energy used heating & cooling a volume of air?

[edi454]

Hi all, this is my first post on the forum. I am a junior mechanical engineer and would appreciate if someone could clarify a problem I'm trying to logically think out.

I am currently using a building energy simulation software Virtual Environment from IES Ltd to run compliance check for CO2 production (Part L2 England & Wales Building Regulations).

The building has been modelled by a drafts person and they have created the internal space (which would be the occupied space to have HVAC) by including both ceiling and floor void as part of the space. Therefore the height of the room is 4.5 meters rather than 2.7 meters. I believe that the ceiling and the floor void should have been modelled in different zones.

What I would like to know is, would a larger room volume affect the energy consumption of the HVAC system? I have thought this out logically and my theory is that a smaller volume of air would mean that less energy is used to either heat or cool than a larger volume of air. Would anyone know how to calculate this?

 

[MintJulep]

Transient, yes.

Steady state, no.

Would anyone know how to calculate this?

Yes, you should. It's a simple energy balance. They still teach that in school, yes?

 

[Dave41A]

If the space is perfectly insulated and has no air exchange to the outside, then you simply need to figure out the mass of air in the room (density *volume) and then multiply this mass by the specific heat of air (assuming constant volume) and multiply that by the temperature change desired to get an approximate figure (specific heat changes slightly with temperature).

However, no room is perfectly insulated and presumably you'll have some air exchange (to keep the inside air fresh).

I agree that you'll probably have a higher a/c load since the total air turnover in the higher-ceiling room will probably be greater. Also, the higher room will have more wall area, so the heat transfer area to the outside will be nearly twice as large, all other things being equal.

It hopefully goes without saying that the selection and quality of installation of doors and windows (if any), insulation, ventilation, and the presence of heat producing devices in the room (computers, lighting, etc.) will all impact the room's a/c energy load.

Hope that helps. Dave41A

 

[edi454]

Thank you for the replies, yes I figured that if I looked only at the room volume and for example had a VRF Fancoil dealing with a room heat gain load and only recirculating the room air to cool (ignoring all other energy paths i.e. solar, conduction and convection through building material, infiltration etc) then I believe that more energy would be used to cool the larger volume than a smaller volume.

I am not sure what "Transient" means would you be able to explain. Thank you.

 

[MintJulep]

Transient means changing with time.

The energy used to heat or cool a room has to balance the loads.

Loads include things like:

Occupants, heat producing equipment, envelope conduction, solar, ventilation, and leakage.

Notice that none of these things contain a term for the volume of the room. Sure, some of them can be estimated based on the volume, but in a proper analysis they are not dependent on the volume of the room.

The only time you need to heat or cool the air that is already in the room is when you want to change the temperature of the room. Like if you turn the temperature down over night and in the morning you need to get the room back to a comfortable level.

 

[edi454]

Ok I'm starting to understand. Thank you for the explanation it's clarifies things for me.

So as I'm using a building environment simulation software it would be best for the ceiling and floor voids be zoned separate to the occupied zone. I have done two test models and there is a difference of 5% in energy use, with the correctly zoned model having less energy consumption.

 

[MintJulep]

Ok, but do you understand why?

Close the software and write out the equations yourself.

 

[Dave41A]

Volume will appear in the equations if ventilation requirements for the room are specified in air changes per hour (ACH).

If you are using non-air conditioned air to achieve this, this will add to your a/c load.