Effect of grain size on the thermoelectric conversion efficiency of semiconductor alloys at high temperature

The theoretical increase in the thermoelectric figure of merit of silicon-germanium alloy at 1000 K which would accompany a reduction in material grain size is calculated. Assuming that acoustic phonon scattering is the dominant scattering mechanism, the improvement in the conversion efficiency compared with single crystal values is estimated to be 14 per cent and 30 per cent for n-type Si₈₀Ge₂₀ alloy with grain sizes of 10 μm and 0·1 μm, respectively.     

A comparative study of an absorber heat recovery cycle for solar refrigeration using NH3-refrigerant with liquid/solid absorbents

This paper presents an investigation on using an ammonia refrigerant with liquid/solid absorbents in an absorber heat recovery cycle where heat released during the absorption process is used to heat up the strong solution coming out of the absorber, thereby reducing the generator heat input and hence improving the coefficient of performance. A comparative thermodynamic study is made with NH₃-H₂O and NH₃-LiNO₃ pairs as working fluids for both conventional absorption and absorber heat recovery systems. It is found that an improvement of about 10 per cent in COP for the absorber heat recovery cycle is achieved over the conventional absorption cycle and the NH₃-LiNO₃ system yields a higher COP than for NH₃-H₂O over a wide range of generator temperatures and condenser/absorber temperatures. A detailed parametric study is also presented in this paper.     

Flexible thermoelectric generator for ambient assisted living wearable biometric sensors

In this work we proposed design, fabrication and functional characterization of a very low cost energy autonomous, maintenance free, flexible and wearable micro thermoelectric generator (μTEG), finalized to power very low consumption electronics ambient assisted living (AAL) applications. The prototype, integrating an array of 100 thin films thermocouples of Sb₂Te₃ and Bi₂Te₃, generates, at 40 °C, an open circuit output voltage of 430 mV and an electrical output power up to 32 nW with matched load. In real operation conditions of prototype, which are believed to be very close to a thermal gradient of 15 °C, the device generates an open circuit output voltage of about 160 mV, with an electrical output power up to 4.18 nW.In the first part of work, deposition investigation Sb₂Te₃ and Bi₂Te₃ thin films alloys on Kapton HN polyimide foil by RF magnetron co-sputtering technique is discussed. Deposition parameters have been optimized to gain perfect stoichiometric ratio and high thermoelectric power factor; fabricated thermogenerator has been tested at low gradient conditioned to evaluate applications like human skin wearable power generator for ambient assisted living applications.     

Improving power density and efficiency of miniature radioisotopic thermoelectric generators

We have built and tested a prototype miniaturized thermoelectric power source that generates 450 μW of electrical power in a system volume of 4.3 cm³. The measured power density of 104 μW cm⁻³ exceeds that of any previously reported thermoelectric power system of equivalent size. This improvement was achieved by implementing a novel thermopile design in which wagon wheel-shaped thermoelectric elements contact the entire circumference of the heat source whereas traditional approaches utilize only one heat source surface. The thermopile consists of 22 wagon wheel-shaped elements (11 P–N thermocouples) fabricated from 215-μm thick bismuth–telluride wafers having ZT = 0.97 at 30 °C. The power source operates on a 150 mW thermal input provided by an electrical resistance heater that simulates a capsule containing 0.4 g of ²³⁸PuO₂ located at the center of the device. Our primary research objective was to develop and demonstrate a prototype thermopile and radioisotopic thermoelectric generator (RTG) architecture with improved power density at small scales. Output power from this device, while optimized for efficiency, was not optimized for output voltage, and the maximum power was delivered at 41 mV. We also discuss modifications to our prototype design that result in significantly improved voltage and power. Numerical predictions show that a power output of 1.4 mW, power density of 329 μW cm⁻³, and voltage of 362 mV, is possible in the same package size.     

Performance analysis of cascaded thermoelectric converters for advanced radioisotope power systems

Advanced radioisotope power systems (ARPSs) for future planetary missions require higher conversion efficiency than the state-of-the-art (SOA) SiGe thermoelectric converter in order to decrease system mass and reduce mission cost. The performance of three cascaded thermoelectric converters (CTCs) for potential use in ARPSs is investigated at heat rejection temperatures of 375, 475 and 575 K and input thermal powers of 1, 2 and 3 W_th. These CTCs have top SiGe unicouples that are thermally, but not electrically, coupled to bottom unicouples having one of the following compositions: (a) TAGS-85 (p-leg) and 2N–PbTe (n-leg); (b) CeFe_3.5Co_0.5Sb₁₂ (p-leg) and CoSb₃ (n-leg); and (c) segmented p-leg of CeFe_3.5Co_0.5Sb₁₂ and Zn₄Sb₃ and n-leg of CoSb₃. The top and bottom unicouples in the CTCs are of the same length (10 mm), but the optimized cross-sectional areas of the n- and p-legs for maximum efficiency are different. The nominal hot junction temperature of the top SiGe unicouples at their peak efficiencies is 1273 K and that of the cold junction is 780 K when the bottom unicouple is of composition (a) and 980 K for compositions (b) and (c). The hot junction temperatures of the bottom unicouples are taken 20 K lower than the cold junctions of the top unicouples, but the input thermal powers to the former are the same as those rejected by the latter. Assuming zero side heat losses and a contact resistance of 150 μΩ cm² per leg in the top and bottom unicouples, the calculated peak efficiencies of the CTCs vary from 9.43% to 14.35%. These efficiencies are 40–113% higher, respectively, than that of SOA SiGe (∼6.5%) when operating at the cold junction temperature of 566 K and the same hot junction temperature (1273 K) and contact resistance per leg. Decreasing this resistance to a realistic value of 50 μΩ cm² per leg increases the peak efficiencies of the CTCs by 0.5–0.9 percentage points to 9.93–15.25%.     

Fiber-based flexible thermoelectric power generator

Flexible thermoelectric power generators fabricated by evaporating thin films on flexible fiber substrates are demonstrated to be feasible candidates for waste heat recovery. An open circuit voltage of 19.6 μV K per thermocouple junction is measured for Ni–Ag thin films, and a maximum power of 2 nW for 7 couples at ΔT = 6.6 K is measured. Heat transfer analysis is used to project performance for several other material systems, with a predicted power output of 1 μW per couple for Bi₂Te₃/Sb₂Te₃-based fiber coatings with a hot junction temperature of 100 °C. Considering the performance of woven thermoelectric cloths or fiber composites, relevant properties and dimensions of individual thermoelectric fibers are optimized.     

Numerical analysis and optimization of a spectrum splitting concentration photovoltaic–thermoelectric hybrid system

This paper presents the numerical modeling and optimization of a spectrum splitting photovoltaic–thermoelectric (PV–TE) hybrid system. In this work, a simulation model is established in consideration of solar concentration levels and several heat dissipation rates. Exemplarily, the performance of a hybrid system composed of a GaAs solar cell and a skutterudites CoSb₃ solar thermoelectric generator (TEG) is simulated. Analysis under different conditions has been carried out to evaluate the electrical and thermal performance of the hybrid system. Results show that the cutoff-wavelength of the GaAs–CoSb₃ hybrid system is mainly determined by the band gap of solar cell, when the solar concentration ratio is ranged between 550 to 770 and heat transfer coefficient h = 3000–4500 W/m² K, the hybrid system has good electrical performance and low operating temperatures. Based on the analysis of the GaAs–CoSb₃ hybrid system, guidelines for the PV–TE system design are proposed. It is also compared with a PV-only system working under the same cooling condition; results show that the PV–TE hybrid system is more suitable for working under high concentrations.     

Development of thermoelectric oxides for renewable energy conversion technologies

Geothermal and solar heat can be directly converted into electricity by using thermoelectric generators. Perovskite-type metal oxides are potential materials to improve the efficiency of these devices. Cobaltates with p-type conductivity and n-type manganates are considered for the development of a ceramic thermoelectric converter.Sintered pellets and thin PLD films with the composition La_1−xCa_xMO_3−δ (x=0, 0.3, 0.4) (M = Co, Mn) were synthesised and characterised concerning their thermoelectric properties in a broad temperature range. It was found that similar to polycrystalline samples the electrical conductivity of LaCoO₃ increases significantly with 40% Ca-substitution due to the formation of Co⁴⁺ ions while the thermopower decreases. The thermopower values of the La_0.8Ca_0.2MnO_3−δ films have a negative sign, but become large and positive at temperatures of 1000 K.     

Analysis on the cooling performance of the thermoelectric micro-cooler

Numerical analysis has been carried out to figure out the performance of the thermoelectric micro-cooler with the three-dimensional model. A small-size and column-type thermoelectric cooler is considered and Bi₂Te₃ and Sb₂Te₃ are selected as the n- and p-type thermoelectric materials, respectively. The thickness of a thermoelectric element considered is 5–20 μm. The effect of parameters such as the temperature difference, the current, the thickness of a thermoelectric element, and the number of thermoelectric pairs on the performance of the cooler has been investigated. The predicted results show that the performance can be improved for the thick element with the large number of thermoelectric pairs or the small cross-sectional area of the element.     

Solar irradiation above various levels for 10-min and 1-hr integrals

In the summer of 1977 several 10 g samples of molybdenite ore containing 5–6 per cent Mo as MoS₂ were heated in one of the 2kW solar furnaces at the Laboratoire des Ultra-Refractaires, Odeillo, Frnace. The end products showed excellent separation with pure yellow acicular crystals of 9⁺ per cent MoO₃ and pure white SiO₂ powder being the two major components. Impurity levels in the MoO₃ did not exceed 6000 ppm while only a small fraction of the molybdenite was entrained along with the SiO₂. All treatment was done in a flowing oxygen atmosphere, and other elements condensed out at specific sites in the gas transport system. Analysis of the products was done by X-ray diffraction and X-ray fluorescence, emission spectroscopy, and wet chemical analysis. The ore was donated by Climax Molybdenum Co., Climax Co. The work is a joint effort of LASL and the Laboratoire des Ultra-Refractaires in Odeillo.     

Vitrinite reflectance geothermometry and apparent heating duration in the Cerro Prieto geothermal field

Vitrinite reflectance measured in immersion oil (R_o) on kerogen extracted from hydrothermally altered mudstones in borehole M-84 at the Cerro Prieto geothermal field exhibit an increase in mean reflectance ( ) from 0.12 per cent at 0.24 km depth to 4.1 per cent at 1.7 km depth. Downhole temperatures measured over this interval increase from about 60° to 340°C. These data plotted against temperature fall along an exponential curve with a coefficient of determination of about 0.8. Other boreholes sampled in the field show similar relationships. A regression curve calculated for temperature and in borehole M-105 correctly predicts temperatures in other boreholes within the central portion of the geothermal system. The correlation between the reflectance values and logged temperature, together with consistent temperature estimates from fluid inclusion and oxygen isotope geothermometry, indicates that changes in are an accurate and sensitive recorder of the maximum temperature attained. Therefore, vitrinite reflectance can be used in this geothermal system to predict the undisturbed temperature in a geothermal borehole during drilling before it regains thermal equilibrium. Although existing theoretical functions which relate to temperature and duration of heating are inaccurate, empirical temperature- curves are still useful for geothermometry.A comparison of temperature- regression curves derived from nine boreholes within the Cerro Prieto system suggests that heating across the central portion of the field occurred penecontemporaneously, but varies near margins. Boreholes M-93 and M-94 appear to have cooled from their maximum temperatures, whereas M-3 and Prian-1 have only recently been heated.Comparison of the temperature- data from the Salton Sea, California, geothermal system indicates that the duration of heating has been longer there than at the Cerro Prieto field.     

Solar gasification of coal, activated carbon, coke and coal and biomass mixtures

Subbituminous coal, activated carbon, coke and a mixture of coal and biomass were gasified using direct solar irradiation in a 23-kW solar furnace located at the U.S. Army White Sands Missile Range, White Sands, New Mexico. The sunlight was focused directly on the coal (or alternate fuel) bed being gasified through a window in the reactor. Steam or CO₂ (in different experiments) was passed through the solar-heated coal bed where it reacted with the coal and thus formed a combustible product gas that contained the energy content of both the coal and the sunlight. More than 40 per cent of the sunlight arriving at the focus external to the reactor was chemically stored as fuel value in the product gas. Since there were considerable solar losses because of the reflectivity of the window and the window aperture being smaller than the focal-spot size, it is estimated that in excess of 60 per cent of the solar energy that entered the reactor was chemically stored. The product-gas production rate increased with increased solar power, and when steam was used for gasification, the product-gas composition and thus heating value were almost independent of solar power. A typical moisture-free gas composition was 54 per cent H₂, 25 per cent CO, 16 per cent CO₂, 4 per cent CH₄ and 1 per cent higher hydrocarbons. Activated carbon and a uniform mixture of coal and biomass were also gasified with similar efficiencies but slightly different product-gas compositions. Coke showed a lower solar conversion efficiency. Solar gasification offers several advantages over conventional oxygen-blown gasifiers: (1) commercial grade oxygen is not required, (2) almost twice as much gas per ton of coal can be achieved because no coal is burned to provide process heat and because the gas contains energy from both coal and the sun, and (3) the system has very low thermal inertia and is insensitive to thermal shock, making it very adaptable to rapidly changing solar conditions such as passing clouds.     

Numerical investigation of thermal and electrical management during hydrogen reversible solid-state storage using a novel heat exchanger based on thermoelectric modules

In order to reduce the costs generated by the hydrogen solid storage tank's accessories such as the heat exchanger, this work was carried out. It shows thermal and electrical investigations of transient hydrogen (H₂) solid storage in a tank filled with porous medium (LaNi₅) to activate a potential PEM automotive fuel cell. For this purpose, we use a novel heat exchanger with a heat sink combined with thermoelectric modules (TEMs). We realize a simulation that helps us verify if thermoelectric exchanger will be an alternative to the conventional ones. The main results are that a thermoelectric cooler and heater with 127 couples of semiconductors coupled with 19 fins heat sink could be used during the reversible hydrogen solid storage. Also, results show that we can avoid the water freezing at negative temperatures when using a conventional heat exchanger by using TEM during hydrogen absorption. Finally, during the endothermic desorption of the hydrogen, TEG use can avoid boiling water used in the heating system. Also, the hydrogen tank will be lighter and compact without fins and water tubes.     

Theoretical estimations of the efficiency of thermophotovoltaic systems using high-intensity silicon solar cells

Efficiencies have been calculated for (a) heat-mirror filter and (b) rear-mirror thermophotovoltaic systems using actual materials values for source emissivity ((λ)), mirror-filter transmissivity (τ_M(λ)) and intensity-dependent cell parameters V_oc and FF. The ratios of heat-mirror and rear-mirror efficiencies to direct sunlight impingement efficiency are estimated to be 1.6 and 1.9, respectively (2500°K thermal source), if mirror and other extraneous absorption are taken to be zero in both cases. The more realistic inclusion, for the first case, of 2 per cent light absorption in the filter/mirror and, in the second case, of 2 per cent absorption of low energy light in the cell and 8 per cent absorption at the rear mirror, plus a 2 per cent thermal loss directly from the hot source (for both), reduces the advantage of both to about a factor of 1.2.     

An Experimental Study of Carbon Dioxide Condensation in Mini Channels

In this study, the local characteristics of pressure drop and heat transfer were investigated experimentally for carbon dioxide condensation in a multi-port extruded aluminum test section, which had 10 circular channels each with 1.31 mm inner diameter. The CO2 was cooled with cooling water flow inside the copper blocks that were attached at both sides of the test section. The temperatures at the outer surface of the test section were measured with 24 K-type thermocouples embedded in the upper and lower surfaces along the length. Local heat fluxes were measured with 12 heat flux sensors to estimate the local enthalpies, temperatures and heat transfer coefficients. Bulk mean temperatures of CO2 at the inlet and outlet of the test section were measured with 2 K-type thermocouples. The measurements were performed for the pressure ranged from 6.48 to 7.3 MPa, inlet temperature of CO2 from 21.63 to 31.33℃, heat flux from 1.10 to 8.12 kW/m2, mass velocity from 123.2 to 315.2 kg/m2s, and vapor quality from 0 to 1. The results indicate that pressure drop is very small along the test section, heat transfer coefficient in the two-phase region is higher than that in the single-phase, and mass velocity has important effect on condensation heat transfer characteristics. In addition, experimental data were compared with previous correlations and large discrepancies were observed.     

High temperature solar collector with optimal concentration: Non-focusing fresnel lens with secondary concentrator

A non-evacuated collector consisting of a linear Fresnel lens and a second stage concentrator of the CPC type is described and tested in detail. Use of a Fresnel lens accomplishes two different objectives simultaneously: it allows for the design of a nearly ideal light collector (of the CPC type) of high concentration and height-to-aperture ratio close to 1 and plays the role of a cover, making the collector less sensitive to the environment than one with exposed reflector surface. The geometric concentration is 15.56 and the acceptance half angle is 3°. The optical efficiency measured with an Active Cavity Radiometer (ACR) is 65.6 per cent and the efficiency at of 0.235 is 48 per cent (ΔT = T_avfluid − T_amb = 200°C, I_ACR = 850 W/m²). Heat loss measurements for double glazed configurations are reported and the resulting efficiency at of 0.3 is predicted to be 48 per cent. These numbers are expected to be raised by 3 percentage points for a next generation of lenses. The collector is mounted with its tracking axis oriented oriented NS since EW tracking axis orientation is impractical for a linear Fresnel lens, but its wide acceptance angle permits tracking by a simple clock mechanism at constant speed. Two different strategies are considered (i) polar mount, (ii) two adjustments of the tracking axis a year (summer and winter); the predicted yearly performance is calculated for four locations and four working fluid temperatures.The projected cost is estimated to be $70.00/m² (1976 dollars), possible because the construction of the collector lends itself to the use of inexpensive materials such as plastic and glass.     

Catalytic combustion of 1-butanol coupled with heat harvesting for compact power

A combustor paired with a heat-harvesting device, such as a thermoelectric or thermal photovoltaic device, can utilize high energy-dense liquid fuels while avoiding direct chemical-to-electrical conversion issues such as electrode and electrolyte poisoning. Therefore, the system is an attractive alternative to batteries and fuel cells for portable power applications. In the current study, a 1-butanol fed catalytic combustor using a Rh/Al₂O₃ catalyst was tested with a heat extractor, in this case being a stainless steel rod with a copper heat sink that was designed to thermally mimic a small thermoelectric module. The effects of residence time, fuel flow rate, and rod size on reactor/extractor temperatures and the energy balance were observed. Fuel-lean equivalence ratios were also studied and shown to have little effect on performance. Residence time does not have a direct effect; however, it does provide a catalytic stability limit for the fuel flow rate. The difference in the hot and cold side temperatures of the rod is dependent on the fuel flow rate and length of the rod. The greatest difference observed in these temperatures was 513 °C using the long-sized (15 cm) rod. The percentage of fuel energy conducted through the rod is only dependent on the rod size, with a maximum around 40% using the short rod. These results provide important design guidelines for the catalytic combustion of energy-dense liquid fuels as an excellent alternative heat source for either direct use or electrical power conversion.     

Treatment of molybdenite ore using a 2kW solar furnace

In the summer of 1977 several 10 g samples of molybdenite ore containing 5–6 per cent Mo as MoS₂ were heated in one of the 2kW solar furnaces at the Laboratoire des Ultra-Refractaires, Odeillo, Frnace. The end products showed excellent separation with pure yellow acicular crystals of 9⁺ per cent MoO₃ and pure white SiO₂ powder being the two major components. Impurity levels in the MoO₃ did not exceed 6000 ppm while only a small fraction of the molybdenite was entrained along with the SiO₂. All treatment was done in a flowing oxygen atmosphere, and other elements condensed out at specific sites in the gas transport system. Analysis of the products was done by X-ray diffraction and X-ray fluorescence, emission spectroscopy, and wet chemical analysis. The ore was donated by Climax Molybdenum Co., Climax Co. The work is a joint effort of LASL and the Laboratoire des Ultra-Refractaires in Odeillo.     

新型发动机排气温差发电器结构探索

根据汽车发动机排气可利用能量的形式,提出了一种新型的置于排气通道内的热电转换系统,使热电偶与热气流直接进行对流/辐射换热,将强化热流密度和转换电流密度、提高系统的温差。在使用现有热电材料的条件下,提高温差发电器的功率密度。     

Evaluation of efficiency factors and internal resistance of thermoelectric materials

It is well known that the figure of merit (ZT) is unreliable in calculating the efficiency (?) of micro thermoelectric generators system level and unrealistic when comparing the performance of thermoelectric (TE) materials in the same metric units. To solve this problem, we have used COMSOL multiphysics to design a single leg of micro thermoelectric generators model for computing efficiency factors (? ) and internal resistance using TE materials' constants, such as electrical conductivity (σ ), TE conductivity (K ), and Seebeck coefficient (α ). The TE materials were placed between two copper electrodes, and the first data analyzed were the voltages per meter and electric currents per meter. The internal resistances were calculated by taking the ration of voltages to electric currents, and at the same time, the electric powers were calculated from the products of electric currents and voltages yielding power per unit area in μW cm^?2. The ? were calculated using changes in power (ΔP ), temperature gradient (ΔT ), and the surface area (A ). The obtained results showed that the TE materials with highest ? when the temperatures are between 375 and 550 K are n‐type SiGe and p‐type SiGe. When the temperatures are between 550 and 780 K, the TE materials with the highest ? are PbTe‐Pbl₂, PbTe‐CdTe, and PbTe‐SrTe‐Na. We noted that the ? obtained from eight TE materials in this work are within the range as those reported in the literature between 0.001 and 0.091 μW cm^?2 K^?2. The TE materials with high internal resistances such as PbS, PbTe, and PbSe have ? that is <0.0001 μW cm^?2 K^?2, and those with low internal resistances have ? in the range between 0.002 and 0.0091 μW cm^?2 K^?2. This work has shown that COMSOL multiphysics is a powerful computational tool that can be used to analyze internal resistances and ? of TE materials in the same temperature ranges. Copyright © 2016 John Wiley & Sons, Ltd.