[Bldg-sim] Glazing heat transfer

[D. Charlie Curcija]

SHGC is calculated as Tsol + inward flowing fraction, which takes into 
account inside temperature and absorbed heat in the glass, so convective 
and FIR radiative gain from solar radiation is incorporated into the 
SHGC.   U-factor that contributes to heat transfer without solar 
radiation is always positive and so delta T where indoor temperature is 
lower than outdoor temperature would result in a net heat gain to the 
space, not the heat loss that you are seeing.  However, note several 
other factors.  Temperature changes over the 24 hours each day, radiant 
temperature of the sky is normally much lower than the air temperature, 
so when you have very small average indoor to outdoor delta T, these 
things can switch direction of heat flow.  I guess, you would need to 
look at each hour and look at the specifics.

Charlie Curcija
DesignBuilder Software USA



-------- Original Message --------
Subject: Re: [Bldg-sim] Glazing heat transfer
From: Raoni Venâncio dos Santos Lima <raoniv at ig.com.br>
To: rgaray at labein.es, bldg-sim at lists.onebuilding.org
Date: 10/13/2009 9:47 AM
> 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 <mailto: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 <http://www.labein.es>
>     --------------------------------------------------------------------------
>     Bilbao, 17-19 de Mayo, 2010
>     Open and Sustainable Building
>     Edificación Adaptable y Sostenible
>     www.OpenBuilding-2010.org <http://www.OpenBuilding-2010.org>
>     --------------------------------------------------------------------------
>
>
>
>
>     ----- Mensaje de Raoni Venâncio dos Santos Lima <raoniv at ig.com.br
>     <mailto:raoniv at ig.com.br>> en Fri, 9 Oct 2009 16:46:37 -0300 -----
>     *Para:*
>     	bldg-sim at lists.onebuilding.org
>     <mailto: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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