Envelop, LLC has designed and engineered R-25, very long service life, high reliability, large structural vacuum insulated glass units for curtain walls and skylights.
Because the spacers are made of an encapsulated very low thermal conductivity thermoplastic, the whole window Uw-value starts out at a very low 0.34 W/m2K or 0.06 Btu/(hr °F ft2) (R-17 hr °F ft2/Btu) as determined by finite element analysis.
This means that for a window to wall ratio (WWR) of 100%, the initial area weighted average façade U-value is 0.34 W/m2K.
With continued pumping and outgassing, the area weighted average façade U-value for a WWR of 100% will approach 0.23 W/m2K (R-25 hr °F ft2/Btu) within time frames allowing a building to operate at this value for more than 95% of its service life.
The gas load from outgassing and permeation is removed by small (one cubic foot in size), continuously operating turbomolecular pumping stations. One turbo station services 250, eight by twelve foot windows that are connected by tubing and specially designed fast actuating protection and mini gate valves. Turbo station power consumption is 250 Watts.
Outgassing occurs at ambient temperatures after installation. Pump down times are estimated using effective pumping speed, surface area, outgassing rates, number of adsorbed layers of water molecules, and time to outgas the bulk materials of the edge seal and spacers.
Outgassing times may be significantly reduced by first venting windows with dry nitrogen to remove water (patent pending). The nitrogen and or windows may be heated during venting. Venting will automatically increase the temperature of the nitrogen. Two dollars' worth of nitrogen will vent 250, eight by twelve foot windows at standard temperature and pressure.
Service life is not limited by gas permeation or outgassing. The medium modulus structural silicone and low viscosity fluid polyisobutene edge seals cannot fail in brittle fracture.
Envelop’s vacuum insulated glass (VIG) makes it easier for glass clad buildings to meet net zero energy standards like those that will be imposed in Europe in 2020. It has been estimated that the revenue from net zero energy buildings will reach $690 billion by 2020 and $1.3 trillion by 2035.
OVERVIEW AND BACKGROUND
Vacuum glazing has a vacuum between its glass sheets; regular insulated glass (IG) has an inert gas. Vacuum glazing operates on the same principal as vacuum insulated thermos bottles: heat conduction and convection is reduced or practically eliminated in a vacuum with the level of reduction depending in part on the level of the vacuum.
Vacuum glazing requires an array of spacers between the glass sheets that maintains the vacuum space by resisting the compressive load of atmospheric pressure. The spacers conduct heat between the glass sheets and tend to negate the ultra high insulating value of the vacuum.
Permanently sealed VIG units, which are evacuated once at time of manufacture in a bake-out and outgassing procedure, use metal spacers because they require ultra-low outgassing materials in order to maintain vacuum service pressures over decades. Polymers cannot attain the low outgassing rates of metals.
Envelop's VIG uses encapsulated cross shaped polymer spacers in a two inch array. The polymer has a thermal conductivity less than one-hundredth that of stainless steel.
Envelop has engineered vacuum glazing that relies on active vacuum pumping of interconnected VIG units while the units are in service (U.S. and foreign patents pending). This system makes it possible to use polymer spacers and edge seals by removing their continuously produced outgassing products (primarily water molecules) that would otherwise cause the vacuum pressure to rise above service pressure.
Common to all vacuum glazing is an edge seal that seals the vacuum from the atmosphere. Envelop has developed very long service life and very low stress viscous edge seals (patented and U.S. and foreign patents pending). Viscous seals eliminate the long-standing performance problems related to edge seal stress that continue to plague other designs and that have forestalled commercial viability of VIG in cold climates and placed limits on its size.
Unlike other seal types, viscous seals allow the large windows, tempered glass, heat strengthened glass, chemically strengthened glass, laminated glass, and glass coatings that are essential for modern glass curtain walls and compliance with international building standards and codes.
Vacuum glazing requires tempered or chemically strengthened glass in order to prevent breakage from thermal stress and to take advantage of insulated deep inset interior edges that minimize edge conduction on large units. Insulated inset interior edges increase temperature gradients across the face of the inboard lite and reduce those across the outboard lite. Given the exceedingly low U-value of the VIG, inset interior edges that minimize edge conduction tend to make spandrel space counterproductive for minimizing area weighted average façade U-value, regardless of the method used to insulate the spandrel space. Therefore, the full thermal advantage of the windows is attained with an uninterrupted total glass exterior.
Envelop’s vacuum glazing includes banks of up to 250 large VIG units all connected to one small continuously operating turbomolecular pumping station by one to four inch diameter 6061-T6 extruded aluminum tubing with a one-eighth inch wall and thin internal oxide layer. Alternatively, low coefficient of thermal expansion borosilicate glass tubing may be used (patent pending).
The vacuum gap between the glass sheets is 0.32 inch and maintained by the cross shaped polymer spacers, which undergo less than one percent strain in a two inch array. The spacers are encapsulated in a low emissivity and low permeability coating. The polymer is inherently resistant to UV light degradation even though an ionoplast interlayer UV filter makes this unnecessary. The polymer has an ultra-high vacuum rating, high compressive strength, and high creep resistance.
The expected service life of a pumping station exceeds twenty years, and their eventual replacement with newer and increasingly more efficient units with longer service lives must be weighed against the alternative of replacing large IG units with service lives that are limited by permeation or seal failure. Even permanently sealed VIG with metal or glass seals has a service life that is limited by outgassing, making replacement inevitable. This problem is exacerbated for permanently sealed VIG units that are produced at lower assembly temperatures than those that undergo a high temperature bake-out.
The cost of one pumping station distributed over 250 windows that are eight by twelve feet in size is $20 per window. Pumping station power consumption is less than 250 Watts. Specially designed and patent pending fast actuating valves limit pressure rise in the event of a breach or power outage. Mini gate valves have a service life of 50,000 to 100,000 open/close cycles, far in excess of what is required.
If a glass sheet breaks and its ionoplast interlayer is breached, a very high reliability bidirectional shock wave arrestor valve, which is just a very simple light weight flapper at the 25 millimeter tap to the window, is forced closed in less than a millisecond by the atmospheric wave front. This occurs without sensors, springs, electrical energy, or compressed air. If a shock wave arrives from the other direction, the bidirectional valve arrests the shock wave entering the window. A shock wave cannot damage a window or its spacers but is nevertheless undesirable. Any temperature rise in air that rushes into the system can only cause a several degree Celsius temperature rise in the glass sheets because of their very much higher heat capacity. Pressure rise in the system can never exceed atmospheric pressure.
If there is a breach in the vacuum system, service pressures are reestablished by outgassing at ambient temperatures. Depending on the extent of the breach, outgassing times may be significantly less than those at time of installation.
The patented viscous edge seals comprise low molecular weight low viscosity polyisobutene (PIB) constrained between inner and outer gaskets of medium modulus structural silicone adhesive sealant. The 0.32 inch vacuum gap provides low silicone percent elongations for low stress thermal strain between the glass sheets. This gap is over twice the structural glazing minimum glueline thickness for thermal dilatation of the silicone for a twelve foot high window on resting pads and a plus or minus 140 °F temperature differential between inner and outer lites.
The liquid PIB, which is a Newtonian fluid, is not subject to the failure modes inherent in elastic materials and bonds. It accommodates relative movement of the glass sheets by undergoing viscous shear. At 20 degrees Celsius, the highly tacky PIB has the consistency of honey and cannot experience the cohesive, adhesive, or bond failures that can ruin all IG primary seals. This, in combination with the medium modulus structural silicone and ionoplast interlayer laminated glass, makes the entire unit resistant to failure from seismic and extreme climatic events.
A patent pending U-shaped spring-like element made of very thin stainless steel is embedded within the PIB to minimize the area that the PIB presents for gas permeation. The floating spring element maintains two 0.0015 inch gaps filled with PIB between itself and the glass sheets by riding on the glass's roller wave or by stand off means if chemically strengthened glass is used. Because of the minimal use of very thin stainless steel with the bulk of the seal comprising low thermal conductivity polymers, the edge seal represents warm edge technology.
The PIB, silicone, and spring element comfortably accommodate long-term spacer creep and differential expansion and contraction of the polymer.
The silicone and PIB degas out of the bulk relatively fast. Silicone has a high permeation rate for nitrogen so that if nitrogen venting is used, the nitrogen will outgas from the silicone relatively fast.
A patent pending flexible metalized polymer diaphragm at the top of the VIG unit allows the fluid PIB to rise and fall like water in a glass in order to accommodate movement and thermal expansion and contraction of the PIB and unit. The diaphragm seals the PIB from moisture condensation. In the unlikely event the diaphragm fails, the embedded spring element sequesters condensation.
Last updated: April 24, 2014, Wisconsin, USA
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