This raises the need for careful investigation to determine the temperature of the sea water coming out of the CWP, where it is necessary to calculate the temperature distribution of the cold sea water flowing through the CWP. This paper aims to estimate the temperature distribution of cold sea water flowing through the CWP and the increase in temperature of cold sea water leaving the CWP. Analysis based on the principles of mass and heat transfer was carried out in this research, where modelling was carried out numerically using a finite volume approach. For the case considered, the change in sea water temperature at CWP from depth to the surface occurs 1-3°C, which is the accumulation of each change in sea water depth. The results of this research illustrate that designing an OTEC system with a long CWP must consider the temperature distribution of cold sea water flowing through the CWP to produce a more realistic design.

Realization of high output power density in solution-processed flexible α-Cu2Se film by compositional off-stoichiometry and optimization of length

The solution-process method can be easily realized without expensive and complicated equipment, which provides the possibility for cost-effective application of flexible thermoelectric films. Nonetheless, the reported output properties of solution-processed Cu2Se based films are not satisfactory due to the absence of effective compositional adjustment and optimal length. In this work, a series of solution-processed nylon-supported flexible Cu2+xSe films have been fabricated. The excess Cu not only fills the vacancies, but also transforms the β-Cu2Se with disordered Cu atoms into long-range ordered lamellar α-Cu2Se, thereby reducing the carrier concentration and improving the carrier mobility of the films. As a result, a high power factor of 856.59 ± 119.92 μW m−1 K−2 has been achieved in the Cu2.20Se film at 303 K. In addition, by numerical simulation, the ideal length of single Cu2.20Se film was calculated to be 2 mm. With an actual temperature difference of 6.6 K, a maximum output power density of 2.918 μW cm−2 K−2, which is superior to those of other p-type flexible thermoelectric films, is realized in the single Cu2.20Se film. This work provides a feasible idea to promote the output performance of solution-processed flexible thermoelectric films.

Extrapolation method for reliable measurement of Seebeck coefficient of organic thin films

Measurement of the Seebeck coefficient of an organic thermoelectric (TE) thin film necessitates the accurate measurement of the temperature difference across the film. However, direct measurement of temperature difference is difficult for a small-gap organic TE device, which is favored for the high-resistance organic materials. Here, the effect of a nonlinear temperature distribution on the Seebeck coefficient measurement of organic TE films was systematically investigated to enable clear identification of the Seebeck coefficient for an organic TE film. The nonlinear temperature distribution across an organic TE device was analyzed by a finite-element analysis simulation and IR thermal imaging. The nonlinear temperature distribution resulted in a difference between the measured temperature difference and the actual temperature difference across the organic TE film. The conventional interpolation method significantly overestimated the Seebeck coefficient when the temperature distribution was nonlinear. To reduce the measurement error, the ideal Seebeck voltage was extrapolated from a series of Seebeck voltages measured at different position in the edge-contacted device configuration. The ideal Seebeck coefficients of various organic TE films with a wide range of electrical conductivities were determined successfully, and the Seebeck coefficient measured by extrapolation method was not affected by the geometry of the organic TE film. • Nonlinear temperature distribution during Seebeck coefficient measurement is systematically investigated. • An extrapolation method is used for evaluating an ideal Seebeck voltage that directly corresponded to the temperature difference across organic films. • Our method enables accurate measurement of the Seebeck coefficient of an organic thin film with a short-channel architecture and a high-resistance.

Temperature-control and low emissivity dual-working modular infrared stealth fabric

Materials with significant infrared (IR) stealth effect have been of great importance for camouflage of military targets and thermal management of special environments. In this paper, we design a dual-working module IR stealth fabric with electroless silver plated fabric component and heat-absorbing component derived from a phase change material (PCM) coating. The aim of electroless silver plating is to reduce the IR emissivity to 0.692 (1–22 µm), 0.687 (8–14 µm) and 0.655 (3–5 µm), and the PCM coating provides a phase change latent heat of 128.5 J/g to further lower temperature. Temperature measurements show that the PCM coating can create a maximum actual temperature difference of 21.6 °C with untreated fabric on a 65 °C hot plate and maintain the surface actual temperature below 38 ℃ for 300 s. IR camera images show that the as-prepared IR stealth fabric can significantly shield or interfere with the IR heat signature of the target compared to the untreated fabric. In addition, related performance tests show that it can meet the fastness requirements for practical use. Benefiting from the modulation of the dual working modules and the good fastness, this IR stealth fabric has great potential for practical applications.

Experimental study on the influence of Peltier effect and heat transfer boundary condition on the performance of thermoelectric generator

ABSTRACT Thermoelectric generator (TEG) has great potential in waste heat recovery which can convert heat into electricity based on the Seebeck effect, while the Peltier effect is negative and inevitable during the energy conversion process. The heat transfer boundary condition and Peltier effect influence the temperature difference which is the main factor determining the power generation performance of TEG. In this paper, to figure out how the heat transfer boundary condition and Peltier effect influenced the power generation performance of TEG, an experiment system was established. In the experiment, the initial hot side temperature of TEG was controlled by a heater, the cold side was cooled by water which belongs to the convective heat transfer boundary. The temperature variation of the hot and cold side of TEG caused by the Peltier effect under different initial hot side temperatures was analyzed firstly. Then, the temperature, internal heat flow, and power generation performance of TEG under different cooling conditions were investigated. The results showed that the Peltier effect reduced the actual temperature difference and resulted in the difference between the experimental and theoretical power generation performance. In addition, the enhancement of cold side heat transfer boundary conditions could raise the actual temperature difference and thus enhance TEG’s power generation performance. Besides, when the Peltier effect and heat transfer boundary conditions were both considered, the enhancement of the cold side heat transfer boundary condition can reduce temperature loss and thus the relative power generation performance loss caused by the Peltier effect. When the load resistance is 0.1 and the water flow rate increases from 0.1 to 7 , the relative power loss decreases from 29.01% to 20.20%, and the relative efficiency loss decreases from 20.49% to 13.60%. The findings of this work may provide a reference for the study of the heat transfer process of TEG.

Relative importance of temperature-dependent properties in non-Newtonian natural convection around curved surfaces

Natural convection of non-Newtonian power-law fluids around curved surfaces is a ubiquitous phenomenon in industrial applications and research. In this context, the importance of temperature-dependent properties combined with the power-law behavior of the fluid was studied numerically to elucidate the extent and significance of non-Boussinesq effects. An order-of-magnitude-analysis (OMA) indicated that both the thermal expansion coefficient and consistency index had significant temperature dependencies for the class of fluids considered. These dependencies were incorporated in the governing equations applicable over practical temperature ranges. It was found that the effect of curvature on the local shear rate distribution for flow around the object lends additional significance to the non-Oberbeck-Boussinesq (NOB) effects augmented or diminished by power-law behavior. The importance of using actual temperature differences in the analysis because of a complete reversal of trends at very low temperatures was demonstrated. The influence of NOB effects in studies where the fluid and geometrical parameters are optimized was found to be significant. Comprehensive numerical studies to estimate the extent of NOB effects due to the inadequacy of OMA were found necessary. The solutions provided additional insight to the relative importance of thermo-dependent viscosity and thermal expansion coefficient for the class of power-law fluids considered.

Experimental study of the effects of the thermal contact resistance on the performance of thermoelectric generator

Semiconductor thermoelectric generation technology is promising for application to waste heat recovery and is increasingly attracting research interest. The objective of the present study was to determine how the performance of a thermoelectric generator (TEG) was affected by the thermal contact resistance. The effects of the interface material and loading pressure—factors that determine the thermal contact resistance—on the TEG performance were experimentally investigated and analyzed. It was found that, for a given temperature difference between the cold and hot sources, the actual temperature difference of the thermoelectric module and the output power significantly increased when an interface material was applied to the contact surface. In addition, with increasing loading pressure, there was a decrease in the thermal contact resistance between the two sides of the thermoelectric module, and an increase in both the actual temperature difference across the module and the output power. Further, for a given loading pressure, an increase in the thermal resistance of the thermoelectric module was accompanied by an increase in the average temperature of the module, a decrease in the ratio of the thermal contact resistance to the total thermal resistance, and an increase in the output power.

Single Electrode Entropy Change for LiCoO2 Electrodes

Heat effects and temperature are known to be of great importance to a Li-ion battery’s performance and lifetime expectancy. In this context the reversible heat effects, i.e. effects that are associated with charge transfer and electrode reaction entropy, play an important role [1, 2]. These effects are however scarcely investigated, especially with respect to the single electrode reaction entropy. These values can be obtained when measuring a symmetric cell under a temperature gradient, i.e. two similar electrodes at zero current and at different temperatures [3-4]. We report values of the thermoelectric potential of the symmetric cell consisting of two identical LiCoO2electrodes separated by an electrolyte: Al(s,T) | Li(1-x) CoO2 (s,T) | EC, DEC, LiPF6(diss) | Lix CO2(s,T + ΔT) | Al(s,T+ ΔT) The electrodes are commercial LiCoO2 electrodes on an aluminium current collector, each kept at temperatures; T and T + ΔT. The electrolyte is 1 M lithium hexafluorophosphate (LiPF6) in a EC DEC mixture (1:1, volume wise) soaked in a separator. The cell is assembled as a pouch cell, and the electrodes are thermostatted to the specified temperatures. We show that there are two significant contributions of opposite sign to the thermoelectric signal relative to the temperature difference; commonly known as the Seebeck coefficient. The contribution from the Peltier heat is obtained from the steady state value after 3-5 days. The second contribution, which manifests itself within hours, is a Soret effect. From these results, we report the transported entropy of Li+ and estimate the heats of transfer for the electrolyte components. The attached figure shows a cross section of the cell used in these measurements, where one side is heated by hot water and the other cooled by cold water. The pouch cell is sandwiched between these two sides and layers of copper containing thermocouples, so that the actual temperature difference on the surfaces of the pouch cell is known and uniform. References; Takano, K., et al., Entropy change in lithium ion cells on charge and discharge. Journal of Applied Electrochemistry, 2002. 32(3): p. 251-258.Viswanathan, V.V., et al., Effect of entropy change of lithium intercalation in cathodes and anodes on Li-ion battery thermal management. Journal of Power Sources, 2010. 195(11): p. 3720-3729.S. K. Ratkje, M. Ottøy, R. Halseid, M. Strømgård, Thermoelectric power relevant for the solid-polymer-electrolyte fuel cell, Journal of membrane science 107 (1995) 219–228.S. Kjelstrup, P. J. S. Vie, L. Akyalcin, P. Zefaniya, J. Pharoah, O. S. Burheim, The seebeck coefficient and the peltier effect in a polymer electrolyte membrane cell with two hydrogen electrodes, Electrochimica Acta 99 (2013) 166-175. Figure 1

Removing the relocation bias from the 155‐year Haparanda temperature record in Northern Europe

ABSTRACTThe village Haparanda in northern Sweden hosts one of the longest meteorological station records in Europe depicting climate conditions in the subarctic. Since the station was relocated several times, moving gradually from urbanized to more rural areas, the record is likely biased by anthropogenic influences. We here assess these influences and demonstrate that even in villages urban heat island biases might affect the temperature readings. We detail a method to quantify this bias and remove it from the long Haparanda station record running since 1859. The correction is based on parallel temperature measurements at previous station locations in Haparanda. These measurements revealed a distinct urban warming pattern, largest in minimum temperatures during summer, since urban geometry and its heat capacity amplify additional warming and night‐time heat release, especially during a period of high insolation and calm conditions. Due to the station movement from the village centre to the outskirts, the net correction results in an additional warming trend over the past 155 years. The trend increase is most substantial for minimum temperatures (+0.03 °C 10 years−1). Maximum and mean temperatures are less affected (+0.01 °C 10 years−1). An increase in trend is even more severe if the 20th century is regarded exclusively, displaying a rise in annual mean temperature trend by +0.03 °C 10 years−1 and +0.07 °C 10 years−1 in annual minimum temperatures, respectively. Our approach of using actual temperature differences between historical station locations did not take into account other factors that might have influenced the data such as changes in instruments or observers. The presented adjustment for temperature residuals caused by a change in historical station locations might be considered as a minimum approach to improve long‐term temperature readings. The adjustment of the Haparanda station record results in an increased warming trend, thereby adding critical information to the evaluation and explanation of 20th century anthropogenic warming.

On the enigmatic similarity in Greenland δ18O between the Oldest and Younger Dryas

AbstractThe last deglaciation (20.0–10.0 kyr B.P.) was punctuated by two major cooling events affecting the Northern Hemisphere: the Oldest Dryas (OD; 18.0–14.7 kyr B.P.) and the Younger Dryas (YD; 12.8–11.5 kyr B.P.). Greenland ice core δ18O temperature reconstructions suggest that the YD was as cold as the OD, despite a 50 ppmv increase in atmospheric CO2, while modeling studies suggest that the YD was approximately 4–5°C warmer than the OD. This discrepancy has been surmised to result from changes in the origin of the water vapor delivered to Greenland; however, this hypothesis has not been hitherto tested. Here we use an atmospheric circulation model with an embedded moisture‐tracing module to investigate atmospheric processes that may have been responsible for the similar δ18O values during the OD and YD. Our results show that the summer‐to‐winter precipitation ratio over central Greenland in the OD is twice as high as in the YD experiment, which shifts the δ18O signal toward warmer (summer) temperatures (enriched δ18O values and it accounts for ~45% of the expected YD‐OD δ18O difference). A change in the inversion (cloud) temperature relationship between the two climate states further contributes (~20%) to altering the δ18O‐temperature‐relation model. Our experiments also show a 7% decrease of δ18O‐depleted precipitation from distant regions (e.g., the Pacific Ocean) in the OD, hence further contributing (15–20%) in masking the actual temperature difference. All together, these changes provide a physical explanation for the ostensible similarity in the ice core δ18O temperature reconstructions in Greenland during OD and YD.

Indicated diagrams of a low temperature differential Stirling engine using flat plates as heat exchangers

Indicated diagrams of a low temperature differential (LTD) Stirling engine (SE) were obtained. The evaluation of LTDSE performance was carried out using the results. The heat source temperatures were 75, 80, 85, 90 and 95° Celsius. The working fluid was air with a mean pressure of atmospheric pressure. The shape of the heat exchangers was flat. The stroke volume of the power piston was 3.9 cc, and the stroke volume of the displacer was 238 cc. The LTDSE did not have any regenerators. The maximum indicated power was 3.34 mW. The polytropic exponents becomes larger than 1.4 before the displacer reached dead center, and the working fluid temperatures fluctuated. These behaviors suggest that the heat exchanger did not work effectively. The evaluation of LTDSE performance was carried out using “the maximum fluctuation of ensemble averaged working fluid temperature”. The value is the fluctuation of the internal energy per the heat capacity, and has dimensions of temperature. The obtained values were from 3.2 to 4.7 °C. The comparison of these values with actual temperature differences suggests that the indicated work of conventional LTDSE was much lower than the thermodynamic upper limit.

Model of thermal infrared image texture generation based on the scenery space frequency

Infrared texture is an important feature in identifying scenery. To simulate infrared image texture effectively at different distances, we propose a model of infrared image texture generation based on scenery space frequency and the image pyramid degradation principle. First, we build a spatial frequency filter model based on imaging distance, taking into account the detector’s maximum spatial frequency, and use the filter to process a “zero” distance infrared image texture. Second, taking into consideration the actual temperature difference of the scenery’s details due to variation of the imaging distance and the effect of atmospheric transmission, we compare the actual temperature difference with the minimum resolvable temperature difference of the thermal imaging system at a specific frequency and produce a new image texture. The results show that the simulated multiresolution infrared image textures produced by the proposed model are very similar (lowest mean square error=0.51 and highest peak signal-to-noise ratio=117.59) to the images captured by the thermal imager. Therefore, the proposed model can effectively simulate infrared image textures at different distances.

In situ temperature measurements for selective epitaxy of GaN nanowires

AbstractWe demonstrate with spatially resolved, in situ temperature measurements and ex situ reflectance measurements that differences in appearance for masked and unmasked surfaces on patterned growth substrates arise from wavelength‐dependent emissivity variations and not from actual temperature differences. For this study, Si(111) substrates were coated with thin AlN buffer layers (about 40 nm thick) and then further coated with a SiNx mask layer (about 60 nm thick). “Blackbody” emission spectra were collected from homogeneous regions at growth temperatures near those that allow the selective epitaxy of catalyst‐free GaN nanowires on the AlN by molecular beam epitaxy. The data were acquired with a commercial in situ diagnostic system that incorporates optical fiber collection to limit the sampling area to a circle about 5 mm in diameter on the substrate. The emittance spectra are estimated from room‐temperature reflectance measurements. The implication for selectivity mechanisms and temperature measurements in molecular beam epitaxy in general are also discussed. (© 2014 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Preparation and Characterization of Bi2Te3/Sb2Te3 Thermoelectric Thin-Film Devices for Power Generation

A device based on a new double-layer-leg thin-film concept has been successfully fabricated by flip-chip bonding of 242 pairs of n-type Bi2Te3 and p-type Sb2Te3 thin-film legs electrodeposited on top substrates to those processed on bottom substrates. Based on the output voltage–current curve, the internal resistance of the double-layer-leg thin-film device was measured to be 3.47 kΩ at an apparent temperature difference of 25.9 K across the device. The actual temperature difference across the thin-film legs was estimated to be 3.51 K, which is ∼13% of the apparent temperature difference ΔT of 25.9 K applied across the thin-film device. The double-layer-leg thin-film device exhibited an open-circuit voltage of 0.43 V and maximum output power of 13.1 μW at an apparent temperature difference ΔT of 25.9 K.

Thermoelectric Power Generation Characteristics of a Thin-Film Device Consisting of Electrodeposited n-Bi2Te3 and p-Sb2Te3 Thin-Film Legs

A thermoelectric thin-film device of the cross-plane configuration was fabricated by flip-chip bonding of the top electrodes to 242 pairs of electrodeposited n-type Bi2Te3 and p-type Sb2Te3 thin-film legs on the bottom substrate. The electrodeposited Bi2Te3 and Sb2Te3 films of 20-μm thickness exhibited Seebeck coefficients of −59 μV/K and 485 μV/K, respectively. The internal resistance of the thin-film device was measured as 3.7 kΩ, most of which was attributed to the interfacial resistance of the flip-chip joints. The actual temperature difference ΔTG working across the thin-film legs was estimated to be 10.4 times smaller than the apparent temperature difference ΔT applied across the thin-film device. The thin-film device exhibited an open-circuit voltage of 0.294 V and a maximum output power of 5.9 μW at an apparent temperature difference ΔT of 22.3 K applied across the thin-film device.

Measuring fetal and maternal temperature differentials: a probe for clinical use during labour

The healthy fetus maintains a higher temperature than that of its mother during gestation and labour. This results from the thermal balance between the heat generated by the fetus and the heat loss to its maternal surroundings. The heat loss can be by heat exchange via blood flowing in the umbilical cord and placenta, and via conduction through the fetal skin and amniotic fluid to the maternal wall. The temperature difference between the fetal and maternal tissue may reflect the metabolic state of the fetus and the magnitude and changing patterns of placental blood flow during labour. Physiological changes, such as those induced by epidural analgesia, and fetal infection have been shown to exhibit an increase in the absolute temperature. An intrauterine probe, previously used for non-invasive ECG detection, has been equipped with temperature sensors that measure fetal and maternal skin temperature in utero. Laboratory tests to characterize the performance of the probe reveal that absolute and differential temperatures can be resolved to around 0.01° C with a thermal time constant of approximately 9 s. Ideally the probe body should have infinite thermal insulation or thermal shunting across the probe will occur reducing the measured temperature difference. In this initial probe design, a high thermal isolation between sensors has been achieved but is not perfect, resulting in around 85% of the actual temperature difference across the probe being registered. Average feto-maternal differences of 0.2° C have been measured in a clinical investigation.

Effects of diffusional modification of garnet growth zoning on P-T path calculations

The effect of intragranular diffusion on chemical zoning in garnet and on P-T paths calculated from that zoning was evaluated using a numerical model of multicomponent diffusion in combination with simulations of garnet growth. Syn-and post-growth diffusion of Mg−Fe−Mn−Ca species in garnet was calculated for a model pelitic assemblage over a range of temperatures from 485 to 635°C. Compositions from zoned garnet, as modified by diffusion, hypothetical inclusions of plagioclase within garnet and matrix phases were used to reconstruct pressure-temperature (P-T) paths from isobaric and polybaric model histories. P-T path calculations, based on numerical simulations conducted over an input isobaric heating path that reached peak temperatures between 585 and 635°C, show that relaxation of garnet compositional gradients by diffusion can induce modest to appreciable curvature in the inferred paths. Retrieved paths also indicated somewhat smaller overall temperature changes relative to the actual temperature difference of the input path. The magnitude of these distortions is shown to depend upon the heating and cooling rate and garnet crystal size as well as the actual peak temperature condition. The effect of diffusion on path trajectories in simulations with thermal histories that also included cooling were comparable to heating-only models that reached peak temperatures approximately 15–30°C higher. Compositions of garnets with radii less than 1 mm, that reached actual peak temperatures of 605°C along temperature-time histories characteristic of regional metamorphism, experienced sufficient diffusional relaxation to introduce errors of hundreds of bars to in excess of one kilobar in path trajectories. Path distortions were significant at heating/cooling rates less than 10°C/Ma, but rapidly diminished for rates faster than this. In polybaric simulations diffusion effects were least noticeable when the actual pressure-temperature conditions changed in a clockwise sense (i.e., convex to higher P and higher T), but apprecciable modification was seen in path models that underwent counterclockwise changes in P and T. Reequilibration of garnet rim compositions occurred during cooling on all paths, and temperature maxima obtained from garnet-biotite geothermometry underestimated actual peak conditions by 40 to 70°C. Calculations suggest that P-T path trajectories calculated from garnets of at least 1 mm size, and that experienced actual thermal maxima below 585°C, are not likely to be distorted by diffusional effects during regional metamorphism. However, P-T path reconstructions based on garnet zonation with smaller grains or higher temperatures may lead to misinterpretation of crystallization history. The partitioning record of peak metamorphic temperatures may be destroyed by diffusional reequilibration of garnet rim compositions during cooling, seriously complicating the task of quantitatively estimating diffusion effects on path calculations.

Effect of fuel pellet eccentricity with cladding on fuel rod thermal behavior under reactivity initiated accident condition.

Fuel rod behavior under Reactivity Initiated Accident (RIA) conditions has been studied in the Nuclear Safety Research Reactor (NSRR), JAERI. In the experiments, cladding thermal behavior was observed to be influenced by the fuel pellet eccentricity to produce large azimuthal temperature variation in the cladding. The maximum azimuthal cladding temperature difference was measured to be as large as 150°C by thermocouples attached to opposite sides of the cladding around the circumference, though the thermocouples did not always detect the maximum temperature difference around the circumference. The actual temperature differences in the fuel rods subjected to less than 290 cal/g•UO2 were estimated to be 350°C at maximum based on metallographies. A simple calculation considering gap conductance variations also showed that the maximum temperature difference became 350°C under fully eccentrical condition in the fuel rod subjected to 260 cal/g•UO2. Moreover, as the rod damage such as cladding deformation, melting and failure occurs unevenly around the circumference due to the fuel pellet eccentricity in general, the fuel pellet eccentricity should influence the fuel rod failure under RIA conditions.

A Tau-Meter for Rapid-Response Temperature Recording Systems

The equipment described is suitable for measuring lag coefficients, as small as 0.1 second, of temperature recording systems. The sensing element is suspended alternately in two air streams of different temperature. The ratio of indicated to actual temperature difference is used to compute the lag coefficient. Measurements indicate that the lag of the amplifier-recorder is small compared to that of the elements tested.