Keeping cool without air conditioning

How does natural ventilation work?
17 March 2020

Interview with 

Shaun Fitzgerald, University of Cambridge,


While we could reuse waste heat underground to grow crops or power our houses, we could use less heating in the first place by using something called natural ventilation.  Shaun Fitzgerald, visiting professor of sustainable buildings at the University Cambridge spoke to Georgia Mills about how we could learn from past building design to reduce our dependence on heating and cooling systems...

Shaun - Well I do have a demo and it's a demo that we can do at home in the kitchen and I've got two glasses of orange juice here and some Grenadine. So I'm going to suggest that we think about actually different ventilation strategies using my orange juice, and one is going to be for summer and one is going to be for winter. And the summer one is going to be our tequila sunrise mocktail where we have the Grenadine and we're going to pour it into the glass with the orange juice. But if you pour it in nice and slowly, just down the side of the glass, what you will find is that at the bottom of the glass we have our lovely syrup and this is showing what we call a stratified environment where the Grenadine represents your cool fresh air at the bottom, which is the occupied zone. And the orange juice is the warm air. So this is a great summer drink for a summer ventilation strategy.

Georgia - Right, so the air is sitting on top of each other. They're not mixing.

Shaun - Exactly. So we've got the warm air at the top that we don't like, but we've got the nice cool air that we brought in and that's surrounding us. Whereas in the winter, we don't want actually that classic tequila sunrise. What we want to do is to put it in really quickly and it's all mixed up.

Georgia - Oh yeah, that's terrible. If you served me that in a bar, I'd be furious.

Shaun - So it's a terrible tequila sunrise, but it is a fantastic drink for winter. And that's really the purpose of how we use different ventilation strategies, different natural ventilation strategies. And it's critical that you do different strategies between the seasons in order to derive the benefits of not using fan power, which is the fundamental reason why we like natural ventilation to reduce the energy load.

Georgia - Right, so at the moment in summer when it's warm, we're blasting the air con and that's using loads of energy. In winter, we're super cold, we've got the heating on. You're arguing that there's a better way of doing this. And that's by thinking carefully about how we mix the air, basically. How we shake our cocktails?

Shaun - We are. And indeed, the previous way of doing natural ventilation was to always just bring the cold air in at low level and have a stratified environment. So that in the winter, actually people don't have to use heaters to preheat the incoming cold air. Whereas if you have my bad tequila mocktail here, then I can use all the heat gains that Ruchi alluded to from within the building itself, to actually provide the heating for the cold air. So this is a much lower ventilation energy strategy by using mixing in the winter.

Georgia - Right. And so in the sort of metaphor of these drinks, are we at the bottom of the drink? So where the cold air sits in the summer, that's where we'd be?

Shaun - Exactly. We sit on the bottom. Gravity means that we're going to sit on the floor.

Georgia - Good. Excellent. As it should be. So how do we do this in real life, outside of the drink?

Shaun - So how do we do this in real life? We ensure that in the winter we bring the cold air in at high level, so that when it comes in, it mixes with enough of the warm room air naturally rather like an inverted volcano really. When you have a volcano erupting, what happens is the hot gases pull in, all of the cold air and you then have mixed air in the middle of the volcanic plume five kilometers up. And that's exactly what we want to do in reverse, in the winter. We want to bring cold air in at a high level, make sure it mixes with enough warm room air and you no longer have a cold draft, but you've used the heat from people, IT, lights, to provide the heating source for that cold air. And in the summer you want to do something different. You want to bring the warm air - so it's not as cold in the summer of course, but it's cooler. It's sufficiently cool to be nice - we want to bring that in at low level directly onto the occupants. So you open the windows at low level, to provide inlet ventilation in the summer and close them in the winter and bring the air in from high level. That's it. And that's what they were doing a long time ago with sash windows in Victorian houses in the UK. But they clearly didn't understand actually getting the apertures correctly and all the strategies being automatically controlled.

Georgia - How much energy savings could there be?

Shaun - Well, naturally ventilated buildings can consume a factor of two less than mechanically ventilated buildings overall. Not all of it's to do with just the reduction in fans and pumps, but the way that they're built, they need to be shallower planned so that they have lower lighting loads as well. So factor two reduction. But by applying the right natural ventilation strategy, you can reduce the energy even further. So with this new strategy of changing how you bring the air in between summer and winter, we can save heating bills by a factor of three from one strategy to the other. So it's enormous.

Georgia - So has this been brought in then? This sounds like a great idea.

Shaun - So we discovered this in about 2005 and it has now not only been adopted by buildings with products now in them, but it was adopted by school regulations in 2018. So any new or refurbished school that's going to be ventilated, needs to be thought of as wherever you can, you'd naturally ventilate. And if you're going to naturally ventilate, you need to use my mocktail example here, making sure you change the strategy between winter and summer, so that you mix the incoming cold air with warm room air in the winter to save your heating bills.

Georgia - Excellent. And so this has been actually brought in and implemented for all new schools. That's brilliant. Is it going to be used elsewhere or in other countries? How much could this be applied basically?

Shaun - So other countries with similar climates to the UK are definitely looking at this. We're in touch with those. But it does not apply to all countries. And the reason being is that if you are in a hot, humid summer environment, so if I was in Florida, I can't dehumidify the air naturally and therefore you have to mechanically ventilate in those climates. But a lot of people live in temperate zones. We've gravitated to quite nice places in the world. The North Western seaboard of the US, all of the coastal areas of Europe, they could all use this natural ventilation strategy. And that's what many people are now thinking about doing. So.


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