Parasitic Heat Research Articles

Optimization Strategies for a Portable Thermoelectric Vaccine Refrigeration System in Developing Communities

The traditional approach to determine an optimum current for thermoelectric cooling assumes that a refrigeration chamber is insulated and has no thermal resistance to a thermoelectric module. As a result, minimum temperature occurs when Peltier cooling matches with parasitic heat transfer and Joule heating. In practical application, minimum temperature happens when heat addition from the environment is matched with heat extracted by a thermoelectric module, and the optimum current differs from that anticipated by the traditional approach. Hence, consideration for insulation and thermal resistances via thermoelectric module should be made to achieve desirable cooling performance/refrigeration temperature. This paper presents a modeling approach to determine the optimum current as well as the optimum geometry to power a small thermoelectric vaccine delivery system for developing communities under the World Health Organization requirements. The model is derived from three energy conservation equations for temperatures at both ends of the thermoelectric materials within a module, as well as the refrigeration chamber temperature. A prototype was built and demonstrated a minimum temperature of 3.4°C. With optimized module geometry, the system is estimated to reduce power consumption by over 50% while achieving twice the temperature difference.

Characterization of various losses in a cryogenic counterflow heat exchanger

A detailed assessment of irreversibility, predominantly heat in-leak and axial wall conduction, is essential in accurately predicting the performance of high effectiveness heat exchangers employed in cryogenic applications. Integration into a refrigeration system as well requires consideration of parasitic heat loss by conduction from exchanger cold end to the adjacent components. Governing equations incorporating these effects in a counterflow exchanger are solved numerically and the model predictions evaluated for heat exchanger ineffectiveness and heat loss by conduction. The optimum performance mandates minimization of both. Although ineffectiveness decreases at higher longitudinal conduction, cold end loss increases with deterioration of the overall performance. Utilizing lower heat capacity rate hot fluid, nevertheless, reduces the cold end loss. Heat in-leak is relatively high with concurrent consideration of axial wall conduction and has adverse consequence on heat exchanger effectiveness. Analysis of the net heat transferred to lower stages of refrigeration reveals a critical NTU.

Determination of the Voltage-Dependence of Parasitic Heat Flow in Lithium Ion Cells Using Isothermal Microcalorimetry

Parasitic reactions that occur in lithium ion batteries are well known to result in cell failure [1], and therefore being able to measure and reduce these reactions is of utmost importance. Isothermal microcalorimetry has been previously used to determine the relative contribution of parasitic heat flow between cells varying in electrolyte composition [2]. However, the absolute magnitude of the voltage-dependent parasitic heat flow for individual cells has not previously been determined. Here, by varying the current over narrow voltage ranges, the relative contributions of each of the heat flow sources as a function of state of charge can be isolated. When a current is applied to a cell there are three sources of heat flow [3]: Joule heating due to polarization, changes in entropy, and parasitic reactions. Polarization produces a non-reversible heat flow that is proportional to the square of the current, changes in entropy during intercalation and deintercalation produces a reversible heat flow that is proportional to the current, and parasitic reactions produce a heat flow that is thought to be independent of the current. The relative effect of entropy, polarization, and parasitics can therefore be determined over small voltage ranges by varying the current. The data can be fit using a simplistic model where each contribution is modeled with a simple function, typically linear, of the state of charge. The fitting results then give the relative contributions of each term, with particular importance to the function associated with the parasitic heat flow for an individual cell, allowing for the extraction of the voltage-dependence of the parasitic heat flow for an individual cell.Figure 1 shows an example of such an analysis for a 180 mAh machine-made high voltage-LiCoO2/graphite pouch cell with a 1 M LiPF6 in 3:7 EC:EMC electrolyte. The heat flow was measured using a TA instruments TAM III isothermal calorimeter equipped with twelve microcalorimeters with an accuracy of < ±1 mW. There is an excellent agreement between the simplistic model and the experimental data, showing the ability of isothermal microcalorimetry to accurately extract the voltage-dependence of parasitic heat flow in individual cells.This analysis is particularly useful for drawing conclusions about the behaviours of electrolyte additives across different cell chemistries where the method of relative differences in heat flow cannot be used. Results will be presented for three different machine-made pouch cell chemistries: high voltage-LiCoO2/graphite to 4.4 V, Li[Ni0.33Mn0.33Co0.33]O2 (NMC)/graphite to 4.2 V, and NMC/graphite to 4.4 V. The effect of a variety of different additives such as vinylene carbonate, methylene methanedisulfonate, ethylene sulfate, trimethylene sulfate, etc, will also be explored.

Uncertainty analysis for common Seebeck and electrical resistivity measurement systems.

This work establishes the level of uncertainty for electrical measurements commonly made on thermoelectric samples. The analysis targets measurement systems based on the four probe method. Sources of uncertainty for both electrical resistivity and Seebeck coefficient were identified and evaluated. Included are reasonable estimates on the magnitude of each source, and cumulative propagation of error. Uncertainty for the Seebeck coefficient includes the cold-finger effect which has been quantified with thermal finite element analysis. The cold-finger effect, which is a result of parasitic heat transfer down the thermocouple probes, leads to an asymmetric over-estimation of the Seebeck coefficient. A silicon germanium thermoelectric sample has been characterized to provide an understanding of the total measurement uncertainty. The electrical resistivity was determined to contain uncertainty of ±7.0% across any measurement temperature. The Seebeck coefficient of the system is +1.0%/-13.1% at high temperature and ±1.0% near room temperature. The power factor has a combined uncertainty of +7.3%/-27.0% at high temperature and ±7.5% near room temperature. These ranges are calculated to be typical values for a general four probe Seebeck and resistivity measurement configuration.

Invited Presentation: The Impact of High Voltage Electrolyte Additives and Solvents on Parasitic Heat Determined Using Isothermal Microcalorimetry

Parasitic reactions that occur in lithium ion batteries are well known to ultimately cause cell failure, and therefore being able to measure and reduce these reactions is of utmost importance. The use of electrolyte additives in can extend cycle life, increase coulombic efficiency, and reduce these parasitic reactions [1]. The technique of isothermal microcalorimetry has been previously used to determine the relative contribution of parasitic heat flow between cells varying in electrolyte composition [2]. Here, this technique is used to determine the impact of high voltage electrolyte additives and solvents as a function of voltage with a variety of cell chemistries. Additionally how these additive combinations compare to commercially available “high-voltage electrolytes” will be shown.Transitioning to increasingly higher upper cutoff voltages has proved difficult as many additives and solvents are unstable at such high potentials and the parasitic degradation of these components result in severely decreased lifetimes. The voltage-dependent impact of electrolyte additives, additive blends, and solvents that have shown to result in high coulombic efficiencies, reduced electrolyte oxidation, and other parasitic reactions, will be explored. Such components include additives such as vinylene carbonate (VC), methylene methanedisulfonate (MMDS) [3], tris(trimethylsilyl)phosphite (TTSPi) [4], and others; commercially available high voltage electrolytes from two electrolyte suppliers, A and B; as well as fluorinated solvent systems.By comparing the heat flow of cells that vary only in electrolyte composition during cycling, the effect of the additive on the parasitic heat for one cell chemistry during the entire potential range is obtained in a short, simple experiment. Furthermore, simple modeling of the heat flow response results in the determination of the absolute magnitude of the voltage-dependent parasitic heat flow for individual cells, allowing for comparisons across different cell chemistries [5]. A TA instruments TAM III isothermal calorimeter equipped with twelve microcalorimeters was used for these measurements. The accuracy of the microcalorimeter used (< ±1 mW) allows for a highly sensitive differentiation between cells.Figure 1 shows an example of such a heat flow measurement for five machine-made 220 mAh Li[Ni1/3Mn1/3Co1/3]O2/graphite pouch cells cycling at 10 mA (C/22) at 40.0oC with various additive blends. At this current, the contributions of entropy and polarization to the total heat flow are nearly identical between cells, such that any differences in measured heat are attributable to differences in parasitic heat. The bottom panel of Figure 1 shows the differences when subtracting the heat flow of the control cell (no additive) from the heat flow of the additive-containing cells as a function of potential. These differences are good measures of the reduction in parasitic heat due to the addition of an additive. This figure shows that the inclusion of electrolyte additives can greatly reduce the parasitic reactions, especially so at high voltage. The effect of a variety of additive systems on a variety of cell chemistries such as high-voltage LiCoO2/graphite operating to 4.4 V, and Li[Ni0.4Mn0.4Co0.2]O2/graphite operating to 4.5 and 4.7 V will be presented.While these electrolyte additives and solvents can reduce the heat due to parasitic reactions, the overall parasitic heat is still substantial at high potentials, as seen in Figure 1, for example, by the dramatically increasing heat flow above 4.3 V. The results presented here clearly show the impact of high voltage electrolyte additives and solvents and directly determine their optimal potential ranges to minimize parasitic reactions and therefore extend cell lifetimes. The feasibility of operating cells above 4.4 or 4.5 V will also be discussed in this context.

The Impact of High-Voltage Electrolyte Additives and Solvents to 4.7 V Determined Using Isothermal Microcalorimetry

The use of electrolyte additives in lithium ion batteries can extend cycle life, increase coulombic efficiency, and reduce parasitic reactions [1]. Here, high resolution isothermal microcalorimetry [2, 3] is used to determine the impact of high-voltage electrolyte additives and solvents as a function of voltage up to 4.7 V in Li[Ni0.4Mn0.4Co0.2]O2(NMC442)/graphite pouch cells.Transitioning to increasingly higher upper cutoff voltages has proved difficult as many additives and solvents are unstable at such high potentials and the parasitic degradation of these components result in severely decreased lifetimes. The voltage-dependent impact of electrolyte additives, additive blends, and solvents that have shown to result in high coulombic efficiencies, reduced electrolyte oxidation, and other parasitic reactions, will be explored. Such components include additives such as vinylene carbonate (VC), methylene methanedisulfonate, ethylene sulfate, trimethylene sulfate, and others, as well as a variety of fluorinated solvents. By comparing the heat flow of cells that vary only in electrolyte composition during cycling and open circuit conditions, the effect of the additive on the parasitic heat during the entire potential range is obtained in a short, simple experiment. A TA instruments TAM III isothermal calorimeter equipped with twelve microcalorimeters was used for these measurements. The accuracy of the microcalorimeter used (< ±1 mW) allows for a highly sensitive differentiation between cells.Figure 1 shows the heat flow versus voltage for four machine-made 260 mAh NMC442/graphite pouch cells cycling at 10 mA (C/26) at 40oC with different additive blends. At this current, the contributions of entropy and polarization to the total heat flow are nearly identical between cells, such that any differences in measured heat are attributable to differences in parasitic heat. Figure 1 shows exceptional differences in the heat flow between the first charge (left) and subsequent charges (the fourth charge is shown as an example, right). The cells which produce the most heat during the first charge to 4.7 V have the smallest heat flow at the same potential for subsequent cycles. This is attributed to passivation-type reactions which, while initially producing vast amounts of heat, result in a reduction of parasitic reactions over the remainder of the cell life. Figure 1 also shows that the electrolyte blends with sulfur-containing additives reduce parasitic reactions compared to 2% VC-containing cells above 4.4 V, and continue to reduce these reactions with increasing state of charge. Similar results will be presented for a variety of high-voltage additives and solvents.While these electrolyte additives and solvents can reduce the heat due to parasitic reactions, the overall parasitic heat is still substantial at high potentials, as seen in Figure 1 by the dramatically increasing heat flow above 4.4 V. The results presented here clearly show the impact of high-voltage electrolyte additives and solvents and directly determine their optimal potential ranges to minimize parasitic reactions and therefore extend cell lifetimes.

Invited Presentation: 4.7 V Li-ion cells: Nonsense or Possibility

One way to improve the energy density of Li-ion cells is to use high voltage positive electrode materials like LiNi0.5Mn1.5O4 [LNMO] or simply increase the upper cutoff potential when positive electrodes like Li[NixMnxCo1-2x]O2 [NMC] are used. This sounds simple, but there are numerous problems to overcome before high voltage Li-ion cells are a reality.Many literature reports of Li/LNMO and LTO/LNMO cells present very promising behavior. However, most of the reports show charge discharge cycling at high rates, where many cycles can be accumulated in a short period of time. In such experiments, authors are able to “beat the clock” on parasitic electrolyte oxidation reactions which occur in cells. We recently showed using storage experiments on LTO/LNMO cells that electrolyte oxidation, as evidenced by rapid self discharge, is a huge problem [1, 2] that is ignored in most literature reports.In this paper we discuss a number of the problems that occur in LTO/LNMO cells and high voltage graphite/NMC cells. These include:1. Impedance increase at the positive electrode as the potential increases due to parasitic reactions.2. Increases in parasitic heat as a function of cell voltage as measured using isothermal battery microcalorimetry3. Increases in gas evolution as a function of cell potential, even when fluorinated solvents are used.4. Decreases in coulombic efficiency as cell potentials increase.5. Etc.The results above paint a bleak picture about the future of Li-ion cells that can operate at 4.7 V. Next, partial solutions to the problems above will be presented:a) Electrolyte additives that can stabilize positive electrode impedance increases during high potential cycling will be discussed. b) Electrolyte additives that can dramatically reduce parasitic heats evolved at high potentials will be discussed. c) Materials that can dramatically lower electrolyte oxidation reactions at the surface of the positive electrode and are suitable as “shells” in core-shell materials will be discussed.d) Electrolyte additives that can reduce gas evolution at high potential will be discussed. e) Etc.These encouraging results provide hope that real solutions can be found.

Determination of the Voltage-Dependence of Parasitic Heat Flow in Lithium Ion Cells Using Isothermal Microcalorimetry

Parasitic reactions that occur in lithium ion batteries are well known to result in cell failure [1], and therefore being able to measure and reduce these reactions is of utmost importance. Isothermal microcalorimetry has been previously used to determine the relative contribution of parasitic heat flow between cells varying in electrolyte composition [2]. However, the absolute magnitude of the voltage-dependent parasitic heat flow for individual cells has not previously been determined. Here, by varying the current over narrow voltage ranges, the relative contributions of each of the heat flow sources as a function of state of charge can be isolated. When a current is applied to a cell there are three sources of heat flow [3]: Joule heating due to polarization, changes in entropy, and parasitic reactions. Polarization produces a non-reversible heat flow that is proportional to the square of the current, changes in entropy during intercalation and deintercalation produces a reversible heat flow that is proportional to the current, and parasitic reactions produce a heat flow that is thought to be independent of the current. The relative effect of entropy, polarization, and parasitics can therefore be determined over small voltage ranges by varying the current. The data can be fit using a simplistic model where each contribution has one linear term and one that scales with the state of charge. The fitting constants then give the relative contributions of each term, with particular importance to the parameters associated with the parasitic heat flow for an individual cell, allowing for the extraction of the voltage-dependence of the parasitic heat flow for an individual cell.Figure 1 shows an example of such an analysis for a 225 mAh machine-made LiCoO2/graphite pouch cell with a 1 M LiPF6 in 3:7 EC:EMC electrolyte. The heat flow was measured using a TA instruments TAM III isothermal calorimeter equipped with twelve microcalorimeters with an accuracy of < ±1 mW. There is an excellent agreement between the simplistic model and the experimental data, showing the ability of isothermal microcalorimetry to accurately extract the voltage-dependence of parasitic heat flow in individual cells.This analysis is particularly useful for drawing conclusions about the behaviours of electrolyte additives across different cell chemistries where the method of relative differences in heat flow cannot be used. Results will be presented for four different machine-made pouch cell chemistries: LiCoO2/graphite to 4.2 V, Li[Ni0.33Mn0.33Co0.33]O2 to 4.2 V, Li[Ni0.33Mn0.33Co0.33]O2 to 4.4 V, and Li[Ni0.4Mn0.4Co0.2]O2 to 4.7 V. The effect of a variety of different additives such as vinylene carbonate, methylene methanedisulfonate, ethylene sulfate, trimethylene sulfate, etc, will also be explored.

Two dimensional model for multistream plate fin heat exchangers

A model based on finite volume analysis is presented here for multistream plate fin heat exchangers for cryogenic applications. The heat exchanger core is discretised in both the axial and transverse directions. The model accounts for effects of secondary parameters like axial heat conduction through the heat exchanger metal matrix, parasitic heat in-leak from surroundings, and effects of variable fluid properties/metal matrix conductivity. Since the fins are discretised in the transverse direction, the use of a fin efficiency is eliminated and the effects of transverse heat conduction/stacking pattern can be taken care of. The model is validated against results obtained using commercially available software and a good agreement is observed. Results from the developed code are discussed for sample heat exchangers.

Cold-induced vasoconstriction may persist long after cooling ends: an evaluation of multiple cryotherapy units

Localized cooling is widely used in treating soft tissue injuries by modulating swelling, pain, and inflammation. One of the primary outcomes of localized cooling is vasoconstriction within the underlying skin. It is thought that in some instances, cryotherapy may be causative of tissue necrosis and neuropathy via cold-induced ischaemia leading to nonfreezing cold injury (NFCI). The purpose of this study is to quantify the magnitude and persistence of vasoconstriction associated with cryotherapy. Data are presented from testing with four different FDA approved cryotherapy devices. Blood perfusion and skin temperature were measured at multiple anatomical sites during baseline, active cooling, and passive rewarming periods. Local cutaneous blood perfusion was depressed in response to cooling the skin surface with all devices, including the DonJoy (DJO, p = 2.6 × 10(-8)), Polar Care 300 (PC300, p = 1.1 × 10(-3)), Polar Care 500 Lite (PC500L, p = 0.010), and DeRoyal T505 (DR505, p = 0.016). During the rewarming period, parasitic heat gain from the underlying tissues and the environment resulted in increased temperatures of the skin and pad for all devices, but blood perfusion did not change significantly, DJO (n.s.), PC300 (n.s.), PC500L (n.s.), and DR505 (n.s.). The results demonstrate that cryotherapy can create a deep state of vasoconstriction in the local area of treatment. In the absence of independent stimulation, the condition of reduced blood flow persists long after cooling is stopped and local temperatures have rewarmed towards the normal range, indicating that the maintenance of vasoconstriction is not directly dependent on the continuing existence of a cold state. The depressed blood flow may dispose tissue to NFCI.

A simple differential steady-state method to measure the thermal conductivity of solid bulk materials with high accuracy

Accurate measurements of thermal conductivity are of great importance for materials research and development. Steady-state methods determine thermal conductivity directly from the proportionality between heat flow and an applied temperature difference (Fourier Law). Although theoretically simple, in practice, achieving high accuracies with steady-state methods is challenging and requires rather complex experimental setups due to temperature sensor uncertainties and parasitic heat loss. We developed a simple differential steady-state method in which the sample is mounted between an electric heater and a temperature-controlled heat sink. Our method calibrates for parasitic heat losses from the electric heater during the measurement by maintaining a constant heater temperature close to the environmental temperature while varying the heat sink temperature. This enables a large signal-to-noise ratio which permits accurate measurements of samples with small thermal conductance values without an additional heater calibration measurement or sophisticated heater guards to eliminate parasitic heater losses. Additionally, the differential nature of the method largely eliminates the uncertainties of the temperature sensors, permitting measurements with small temperature differences, which is advantageous for samples with high thermal conductance values and/or with strongly temperature-dependent thermal conductivities. In order to accelerate measurements of more than one sample, the proposed method allows for measuring several samples consecutively at each temperature measurement point without adding significant error. We demonstrate the method by performing thermal conductivity measurements on commercial bulk thermoelectric Bi2Te3 samples in the temperature range of 30-150 °C with an error below 3%.

Effects of axial heat conduction, property variation and parasitic heat in-leakon performance of compact plate-fin heat exchangers

Modern helium liquefaction/refrigeration systems employ compact plate-fin heat exchangers having very high effectiveness (>0.95). Performance of such systems is a strong function of effectiveness of heat exchangers used. The calculation of heat exchanger effectiveness in such cases needs considerations of several secondary parameters apart from basic fluid film resistance. In the present paper, the combined effects of secondary parameters like axial heat conduction through heat exchanger matrix and parasitic heat in-leak from the surroundings has been studied numerically. Large temperature changes in cryogenic heat exchangers may result in correspondingly larger changes in fluid properties and metal matrix conductivity, which shall be taken care of during numerical calculations. Numerical model developed in the present work is based on the one given in the literature. Numerical technique to solve the system of equations is implemented in MATLAB®. Real properties of helium at each node are evaluated using HEPAK®, which is linked to the developed code. Using this model, performance of heat exchangers is studied at four different temperature levels of (300-77)K, (77–20)K, (20-8)K and (10-5.1)K. The results highlight the effects of each of the above secondary parameters.

Determination of the Voltage Dependence of Parasitic Heat Flow in Lithium Ion Cells Using Isothermal Microcalorimetry

Isothermal microcalorimetry has been previously used to determine the relative contribution of parasitic heat flow between cells varying in electrolyte composition. However, the absolute magnitude of the voltage-dependent parasitic heat flow for individual cells has not previously been determined. Here, by varying the current over narrow voltage ranges, the relative contributions of each of the heat flow sources (polarization, entropy and parasitic reactions) as a function of state of charge is isolated. The measured heat flow data is fit using a model where each contribution is a function of state of charge (typically linear over a narrow voltage range), allowing for the extraction of the voltage dependency of the parasitic heat flow for an individual cell. The parasitic heat flow as a function of voltage is determined for LiCoO2/graphite and Li[Ni1/3Mn1/3Co1/3]O2(NMC)/graphite pouch cells. The effect of the addition of 2% VC to the electrolyte in NMC/graphite cells is also shown, where a significant reduction in parasitic heat flow is observed. The absolute parasitic heat flow for individual cells also allows for cross-chemistry comparisons, where it is observed that at high potentials, the parasitic heat flow for the LiCoO2/graphite cells is reduced compared to NMC/graphite cells.

Sub-picowatt/kelvin resistive thermometry for probing nanoscale thermal transport

Advanced instrumentation in thermometry holds the key for experimentally probing fundamental heat transfer physics. However, instrumentation with simultaneously high thermometry resolution and low parasitic heat conduction is still not available today. Here we report a resistive thermometry scheme with ~50 μK temperature resolution and ~0.25 pW/K thermal conductance resolution, which is achieved through schemes using both modulated heating and common mode noise rejection. The suspended devices used herein have been specifically designed to possess short thermal time constants and minimal attenuation effects associated with the modulated heating current. Furthermore, we have systematically characterized the parasitic background heat conductance, which is shown to be significantly reduced using the new device design and can be effectively eliminated using a "canceling" scheme. Our results pave the way for probing fundamental nanoscale thermal transport processes using a general scheme based on resistive thermometry.

PACS photometer calibration block analysis

The absolute stability of the PACS bolometer response over the entire mission lifetime without applying any corrections is about 0.5% (standard deviation) or about 8% peak-to-peak. This fantastic stability allows us to calibrate all scientific measurements by a fixed and time-independent response file, without using any information from the PACS internal calibration sources. However, the analysis of calibration block observations revealed clear correlations of the internal source signals with the evaporator temperature and a signal drift during the first half hour after the cooler recycling. These effects are small, but can be seen in repeated measurements of standard stars. From our analysis we established corrections for both effects which push the stability of the PACS bolometer response to about 0.2% (stdev) or 2% in the blue, 3% in the green and 5% in the red channel (peak-to-peak). After both corrections we still see a correlation of the signals with PACS FPU temperatures, possibly caused by parasitic heat influences via the Kevlar wires which connect the bolometers with the PACS Focal Plane Unit. No aging effect or degradation of the photometric system during the mission lifetime has been found.

1D model of an active magnetic regenerator

The 1D active magnetic regenerator model developed at the University of Applied Sciences of Western Switzerland is described. The system of two partial differential equations is discretized with the finite differences method backward in time and solved with the tridiagonal matrix algorithm. New features, not found in the literature in 1D models, are thermal losses in the regenerator, parasitic heat exchange, and the calculation of the AMR cycle output power in steady state. The model is implemented in MATLAB and it has a graphical user interface.

Numerical investigation of the thermohydraulic behaviour of a complete loop heat pipe

A complete steady-state model has been developed to determine the thermohydraulic behaviour of a loop heat pipe. The model combines a fine discretization of the condenser and the transport lines with a 2-D description of the evaporator. These original features enable to take into account heat losses to the ambient and through the transport lines as well as to evaluate the parasitic heat flux through the wick and the evaporator body. The present numerical simulations may improve the understanding of the physical mechanisms operating in an LHP evaporator, and their coupling with the other parts of the LHP, and provide guidance for the LHP design, aiming to reduce the thermal resistance of the system. The comparison between experimental data of a flat disk-shaped evaporator from the literature and theoretical simulations validates the proposed model. Simulations show the significance of heat conduction through the liquid line. Additionally, results show the major influence of the evaporation coefficient and of the wick thermal conductivity on the LHP operating temperature. When longitudinal heat losses are not significant, the competition between the parasitic heat flux through the wick and the heat transfer to the evaporation zone leads to an extremum for which the operating temperature is maximal. With a thermal conductive evaporator casing, the longitudinal parasitic heat flux strongly influences the LHP operational temperature and the evaporator energy balance.

Reexamination of thermal transport measurements of a low-thermal conductance nanowire with a suspended micro-device

An increasingly used technique for measuring the thermal conductance of a nanowire is based on a suspended micro-device with built-in resistance thermometers. In the past, the technique has been limited to samples with thermal conductance larger than 1 × 10(-9) W/K because of temperature fluctuations in the sample environment and the presence of background heat transfer through residual gas molecules and radiation between the two thermometers. In addition, parasitic heat loss from the long supporting beams and asymmetry in the fabricated device results in two additional errors, which have been ignored in previous use of this method. To address these issues, we present a comprehensive measurement approach, where the device asymmetry is determined by conducting thermal measurements with two opposite heat flow directions along the nanowire, the background heat transfer is eliminated by measuring the differential heat transfer signal between the nanowire device and a reference device without a nanowire sample, and the parasitic heat loss from the supporting beams is obtained by measuring the average temperature rise of one of the beams. This technique is demonstrated on a nanofiber sample with a thermal conductance of 3.7 × 10(-10) W/K, against a background conductance of 8.2 × 10(-10) W/K at 320 K temperature. The results reveal the need to reduce the background thermal conductance in order to employ the micro-device to measure a nanowire sample with the thermal conductance less than 1 × 10(-10) W/K.

The Impact of Electrolyte Additives Determined Using Isothermal Microcalorimetry

Isothermal microcalorimetry is a quick and simple method of determining the effect an additive or additive combination has on the parasitic reactions occurring as a function of state of charge. It can easily discriminate exactly where in the voltage range the additive is providing a benefit with just a single cycle. As a demonstrative example, the effect of varying concentrations of vinylene carbonate (VC) on a LiCoO2/graphite cell is examined. Machine-made pouch cells were used such that the cells were nominally identical except for the concentration of VC. The measured heat flow for the different cells is then identical except for the heat flow that results from parasitic reactions. It is shown that the presence of VC reduces parasitic reactions above 3.9 V, and continues to reduce these reactions with increasing state of charge. The heat flow during open circuit conditions at the top of charge also shows that the presence of VC dramatically reduces the heat due to parasitic reactions. Lithium-ion batteries are being used in an increasing number of applications that are demanding higher energy densities and longer lifetimes. The use of electrolyte additives is a common method that has shown to extend calendar and cycle life and reduce parasitic reactions such as the consumption of active lithium in forming and repairing the solid electrolyte interphase (SEI), electrolyte oxidation, transition metal dissolution, electrode damage, etc. 1 However, it is not very well understood how these additives are functioning and exactly where in the charge-discharge cycle they prove advantageous. One such example is with the popular additive, vinylene carbonate (VC). Several studies 2‐7 report that the increase in full cell lifetime is caused by VC being reduced on the surface of graphite, which forms a more stable SEI. More recent studies 8‐13 have challenged this thought, reporting that the benefit of VC arises primarily from limitingelectrolyteoxidationandpossibletransitionmetaldissolution reactionsatthepositiveelectrode.Therefore,itisofdistinctinterestto be able to determine the voltage-dependent advantage of a particular additive or additive combination, which can aid in the understanding of how these additives are extending lifetimes. Recently the technique of isothermal microcalorimetry has been combinedwithelectrochemicalmeasurements,whichhasbeenusedto examine the thermal behavior of several lithium-ion chemistries. 14‐21 More recently, Krause et al. 22 showed how to use this technique to separate the various contributions to the thermal power and isolate parasitic energy. In this paper, this technique is used to qualitatively compare the heat flow between cells that only vary in concentration of additive. In this case, with all other sources being identical, the measured difference in heat flow arises from differences in parasitic heat. This is done as a function of state of charge, providing a simple and quick method of determining exactly where and to what extent the additive is reducing parasitic reactions. As a demonstrative example, the effect of varying concentrations of VC on a LiCoO2/graphite cell is examined. Experimental General.— Machine-made 225 mAh LiCoO2 (LCO)/graphite pouch cells 0.4 cm thick × 2 cm wide × 3 cm high (obtained from Pred Materials Co.) were provided dry. The pouches were then filled with 0.75 g of 1M LiPF6 in 3:7 ethylene carbonate (EC):ethylmethyl carbonate (EMC) (Novolyte Technologies, now BASF) with various amounts of VC (Novolyte Technologies, now BASF) additive (0%, 0.5%, 2%, and 4% by weight) and then vacuum sealed. The electrodes

Protocols for the high temperature measurement of the Seebeck coefficient in thermoelectric materials

In Seebeck coefficient metrology, the present diversity in apparatus design, acquisition methodology and contact geometry has resulted in conflicting materials data that complicate the interlaboratory confirmation of reported high efficiency thermoelectric materials. To elucidate the influence of these factors in the measurement of the Seebeck coefficient at high temperature and to identify optimal metrology protocols, we measure the Seebeck coefficient as a function of contact geometry under both steady-state and transient thermal conditions of the differential method, using a custom developed apparatus capable of in situ comparative measurement. The thermal gradient formation and data acquisition methodology, under ideal conditions, have little effect on the measured Seebeck coefficient value. However, the off-axis 4-probe contact geometry, as compared to the 2-probe, results in a greater local temperature measurement error that increases with temperature. For surface temperature measurement, the dominant thermal errors arise from a parasitic heat flux that is dependent on the temperature difference between the sample and the external thermal environment, and on the various thermal resistances. Due to higher macroconstriction and contact resistance in the 4-probe arrangement, the measurement of surface temperature for this contact geometry exhibits greater error, thereby overestimating the Seebeck coefficient.