Vacuum Glass U value analysis
Heat transfer mechanism of glass
In previous post regarding thermal performance of vacuum glass,we got conclusion that the vacuum space in vacuum glass eliminate the possible heat convection,and vacuum glass U value is decided by the heat transfer and heat radiation,including the residual air, low-E coating emissivity,and the supporting pillars.
Considering that the final thermal transfer not only include the glass center,but also the glass edge,which is decided by the edge sealing total length,and glass area,linear heat transfer coefficient.
Here we summarized 11 factors that vacuum glass final U value:
|1||Contact area between support pillars and glass|
|2||Pillar arrangement mode||Triangle,rectangle,diamond etc|
|6||Pillar materials||Stell, ceramics|
|7||Edge sealing materials||Glass powder,metal|
|8||Edge sealing totla length||VIG perimeter|
|9||Vacuum glass area||Decided by window size|
|10||Glass Emissivity||Single,dual,triple silver low-E coating|
|11||Residual air conductivity||Residual air content|
1.Residual air inside VIG.
The residual air is related to the VIG sealing technology – Whether the glass is sealed under high vacuum condition,or air been evacuated through the evacuation holes,but generally the vacuum degree is below 0.1 Pa,the residual air conductivity is only about 0.00789W/M2.K,it can be ignored .
There’re 6 factors that related to the micro support pillars, all these decide the final total contact area between pillars and glass,as well as the heat transferred through the pillars.As every factory has their unique design and production method,decided the U value difference.That’s also the reason why the best possible VIG U value 0.4W/M2.K while others reach 0.6 and even 0.7W/M2.K.
Glass emissivity is usually 0.84,while with the help of low-E coatings,emissivity can reach 0.12, 0.08 and even 0.03 with different coatings.Low-E coating plays important role in building energy efficient glass,below tables shows VIG U value with different coating.
|Glass combination||Coating emissivity||U -Summer||U-Winter||U-EN673|
|5 C+0.3V+5 C||0.84||2.444||2.171||2.331|
|5D80 +0.3V+5 D80||0.034/0.034||0.425||0.406||0.416|
4.Edge thermal transfer.
There’s no detailed research about this part,also bring difficulty in calculating the final windows U value.but the formula:
U =L/A ψ
U=Thermal transferred through glass edge
L=Total length of glass edge
Ψ =linear heat transfer coefficient
For 1000*1000mm vacuum glass,the sealing edge area account only 4% of glass total area,which will cause 0.1-0.2W/m2.k overall U value,so it’s suggested to increase the vacuum glass single panel area to decrease the negative effect of glass edges,or add extra double glazing(especially warm edge spacer,TPS warm edge ) to decrease the heat transferred through the edge.
5.Further processing -Lami or DGU process-Hybrid VIG.
Double glazing process can help decrease the edge thermal transfer,but will extra low-E coatings on the lami or insulating process help decrease U value again?
Below table shows the calculation result based on different complicated VIG products:
|5 D80+0.3V+ 5C||0.508||0.471||0.488|
- Add on insulating process with double silver low-E coating can decrease IG U value by 0.1W/M2.K,but will cause decrease light transmittance.
- Extra argon inside DGUwon’t help much further decreasing U value.
- Lami process can only increase glass safety performance,but won’t help much on U value.
- Residual air won’t affect much in overall glass
- Manufacturer can decrease the overall contact area between pillars and glass to further increase VIG performance and visual effect.
- Low-E coating is the most important part of VIG to achieve 0.5 W/M2.K U value .
- Through further DGU process,or larger VIG panels can decrease the negative effect of linear thermal transfer.
- DGU& extra low-E coating will also further decrease vacuum glass U value by 0.1W/M2.K.
HaanGlas is your turnkey vacuum glass manufacturer and supplier in China,we have world class production technology and will help your project more energy efficient.