[Bldg-sim] Glazing heat transfer

[Raoni Venâncio dos Santos Lima]

This was my hypothesis, but none of my DesignBuilder students bought that.
Indeed, the glass surfaces temperatures (internal and external) were higher
than the outside temperature. On the other hand, the internal temperature
was lower than outside temperature. Apparently the glass tends to lose heat
from its external surface to the outside air (probably maximized by
convection) and to gain some heat due to conduction (from outside air to
internal air). In this case, my glass is losing more heat.

I appreciate your reply,
Best regards,

Raoni.




2009/10/13 <rgaray at labein.es>

>
> Dear Raoni,
>
> I´m not completely  of the calculation method used by Energy Plus, There
> are several reasons an outward heat flux in the outer surface of a glazing
> system:
>
> The usual one is when the outside temperature is colder than the one
> inside. As you tell in your post, this is not the case in your simulations.
>
> There is a second reason that only happens when both internal and external
> temperatures are nearly the same (even beein the internal one a little bit
> lower than the external one): The glazing is heated by the incoming solar
> radiation and its temperature rises over the external temperature. This
> creates an outward heat flux. For a clear glass, the usual absorptance is
> very low (1-4%) so this phenomenom is usually very rare.
>
> I don´t know is this info will solve your problem.
>
> Kind regards,
>
> --------------------------------------------------------------------------
> Roberto Garay Martinez
> Labein-Tecnalia
> Unidad de Construcción y Desarrollo del Territorio
> c/Geldo. Parque Tecnológico de Bizkaia
> Edificio 700
> 48160 Derio (Bizkaia)
> Tel.: 34 94 607 33 00
> Fax: 34 94 607 33 49
> www.labein.es
> --------------------------------------------------------------------------
> Bilbao, 17-19 de Mayo, 2010
> Open and Sustainable Building
> Edificación Adaptable y Sostenible
> www.OpenBuilding-2010.org
> --------------------------------------------------------------------------
>
>
>
> ----- Mensaje de Raoni Venâncio dos Santos Lima <raoniv at ig.com.br> en Fri,
> 9 Oct 2009 16:46:37 -0300 -----  *Para:*
> bldg-sim at lists.onebuilding.org  *Asunto:*
> [Bldg-sim] Glazing heat transfer
> Dear colleagues,
>
> I've noticed a slight heat loss through glazed surfaces during the whole
> year. I'm usind DesignBuilder and the situation is the following:
> 1) climate: Brasilia (Brazil). Average temperatures around 22°C, but during
> occupied periods (daytimes) the temperatures are higher (around 26°C).
> Solar
> radiation is considerable.
> 2) Conditioned zone with 24°C set point temperature
> 3) Using a single pane glass 3mm (SHGF around 89%) form the EnergyPlus
> dataset.
>
> I wonder why the zone is losing heat through glazing even when internal
> temperatures are lower than external (conditioned zone). Heat gains due to
> solar radiation are ok, but I'm not sure the glazing results are fine. The
> interesting thing is that the zone never gains heat from the glazing
> (except
> due to solar radiation).
>
> Is this a common thing? Any thoughts?
>
> Thanks in advance,
> Raoni
>
>
>
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