Heat transfer mechanism of VIG

Vacuum insulating glass (VIG) is famous for its extremely low thermal conductivity and excellent anti-dew performance,while people don’t know exactly what caused the heat transfer mechanism of vacuum insulating glass,this post will analysis the reasons that why VIG has R12 and even R14 U value.

Typical Glass Combination	6mm	6+1.52+6	6-12A-6	6 LE -12A-6	6LE-12A-6LE-12A-6	5+0.3V +5
Glass Type	Tempered	Laminated	Double glass unit	Double glass unit	Triple glass unit	Vacuum glass
Glass thickness	6MM	13.52MM	24MM	24MM	42MM	10.3MM
Glass weight	15 kgs/sqm	30 kgs/sqm	30 kgs/sqm	30 kgs/sqm	45 kgs/sqm	25 kgs/sqm
Glass U value	5.8 w/m2.k	5.2 w/m2.k	2.7 w/m2.k	1.6 w/m2.k	0.6 w/m2.k	0.47 w/m2.k

Basic knowledge of heat

1. What are the components of solar radiation?

It consists of three parts: ultraviolet radiation, with a wavelength range of 0.01 to 0.38 microns. The visible light wavelength range is 0.38 to 0.75 microns. Near-infrared radiation, the wavelength range is 0.75 to 3 microns.

2.How to distinguish far-infrared thermal radiation and near-infrared radiation?

Near-infrared radiation comes directly from the sun, and its heat sensation is not obvious, but it can be converted into heat after being absorbed by objects.The converted heat is called far infrared.

Far-infrared thermal radiation cannot be directly transmitted through ordinary glass, but can only be absorbed or reflected by glass.

After the glass absorbs this part of the energy, the temperature will rise, and the part of the energy will be dissipated through convection conduction with the air and thermal radiation to both sides.

3.What are the components of the total thermal energy transferred through glass?

It consists of two parts: direct solar radiation heat transfer and convective heat transfer.

Direct solar radiation include UV,visible light and near-infrared radiation.

Convective heat transfer:When glass absorb the near infrared radiation,glass temperature increase and then glass itself becomes heat source, the heat transferred through convection or conduction.

4. What’s glass SC and U value ?

Shading coefficient Sc: Under the same conditions, the ratio of solar radiation energy through glass to solar radiation energy through 3mm transparent glass,this evaluate the direct solar radiation.

U value:The heat energy transmitted through the glass due to convection & conduction, including the heat energy absorbed by the glass and then radiated outward. Therefore, the lower glass emissivity E ,the lower  U value.

Learn more: 20 FAQ about low emissivity glass-Glass Guidance

Why we use low-E in vacuum insulating glass?

Low emissivity glass -low-E glass,is to have a multi layer coating composing metalic &chemical on glass surface to decrease the glass emissivity ratio from 0.84 to below 0.15,or even only 0.03.

The low emissivity coating can reflect the far infrared radiation directly,especially the silver layer inside the soft coated low-E coating can reflect 98% far infrared radiation like mirrors.The coating has little capability absorbing the heat,which will control the glass surface temperature very well.

The low emissivity means that glass also has limited capability to re-radiated out.

Below test shows the difference between HaanGlas Pro and normal clear glass.

 

 

Glass Type	Thickness	U value	Starting Temp	60s Heating	120s Heating	180s Heating	240s Heating
HaanGlas VIG PRO	10.3	0.47W/M2.K	23.5℃	26.2℃	27.5℃	28.5℃	29.5℃
10 Clear Glass	10	5.75W/M2.K	23.5℃	48.3℃	61.5℃	71℃	79.1℃

By applying low emissivity coating in vacuum insulating glass,the low-E coating reflect most heat out,meanwhile after glass absorb heat ,it re-radiated limited heat through the pillars and conduct the heat to interior side.

On the contrast,normal clear glass emissivity is 0.84,after glass absorb heat,it re-radiate the heat very soon and front side temperature increased much.

The heat convection & conduction difference between VIG &IGU

Below test shows the different heat convection & conduction between VIG and IGU.

Both glass contain same emissivity coating ,with same SC and SHGC,which means that the back side has the same capability to reflect far-infarred and get the same heat,meanwhile the glass re-radiated same energy and heat,while vacuum insulating glass and dual silver IGU has different back temperature.

 

 

Glass Type	Thickness	U value	Starting Temp	60s Heating	120s Heating	180s Heating	240s Heating
HaanGlas VIG PRO	10.3	0.47W/M2.K	23.5℃	25.5℃	26.9℃	28.1℃	29.3℃
4-9-4 IGU	17	1.9W/M2.K	23.5℃	26.3℃	28.4℃	30.7℃	33.1℃

Based on above basic principle ,we can learn that the temperature difference is caused by the thermal conductivity and convection.

1.As the test only tested glass center area, limited heat can be transferred through glass edge,so heat transfer through conduction can be ignored.While for vacuum insulating glass, there’re pillars between panels,so heat can still been transferred through pillars.

2.Different from normal insulated glass with air inside,Vacuum insulating glass has high vacuum degree inside,so heat transfer caused by air convection is eliminated.

So the test shows vacuum insulating glass pillar conduction capability +remained air convection capability is much less than normal insulated glass air convection capability.

This is just 240 seconds test, imagine in hot summer, long day lighting time,exterior glass panel will keep high temperature for over 10 hours, the heat transferred glass is huge and need much energy to keep indoor comfortable.

And in winter times when indoor&outdoor temperature reach over 30℃, heat loss through windows & doors will consume more energy and in some European areas,more firewood to keep warm.

Learn more:Vacuum  glazing  VS  Double  glazing

Vacuum insulating glass can not only decrease the energy to keep home comfortable during its 20-30 years lifetime,its thin and light will also help customers reducing the costs on thick and heavy windows systems,so whether initial investing or following consumption ,vacuum insulating glazing is your ideal choice.

For windows renovation project, glass can be replaced without changing original windows frame,people’s daily life won’t be affected but enjoying the newest glass technology.

HaanGlas is your turnkey vacuum glazing manufacturer and supplier,with SGCC/IGCC certified vacuum glazing products,HaanGlas will continuously contribute to net-zero world.