Passive and active cooling with concrete floors - The Concrete Centre
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Introduction Concrete Floor Solutions for Passive and Active Cooling
Hello, my name is Tom De Saulles and I’m a building services engineer working at the concrete centre. I’m going to talk to you today about heavyweight high thermal mass concrete floors and how they can be used as part of a passive cooling strategy in buildings. Im going to run through the main system options and also show you some examples of buildings that use these systems. Now everything I'm going to talk about his covered in this publication called concrete floor solutions for passive and active cooling. If you want to copy of that, you can download it from our website and address for that is www.concretecentre.com.
Now the use of heavyweight’s concrete floors for cooling, started to become popular back in the mid to late 90s, when a series of under occupied fairly prestigious headquarters buildings were constructed. And these all had exposed concrete soffits which an integral part of the cooling strategy. And since then the technique has started to find its way into more mainstream construction, including speculative offices. There a few examples in this slide.
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Benchmark CO2 Emissions from Office Building
And really the reason why its growing in popularity is because of course enhances the public energy efficiency which in turn, cuts the CO2 emissions quite significantly. And this graph I think makes the point quite well and what it shows you is the emissions from a typical air conditioned office, also for good practice office with energy efficient shares and good solar shading etcetera. And then finally you can see on the graph the type of building that I'm talking about which is the heavyweight high thermal mass office which incorporates some form of passive or active cooling system. And you can see emissions for that type of building are considerably lower.
Now going back a few years again there really only two options to choose from you could go for a Bespoke Concrete Floor made with in situ ready mixed concrete and it might be a coffered profile like the image to the left of this slide. And that might also be combined in under floor ventilation, this approach is very popular in the headquarter buildings I mentioned just now.
The only alternatives that really was the TermoDeck system which is an off the shelf solution that uses 3 carts hollow core concrete slabs that are combined with a mechanical ventilation system.
Now these both excellent options still available today but what's changed over the years is that we've had a new systems come on to market which you can see here. So I’m going to briefly run through these. Before I do that just a few words on why it is, and sockets are very good at providing cooling. And it's essentially because we lose about 60% of our body heat by radiation. And in order to be able to do that effectively, we need to go to physically see another surface that is cooler than we are and of course the soffit designing and ideally placed to do that it's never very far away, distance is also important closer to services the better. It doesn't tend to get covered up like floors and walls do, and it has a course it has a very large service area which helps. And it is made of concrete and it has a high thermal capacity so that means it can absorb an awful lot of heat during the day, whilst maintaining a pretty stable surface temperature so it's always a lot cooler than we are. So that's why we tend to use heavyweights’ soffits for cooling.
Exposed Soffit, Natural Ventilated Building
Now the simplest option is just to have a plain flat concrete soffit, could be precast, could be in situ doesn't really matter and that's in a building that's naturally ventilated so from windows that can be opened and importantly at night because of course we need to get rid of excess heat that's built up in the fabric during the day. Then in that building will probably get around 15 to 20 watts per metres squared of cooling directly from soffit and that's not a huge amount but it's enough to ensure that you won't have overheating problem in buildings with low internal heat gains and a low occupant density.
Now I should just point out, on top of that 15 to 20 watts you can also get up to 40 Watts of cooling from the natural ventilation depending of course on the weather conditions. I'll show you an example of a building that uses this approach.
Cambridge Federation of Women's Institutes
This is the Cambridge Federation of Women's Institutes building which is actually a converted farm building and they've replaced the roof with precast slabs to form a simple monpitched roof that insulated on the outside, so soffits exposed on the inside which is of course very important for the thermal mass. It's well ventilated lots of openings on the front and it's got ventilation grilles on the back wall to provide secure night time ventilation.
So very simple designs are very effective, and the occupants are extremely happy with performance of this building.
National Trust headquarters in Swindon
Here’s another example. This is the National Trust headquarters in Swindon slightly more sophisticated in as much as it's got mechanical ventilation system but primarily relies on natural ventilation during the summer. And here you can see a view of the soffit in one of the office areas again just ordinary precast concrete planks painted with white emotion very simple finish but I think the most interesting feature of this building is actually a trim where they've incorporated precast panels concrete panels in the roof structure and there simply there to provide additional thermal mass so they're not structural in anyway.
Woodland Trust Headquarters, Lincolnshire
Now another building uses a similar approach is the Woodland Trust headquarters same architect filled in playground lee and unsurprisingly this is a timber buildings actually made with cross laminated timber so it's fairly lightweight construction and in order to ensure that it doesn't overheat during hot conditions they've incorporated some precast concrete panels on the soffit there simply bolted to the cross laminated timber soffit and these are painted white which you can see here in this photo.
Exposed Soffit, Underfloor Mechanical Ventilation
Right the next system is where you have a coffered slab with under floor ventilation knuckles coffers are used to reduce the weight of the floor so that you can increase the span but they also have the added benefit of increasing the surface area so you got more area to get heat in and out of the soffit which in turn improves the cooling performance if you combine that with under floor insulation so you've got air in contact with the top of the slab as well you've now effectively doubled the surface area through which you can get heat in and out of the concrete slab so really your making maximum use of lethal mass and that's why you can get up to 35 Watts per metre squared cooling with this system.
Canon Headquarters in Surrey
Here’s an example of building that uses this technique this is the Canon Headquarters in Surrey and I could have also shown you the Toyota Headquarters in the UK or the Powergen building which is actually the first to use this approach all very similar they are all built over three floors for that a large central atrium. They have printer windows that can be opened and importantly they have an open balcony arrangement so that air from those windows can travel freely across the soffit and into the atrium and as it warms up becomes more buoyant rises up and leaves the building at high level through the openings you can see in this slide. And incidentally these openings quite often also double up as part of the smoke extract system so it's quite an efficient design.
Now they’ve all got some mechanical ventilation well under floor mechanical ventilation that tends to get used mostly in the heating season when you want to keep the windows shut and keep your ventilation to a minimum but it can also be used during the summer when you need some additional ventilation perhaps on a hot summer night when conditions are quite still and you need to get some air moving through the building.
Exposed Soffit, Hollowcore Slabs with Mechanical Ventilation (TermoDeck)
The next system is the TermoDeck system which is very popular in the UK being used quite a large number of buildings been around for a long time and it essentially uses precast hollow core slabs that are linked to mechanical ventilation system so you've got air passing through those cores and I’ll show you how that works in this slide so here we’ve got a cutaway of one of the slabs and what they do is to drill out the webs the concrete webs between adjacent cores so that air can get from one core to the other and then it travels up and down cores in a Serpentine arrangement before it enters the occupied space through a diffuser on the underside of the soffit so you've got very good contact between the air and the concrete so very good heat transfer and that's why you can get up to 40 Watts per metre squared of cooling.
Now, it also works really well in heating season because what you can do is at the end of the day you can recirculate the warm air in the building through the slab and the concrete picks up that heat hangs onto it until the next day and when the occupants come back into the building and the ventilation switched on so you've got cold fresh air coming into the building, the heat in this soffit warms that fresh air and of course reduces the load on the heating system.
Jubilee Library, Brighton
Now here’s an example building that uses TermoDeck system as I said earlier there's a very large number of buildings in the UK that used TermoDeck this one is Juliee Library in Brighton very heavyweight construction. Here you can see the South facade she's high delays so very good day lighting but also very good for some passive solar gains during the heating season. And I’ll just show you one of the perimeter reading rooms which uses the TermoDeck system.
Pretty unremarkable just plain flat concrete soffit painted with white emulsion, you can see in the shot one of the supply diffusers and we've got suspended light fittings with acoustic wings. That’s a very standard featuring in all of these buildings because of course you've got a hard concrete soffit and you need to control the reflected sound off that soffit and acoustic wings work extremely well.
Exposed Soffit, Insitu or Composite Slab with Cast in Aluminium Air Ducts (Concretcool)
The next system is called Concretcall this is a German system being used for number of years in Germany and to date I think it’s being used in around quarter of a million square metres of office space. Very similar to TermoDeck in the way it works the key difference is that this can be used with in situ concrete slabs if you remember the TermoDeck system uses precast slabs and what you do is to embed these circular aluminium air ducks in the lab. These come in two sizes 60 and 80mm. And they’re made by a German air conditioning company called Kiefer. I have a sample here to show you and it has a lot of internal fins to maximise the service area so you get really good heat transfer between the air and the aluminium duct and in turn the concrete. And that's why you can get up to 65 Watts per metre squared of cooling with this system.
4 West Building, Bath University
And the only building this is being used in so far in the UK is the 4 West Building at Bath University which is 6 floors of office space and teaching areas and when it was completed back in 201,0 it got a BREEAM excellent award.
I’ll show you one of the floors slabs being constructed and you can see the orange formwork with the reinforcing mesh placed on top of that and then we've got the Kiefer aluminium air ducts. These incidentally here all pre-cut at the factory and when they delivered to site all you’ve gotta do is just assemble them, which is very easy job. There's still some reinforcing to go on top of the aluminium ducts. So the ducts themselves are actually in the centre of the slabs so they’re not in any areas of tension or compression so they’re not affecting the slab structurally in anyway at all.
The next system I want to show you uses water rather than air to regulate the soffit temperature and that has a number of advantages not least of which is that you can control the soffit temperature 24 hours a day if you need too so you’re not reliant or night time cooling to regulate temperature which is of course subject to the vagaries of the weather.
Now quite often with this system the water is supplied from a borehole going down to aquifer and that brings water up at around 13 or 14 degrees which is ideal and will give you the maximum cooling output possible from your soffit. You don't actually want to add water at a lower temperature than that because you run the risk of getting condensation forming on the soffit and another thing you can do in the system is to connect it to heat pump so you can boost the temperature of that water coming out of the aquifer and use it to heat the slab during the heating season and provide some radiant heating.
Exposed Soffit, Composite Lattice Girder Soffit Slab with Embedded Pipework
Now the side you can see at the moment shows a composite lattice girders soffit slab which gives quite a high quality finish to the soffit and I'm going to show you an example building that uses this approach.
Manchester Metropolitan University Business School
This is the Manchester Metropolitan University Business School very large project completed in 2011 and here you can see one of the floor units being craned into position they’ve actually used rectangular polystyrene void formers and this what the units look like once they’re craned into position.
If you look to the right of this slide you can see hopefully that the columns are reflected in the soffit which just shows you how good the surface finish is and on this particular project they’ve achieved it using a blend of 75% white cement and 25% ordinary Portland cement and although that increased the cost of the precast units they actually found that it saved money overall because of course they didn't have to paint the soffits and this shows you the finished result I'm just wanting to point out in this slide is the way they've used the rebate between precast floor units to concealed the wiring for the lighting which is quite a neat touch.
Exposed Soffit, Hollow Core Slabs with Embedded Pipework
The next system I want to show you is very similar in terms of cooling output which is around 65 Watts per metre squared. The only difference is that now we've got a precast hollow core slabs with embedded pipe work instead of a lattice girders slab it's a slightly cheaper option and it has a slightly more utilitarian surface finish but once it's been painted is absolutely fine. Now the cover over the pipe work is about 60MM so it's very well protected but just to be sure there's no damage on site and what tends to happen is that the contractors are issued with shanked drill bits which will ensure if they’re drilling into the soffit to mount something the drill bit cannot reach the pipe work. Now with both systems the pipe work is pressure tested at the precast factory so once it's delivered to site it just has to be connected up to a distribution manifold and it's ready to go.
Vanguard House, Daresbury Science and Innovation Centre
Vanguard House which is a research facility in Cheshire uses the hollow core option. This got a BREEAM excellent rating when it was completed in 2011. Here you can see the hollow core units in position.one thing to mention about this option is that the hollow core units have top strand reinforcing which ensures that the soffit is absolutely flat. Now typically hollow core slabs don't have top strand reinforcing and as a consequence of that sometimes you can see a slight camber when you look up at the soffit but that's not the case here. Now if you look to the left of the photo you can see all of the pipe work sticking out the soffit and this is Polly beauty Lane or pecks pipe work and it's ready to be connected up to the manifold distribution manifold that'll be concealed in a bulkhead along with the steel beam. And this shows you the finished result after it's been painted with white emulsion pretty good surface finish and once again you can see the use of acoustic wings to control the reflected sound and the only thing really to say about this building is that it also uses night time ventilation to cool the soffit down and they’ve done that to maximise the fabric energy efficiency of the building.
Chilled Beams with an Exposed or Partially Exposed Soffit
The final system I want to show you is when you use a combination of chilled beams with an exposed concrete soffit. Now the to work very well together because the output from the beams is nearly all convective whereas the cooling output from the soffit is mostly radiant and you need a combination of the two to achieve comfortable conditions. And the beam itself can give you 160 Watts for metres squared or more of cooling, which is very high. The soffit will give you around 20/25 watts metres squared of cooling and this approach is popular in new build but it’s also popular in refurbishment projects, particularly where you've got concrete frame office buildings from 1950s and 1960s. And these buildings tend to have fairly poor slab to slab heights by modern standards and if you want to put in a raised floor which invariably do, for your cabling then that just compounds the problem. But what you can do is strip out the full ceiling and make good any blemishes in concrete give it a coat of white emulsion and put in some low profile multiservice chilled beams, that will provide the cooling, the lighting and other services such as fire alarms and smoke detectors.
Conquest House, London WC1
Here's an example of building that uses this approach is Conquest House in London which is an old 1950s concrete frame building now occupied by an international Business School. And as I described they’ve essentially stripped out full ceilings painted the concrete white emulsion it was fairly good surface finish so that was an acceptable thing to do put in some chill beams and also put in a raised floor. And I think you’ll agree that it’s a pretty good standard of finish.
Right that’s the end of my talk as I mentioned at the beginning everything I covered is included in this publication and you can download from our website which is www.concretecentre.com and it you'll find this graph which summarises all of the systems I’ve spoken about in terms of the cooling output and has also got a few additional systems in there and its just quite a good way of comparing performance.
Finally also if you go to our website specifically the publications area you can find the publication on the right called thermal mass explained and if you're not familiar with thermal mass it will give you a pretty good introduction to what it is and how it could be used.