Hybrid VIG thermal performance analysis
Even though vacuum insulated glass has superior thermal performance than insulated glass,it help decreasing the popular 1.6W/M2.K IGU value to below 0.6W/M2.K and HaanGlas VIG PRO even make it 0.47w/m2k,while many customers still pursue the extremely low U value,this post will analysis the possibility to further increase VIG thermal property.
VIG Glazing thermal property
The thermal transfer through VIG contain 3 parts:
Heat transfer through micro pillars.
The pillars distance,pillar diameter decide the heat conduction through VIG.The only way to decrease heat conduction ,is to increase the distance and decrease pillar size to the minimum. But,the pillars main function is to support the glass,larger pillar distance and smaller size will increase the pressure and easy to cause glass breakage, even though most VIG are produced with tempered glass,the pillars also has huge stress on glass surface.Current pillars design fit the glass perfectly and any change will increase the breakage risks.
Heat convection through remaining air inside VIG.
The less air,the less heat convection through glass.The vacuum degree has decreased from initial 10-2Pa to now 10-4 Pa,U value also decreased from initial 1.0W/M2.K to now below 0.5w/m2.k.
To further increase vacuum degree will take more costs and ,the U value improvement has low costs performance.
High performance low emissivity coating.
Low emissivity coating is the deciding point of VIG.Without the coating that can reflect infarred,VIG thermal U value is far from expected,below table shows the difference between single silver low-E,dual silver low-E and triple silver low-E.
|low-E smissivity||HANGLAS BASIC||HAANGLAS PRO|
|4 CL184+ V +4||0.099||0.67||0.72|
|4 S1.16+ V +4||0.076||0.58||0.65|
|4 D80+ V +4||0.034||/||0.48|
|4 T70+ V +4||0.033||/||0.48|
Compared with dual silver LOW-E, Triple silver low-E has slightly lower emissivity and not much difference in final U value,even though new more advanced quad silver low-E was invented but cannot be applied in VIG production.
Bother triple and quad silver low-E has high and very strict production skills and require short time from unpacking to making into finished IGU or VIG,while VIG production time can be 48-72 hours as it contains much more procedures and the time don’t allow VIG manufacturers to adopt high performance triple and quad silver low-E.
With above analysis,we got conclusion that now VIG thermal property has almost reached the limitation.Any changes,whether pillars,low-E coating,and other points changes will lead to unstable products and high costs.Then we can only find other methods to enhance final U value,for example ,hybrid vacuum glazing.
Hybrid VIG Glazing thermal property
The benefit of making VIG into hybrid VIG include:
- Decrease VIG edge thermal conductivity. The edge thermal conductivity is the weakest point of VIG as the edge low-E coating will be removed before sealing VIG ,and the sealing materials ,whether it’s metal or glass solder,has high conductivity and will bring up the overall glass U value.
- Increase glass impact resistance.Even though VIG is safety glass and will break into particles that is safe enough to avoid serious injury,while when applying large panel VIG into curtain wall,it’s always suggested to add extra tempered panel outside to improve glass wind resistance .
- With extra low-E coating and argon filled insulating space,VIG thermal performance will increase further.
Factors affect hybrid VIG performance
Below table shows the difference based on same aluminium spacer,and same glass combination,but difference spacer thickness.
|VIG PANEL||6C+AIR+8.3 D80 VIG||9A||12A||16A||20A|
Making single VIG into VIG insulated glass can only increase 0.02-0.03W/M2.K U value.
in normal insulated glass,fill argon to replace air will help increasing about 0.2W/M2.K U value,then how about hybrid VIG?
|VIG PANEL||HYBRID VIG||9A||12A||16A||20A|
|0.47W/M2.K||6C+AIR+8.3 D80 VIG||0.45||0.45||0.44||0.44|
|0.47W/M2.K||6C+ARGON+8.3 D80 VIG||0.45||0.44||0.44||0.44|
Obviously no meaning….
Adding extra low-E coating.
By adding extra low-E coating, the overall IGU U value decreased by 0.05-0.1W/M2.K.
|VIG PANEL||6D80+AIR+8.3 D80 VIG||9A||12A||16A||20A|
Warm edge spacer.
Usually the U value we calculated are glass center U value,but when applying glass in windows,doors and facade,the linear edge U value will affect final windows& doors U value.Below table shows test result from Ms Hou Yuzhi’s article-vacuum sealed windows thermal performance analysis ,shows the difference by using warm edge spacer.
|Type||Glass U value||Glass Combination||Spacer type||Windows U value|
|Thermal break aluminium window||0.466||5+16A+5 LE+V+5||Aluminium spacer||1.174|
|Thermal break aluminium window||0.466||5+16A+5 LE+V+5||Technoform warm edge Spacer||1.078|
|Thermal break aluminium window||0.466||5+16A+5 LE+V+5||Technoform Super warm edge spacer||1.054|
Even though warm edge spacer don’t affect glass center U value,but overall windows U value will decrease by about 0.1W/M2.K.
- Current VIG structure and design has realized the best U value,any changes will affect glass safety and increase risks after installation.
- By making VIG into hybrid VIG,will help decrease glass edge thermal conductivity,and increase glass overall impact resistance.
- Filling argon and clear glass will not affect much on finishing glass U value.
- The best method to further increase hybrid VIG thermal performance ,is to use warm edge spacer and add extra low-E coating,this will help decrease windows overall U value by 0.15W/M2.K.
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