White Paper - Vacuum Insulated Panels

Watlow's Single Iteration division has made effective use of Vacuum Insulated Panels as part of a recent development project. The use of this technology can help Watlow provide benefit to our customers in other situations as well. This article will provide a basic understanding of this technology to help the reader recognize these value-creating situations and to use them to create a competitive advantage for Watlow.

What is VIP and why is it useful?

Vacuum Insulated Panels, commonly called VIP, are insulating structures that take advantage of the superior insulating properties of an evacuated space while avoiding the need for pressure vessels.

In typical vacuum gap insulation systems, such as a thermos bottle or nitrogen dewar, the space between an inner and outer enclosure is evacuated and sealed. With virtually no matter inside the vacuum space to support conduction or convection these two modes of heat transfer are virtually eliminated across the evacuated space itself. Radiation, the only other mechanism for transfer of heat through the space, can be minimized by low emissivity surfaces, multi-layer "blankets", or opacified fillers. In typical vacuum gap insulation systems the filler, if any, is not load bearing. This requires the inner and outer enclosures to be pressure vessels to support atmospheric pressure. Pressure vessels are heavy and can impose shape restrictions. Cylindrical, ellipsoidal or conical shapes are commonly used to limit the weight of the pressure vessel enclosures. These shapes do not make efficient use of an available volume, limiting widespread use of typical vacuum spaces for insulation.

VIP technology takes advantage of the insulating properties of an evacuated space but incorporates a load bearing core (i.e. filler) material to eliminate the need for pressure vessel enclosures. Since the structural core supports loads due to atmospheric pressure, vacuum insulated panels can be made flat to accommodate rectangular or faceted shapes. While the relatively high density of the structural core material compromises the thermal performance of VIP versus the best performing vacuum gap insulation systems, the thermal performance of a VIP system can be several times better than the best equivalently-sized closed cell foam insulation and up to 20 times better than fibrous insulation. The shrinking wall thickness of modern refrigerators has been made possible in large part through the use of VIP systems. In insulating systems of equivalent thickness, VIP technology can cut energy (or power) consumption by 50% or more over foam insulation depending on the size of the enclosure. In addition to the savings associated with the cost of electricity, the reduced power consumption may allow the use of smaller heaters and power switches, lowering initial cost.

The Structure of a typical Vacuum Insulated Panel is illustrated by the typical cross-section below:

The core material is porous and it occupies an evacuated space so that almost no air remains in the pores. The core material is often open cell polyurethane or polystyrene foam. Silica aerogel pressed into a board structure is a relatively recent and high performance alternative to foam. The vapor barrier maintains the vacuum in the space occupied by the core. Thin polymer films are common choices for barrier materials. An aluminized layer is commonly used on the barrier to minimize vacuum degradation due to diffusion (Polymer materials diffuse water vapor too readily). To delay vacuum degradation due to any leaks that remain, a getter (for absorbing gas molecules) or a desiccant (for absorbing water vapor) is often used. Typically a heat seal is used to hermetically join the barrier material around the perimeter of the panel. For high temperature applications, foil replaces the aluminized polymer barrier and a weld replaces the heat seal.

The flanges created by the heat seal are usually folded so that they are parallel to the surface much like a Christmas present. This folding allows the panel to fit more tightly with adjacent panels or structure. Any remaining gaps between panels are often filled with a foaming sealant.

Capabilities of Commonly Available VIP

Thermal Conductivity

The thermal conductivity through the center of VIP currently on the market ranges from 0.0025 W/mK to about 0.01 W/mK. This compares with 0.02 to 0.05 W/mK for closed cell foam insulation. However, while VIP manufacturers will usually quote the thermal conductivity at the center of a panel, the heat leak through the center of the panel may not be the dominant heat leak. The aluminized layer on the barrier will conduct heat around the evacuated core and compromise the performance of the panel to some degree. The deleterious effect of these "edge losses" depends on the size of the panel. For small panels, the edge losses may be greater than the heat leak through the rest of the panel. In fact, panels smaller than about 3" square offer no performance advantage over high performance foam insulation. For larger panels the edge losses become less significant.

Geometry

VIP is available in flat panels ranging in size from about 3 inches square up to about 35 inches square depending on core material (larger for foam cores). Thicknesses from about 0.125 up to 1.00 are readily available. Flat panels can be arranged to line the interior of an enclosure to provide good thermal performance. To minimize edge losses and provide better heat leak performance, some manufacturers offer hollow cylinders or rectangular "shoe box-shaped" enclosures. The rectangular enclosures can be used in pairs or with a flat panel to enclose a space with a minimum of edges to conduct heat around the core.

Flat Panels with non-rectangular shapes are possible as well. But inside corners on a flat panel should be avoided, if possible, due to problems with folding the seal flange. Holes and slots are also possible; however, as with inside corners folding the heat seal flange is not possible adjacent to these edges. Since the heat seal flanges are often 0.5 inches wide, the hole through the VIP core is much larger than the useful size of the hole. This leads to a heat leak and compromised thermal performance. Some sort of auxiliary insulation (such as a foaming sealant) is usually required to partially mitigate the heat loss problem near any holes through the insulation. As a result of the performance degradation associated with holes and inside corners, the best designs minimize the number of these features required in a given application.

Temperature

While VIP with silica aerogel core material and a welded foil vapor barrier can be used in applications up to 800C, commonly available configurations using a heat sealed aluminized polyester barrier are limited to about 50C depending on the life required of the panel. At high temperature, the heat seal itself diffuses gas and water vapor to the inside of the panel and causes the vacuum to degrade over time. The diffusion is a strong function of temperature so that the life of the panel is strongly effected by exposure to high temperatures. In the Single Iteration application mentioned in the introduction, the panels are intermittently exposed to temperatures as high as 85C. Since the exposure time at 85C is a small percentage of the total (approximately 1%), the panels will retain their vacuum well enough to outperform foam for several years.

Thermal Performance Estimation

Standard methods of calculating the heat loss through a given thickness of insulation using the thickness, area, temperature difference and conductivity are insufficient for VIP systems since they neglect the edge losses. Since edge losses are important heat leaks in a VIP insulation system, a method of calculating the expected heat loss is needed. The figure below illustrates a relatively simple calculation method developed by Single Iteration for estimating the heat loss from a VIP insulated enclosure:

This method uses a simplified model of the insulation system to facilitate the approximate calculation of heat transfer through the insulation system. The simplified system includes two layers. The first is a layer of auxiliary foam insulation. This layer may be needed to protect the VIP from temperature extremes or other environmental factors. The second layer includes the vacuum insulated panels, their barrier layers and auxiliary foam insulation filling any gap between the VIP. While the barrier material across the thickness of the VIP has significant impact on the thermal performance, the barrier material over the top and bottom faces does not. Therefore, it is not represented in the simplified system.

A mathematical model of the simplified system is created using an electrical analogy. A resistor represents each component in the simplified system. R-a represents the thermal resistance of the auxiliary foam insulation and can be set to zero if no auxiliary insulation is used, R-b represents the thermal resistance of the aluminized barrier, R-g represents the gap resistance and R-V the resistance of the evacuated core. The total resistance can then be calculated as R-total = R-a + 1/(1/R-b + 1/R-g +1/R-V). Together with a temperature difference (analogous to electrical potential), this total resistance allows the calculation of heat flow (analogous to current in an electrical circuit).

An example set of calculations are illustrated in the table below:

Heat Loss for VIP Insulated Enclosure

Area of enclosure	= 6435 Sq. cm
Metal Barrier Thickness	= 0.0006 cm
Metal Barrier Conductivity	= 150 W/m C
Aux. Insulation Conductivity	= 0.035 W/m C
VIP Conductivity	= 0.0025 W/m C

Gap Length Avg Gap Width VIP Thickness Aux Ins Thickness Temp Diff Total Thickness R-b R-V R-g R-a RT Q

(cm) (cm) (cm) (cm) (C) (cm) (C/W) (C/W) (C/W) (C/W) (C/W) (W)

177 1 1 1 30 2 3.14 6.39 16.14 0.44 2.31 13.01

177 1 1.333 0.667 30 2 4.18 8.52 21.52 0.30 2.78 10.80

177 1 1.5 0.5 30 2 4.71 9.59 24.21 0.22 3.02 9.95

177 1 1.9 0.1 30 2 5.96 12.14 30.67 0.04 3.58 8.37

177 1 2 0 30 2 6.28 12.78 32.28 0.00 3.72 8.06

177 0.5 2 0 30 2 6.28 12.61 64.57 0.00 3.94 7.62

0.00 0.00 0.00 8.8 30 8.80 555.56 0.00 2.86E+07 3.91 3.91 7.68

The highlighted areas in the table represent inputs that can be varied to accommodate changes to the configuration of the enclosure or the VIP system. The first 5 rows illustrate that any thickness of auxiliary insulation outside the VIP layer is detrimental to thermal performance assuming a fixed total thickness. This finding refutes a common misconception that such insulation helps to block edge losses. One would be better served to simply increase the thickness of the VIP layer if thermal performance is the only issue. Comparison of the last two rows shows that a foam insulating layer would need to be 4.4 times as thick as the VIP in this example to achieve the same thermal performance.

Summary Insulation can have a significant impact on the design and performance of a thermal system. Vacuum Insulated panels offer better performance than other insulation systems and thermal systems that employ VIP can thereby offer lower life cycle cost. Watlow often provides technical assistance to customers in selecting heaters, sensors and controls and often offers advice on the design or selection of other components in the thermal system. Through the Single Iteration division, Watlow also designs and builds complete thermal systems that incorporate a variety of components for use with heaters, sensors and controls. By understanding the other components in the thermal system, like insulation, and utilizing them to create superior thermal system designs or recommendations Watlow can strengthen its position as a complete thermal solution provider and win new business.

For more information on Vacuum Insulated Panels contact the author or see the following web sites:

Vacuum Insulated Panels