Vacuum Insulation Panel Technology
Vacuum insulation panel technology has existed for many years, but only recently has it become commercially viable. This is mainly due to the development of lower cost, higher performing materials such as Vacupor®, Porextherm’s proprietary microporous material based core. Traditionally Vacuum Insulation Panels (VIPs) have been seen as a tool to increase energy efficiency in various applications. However, because of their performance and design flexibility, they provide many more valuable application opportunities. These include increased volume in commercial and domestic refrigerators, increased shipping times for temperature controlled transportation systems, and reduced package size and weight for insulated shipping containers. Even applications in the building and construction sector are becoming more and more important, due to rising requirements to fulfill energy standards. Major competitive advantages can therefore be achieved by incorporating panels in the design of your products and systems.
Porextherm® has a huge experience in serving customers with insulating material technologies and design capabilities.
What is vacuum insulation?
It has been known for a long time that the insulation values of some materials can be significantly improved by maintaining them in an evacuated environment. The choice of insulation materials in combination with the degree of vacuum applied to them, very much determine the final insulation values of a VIP. In order to understand where the extreme insulation values of a VIP are derived from, it is worth reviewing the mechanisms of heat transfer, first.
Conduction is mostly associated with heat transfer in solids. An example is when one end of a metal rod is heated, the heat is conducted to the other end. However, heat can also move through gases via conduction when the hotter and faster moving molecules collide with the colder, slower moving ones. Solid materials have intrinsic thermal conductivity properties dependent on their atomic structures. Metals, for example, are good conductors whereas glass and other silica based materials are poor conductors. Smaller gas molecules such as hydrogen are better conductors than larger ones such as oxygen or nitrogen.
Convective heat transfer is only found in fluids. It is based on the principle that as a fluid heats, it expands as its density is reduced. In the case of a gas such as air, this will cause the warm air to rise. A practical application is a hot air balloon.
Radiation is the transfer of energy by electromagnetic waves and is the mechanism by which the sun heats the earth. A body’s ability to both emit and absorb radiation is determined by its atomic structure.
Vacuum technology can be used to inhibit all three heat transfer mechanisms. The “ultimate” example of vacuum insulation is the Dewar's Flask, commonly known as a “Thermos bottle”. In a Dewar’s Flask the space between the dual walls of a cylinder is completely (99.999999%) evacuated. With virtually no molecules of gas available heat transfer by conduction and convection are almost eliminated and therefore thermal conductivities are extremely low: 0,005 W/(m · K) (R 30) or better.
Nevertheless, it is mechanically difficult to support such a pressure differential between the outside and inside of the flask. This certainly limits the structural configurations and the choice of materials for fabrication. Additionally, since even a few molecules of gas will destroy its insulation value, the cylinder walls must be absolutely impermeable to gas and moisture. Also, because radiation travels best through a vacuum where there is nothing to hinder its path, the wall materials are limited to either specially treated glass or metal. Both have a tendency to conduct considerable amounts of heat at areas where the walls are joined together.
Comparison of thermal conductivity values between Vacupor® and alternative insulation products
What are Vacuum Insulation Panels?
Vacuum insulation panels, or VIPs, consist of a filler material called a “core” that is encapsulated in a barrier film. The encapsulated system is then evacuated to a vacuum between 0.001 and 1 Torr (0.0013 and 1.3 mbar) and sealed thereafter. The core material serves three main purposes:
First, the core supports the panel walls. Atmospheric pressure exerts 14.5 psi (pound per square inch) of pressure on the evacuated panel. This means that 2,000 lbs force are acting on a one square foot panel.
Second, the core material also inhibits the movement of the remaining gas molecules. The smaller the core pore size, the more likely it is that the gas molecules will collide with the branched network of the filler material rather than each other. This essentially traps the molecules and any heat that is conducted to the solid core material is required to pass through a tortuous branch network where it is mostly dissipated. Porextherm’s Vacupor® core that is microporous material based has the smallest pore size and hence best insulating performance of any filler material.
Third, the core materials provide a barrier against heat transfer by radiation and often include special opacifying materials that scatter or absorb infrared radiation. A comparison between Vacupor® vacuum panels and conventional insulation materials shows their significant superior performance.