Gaosheng Wei

Thermal conductivity measurements on xonotlite-type calcium silicate by the transient hot-strip method

Abstract The experimental results of the thermal conductivities of xonotlite-type calcium silicate insulation materials were presented at different temperatures and pressures. Two appropriative surroundings, i.e. an elevated temperature surrounding from ambient temperature to 1450 K and a vacuum surrounding from atmosphere pressure to 10−3 Pa, were designed for the transient hot-strip (THS) method. The thermal conductivities of xonotlite-type calcium silicate with four densities from ambient temperature to 1000 K and 0.045 Pa to atmospheric pressure were measured. The results show that the thermal conductivity of xonotlite-type calcium silicate decreases apparently with the fall of density, and decreases apparently with the drop of pressure, and reaches the least value at about 100 Pa. The thermal conductivity of xonotlite-type calcium silicate increases almost linearly with T3, and increases more abundantly with low density than with high density. The thermal conductivity measurement uncertainty is estimated to be approximately 3% at ambient temperature, and 6% at 800 K.

Theoretical Study on Transient Hot-Strip Method by Numerical Analysis

This paper presented the effects of finite dimensions of the sample and nonzero heat capacity of the strip on thermal conductivity determination with the transient hot-strip method. Through the numerical analysis of the temperature field within the system composed of the samples and the strip, the temperature transients at the strip surface were obtained to calculate the thermal conductivities of materials, which were compared with the exact values. The effect of heat losses out of the external surfaces of the sample and the heat capacity of the strip on thermal conductivity determination were then analyzed comprehensively. It is shown that the sample finite dimensions have a great effect on thermal conductivity determination, especially on the materials with relatively higher thermal diffusivities, and the measured thermal conductivity is always lower than the exact value due to the lower convective heat transfer coefficient out of the external surfaces of the sample. The measurement error is estimated to be less than 2.1% for the material with thermal diffusivity less than 4.0 × 10 -6 m 2 /s with the sample dimensions of 120 × 60 mm 2 (width × thickness) and the fitting time interval of 20-300 s. The nonzero heat capacity of the strip has a great effect on thermal conductivity determinations of the materials with relatively lower thermal diffusivities. The measurement error is estimated to be less than 5% for the material with thermal diffusivity larger than 0.8 × 10 -7 m 2 /s with Cr20Ni80 alloy as the strip.

Thermal Radiation in Silica Aerogel and its Composite Insulation Materials

This paper engages in experimental measurements on thermal radiative transfer in silica aerogel and its composite insulation materials (xonotlite-aerogel composite and ceramic fibre-aerogel composite). The samples of silica aerogel, xonotlite-type calcium silicate, and ceramic fibre insulation materials are all considered as a semi-transparent medium capable of absorbing, emitting and scattering thermal radiation. The spectral transmittances are then measured at different infrared wavelengths ranging from 2.5 to 25μm with a Fourier transform infrared spectrometer (FTIR), and subsequently used to determine the specific spectral extinction coefficient and the specific Rossland mean extinction coefficient of the sample. The radiative conductivities deduced from the overall thermal conductivities measured with the transient hot-strip (THS) method are compared with the predictions from the diffusion approximation by using the measured spectral extinction coefficient. The results show that the spectral extinction coefficients of the samples are strongly dependent on the wavelength, particularly in the short wavelength regime (<10μm). The total Rossland mean extinction coefficients of the samples are all decreasing with the temperature increasing. The radiative conductivities are found almost proportional to the cubic temperature, and decreases as the sample density increases.Copyright

Recent advances in the PV-CSP hybrid solar power technology

Photovoltaic - Concentrated Solar Power (PV-CSP) hybrid technology is considered to be an important future research trend in solar energy engineering. The development of the PV-CSP hybrid technology accelerates in recent years with the rapid maturation of photovoltaics (PV) and concentrated solar power (CSP). This paper presents the recent advances on PV-CSP technology, including different technologies based on new dispatch strategies, Organic Rankine Cycles, spectral beam filters and so on. The research status and the hybrid system performance of the recent researches are summarized, aimed to provide an extended recognition on the PV-CSP hybrid technology. The advantages and limitations of the hybrid system are concluded according to the researches reviewed.Photovoltaic - Concentrated Solar Power (PV-CSP) hybrid technology is considered to be an important future research trend in solar energy engineering. The development of the PV-CSP hybrid technology accelerates in recent years with the rapid maturation of photovoltaics (PV) and concentrated solar power (CSP). This paper presents the recent advances on PV-CSP technology, including different technologies based on new dispatch strategies, Organic Rankine Cycles, spectral beam filters and so on. The research status and the hybrid system performance of the recent researches are summarized, aimed to provide an extended recognition on the PV-CSP hybrid technology. The advantages and limitations of the hybrid system are concluded according to the researches reviewed.

Parameter optimization of a hybrid solar concentrating photovoltaic/concentrating solar power (CPV/CSP) system

Parameter optimization of a hybrid CPV/CSP system is investigated using energy analysis with a steady-state physical model. Two kinds of solar radiation flux density distribution, the Gaussian distribution (GD) and the trapezoidal-shaped distribution (TSD), are employed in the analysis. Three influencing parameters including the average optical concentration ratio, the outlet temperature of R134a from the solar thermal receiver, and the saturation vapor temperature of R134a are taken into consideration. The results show that, increasing the average optical concentration ratio, decreasing the outlet temperature of R134a from the thermal receiver, and selecting appropriate saturation vapor temperature are the effective methods for increasing the power output of the proposed hybrid system.

Experimental investigations on radiative properties of two kinds of open-cell porous ceramic materials

Open-cell porous ceramic is an ideal volumetric heat absorbing material, and understanding in detail the thermal properties of the material, particularly its radiative properties, is of primary importance for the design and improvement of volumetric solar receivers. This work investigates the radiative properties of open-cell porous ceramic through experiment. Fourier transform infrared spectroscopy (FTIR) was applied to measure the spectral transmittance of open-cell porous ceramic samples with different porosities and cell densities in infrared wavelengths between 2.5 and 25 µm. The results were analyzed to determine the spectral extinction coefficient and Rosseland extinction coefficient, which showed that the radiative properties of porous ceramic are strongly dependent on its microstructure parameters, while the type of material has little influence. The spectral extinction coefficient and Rosseland extinction coefficient both increased with increasing cell density and decreasing porosity. Based on the experimental results, two empirical correlations related to the window diameter and porosity were proposed to predict the Rosseland extinction coefficient of open-cell porous ceramics. Finally, the radiative thermal conductivities of different porous ceramics were studied.Open-cell porous ceramic is an ideal volumetric heat absorbing material, and understanding in detail the thermal properties of the material, particularly its radiative properties, is of primary importance for the design and improvement of volumetric solar receivers. This work investigates the radiative properties of open-cell porous ceramic through experiment. Fourier transform infrared spectroscopy (FTIR) was applied to measure the spectral transmittance of open-cell porous ceramic samples with different porosities and cell densities in infrared wavelengths between 2.5 and 25 µm. The results were analyzed to determine the spectral extinction coefficient and Rosseland extinction coefficient, which showed that the radiative properties of porous ceramic are strongly dependent on its microstructure parameters, while the type of material has little influence. The spectral extinction coefficient and Rosseland extinction coefficient both increased with increasing cell density and decreasing porosity. Based on t...

Gaseous Conductivity Study on Silica Aerogel and Its Composite Insulation Materials

This paper presented theoretical and experimental study on gaseous conductivity of silica aerogel and it’s composite insulation materials. The samples of silica aerogel, xonotlite-type calcium silicate, xonotlite-aerogel composite and ceramic fibre-aerogel composite insulation materials were prepared firstly. The gaseous conductivities of the prepared samples were measured from 0.045 Pa to atmospheric pressure with the transient hot-strip (THS) method. The gaseous conductivity expressions based on the kinetic theory were then compared with the experimental results. It is shown that both the gaseous conductivity of xonotlite-type calcium silicate and silica aerogel decreases significantly with the drop of pressure. The gaseous conductivity of xonotlite-type calcium silicate reaches to zero at about 100 Pa and the gaseous conductivity of silica aerogel reaches to zero at about 104 Pa. The theoretical gaseous conductivity expressions match well with the experimental results of xonotlite-type calcium silicate and silica aerogel respectively, but do not match with the experimental results for the composite insulation materials. It indicates that the aerogel does not fill the two kinds of composite insulation materials entirely, and some micro level pores still exist in them.Copyright