Vacuum insulation board
Vacuum insulation panels (also called Vacuum Insulated Panels or Vacuum Insulation Panels, VIP for short) are highly efficient thermal insulation materials that use the principle of vacuum thermal insulation . They generally consist of a porous core material, which, among other things as a support body for the present in the vacuum insulating vacuum is used, and a high-density shell which prevents gas entry into the insulation board. With vacuum insulation panels, thermal conductivities of less than 0.004 W · m −1 · K −1 can be achieved; a vacuum insulation panel with a thickness of 2 cm can replace a Styrofoam panel with a thickness of 20 cm.
construction
Vacuum insulation panels consist of an open-pored support core, a high-density envelope system and, if necessary, a material that serves as a dryer or getter to bind gas molecules. The different composition of these components influences the thermal conductivity and the service life of the vacuum insulation panel.
Support core
The support core of a vacuum insulation board must meet various requirements. On the one hand, it must be able to withstand the air pressure on the surface of the vacuum insulation board (approx. 100,000 N · m −2 ). On the other hand, it must be evacuable, i.e. made of an open-pored material. The larger the pores of the material, the higher the requirements for the applied vacuum in order to achieve the lowest possible thermal conductivity. The requirements for the applied vacuum are characterized by the half-value pressure, that is the pressure at which the thermal conductivity of the air in the material is exactly half the thermal conductivity of static air (0.026 W m −1 K −1 ). A support core is more suitable for vacuum insulation panels, the higher its compressive strength, the higher the half-life pressure it may be, and the lower the heat transfer through solid-state heat conduction and thermal radiation through it. Different classes of material are suitable as a supporting core for the production of vacuum insulation panels:
| Material class | Half-life pressure in mbar | Typical thermal conductivity in W m −1 K −1 (in a vacuum) |
|---|---|---|
| Open-cell plastic foams | 0.5 | 0.008 |
| Microfiber materials | 1 | 0.003 |
| Fumed silicas | 600 | 0.004 |
| Perlite | 2 | 0.006 |
The specified thermal conductivities are typical values. With suitable configurations, values of 0.0035 W · m −1 · K −1 can be achieved using pyrogenic silicas, for example . When using glass fibers as the core material, values of less than 0.0025 W · m −1 · K −1 can be achieved. All values here relate to measured values after manufacture without aging effects. Glass fiber cores and plastic foams in particular show faster aging compared to pyrogenic silica. The increase in pressure inside the core leads to poorer insulation properties much more quickly.
Other designs
Instead of a porous, rigid foam- like support core or a granulate filling , a chamber structure , for example a honeycomb- shaped part, can also be embedded as the support core .
Shell
The service life of a vacuum insulation board depends crucially on the quality of its covering. The more gas particles (atoms, molecules) diffuse through the envelope, the faster the pressure rises in the vacuum insulation panel, which worsens the insulation properties. In particular, the shell must also prevent water vapor from diffusing in, since this also makes a contribution to heat transfer and also condenses out when the saturation vapor pressure is exceeded. In addition to the low gas and water vapor permeability, the shell must not be too large a thermal bridge . When using aluminum composite foils with a thickness of a few micrometers for the aluminum layer, the heat losses via the foil at the edge of the panel are already of the same order of magnitude as the heat flow through the vacuum insulation panel. As a standard, therefore, metallized plastic films are used as the covering material. These are vapor-deposited with several layers of a few nanometers of aluminum (approx. 20-100 nm). The thin aluminum layer creates so-called pin holes and nanogaps. This means that several layers (usually three) are used, as there is little chance that the nanogaps and pinholes will appear in the same place. The highest quality of these metallized foils allow a service life of several decades for support cores made of pyrogenic silica. If the shell of a vacuum insulation panel is damaged, the vacuum breaks down and the thermal conductivity of the panel increases dramatically, making it practically unusable.
Dryer and getter
In the case of vacuum insulation panels made of microfibers or plastic foams, the service life can be significantly extended if a dryer or getter is integrated in the vacuum insulation panel. Dryers bind water vapor that passes through the shell, getters bind the gas molecules contained in the air, primarily nitrogen and oxygen , chemically, but not the noble gas atoms , especially argon . Barium and lithium are used to bind gases such as nitrogen, oxygen or carbon dioxide. Calcium oxides, barium oxides and cobalt oxides are used as drying agents. Getters and desiccants are usually housed in so-called containers.
The type and amount of getter material or drying agent must be carefully matched to the amount of gas molecules to be absorbed. The following aspects are important:
- Core material
- Wrapping foil
- Panel dimensions
- required service life
Opacifiers
In order to reduce the heat transport through infrared radiation, so-called opacifiers can be introduced into the core. Opacifiers absorb and reflect the infrared radiation, whereby the value of the radiation transport can be reduced to below 0.001 W / (mK). Common substances for opacifiers are carbon black, iron oxide, titanium oxide and silicon carbide.
Areas of application
Vacuum insulation panels are used wherever there is little space available, but good thermal insulation is still required. The vacuum insulation board must be integrated into the application in such a way that the shell of the panel is protected in order to avoid a breakdown of the vacuum and thus a drastic increase in thermal conductivity. When used in buildings, it must be ensured that the panel has a corresponding building authority approval. Although non-combustible core materials are available, the combustible shell must also comply with fire protection requirements.
Areas of application are:
- Cool boxes or containers , mostly in conjunction with a latent heat storage system
- Refrigerators and freezers
- Boiler and other liquid storage
- Cold stores
- Building insulation for ultra- energy saving houses
- Old building renovation because of the possible preservation of monumental elements
- Building insulation when there is no space for conventional insulation
The price of a vacuum insulation panel is currently far higher than the price of conventional thermal insulation with comparable heat transfer.
quality control
The thermal conductivity of a vacuum insulation panel depends crucially on the internal pressure of the panel. In order to be able to guarantee a certain maximum thermal conductivity, it must be possible to control it. In critical applications where a sharp increase in thermal conductivity has serious consequences (e.g. the temperature-controlled shipping of biopharmaceuticals ), it may be necessary to test the vacuum insulation panels of a transport container before each use. An initial visual inspection of the panel can already be meaningful. Damaged or poor quality vacuum insulation panels can be recognized by a loosely attached covering film. Intact vacuum insulation panels have a tight-fitting film.
The gas pressure in the insulating element can be monitored using various measurement methods: The application of an increasing negative pressure from the outside by means of a small suction bell has a very direct mechanical effect: From the moment when it falls below the internal pressure, the envelope membrane of the insulating element is curved outwards, which is Shadowing of a surface-parallel laser beam can be detected. Another method works with the introduction of a heat pulse into a metal plate inside the vacuum insulation panel, which serves as a pressure-dependent heat sink. For the third method, a pipe with a steel ball must be built into the insulating element: The ball is made to rotate magnetically, the delay is an indicator of the gas pressure inside.
See also
Individual evidence
- ↑ Treml, Sebastian, Engelhardt, Max, Simon, Holger, Kagerer, Florian, Fraunhofer IRB-Verlag: Vacuum insulation panels (VIP) in building applications: from insulation material to insulation system. Processing, fastening, durability. Stuttgart 2018, ISBN 3-7388-0023-9 .
- ↑ Chamber structure instead of porous core: see e.g. B. European patent EP2480407
- ↑ F. Pacheco Torgal, Cinzia Buratti, Siva Kalaiselvam, Claes-Göran Granqvist, Volodymyr Ivanov: Nano and biotech based materials for energy building efficiency . Springer International Publishing, 2016, ISBN 978-3-319-27503-1 , pp. 167-214 .