Question regarding cooling design

Dirk.Rilling · Posts: 2 Location: Malacca

Dear Andy,

I have found some interesting results while simulating a exisiting building regarding the cooling design.

Me and some students analyze buildings regarding their cooling load here in Malaysia. For an exisiting bulding, transferred to a DB model, we have found out that, e.g. by increasing the insulating quality of the glazing (from single-uninsulated to up double-glazed and Argon filled), the power requirement regaring the air conditioning increases as well. Especially the solar gains increase.

I thought it would decrease, since, well, you know...why should I buy (here in this region) the very expensive double glazing when at the same time my air conditioning has to run all day long at higher load... Shocked

Then I made a counter calculation by just defining a simple cube without any glazing, without HVAC and subjected it to cooling design simulations (design day: July 15th) with increasing insulation quality (from uninsulated medium weight to best practise medium weight). The results are:

* the heat gains and losses decrease (as expected) and are around 0 kW fo best practise.
* the inside temperature are increasing the better the insulation quality is. Hmm...

This is, ehm, interesting. Or am I subjectd to a error of reasoning? I thought it would be easy to show that with increasing insulation quality the cooling load would decrease and hence no need to run my air conditioning at 100% the whole day. Or am I wrong or is it a DB or E+ thing...?

I've attached the simple model of my "cube" as well as the result pictures regaring the temperatures.

Thanks for your support!

Best regards from Malacca/Malaysia,
Dirk Rilling

Andy Tindale · Posts: 1196 Location: Stroud

Dear Dirk,

Relationship between solar gain and number of layers of glazing.
You should find that solar gain is smaller the greater the number of panes of glazing. Single glazing transmits more solar radiation, triple glazing transmits less. The reasons for this are fairly obvious and you can test it within DesignBuilder. Make sure that you compare like with like though. For example reflective single glazing and may transmit less solar radiation than clear double, so be sure to use clear glazing in your tests.

Relationship between cooling load and wall/window/roof insulation levels
If the temperature within the space is higher than outside then clearly high levels of insulation will keep the heat in. This is true during the day but also during the night, so I would expect highly insulated spaces to have much more constant temperatures and so the average temperature may well be higher. Are you considering what happens at night-time? Are you looking at times series temperatures or daily average temperatures?

I've downloaded your test model and simulated it for various constructions and got results which seem reasonable to me - the more the insulation are more constant the temperature with cooler daytime temperatures and warmer night-time temperatures.

I guess ideally you should insulate the building during the daytime to keep the daytime heat out and remove insulation at night-time to provide free cooling! a common approach to reducing daytime cooling loads in the UK is to force cool night-time air over heavyweight concrete slabs which then absorb excess daytime heat thus reducing cooling loads/maximum temperatures.

I hope this helps.

Best regards
Andy

Dirk.Rilling · Posts: 2 Location: Malacca

Dear Andy,

Thanks for your answer regarding our problem(s) here which were very helpful.

However two "problems" remain:
(1) It is not that much the relationship between solar gain and numbers of glazing but the HEAT GAIN BY GLAZING from the windows.

From the Simple Model that I'd create, it was found that the Heat Gain of Triple Glazing+Clear windows is higher than the Single glazing+clear window.

I compare 4 types of glazing templates which are:

1. Single+clear+no shading
2. Double+clear+no shading
3. Triple+clear+no shading
4. Double+clear+LoE+argon filled

The result shows that the AirTemperature of TYPE 4 glazing was the lowest, and TYPE 1 was the highest. But the Heat Gain by Glazing shows that, TYPE 4 Glazing has highest heat gain while TYPE 1 glazing has lowest heat gain, which is exactly the opposite of our insulation purposes here. *looking puzzled*

(2) Another problem occurs at the floor: the better the insulation standard the more more cooling requirement is needed to keep the house at a certain temperature level. *looking puzzled^2*
Actually I have expected the other way around: if I insulate my floor better I provide a higher thermal resistance, so my "cool air" will be kept inside rather than to volatilise through the floor/ground, isn't it? In result, my cooling load is decreased. But what we see here is quite the opposite.

Thanks again for your support!!

With best regards,
Dirk Rilling

Andy Tindale · Posts: 1196 Location: Stroud

Dirk,

Sorry about the delay in answering this.

1) Glazing Gains
You have larger "glazing gains" with triple glazing because the inner pane transfers more heat into the building through, convection and longwave radiation. The shortwave solar radiation transmission is less of course and this is recorded in the "Transmitted solar". You will get a better picture if you add the transmitted solar gains to the window gains giving a total contribution to the zone heat balance from the windows. Please bear in mind that the Transmitted solar data does not include diffuse solar radiation retransmitted back out of the window and so is not the same as the "Absorbed solar".

Here is an extract from the new online help:

Login