Hans-Peter Ebert

Permeation of Different Gases Through Foils used as Envelopes for Vacuum Insulation Panels

Vacuum insulation panels (VIPs) are distinguished by their outstandingly low thermal conductivity. In the evacuated state, the VIPs being examined in this study (which have fumed silica as a core material) have a thermal conductivity of 4 10 3 W/(m K). Gases (N2, O2, H2O,...), which penetrate the foil cover cause an increase in pressure and water content and hence, an increase in the thermal conductivity. To determine these increases, VIPs have been manufactured with laminated aluminum foils (AlF) and aluminum-coated multilayer foils (MFs). The pressure and mass increases are determined at various temperatures, humidity, and with various panel formats. Large differences in the rates of pressure increases (1 -70 mbar/yr) and in the rates of mass increases (0.02-4 mass%/yr) are recorded, depending on the foil type, climatic conditions, and panel formats. From these measurements, the air and vapor transmission rates of the foil covers and their dependence on temperature, relative humidity, and panel size are derived. Using these gas transmission rates, it is possible to estimate which pressure increases are to be expected for panel formats and climatic conditions occurring in building applications. With laminated Al foils and selected Al-coated multilayer foils, rates of pressure increases below 1-2 mbar/yr are achieved. The rates of mass increase for typical climatic conditions for laminated Al foils are significantly below 0.1 mass%/yr, while with Al-coated multilayer foils, depending on the foil quality, mass increases per time of up to 1 mass%/yr are recorded. Increases in gas pressure per time of 1 -2 mbar/yr lead to relatively small increases in thermal conductivity, allowing applications in the construction sector, where service lives of several decades are required. With respect to the humidity-related increase in thermal conductivity, one has to know the climatic conditions, which have a strong influence on the increase in mass, and, above all, the precise dependence of the thermal conductivity on the humidity in the VIP.

Thermal Bridges in Vacuum-insulated Building Façades

In architecture, the outstandingly low thermal conductivity of vacuum insulation panels (VIPs) of 4 103 W/(m K) allows to realize thin thermal insulation layers. Typical U-values are 0.2 W/(m2 K) for a 2 cm-thick VIP. On the other hand, with vacuum-insulated faç ades the relative effect of thermal bridges is much stronger than that for conventionally insulated buildings. In this work, different thermal bridges are investigated. Especially with VIPs with laminated Al foils (here the aluminum foil is 8 mm thick and laminated on both sides with plastic foils of 15 mm PET and 50 mm PE), strong thermal bridges around the perimeter of the VIPs occur. Also the mounting system can have a strong negative effect on the thermal performance of VIP-insulated walls. As our calculations show, the effect of the thermal bridge depends strongly on the thermal contact of the VIPs with the wall. Therefore, it is necessary to optimize every vacuum-insulated construction in order to make the best use of the low thermal conductivity of VIPs. As an example, we describe how VIPs were effectively integrated into a renovated gable faç ade and into a new ultra-low energy timber building.

Dependence of Thermal Conductivity on Water Content in Vacuum Insulation Panels with Fumed Silica Kernels

The influence of moisture in vacuum insulation panels (VIPs), with fumed silica kernels, on their thermal conductivity has been investigated. The VIPs are produced with different water contents. The thermal conductivities at different water contents are measured under stationary conditions in a hot-plate apparatus with an average temperature of 10°C (plate temperatures are 0 and 20°C). The increase in thermal conductivity is approximately proportional to the water content. The increase is ≈0.5 × 10 -3 W/(m K) per mass% of water. For typical middle European climate, a maximum moisture content of ≈6 mass% can be expected, which corresponds to a maximum increase of thermal conductivity of ≈3 × 10 -3 W/(m K) for VIPs with fumed silica kernels.

Prediction of Service Life for Vacuum Insulation Panels with Fumed Silica Kernel and Foil Cover

For vacuum insulation panels (VIPs) with fumed silica kernels and foils as cover, a calculation model is developed to predict the service life. It is defined as the period during which the thermal conductivity of the VIP has risen 50% due to infusion of air and moisture. Two panel sizes, 50 ×50 × 1 cm3 and 100 × 100 × 2 cm3 are considered. For VIPs with laminated aluminum foils, calculated service lives of many decades are determined. For VIPs with aluminum-coated multilayer foils, shorter service lives still above 20 are calculated. This is due to the higher water vapor transmission through the Al-coated multilayer foils (compared to laminated Al foil) and the humidity-related increase in thermal conductivity. Overall, our model predicts service lives, which are large enough for applications of VIPs in buildings. An open question that remains is the long-term stability of the foil cover.

Predictions for the Increase in Pressure and Water Content of Vacuum Insulation Panels (VIPs) Integrated into Building Constructions using Model Calculations

The climatic conditions (temperature, relative humidity, and water vapor pressure) on both sides of vacuum insulation panels (VIPs) that were integrated into different building constructions are measured every hour. The influence of these conditions on the increase in air pressure and water content within the VIPs is estimated using a calculation model. The results of these model calculations are correlated with the pressure and mass measurements on VIPs, exposed to actual climate but removed for laboratory measurements. First, we find that upon use of the temperature-dependent air permeation rates for VIPs, the linear increase within the VIPs can be predicted reliably. Thus, it is sufficient to use annual average temperatures for these estimates. Second, the mass increase of VIPs due to infusion of water vapor through the barrier foil can be determined using the calculation model. The ‘driving’ force in this case is the difference in vapor pressure across the foil cover, which decreases with time, once the water vapor pressure within the VIP starts increasing. In effect, the water vapor pressure and the water content within the VIPs reach equilibrium. Depending on the climatic conditions, the maximum water content between 3 and 7 m% can be predicted.

Impact of thermal coupling effects on the effective thermal conductivity of aerogels

Nanoporous aerogels are excellent thermal insulation materials with thermal conductivities down to about 0.012 W m−1 K−1 at ambient conditions. So far, it was assumed that the total thermal conductivity of aerogels can be described by a simple superposition of the different individual heat transport contributions. However, recent investigations reveal that thermal coupling effects can result in a gas pressure dependent contribution that may be up to three times higher than expected from just a gas phase thermal conductivity, which is predicted by the Knudsen equation at given porosity and pore size. In this study, we use data from previous publications covering a gas pressure range from 10−5 to 10 MPa and analyze systematically the impact of pore size as well as solid phase and gas phase thermal conductivity on the coupling effect. The goal is to evaluate the data with respect to practical implications for aerogels in general. This means using the gas pressure dependence of the thermal conductivity of aerogels to determine their average pore size as well as allowing for a targeted optimization of aerogel-based insulations for applications at given gas pressure and temperature.Graphical Abstract

Entwicklung und Test eines langwelligen Strahlungsthermometers zur berührungslosen Temperaturmessung in Gasturbinen während des Betriebs

Zusammenfassung Das Ziel dieser Arbeit bestand in der Entwicklung eines langwelligen Strahlungsthermometers zur berührungslosen Messung von Oberflächentemperaturen in stationären Gasturbinen während des Betriebs der Turbinen innerhalb des EU-geförderten Projektes „Sensors Towards Advanced Monitoring and Control of Gas Turbine Engines (Acronym STARGATE)“. Im Rahmen der Arbeit wurden die infrarot-optischen Eigenschaften der Wärmedämmschichten und der vorhandenen Brenngase am ZAE Bayern bei hohen Temperaturen bis 1600 K und Drücken bis 13 bar bestimmt. Mit Hilfe dieser experimentellen Charakterisierungen konnte ein geeigneter Spektralbereich um 10 μm für das langwellige Strahlungsthermometer identifiziert werden. Entsprechend dieser Erkenntnisse wurde zunächst ein Laboraufbau mit geeigneten optischen Bauteilen (Filter, IR-Wellenleiter, etc.) realisiert und verifiziert. Anschließend wurde ein Prototyp für Messungen in Gasturbinen während des Betriebs der Turbinen entwickelt und in einem Turbinenteststand der Firma Siemens AG in Berlin erfolgreich getestet. Abschließend wurde eine Unsicherheitsanalyse durchgeführt, die eine erweiterte Messunsicherheit der gemessenen Temperaturen von etwa ± 30 K ergab.