[EnergyPlus_Support] RE: Modelling Displacement Ventilation

Very useful , thanks. I had initially left out the economiser to see the effects of the DV system alone but I guess this was not very useful. I will add economisers to both models and give feedback . We don’t have humidity control for the climate in question. There is not a lot experience in DV design or modelling in south Africa so this is new territory for us. Thanks for the help.

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Chilufya Lombe
Electro-Mechanical Engineer

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From: alaa kandil [mailto:alaak1974@xxxxxxxxx]
Sent: 02 July 2010 08:55 AM
To: Lombe, Chilufya
Cc: energyplus_support@xxxxxxxxxxxxxxx
Subject: Re: Modelling Displacement Ventilation

I didn't have a chance to look at your model yet. I will do that when I go back to work on Monday, but let me give you an idea about energy savings with displacement ventilation and how to get them:

-Because supply air temperature is high (19 degree c) there will be more chances to use more free cooling if the outdoor conditions are suitable (cooling only savings). Note: to do that you should have an economizer.
-Because of the higher ventilation effectiveness of displacement ventilation, when the outdoor conditions are very hot, less outdoor flow could be used with DV which leads to energy savings as well (cooling and heating). Note: you need to define that in the object (controller:outdoor air).

However .. these savings could be easily canceled, and even more energy use could happen because of:
1- The higher fan power due to higher flow rates with DV. This is because smaller d(t) between supply air and room set point requires higher flow to handle cooling loads. Note that the effect of increasing flow rate on energy use is cubical.
2- If there is a need for humidity control, the air will need to be cooled anyway for de-humidification which will cancel savings due to free cooling even if the outdoor temperature is suitable, unless another way of de-humidification other than cooling is used.
3- In the case of heating with 100% outdoor air, all heating inputs from the lighting are canceled. However, this is an advantage in the case of cooling with 100% outdoor air.

In conclusion if you want energy savings with DV you should :

-Have outdoor conditions suitable for free cooling (dry and below 19 degree c)
-Or/and separate between thermal control and ventilation. e.g., use DV for ventilation only and use chilled ceiling for cooling.
-Or/and have a conditioned space that requires a very high ventilation rate due to high occupancy levels, e.g. class rooms

I hope that was useful

Cheers

Alaa Kandil
Calgary, Canada

From: "Lombe, Chilufya" <Chilufya.Lombe@xxxxxxxxxxxxxx>
To: alaak1974@xxxxxxxxx
Sent: Thu, July 1, 2010 7:00:34 AM
Subject: Re: Modelling Displacement Ventilation

Hi , thanks for replying to my post. I was expecting to get lower energy usage for the displacement system. I have attached my idf files , I followed the example files quite closely and went through the input/output reference as well but I was expecting some kind of reduced energy usage from the DV. To my knowledge , users don’t have control over the air velocity into the room (except for PMV calcs) so the settings I change was only the supply air temp (apart from adding the room air models).

10b.

Re: Modelling Displacement Ventilation

Posted by: "alaa kandil" alaak1974@xxxxxxxxx alaak1974

Thu Jul 1, 2010 12:20 am (PDT)

I have some experience with modeling displacement ventilation with energy plus and I think I can help .. what kind of results did u get ? why do u think results are strange ? what were you expecting?

Alaa Kandil
Calgary, Canada

________________________________
From: chilu32 <chilufya.lombe@xxxxxxxxxxxxxx>
To: EnergyPlus_Support@xxxxxxxxxxxxxxx
Sent: Thu, July 1, 2010 12:07:59 AM
Subject: [EnergyPlus_Support] Modelling Displacement Ventilation

Hi , I am trying to do a simple comparison between a displacement ventilation system and a normal mixed zone system. The energy consumption numbers I get back don't make sense and I am trying to troubleshoot my inputs. Anyone have similar experience?? I added the room air models and changed supply air temp to 18C for the DV model.