Influence of Water on the Total Heat Transfer in ‘Evacuated’ Insulations

Abstract

The impact of water vapor on the thermal conductivity of vacuum insulation panels (VIPs) filled with fumed silica kernels was investigated and found to be surprisingly large. Besides thermal transport by water vapor within the pore space, the question was whether water in the adsorbed phase also contributed significantly to the total heat transfer. In order to quantify these effects, several series of thermal conductivity measurements were performed in an evacuable guarded-hotplate. The fumed silica specimens were first investigated in the dry state. Then water vapor was added into the vacuum system in different quantities (valves closed and no pumping), yielding water content in the VIP-kernel of up to 4% by mass. Most of the water is adsorbed in the specimens; smaller amounts are to be found in the gas phase. The parameter varied was the temperature; accordingly, the partial pressure of water vapor changed. An empirical equation is presented that describes the influence of water vapor and adsorbed water on the total heat transfer.

Vacuum insulation panels for building applications: A review and beyond

“…(9) comes out in the range of 0.0021-0.0034 W/(mK) and that the radiative heat conductivity can be found between 0.001 and 0.004 W/(m K) at a low gas pressure of 1 mbar, depending on the temperature. Considering the application temperatures for building insulation, a linear approach can be made for the temperature dependency of the thermal conductivity [17] taking into account the effect on the solid conduction and the effect on radiation:…”

“…However, the total effect of moisture on the thermal conductivity of a vacuum insulation panel is much more complex than the mentioned linear relationship. Complex non-linear relations between thermal conductivity, relative humidity, water vapour pressure and temperature determine the effect of the moisture content on the overall thermal conductivity of VIPs [18,20,10,21]. @l c @u % 0:05 W=ðmKÞ (13) The total water content in a VIP will not be distributed uniformly in the VIP [18,20,22]: The partial water vapour pressure p wv will vary with the temperature gradient, conform the different p wv,sat for the corresponding temperatures.…”

“…Complex non-linear relations between thermal conductivity, relative humidity, water vapour pressure and temperature determine the effect of the moisture content on the overall thermal conductivity of VIPs [18,20,10,21]. @l c @u % 0:05 W=ðmKÞ (13) The total water content in a VIP will not be distributed uniformly in the VIP [18,20,22]: The partial water vapour pressure p wv will vary with the temperature gradient, conform the different p wv,sat for the corresponding temperatures. This distribution is non-linear but can be approximated as linear for VIPs within a low temperature spread DT < 20 K. The spreading of the moisture is shown in Fig.…”

“…6. Moisture distribution based on numerical simulations in a VIP with a thickness of 20 mm and a total water content of 0.03 kg/kg (redrawn from [20]). Note that the graph should be corrected including gravity.…”

“…(13) and (19). Later on, the dependency has been investigated in detail [50,17,19]. The conclusions on the pressure and moisture dependency of the thermal conductivity can be summarized by Fig.…”