[EnergyPlus_Support] Re: Zone Heat Balance

["Matt Webb" <mspider.webb@xxxxxxxxx>]

Hi Andy,

I appreciate the explanation. I think the distinction between radiative solar gains to surfaces versus actual change to air temperature was missing from my thought processes. I suppose in this instance I was surprised at the high solar gains but low Zone Cooling but as you have clarified, the solar gains don't translate straight to zone cooling requirements.

Regards,
Matt

--- In EnergyPlus_Support@xxxxxxxxxxxxxxx, Andy Tindale <andy.tindale@...> wrote:
>
> Hi Matt,
> 
> Zone heat balances can be quite complex to fully understand and it usually isn't as simple as Solar + internal gains + cooling = 0. This is one reason why accurate HVAC design requires such complex simulation software. There are issues such as heat conduction through walls, floors, roofs etc and diurnal storage of heat in thermal mass, especially in thermally massive buildings to take into account. 
> 
> Also in EnergyPlus the "Zone Transmitted Solar" data from my understanding is probably an over estimate of the solar energy that is actually absorbed by the zone - some solar heat is reflected back out of the zone, especially in highly glazed zones. 
> 
> Other factors such as infiltration should be accounted for.
> 
> Also bear in mind that solar and some internal gains are mostly radiative gains and find their way into the fabric whereas the cooling is to the air. In thermal modelling terms it is not possible to make a simple heat balance of heat which is added to different parts of the thermal network.
> 
> If you analyse your results in DesignBuilder and include the "Surface including solar" output option you should be able to view the heat balance data. Again this is only intended to be approximate but should balance better that 120 vs <60 W/m2. 
> 
> When doing heat balances make sure to view data over a reasonable period (> week) to avoid thermal storage issues.
> 
> I hope it helps.
> 
> Regards,
> 
> Andy
> 
> Andy Tindale
> DesignBuilder Software ltd
> www.designbuilder.com
> Tel: +44 (0)1453 755500
> 
> 
> -----Original Message-----
> From: EnergyPlus_Support@xxxxxxxxxxxxxxx [mailto:EnergyPlus_Support@xxxxxxxxxxxxxxx] On Behalf Of Matt Webb
> Sent: 15 September 2011 06:34
> To: EnergyPlus_Support@xxxxxxxxxxxxxxx
> Subject: [EnergyPlus_Support] Zone Heat Balance
> 
> Hi Team, 
> 
>  I've been doing some detailed analysis of zone heat balances while investigating the use of the chilled ceiling component in the detailed HVAC module of DesignBuilder and then analysing the results using EnergyPlus. However, my query arises from a zone where all cooling is provided by the air-side system. I'm getting ~120W/m² heat gain from the glazing while Zone Sensible Cooling output is <60W/m². This discrepancy raised a bit of a question mark. 
> 
>  I've looked at the results in the E+ sql file and discovered a similar discrepancy which I have no explanation for. The E+ Input/Output references states that the variable "Zone Transmitted Solar" is "The total Window Transmitted Solar of all the exterior windows in a zone" and also "Zone/Sys Sensible Cooling Rate" "represents the sensible cooling rate in Watts that is actually supplied by the system to that zone for the timestep reported". 
> 
>  So my thinking is that the "Zone/Sys Sensible Cooling Rate" should balance out with the zone loads, in particular "Zone Transmitted Solar". Yet this is not the case. 
> 
>  Have I missed something? Another variable to capture additional cooling? 
> 
>  Your assistance is appreciated. 
> 
>  Thanks 
>  Matt
> 
> 
> 
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