Naked Engineering - Natural Ventilation

Meera and Dave explore how the principles of convection can be manipulated to cool down, and warm up, our buildings...
17 October 2010

Interview with 

Alan Short, Andy Woods, BP Institute


Meera -   This week on Naked Engineering, we're going to be looking into how simple principles of physics can be used when designing buildings to cool them down without the need for energy consuming air-conditioning.  To investigate the design of these low energy buildings, Dave and I have come along to the BP Institute here at the University of Cambridge.  Now Dave, to start things off, this all basically uses the principles of convection, doesn't it?

Dave -   Yeah.  Convection is a major thing which drives fluid flows all over the universe in fact, from the sun to the deepest ocean and in a house.  So I've got a nice little experiment here to show you what's going on.  I've got a little bottle filled with red fluid.  It's just some hot water with a little bit of food colouring in it.  When water heats up, it expands.  It becomes slightly less dense.  So we then put it in this small fish tank full of cold water and then take the lid off, the slightly less dense red warm water should float up...

Meera -   Yes, the red food colouring is moving up through the water towards the top.

Dave -   That's right and so, the warm fluid will float upwards and because its place is takenLanchester Library, coventry by cold water, it's pushed up and you get a circulation.

Meera -   Well whilst that colouring spreads through, let's look into how this principle can be used to actually design naturally ventilated buildings.  So joining us this week are Alan Short,  Professor of Architecture here at University of Cambridge, and Andy Woods, Professor of Fluid Mechanics.  Now Alan, how do you actually adapt this when designing buildings and therefore, take into account air movement for the first low energy building?

Alan -   Well we had a wonderful opportunity to make quite a big building in Malta.  A very simple building.  It was an industrial building for a brewery.  We had to make one big space that would stay as cool as possible through the summer in Malta where the temperature cheerfully gets up to 40 degrees centigrade quite often.  The building didn't have very many people in it at any one moment so we didn't need to supply them with much fresh air during the day.  So our strategy was to make a very heavy weight building, very difficult to change its temperature quickly, and then as soon as the sun went over the horizon, we'd open up our chimneys and vent it as fast as we could.  We achieved up to 12 full air changes an hour by about 2 o'clock in the morning and the building starting to actually respire - you could see it breathing and then by dawn in the following morning, when the temperature inside is exactly the same as outside, you shut it up as quickly as you can, and you live off the coolness of the rest of the day.

Meera -   And Andy, you look into the fluid mechanics really behind this flow of air.  So how does it actually work and what controls the flow of air through a building?

Andy -   Well essentially, the dominant principle driving the flow is a combination of wind driving forces and buoyancy forces, and the buoyancy forces arise from the fact that air inside the building is typically warmer than the air outside the building.  And so, that air wants to rise, relative to the air outside the building because it's less dense.  So if you put an opening at the top of the building and the base of the building, the warm air will tend to rise out the upper opening, bringing in cooler air from the outside.

Meera -   And to encourage this, you use stacks on your building?

Andy -   And so stack is really like a large chimney on the top of a building.  It provides an extra vertical distance through which you can have the warm air as it rises out through the building.  And essentially, this increases the pressure driving the flow between the inside and the outside of the building.

Dave -   So this is actually the reason why we have chimneys anyway.  So you have a big column of hot air above a fire which floats upwards very strongly, so you get a big draft going up the chimney which pulls all the smoke out.

Andy -   That's exactly the case.  And of course, with the chimney, you want the smoke to vent out of the building, but in natural ventilation, we're also trying to bring air into the building.

Meera -   So to actually show this in action, you've got a water model here.  So it's a plastic box which represents a room, but it's tipped upside down so the floor is at the top and then the ceiling is at the bottom.  But coming in through the floor, you've got a vent and out of the ceiling, there are two chimneys essentially and you can see a good movement of water through here.  What's actually happening in this model then?

Andy -   Okay, so this is what we call a water bath model of natural ventilation.  We're using salt as an analogue system to model the density changes associated with heating and cooling air.  So, salt water is heavier than fresh water and so, salt water sinks.  And so, if we turn the system upside down as we have in the experiment, we can put a source of salt water to model a heat source, and so we've dyed that red and you can see a turbulent plum starts rising off this little model radiator, and it rises to the ceiling in the tank, taking up a lot of the air in the room as it goes.  We've also got a hole in the floor of the tank and you can see blue fluid is coming through there and this is the cold outside air, providing fresh air into the building.  This is what's called a conventional displacement ventilation system that you'd use in the summer.  This is a very effective way of removing the heat from the building because as you see, this red plum of heat goes all the way to the ceiling.  And even though it does mix, it produces a layer near the ceiling of hot air which goes straight out of the space and the occupants near the floor are actually immersed in this colder layer of outside air that you can see below that interface.

Meera - Now this model is great, it clearly shows the movement of air through a building. But Alan, how do you actually adapt this for different conditions of different buildings?

Manchester Contact TheatreAlan - there are many different recipes for different types of climate, and different times of the year. You can manipulate the air before it gets into the space at the bottom, if you capture it in a plenum first you can pre-cool it or pre-warm it naturally or help it along a bit. If you back up to the top, you can pre-cool air as it comes in and drop it into the space, which is very effective. You can do that in a dry version or you can mist water vapur into it which is even more effective. And you can the exhaust stacks as well just to try and induce a flow.

Dave - So this is increasing the flow because you're making the air in the stacks even hotter than it would be naturally, driving more comvection and more flow.

Meera -   And coming back to your water model Andy, I can see here that the hot air, or hot water in this case, is still moving through the building and out of the stacks at the top. But what about during winter?  Could this be changed to keep a building warm as well?

Andy -   Winter presents a very interesting challenge because if you look at the way this system is running at the moment, the air is coming in through the low level in the building from the outside and then the warm air is venting out through the ceiling.  And so, we're actually losing a lot of heat out through the ceiling to the outside.  In many buildings in the UK today, the heat generated in the building from the occupants of the building who may be using computers, or doing other activities in building, is typically sufficient to actually keep the building warm during the occupied times.  And so, what we don't want to do is actually vent away that heat through the stacks.  And so, in winter, there's a very simple thing we can do to actually change the whole of this process and I'll show you this by just putting this bung in, essentially closing the window at low level, and what you see now is that the inflow at the floor has stopped and the two stacks are now doing something very different.

Meera -   There's actually water flowing out of one and it's coming in through the other one now.

Andy -   That's right because we stopped the air supply at the floor level, but we still have a space which is warmer than the outside, the warm air leaves through one of the stacks and to replace all that air, air has to come in through the other stack. This is very interesting because the air that's coming into the building there develops a cold plume that you can see falling towards the floor.  And as it falls towards the floor, it mixes with the warm air in the space, so by the time it's reached the floor, it's actually much warmer than the outside temperature.  So this is a way of ventilating the building in winter when you've got a lot of heat generated in the building without needing to provide additional heat to preheat the ventilation air.  And so, that provides a low energy solution to actually keeping the natural ventilation in the winter.

Meera -   Now Alan, we've discussed here then that buildings can be cooled down and also a higher temperature can be maintained, but just how much - so what are the actual temperatures we're talking about here?

Alan -   Well, it's surprisingly effective. The library that we all designed in Coventry, the Lanchester library for the university.  I think the temperature there has never been recorded as being above 26 ½ degrees centigrade, but we know that it's got to about 33 ½ outside, so that's quite a lot of free cooling.  It's 110,000 square feet - that building.

Meera -   And as well as monitoring the temperature though, it's a low energy building.  So how much of a reduction is there in the energy consumption say, compared to using air-conditioning?

Alan -   Well air-conditioning is a huge energy guzzler because you refrigerate the air before you heat it back up again, so a naturally ventilated office building should be using about 1/3 of the energy of the full fruit salad air-conditioned thing.

Meera -   But this is all for the design of new buildings.  What about the current buildings that we have?  Would it be possible to retro-fit these buildings in order to give them natural ventilation as well?

Alan -   Yes, it certainly is and in fact, that's much more important than the design of new buildings because most of the buildings that we know now will be here in 50 years time.  We're very interested in '60s and '70s buildings which tend to be concrete framed with lightweight envelopes.  You can easily start adding new things to the facade.  This is a fantastically rich and interesting area of work.

Meera -   Okay, well that's great.  Thank you, Alan and thank you, Andy.  That's it from Dave and I this week, but look out for more of these naturally ventilated buildings in the future.

Dave -   That was Meera Senthilingam who joined me with Alan Short and Andy Woods from Cambridge University's BP Institute.



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