Natural Ventilated Double Facades

DBarreraG · Posts: 13

Hi:
I’m working in the simulation of a natural ventilated building with double facades in north and south sides. You can find the whole building in the attached file.

As you can see the double facades are built from floors 1 to 6, which means they don’t touch the ground and the air is introduced by the space in the base of the facades (holes). Since the fact that horizontal holes do not transmit the airflow accurately, how can I establish a fixed air mass flow rate for the holes in the base of the double facades? I got the data, about the air mass flow rate, from CFD analysis. I’m still working in the best way to connect the facade to each floor; this is because you will notice many things missing in this model.

Thanks in advanced.

Andy Tindale · Posts: 1196 Location: Stroud

Hi DBarreraG,

It is not currently possible in DesignBuilder to set fixed flows when using the calculated natural ventilation option. However perhaps you could vary ventilation opening areas until you get the airflow rates you have calculated from CFD?

Your other option is to export the model in IDF form and add an extractor fan connected to the airflow network. This will allow you to control the airflow more precisely.

Andy

PS It's a very nice model you have posted. Can you allow us to put an image of it on the Visualisation page of our website to show what is possible in DesignBuilder?

DBarreraG · Posts: 13

Ok Andy. About the suggestion of controlling the airflow I have one more question. If you look at the model, you can see I've defined large windows by establishing high % window/wall area. If I defined the openings to ventilate the building as a % glass area opened, I would get a uniform airflow inlet throughout this "windows" in the building, is that correct? In order to control the airflow better, can I delete some of these windows and define some different inlets (either doors or windows)? How can I the airflow trough these openings, so I can modify size of each until I get the desired rate?

You can show the model for sure, it is the Torre de Ingenieria (Engineering Tower) which belongs to the Engineering Institute, UNAM, MÉXICO.

Thanks in advanced.

Andy Tindale · Posts: 1196 Location: Stroud

You can change the maximum openable glazing area using the '% Glazing area opens' edit control.

Andy

DBarreraG · Posts: 13

Sorry, I made a mistake. I wanted to ask: How can I verify the airflow rate trough these openings, so I can modify size of each until I get the desired rate?

Thanks

Andy Tindale · Posts: 1196 Location: Stroud

You will need to use v.1.3 which provides 'air change rate' outputs for each zone.

Andy

PS thanks for the go-ahead in using your model on our website. See:

http://www.designbuilder.co.uk/content/view/33/50/

DBarreraG · Posts: 13

Is version 1.3 currently available? Sorry, I'm always confused about the versions. I just need to update the DB version that I currenly handle?

Thanks.

Andy Tindale · Posts: 1196 Location: Stroud

Yes v.1.3 is available in beta form - there is link to download this under "Latest News" in the top left of the website home page.

Andy

Liefooghe · Posts: 94 Location: Gent

Dear sir,

We are currently simulating for 3 month double facades of all kinds for a specific project in Belgium. This for a Masterplan - office building -15000m².

A/
Generally there are 2 types of double facades, ventilated and not ventilated (passive).
The not ventilated double façade is fully supported by DB. The way to simulate this is found in help under “Passive Trombe wall”. Of course with this type of façade we hade temperatures over 70°C.

B/
So we started to simulated ventilated double facades. There are many possibilities but generally there are 2 types of active double facades:

1 - Ventilation openings towards the outside. Then the interior façade needs to be best insulation quality (glass U 1.1, ZTA 0,4…) and the outside glass can be “cheap” clear glass. Because is the double façade is ventilated to the outside the “front”-glass give no thermal barrier for the building (we are using this option combined with air tight grills in de bottom and in the top, so we can close it in the winter and min temperature that the building will “sense” is 7°C while outside temperature is -8°C)

2 – Ventilation openings towards the inside of the building. Here the front glass needs te be very well insulating glass (U1.1) as it gives the first thermal barrier of the building. In this case the interior glass can be normal (single) clear glass. The air is heated in the winter and can be blown in the offices. In the summer the air is extracted and blown outside via AHU of cooled and recycled. Again many options here.

C/
Like I told you we used the Outside –Outside option but with airtight grills on bottom and top. But the big question is: what will the air do in this double façade?
DB has at this moment not the possibility to simulate the natural draft in a double façade caused by heat difference in bottom and top (chimney-effect)
Unfortunately we haven’t got the CFD program, so we couldn’t predict the natural air flow in the double façade.

To do this we looked into literature and wed found that 8-20ACH is a reasonable amount of natural draft in a double façade. Again this depends on wind, atmospheric pressure, temperature inside the building, temperature outside, direct solar gains in the double façade…

We have simulated the double façade over 2 levels, otherwise we would have problems with the fire brigade. We made a hole in the connecting floor and selected “murge zones with holes” in the option menu.

We have put the zone activity to cavity – but here there is again a danger. If you select cavity the convection coefficient will be set to a low value (standard is approx 4 W/m²K). This is true for passive double facades but not for active double facades.

Once there is air blowing in the double façade the convection coefficient will rise up to 25 W/m²K. In Design builder you can only enter 1 value and this coefficient is will not change due to e.g. higher AHU in the double façade. And therefore will have a deviation will the real heat transfer from ventilated zone (continuous changing) towards building.

In our case we have a bigger problem concerning this coefficient as in the winter we are closing the grills and making a Passive double façade. In this case we would need to have a heat convection coefficient of approx 4W/m²K. In the summer we open the grill (active double façade) and then the coefficient should be approx 25 W/m²K .

The set the ventilation rate in the Double façade we use HVAC – Natural ventilation 20ACH (schedule: summer 1 and winter 0.1 (due to leaks)). There was a bug in the past because he still calculated ventilation although you select no mechanical ventilation and natural ventilation (simple HVAC was then set to FCU), I think the bug is removed now.

D/
Although the assumptions we have to do, we checked the energy results with literature and experience. The result is that the energy consumptions are logic and comparable.

Also comfort is a very import factor in double façade due to the huge solar effect in the double façade. With the simulations now we have an average temperature raise of 5°C in the double façade. Again compared with literature this is a logic value.

E.g. outside design temperature during day 32°C with double façade air temperature 38-40°C. There is also a big difference in East and west side…

At this moment we are cooling the building with a VAV-system. We encounter some inconsistencies concerning HVAC. Sometimes the simulation is heating and cooling at the same time. This is for us still a “grey” zone, why is the model doing this? But we are solving each illogic aspect one by one. So that we are closer to the reality.

E/
An interesting link is the following: http://www.bestfacade.com/
When you go to calculation tool you will find a very rough energy consumption estimation of several double façade systems. Again our model has the approx. equal logic values.

With Kind Regards,

Liefooghe Jan

Project Engineer
Technum NV

DBarreraG · Posts: 13

There's something I don't get about the door operation. If an %Area door opens is defined, and I can schedule the operation (e.g. from 7:00 to 19:00 hrs), why should I need a % Time door is open? I mean, the % Time door is open is not defined yet by the other two parameters?

Thanks in advance.

AOG · Posts: 85

David,

I think this gets you a greater control level, without very complex scheduling. For example, if you schedule the door operation from 7:00 to 19:00 hrs, and specify % Time door is open as 10%, then you are assuming that the door will not be constantly opened during this period, but only for a total time of 1 hour and 12 minutes.

Best regards,

Arturo Ordóñez

Andy Tindale · Posts: 1196 Location: Stroud

The '% Time door is open' defines the average time the door is in the open position accounting for the fact that during occupied periods the door may open and close repeatedly.

The average door opening size used in the simulation is calculated by multiplying the '% Area door opens' by the '% Time the door is open' by the gross door area.

Andy

DBarreraG · Posts: 13

Hi:

Anybody knows why every time I try to define vents in my model, the visualization shows something like a wall. I mean, it might works as a vent (I don't know yet, I haven't run this particular simulation), but it should look like as well.

I have one more problem: I don't get any variation when considering and ignoring the door you can see defined in the top of the Elevators Shaft-BoxHow (0% open vs 100% open). The actual conditions and meassurments don't match with this. Is Design Buillder a robust enough tool to determine what exactly the influence of this zone (door) is? The elevators zone might works as in the "solar chimney example file".
How can I can I paste the images of the graphics in these kind of mails?

Thanks in advanced.

Andy Tindale · Posts: 1196 Location: Stroud

The vent is displaying as wall because the 'Opening' vent type selected uses the 'Brushed flat concrete' texture. Go to Openings > Vents > External, double click on the 'Opening' Vent type to change the texture.

I will change the texture for the 'Opening' vent type to use one of the more realistic vent textures for the next release.

This does not affect the simulations.

Andy

Andy Tindale · Posts: 1196 Location: Stroud

The 'door' opening at the top of the elevator shaft has little or no effect because the only other opening is the window which is set with zero opening area. Did you mean to include other openings to allow the air to flow through?

Andy

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