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Developments in semiconductor thermoelectric materials
Laifeng LI, Zhen CHEN, Min ZHOU, Rongjin HUANG
Frontiers in Energy. 2011, 5 (2): 125-136.
https://doi.org/10.1007/s11708-011-0150-1
A surge in interest in developing alternative renewable energy technologies has been observed in recent years. In particular, thermoelectrics has drawn attention because thermoelectric effects enable direct conversion between thermal and electrical energy, and provide power generation and refrigeration alternatives. During the past decade, the performance of thermoelectric materials has been considerably improved; however, many challenges continue to exist. Developing thermoelectric materials with superior performance means tailoring interconnected thermoelectric physical parameters-electrical conductivities, Seebeck coefficients, and thermal conductivities for a crystalline system. The objectives of this paper are to introduce the recent developments in semiconductor thermoelectric materials, and briefly summarize the applications of such materials.
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Nongray radiation from gas and soot mixtures in planar plates based on statistical narrow-band spectral model
Huaqiang CHU, Qiang CHENG, Huaichun ZHOU, Fengshan LIU
Frontiers of Energy and Power Engineering in China. 2011, 5 (2): 149-158.
https://doi.org/10.1007/s11708-010-0124-8
The nongray behavior of combustion products plays an important role in various areas of engineering. Based on the statistical narrow-band (SNB) spectral model with an exponential-tailed inverse intensity distribution and the ray-tracing method, a comprehensive investigation of the influence of soot on nongray radiation from mixtures containing H2O/N2+soot, CO2/N2+soot, or H2O/CO2/N2+soot was conducted in this paper. In combustion applications, radiation transfer is significantly enhanced by soot due to its spectrally continuous emission. The effect of soot volume fraction up to 1×10-6 on the source term, the narrow-band radiation intensities along a line-of-sight, and the net wall heat fluxes were investigated for a wide range of temperature. The effect of soot was significant and became increasingly drastic with the increase of soot loading.
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Permeability and thermal conductivity of host compressed natural graphite for consolidated activated carbon adsorbent
Bo TIAN, Liwei WANG, Zhequan JIN, Ruzhu WANG
Frontiers in Energy. 2011, 5 (2): 159-165.
https://doi.org/10.1007/s11708-011-0145-y
Permeability and thermal conductivity test units were set up to study the heat and mass transfer performance of the host material, i.e. expanded natural graphite (ENG), for consolidated activated carbon (AC) adsorbent. The permeability was tested with nitrogen as the gas source, and the thermal conductivity was studied using steady-state heat source method. The results showed that the values of permeability and thermal conductivity were 10-15 to 10-12 m2 and 1.7 to 3.2 W/(m·K), respectively, while the density compressed expanded natural graphite (CENG) varied from 100 to 500 kg/m3. The permeability decreased with the increasing density of CENG, whereas the thermal conductivity increased with the increasing density of CENG. Then the thermal conductivity and permeability of granular AC were researched. It was discovered that the thermal conductivity of samples with different grain size almost kept constant at 0.36 W/(m·K) while the density was approximately 600 kg/m3. This means that the thermal conductivity was not related to the grain size of AC. The thermal conductivity of CENG was improved by 5 to 10 times compared with that of granular AC. Such a result showed that CENG was a promising host material for AC to improve the heat transfer performance, while the mass transfer performance should be considered in different conditions for utilization of adsorbent.
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Heat transfer of phase change materials (PCMs) in porous materials
C Y ZHAO, D ZHOU, Z G WU
Frontiers in Energy. 2011, 5 (2): 174-180.
https://doi.org/10.1007/s11708-011-0140-3
In this paper, the feasibility of using metal foams to enhance the heat transfer capability of phase change materials (PCMs) in low- and high-temperature thermal energy storage systems was assessed. Heat transfer in solid/liquid phase change of porous materials (metal foams and expanded graphite) at low and high temperatures was investigated. Organic commercial paraffin wax and inorganic calcium chloride hydrate were employed as the low-temperature materials, whereas sodium nitrate was used as the high-temperature material in the experiment. Heat transfer characteristics of these PCMs embedded with open-cell metal foams were studied. Composites of paraffin and expanded graphite with a graphite mass ratio of 3%, 6%, and 9% were developed. The heat transfer performances of these composites were tested and compared with metal foams. The results indicate that metal foams have better heat transfer performance due to their continuous inter-connected structures than expanded graphite. However, porous materials can suppress the effects of natural convection in liquid zone, particularly for PCMs with low viscosities, thereby leading to different heat transfer performances at different regimes (solid, solid/liquid, and liquid regions). This implies that porous materials do not always enhance heat transfer in every regime.
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