VACUUM INSULATED PANELS (VIPs) TECHNOLOGY : A VIABLE ROUTE TO REDUCE ENERGY CONSUMPTION IN DOMESTIC, INDUSTRIAL AND CIVIL APPLICATIONS.

Manini Paolo, Guido Pastore and Pierattilio Di Gregorio

Saesgetters

The growing attention to the global warming issue is forcing policy makers to take actions to cut CO₂ emission and minimise the greenhouse effect. With this respect, the Kyoto Conference has posed ambitious targets for energy saving, the achievement of which calls for improvements in many fields, from transportation, to refrigeration and building insulation.

Vacuum insulated panels ( VIPs ), having thermal conductivity much lower than conventional insulators like polyurethane and polystyrene foams or glasswool allow the achievement of superior insulation performances. They can be used to partially replace the conventional insulation materials to provide a more efficient insulation structure, which allows energy saving without the need to increase the insulation thickness. Alternatively, vacuum insulated panels can be used in those applications where it is important to reduce the insulation thickness to a minimum value without loosing thermal performances.

Thanks to this, vacuum insulated panels are finding increasing acceptance in a variety of domestic and industrial applications encompassing refrigerators and freezers, vending machine, shipping containers, insulated transportation and building.

1. VACUUM INSULATED PANEL DESIGNS

Vacuum insulated panels ( VIPs ) have thermal conductivity 3 to 7 times lower than conventional insulating materials such as closed cell polyurethane and polystyrene foams or fiber glass, as showed in the following chart.

Vacuum panels can therefore be used to significantly reduce the energy consumption in a variety of domestic, industrial and civil applications. VIPs are manufactured by inserting a micro-porous low thermal conductivity core material in a highly impermeable gas barrier which is evacuated and sealed. Open cell polyurethane [1] and polystyrene foams [2] and fumed/precipitated silica powders [3] are the preferred options.

To fully exploit their insulation performances, panels must be evacuated and kept at a suitable vacuum level during operation. Thermal performances as a function of the vacuum level is showed in the figure for some selected core materials

To limit as much as possible air and moisture penetration in the panel due to gas diffusion from the ambient, extremely good gas barrier envelopes have to be used. Moreover, a gas absorbent (also referred to as “getter”) is generally necessary in most panel designs to remove gases and keep stable pressure value.

The use of stainless steel to envelope the core material is also possible. This panel design is generally very robust and well suited for high temperature conditions or where good structural properties are mandatory (e.g. building or logistics). Being generally heavy and expensive, the adoption of stainless steel panels in refrigerator and freezers has been so far quite limited.

Vacuum panels can be prepared in different shapes, geometry and sizes, as showed in the following pictures

2.VACUUM PANEL APPLICATIONS AND PERFORMANCES

Vacuum Insulated Panels are already being used in a variety of domestic and industrial applications encompassing refrigerators and freezers, vending machines, shipping containers or insulated transportation.

Refrigerators and freezers : for household appliances, depending on the surface coverage of the cabinet and the panel thickness, energy savings from 20% to 30% or more can be achieved [4].

Vending machines : VIPs extensively used in vending machines, which are especially popular in Japan, Korea and the Far East. One or two panels are generally used to separate the hot and cold beverage compartments. In this case, the main driving force for VIP adoption has been the increase of the internal storage volume (up to 20%), without reducing the insulation efficiency.

Bio medical and laboratory freezers : these are very special equipment which are designed to operate at very low temperature (e.g. from –30 °C to –86°C) to store valuable and temperature-sensitive goods, like organs and tissues, biological samples or vaccines. Vacuum panels are used to reduce the thickness of the insulation without loosing performances. Increase of the internal volume as much as 30 to 40%s has been achieved in specific models. VIPs also deliver overall superior passive insulation to the cabinet. This is important in case of power failure, since the rate of rise of temperature is less steep.

Refrigerated trucks, reefers and LNG carriers : studies are ongoing to use VIP in the logistic and transportation business either to increase inner space or improve insulation performance.

Shipping containers : commercial shipping containers to store and deliver pharmaceuticals, drugs, vaccines and other temperature sensitive products, are already using VIPs. Vacuum panels allow longer storage time and safer transportation conditions. Food containers in the catering industry are also starting to adopt VIP is special applications.

Water heaters : both gas and electric water heaters might use VIP to comply with the forthcoming energy regulations and labelling, dictating much lower overall energy consumption. Results of trials indicate that energy savings exceeding 50% can be achieved by replacing partially the conventional foam with vacuum panels.

Insulation in buildings : VIPs already started to be adopted in selected applications like prefabricated roofs, wall constructions, doors and facades. They are very useful in the renovation of old buildings or in applications where the higher VIP cost is fully compensated by the space saving they can deliver.

A more comprehensive review of VIP technology, application and perspective can be found in [5]

References

[1] Hamilton,”An Evaluation of the Practical Application and use of VACPAC panel technology, Vuoto, XXVIII, n°1-2, Jan/Jun 1999, 27.

[2] Pendergast et al.“Characterization and Commercialization of INSTILL Vacuum Insulation Core, a Unique Polystyrene Vacuum Insulation Filler from the Dow Chemical Company”, Vuoto, XXVIII, n° 1-2, Jan/Jun 1999, 27.

[3] Boes et al. “ Vacuum Panels based on nanoporous Fillers “, presented at the VIP Symposium,Baltimore, 1999, SAES Getters Web site, http://www.saesgetters.com

[4] Wacker et al.“Developments of Vacuum Panel Technology Based on Open Celled Polyurethane Foam”, Proceeding of the Polyurethanes Expo ’96, 1996, 35

[5] Paolo Manini “ Recent Developments in Open Cell Polyurethane-Filled Vacuum Insulated Panels for Super Insulation Applications”, Advances in Urethane Science and Technology, Chapter 4, Rapra Technology Editor, 2001