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Heat and Humidity Resistant Vacuum Insulation Panel Envelopes

Keeping things cool is expensive. It is estimated that the energy needed to cool large refrigerated shipping containers accounts for over 66% of their total cost of ownership. Refrigerated trucks can use up to one-half gallon of fuel per hour just keeping the cargo cold, and 91% of temperature controlled freight is transported by truck in the United States. An important method for minimizing these costs is effectively insulating spaces requiring controlled temperatures.

Nearly 40% of total U.S. energy consumption in 2012 was consumed in residential and commercial buildings, or about 40 quadrillion British thermal units. EIA has estimates for total monthly and annual energy consumption by the residential and commercial sectors, which is nearly all "building-related."  Those sectors account for nearly all the U.S. building-related energy consumption. (eia.gov). 

Insulation works by inhibiting heat transfer from a warm region, known as the heat source, through the walls of a space into a cooler region, referred to as the heat sink. In the case of refrigerated materials, the refrigerated space is the heat sink, and the environment external to this space is the heat source.

Insulation inhibits the flow of heat between source and sink by providing a barrier between them that has a low thermal conductivity. This low heat transfer capability is caused largely by the presence of pockets of stagnant air constructed into the insulation. Air, and gases in general, are very poor conductors of heat. Also, since the air in these pockets is essentially stagnant, convection heat transfer is largely eliminated. The other materials in the insulation are chosen such that they are poor conductors and radiators of heat to further reduce the rate of heat transfer.

Inspired initially by research in the 1930s into the extremely lightweight, porous solids known as aerogels, a new type of insulation is beginning to make its mark in the energy products market. This insulation is known as a vacuum insulation panel or VIP. The general principle behind a VIP is the same as for standard insulation: provide a low conductivity barrier containing voids between the heat source and heat sink. However, in a VIP, the voids are truly voids. The core of a VIP has been evacuated to create a vacuum. Since these voids are virtually empty, both conduction and convection heat transfer across the voids are eliminated. The remaining mode of heat transfer, radiation, can be minimized by using multilayer blankets or low emissivity or opacified fillers. VIPs have demonstrated thermal conductivities over five times lower than those of standard insulation.

In addition to excellent thermal performance, VIPs are much more compact than standard insulation blankets resulting in considerable volume savings. Currently, VIPs are commonly used in commercial applications such as temperature controlled shipping, major appliances, and walk-in freezer and refrigeration units. Standard VIPs can operate at temperatures between -30°F (-34°C) to around 150°F (65°C). Custom VIPs can be designed and built to operate in temperatures outside this range.

However, VIPs are not without their problems, and these problems are critical enough to their efficient application that Sympretek has focused research on addressing these issues. A typical VIP consists of an airtight envelope around an evacuated core. The cores are load bearing to eliminate the need for a pressure vessel enclosure. Some typical core materials include polyurethane and polystyrene foams, and fumed silica. Chemicals that absorb gases, known as getters, are also added to the cores to help minimize the effects of gaseous diffusion into the evacuated core. Thin polymer films are commonly used for the envelopes around the cores. These films are covered with an aluminized layer to minimize gaseous diffusion through the envelope. These envelopes are sensitive to heat and humidity which can significantly reduce the useful lifespan of a VIP.

Sympretek is developing improved VIP envelopes that will resolve these issues and significantly lower the cost of these insulation panels. These improved envelopes will be available to manufacturers such as commercial roofing insulation providers.