Introduction:
Playing against sun, what is G value and how it impacts Cooling/Heating comfort? The term has been of immense importance for skylight and façade specially in hot climate locations and in other context instead of G-value or SHGC, a reference to SC Shading Coefficient is made yet they are quite close in value SHGC=0.87SC, identical in usage , and SHGC is notoriously referred now a days as G-value. In simple words it’s the shading performance when we play against sun(in technical words the amount of sun energy allowed to enter mainly due to radiation).
Definition:
G-value is the fraction of solar energy transmittance through glazing, which is expressed between the values zero (0.0) i.e., black body, to one (1.0) i.e., 3mm annealed clear glass. The higher the number, the more energy transmitted through the glazing, and more the inner space or zone of structure envelope will get heated up.
ETFE G-value impact in all year hot climate locations:
Focusing on ETFE, ETFE cushion can achieve a G value of as less as 0.22 and as max as 0.95, furthermore since ETFE practically isn’t opaque and accordingly is transmissive to direct solar heat the G value turns to be the most significant factor in calculating the needed cooling load against transmitted heat, that is identical to glass windows or skylights too. Nevertheless, we may go bolder here and state that in all year hot climate locations(always cooling is needed), in terms of heat transfer the conductive part of it(i.e. U value) is not the main driving factor of determining cooling load once ETFE applied as air filled cushion but the transmission part of it is what matters, therefore in all ETFE skylights or facades in such locations the cooling load needed is mainly determined by transmission heat load which is by far higher than conductive one, it could be from 10 to 20 folds and even higher than that. However, this does not mean that conductive part should be ignored, no it must be calculated and considered but what is to be carefully noticed here is that feasibility of reducing G value shall be more highlighted by adding frits, color of frits (ink darkness, reflectance), adding special films etc., rather than wasting money in reducing U value by adding more ETFE layers. Remember carefully, this is not true in cold winter locations where U value/ the conductive and convictive part plays the main driving factor in heating load determination specially in night and early morning. Not only this, in cold locations the increase in G value could be recommended to receive such free natural heating loads depending on the envelope orientation.
Why ETFE G-value is ignored in cold winter:
Heating load calculations in cold winter are carried out to estimate the heat loss from the building in winter to arrive at required heating capacities. Normally during winter months, the peak heating load occurs before sunrise and the outdoor conditions do not vary significantly throughout the winter season. In addition, internal heat sources such as occupants or appliances are beneficial as they compensate some of the heat losses. As a result, normally, the heat load calculations are carried out assuming steady state conditions (no solar radiation or transmission and steady outdoor conditions) and neglecting internal heat sources. This is a simple but conservative approach that leads to slight overestimation of the heating load or demand. For more accurate estimation of heating loads, one must consider the thermal capacity of the walls and internal heat sources, which makes the problem more complicated. Nevertheless, note that G-value plays no role in night and in estimation is ignored conservatively, here the only player in ETFE part is the thermal conductivity and conviction in the ETFE envelop.
On the other hand, for estimating cooling loads, one must consider the unsteady state processes, as the peak cooling load occurs during the daytime and the outside conditions also vary significantly throughout the day due to solar radiation. In addition, all internal sources add on to the cooling loads and neglecting them would lead to underestimation of the required cooling capacity and the possibility of not being able to maintain the required indoor conditions. Thus, cooling load calculations are inherently more complicated.
Hence, in determining the heating load, credit for solar heat gain or internal heat gains is usually NOT included and the thermal storage effects of building structure are generally ignored. Whereas in cooling load calculations, the thermal storage characteristics of the building play a vital role because the time at which the space may realize the heat gain as a cooling load will be considerably offset from the time the heat started to flow.
What ETFE G-value impact on sky visibility:
As previously mentioned, reducing G-value by adding frits reduces significantly cooling load, however this comes on the account of sky visibility if ETFE cushion G-value must go low typically below 0.25 the ETFE turn opaque to sky view. An optimal G-value where you can see the sky and at the same time maintain reasonable cooling load would be in the range of 0.33 to 0.40. To know more how it look like intuitively please visit our Toolbox free web site and select ETFE G-value from below link(wait maximum 60 second for web to upload):
Also refer to our blog for other pillars of skylight planning for G-value:
Conclusion:
In Summer, reducing G-value reduces significantly the cooling load, On the other hand, that is not the case in cold winter where conductive and convictive part(U-value) is the most significant driver in determining heating load and G-value can be ignored or doesn't have significant weight in heating load calculation specially in night and right before early morning.
This blog is concerned about ETFE G-value contribution to cooling or heating comfort, there are many other factors but not related to ETFE plays important part in HVAC loads determination, however, they are beyond the scope of this blog, and to know more about them you can refer to ASHRAE Standard
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