Andreas Beck

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.

Optimized finned absorber geometries for solar air heating collectors

Solar air heaters are limited in their thermal performance due to the low density, the small volumetric heat capacity and the small heat conductivity of air. For high solar gains, an efficient thermal coupling between absorber and fluid is required, while the electrical power for the fan operation ought to be as small as possible. As heat transfer augmentation usually increases friction losses, an optimum fin geometry has to be found. For a solar ventilation air preheater, mounted on a south facing facade in Wurzburg, Germany, optimized geometries have been derived. For a specific air mass flow rate of 70 kg/(m2 h) the air gap for smooth absorbers without fins should be about 7 to 8 mm to get a maximized yearly net energy output of about 680 MJ/m2 during the heating season from October through April. Finned absorbers perform much better. Continuous aluminum-fins, 0.1 mm thick and spaced about 6 mm apart in a 30-mm-wide air gap yield about 900 MJ/m2 yearly net output. Offset strip fins do not show an improved performance compared to optimally spaced continuous fins, due to the larger electrical power for this geometry. However, offset strip fins yield high net energy gains for large fin spacings.

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.

Thermal Transport in Straw Insulation

An evacuable guarded hot plate for thermal conductivity measurements between 200 and 800 C was used to investigate the heat transfer in barley straw. The different thermal transfer mechanisms (solid, gaseous conduction and infrared-radiative heat transfer) as well as coupling effects were separated. The measured thermal conductivities (λ = 0.041 W m−1 K−1) are similar to those of conventional insulation materials such as foams, glass or mineral fibres which are widely used as building insulation materials. Straw from barley or wheat, which is a low-cost, renewable material readily available world-wide, is therefore an interesting alternative to conventional insulation materials.

Making better use of natural light with a light-redirecting double-glazing system

The subject of the present paper is a fixed daylighting system known as the LIF light guide. The system consists of a fixed arrangement for capturing sunlight incident at various angles and re-directing it by means of an optical wave-guide which provides diffuse ceiling illumination. The light guide is generally sandwiched between two glass panes and mounted above the windows to avoid obstructing visibility. Its intended function is to furnish offices and other working areas with a uniform level of natural illumination while avoiding glare.

Thermochromic gels for control of insolation

Abstract Thermochromic gels consist of a mixture of water, gelling agent, and a polyether reaction compound. They show a drastic increase of scattering when a characteristic switching temperature is surpassed. The hemispherical transmission consequently decreases from about 90 to 50% for a 1-mm-thick layer sandwiched between two glass panes. The increase in scattering is caused by a dramatic increase in number density and particle size of created scattering centers. The latter consists of agglomerated polyether chains with reduced water content, i.e., increased index of refraction. Our measurements cover the directional-directional transmission, as well as the directional-hemispherical transmission and reflection, using a double-beam spectrophotometer with an integrating sphere. For structural information a light-scattering apparatus was employed. Multiflux calculations allow predictions of the switching behaviour for nonvertical incidence and for arbitrarily thick layers. The thermochromic material is a low-cost, nontoxic product. The achieved switching action is reliable; however, it may need improvement to allow application in solar architecture.

Optical properties of monolithic and granular SiO2-aerogels

The optical and thermal properties of aerogels play an important role for their application in transparent insulation (TI) or daylighting systems. In this paper the directional-directional transmission tdd as well as the directional-hemispherical transmission tdh are presented for 20 mm thick aerogel specimens. For optimized monolithic aerogels we obtained solar averaged values of -tdh,sol equals (91 +/- 1/4)% and -tdh,vis equals (84 +/- 2)% in the solar and visible spectral region, respectively. The investigated granular aerogels show a visual transmission of -tdh,vis equals (58 +/- 4)% for waterglass as a precursor and of -tdh,vis equals (78 +/- 4)% for tetramethoxysilane. In order to correlate the structural build-up of the SiO2-network of the aerogels with their light scattering properties angular-dependent light and X-ray scattering measurements are performed. To characterize the visual image transmission the modulation transfer function (MTF) was measured for 20 mm thick monolithic layers. In order to explain the measured data a Monte Carlo based simulation model was developed. The analysis yields that the MTF is mainly effected by surface scattering.

Pearl luster pigments as overheating protection in transparently insulated solar façades

Abstract Transparently insulated (TI) walls are an efficient means to reduce the heating demand of buildings. However, large scale use of TI is still not realized, basically due to overheating problems. Mechanical shading devices are expensive and may malfunction. Furthermore, the lack of acceptance from architects because of the missing color-designability remains an unsolved problem. A promising solution for both mentioned points is a coating of pearl luster pigments on the TI wall. These pigments show an angle-dependent transmittance and reflectance and their color is variable. The pigments are structured similarly to natural pearls, where the color is caused by an interference effect of multiple layers with alternating refractive indices. A model for radiation transfer in the pigments has been derived and verified experimentally. The model can be used to design pigments with tailor-made properties. Based on insolation data from the test reference year for Wuerzburg, Germany (TRY05), the ideal cut-off angle which depends on the latitude has been calculated for a middle-European location, considering direct and diffuse solar radiation. This allows to derive the energy savings potential of TI walls covered with optimized pearl luster pigments. Finally, a 1.6×2.4 m 2 glass cover, equipped with selected pigments, was installed at the ZAE test facility in Wuerzburg. These experiments show that the heat flux into the room in summer is reduced drastically and overheating can be prevented.

Preparation and characterization of SiO2 two-step aerogels: Code: HP15

Aerogels are well suited as transparent insulation materials in solar architecture and collector systems. Their nanoporous structure provides a high solar transmittance and a low thermal conductivity, generally below 0.02 W m−1 K−1. Transparent aerogels with densities above 80 kgm−3 can easily be prepared at room temperature via a one-step sol-gel process with subsequent supercritical drying. Separating hydrolysis and condensation via a two-step method allows the preparation of transparent ultra-low density SiO2-aerogels. To optimize the optical properties, characterized by the scattering coefficient of the gels, we have investigated the influence of preparation parameters, such as pH-value of the sol-gel starting solution and macroscopic density, on the gel structure. To determine the nanostructure we performed spectral light scattering as well as small angle X-ray scattering (SAXS) measurements.