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Research Papers: Conduction

# Heat Transport in Evacuated Perlite Powders for Super-Insulated Long-Term Storages up to 300 °C

[+] Author and Article Information
Thomas Beikircher

e-mail: beikircher@muc.zae-bayern.de

Matthias Demharter

Bavarian Center for Applied Energy Research (ZAE Bayern),
Division 1: Technology for Energy Systems and Renewable Energy,
Walther-Meißner-Straße 6,
85748 Garching, Germany

Contributed by the Heat Transfer Division of ASME for publication in the JOURNAL OF HEAT TRANSFER. Manuscript received March 30, 2012; final manuscript received January 6, 2013; published online April 11, 2013. Assoc. Editor: Jose L. Lage.

J. Heat Transfer 135(5), 051301 (Apr 11, 2013) (11 pages) Paper No: HT-12-1141; doi: 10.1115/1.4023351 History: Received March 30, 2012; Revised January 06, 2013

## Abstract

Vacuum super insulation (VSI) with expanded perlite powder is commonly used at cryogenic temperatures, but principally can also be adapted to applications at higher temperatures, such as the long-term storage of hot water in solar thermal systems. Due to the lack of experimental data in the respective temperature range, especially without external load, thermal conductivity measurements have been performed with commercial perlite powder up to $150°C$ mean sample temperature, corresponding to storage temperatures of around $300°C$. Two different experimental geometries have been used: a guarded hot plate (GHP) setup and a cut-off concentric cylinder (CCC) apparatus. Furthermore, the radiative heat transport has been determined separately by extinction measurements using Fourier transform infrared (FTIR) spectroscopy. In addition to the laboratory experiments, a real-size prototype of a solar VSI-storage tank with 16.4 m3 water storage volume has been constructed, and the effective thermal conductivity of the perlite insulation has been determined from a heat loss measurement. The heat transport in evacuated perlite has also been treated theoretically using common models and approaches for gas heat conduction, solid-body conduction and heat transfer by thermal radiation. For the coupling between solid-body and gas conduction which occurs in the intergranular spaces of a powder material, a simple model has been developed. The total effective thermal conductivity $λeff$ of a vacuum super insulation with dry, evacuated perlite powder ($p≤0.01 mbar,ρ≈60 kg/m3$) amounts to 0.007–0.016 W/mK for mean sample temperatures between $50°C$ and $150°C$, compared to 0.003–0.005 W/mK at cryogenic temperatures. For the real-size storage prototype, the value $λeff=0.009 W/mK$ has been obtained at $T=90°C$ (storage temperature), p = 0.08 mbar and $ρ=92.4 kg/m3$, which compares to 0.03–0.06 W/mK for dry conventional storage insulations. With the applied theoretical models and approaches, the effective thermal conductivity of evacuated perlite and its individual contributions can successfully be described at different densities ($55-95 kg/m3$), compression methods, vacuum pressures ($10-3-1000 mbar$) and filling gases (air, Ar, Kr) up to mean sample temperatures of $T=150°C$. With regard to practical purposes, it has shown that vacuum super insulation with perlite is a suitable and economic method to achieve low thermal conductivities also at medium storage temperatures.

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## Figures

Fig. 1

SEM (top) and optical microscope (bottom) pictures of Technoperl®- C 1,5, made at KIT [5] (top) and ZAE Bayern (bottom)

Fig. 2

Schematic illustration of solid-body conduction in porous powder materials (a) and the coupling effect (b), where the structural thermal resistances are shorted by gas molecules, and additional pathways occur. The indicated pore and grain dimensions do not represent realistic values for perlite.

Fig. 3

Schematic layout of the cut-off concentric cylinder apparatus with vacuum pump (1), pump valve (2), valve for gas inflation (3), gas reservoir for air, argon or krypton (4), safety valve (5), pressure sensor for 1 mbar ≤p≤ 100 mbar (6), pressure sensor for 0.01 mbar ≤p≤ 1 mbar (7), three-zone cartridge heater (8), perlite sample (9), centering facilities (10), tube with vacuum flange on both edges (11), thermal insulation (12), and electrical power supplies (13). The figure also indicates the positions of the temperature sensors T1 to T10.

Fig. 4

Photographs of the cut-off cylinder apparatus: overview of the device with vacuum pump, valves, tube with thermal insulation and pressure sensors (a), gas inflation with argon bottle and pressure reducing valve (b), and mounting of the heating element (c)

Fig. 5

Photograph of the real-size prototype storage tank

Fig. 6

Effective spectral mass-specific extinction coefficient of Technoperl®- C 1,5 at wavelengths between 1.4 and 18μm

Fig. 7

Total effective mass-specific extinction coefficient of Technoperl®- C 1,5 as a function of radiation temperature between 300 and 450 K. The uncertainty of the measurement is Δe* = 0.004m2/g.

Fig. 8

Solid thermal conductivity of Technoperl®- C 1,5 as a function of density

Fig. 9

Sum of gaseous conduction λg and coupling effect λc as a function of pressure for air, argon and krypton, measured in the cut-off concentric cylinder apparatus at ρ = 68.2kg/m2 and T = 64°C

Fig. 10

Calculated sum of radiative and solid thermal conductivity as a function of density

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