The Stack Effect is when air is moved into and out of buildings by means of a difference in the buoyancy of the air on the inside and on the outside – between air that is colder and air that is warmer, in other words. Buoyancy can be either positive or negative – in the sense that warmer air can rise to be replaced at the bottom by cooler air, or cooler air can sink to replace warmer air at the bottom. Positive buoyancy is when warm air rises, and negative is when cool air something sinks. Got that? Both are good, if you’re trying to keep a building cool.

Or warm. Before I go any further, let’s not forget chimneys, sometimes called chimney stacks. Here’s a link to the history of the chimney. The first known English example of a chimney is at Conisbrough Castle from the 12th century, but they didn’t become common in houses until the 16th. Until then, life without chimneys was very very unhealthy.

It still is, in many parts of the world. According to a World Health Organization report, “Nearly 2 million people die prematurely from illness attributable to indoor air pollution from household solid fuel use.”

Early chimneys were an improvement, but weren’t perfect. Count Rumford, whom you’ve already met, invented the Rumford Fireplace in 1797. He made two important improvements – one was to change the shape to direct more radiant heat back into the room, and the other was to restrict the size of the chimney at the bottom to increase the speed of the smoke going up the chimney. The streamlined smoke flow generating the stack effect had the added advantage of sending the non-smoky but heated air back into the room.  Importantly, existing fireplaces could be easily modified.

To many, these new and better fireplaces looked strange. They were shallow and their sides were angled and they were taller than they were wide.

Lloyd Alter, over at treehugger, can tell you more about modern Rumford fireplaces. Interestingly, a fireplace has an optimum burn rate that is set to something like “roaring” – the heat should be burning off the soot at the front of the fireplace, as in the photo above. (Thanks for that Lloyd.)

Finally, to end this introduction to the stack effect, here’s a photograph you’ve seen before, of some chimneys that may have been admired by Walter Gropius for their stack effect. Or maybe not.

Conventional chimneys make use of positive buoyancy from the bottom, as do some solar chimneys that have been recently proposed to generate electricity.

Here’s a schematic configuration of a solar chimney power generation plant. [Thanks climateandfuel.] It’s usually proposed that the space beneath the heat trap be used to grow tomatoes. Whatever.

Other solar chimneys use the sun to create a positive buoyancy in the air at the top of a chimney and thereby draw cooler air into the building to replace it.  Here’s a CAD model that’s easy to understand. Chimney heats air, causing it to rise and draw air through the building albeit at different rates for each floor.

This principle is used by the University of Washington’s University of Phase 1 Molecular Engineering Building (MEB) except that the rates are the same as each floor now has its own solar chimney. You’ll find more facts and performance data here. These stacks are really turbine ventilation inducers in which

a glazed panel was incorporated in the west-southwest orientation of the stack, inducing a solar assist to airflow by increasing the buoyancy of exhaust air during peak summer months.

Here’s a schematic of a building that uses passive energy to draw passively cooled air through a building.

It describes exactly the same principles known in Persia in the first millennium BC.

Here, this windcatcher is coupled with a qanat which, you’ll remember from the post on yakhchal, is an underground canal (aka underground heat exchanger). Windcatchers come in many types depending upon their function and the direction or directions of the wind. They also trap a lot of heat because the µ-value of mud brick isn’t that high. This means that they function as solar chimneys when there’s no wind.

A windcatcher doesn’t have to be made out of mud brick to catch wind. However, if mud brick is what you have to build with, then you also have a solar chimney and a stack effect. This stack effect still exists when the tower is wind-driven but is insignificant compared with air movement generated by the pressure difference caused by the Bernoulli Effect. Given their names, it’s unsurprising that the stack effect of windtowers and windcatchers is overlooked but, even if wind had nothing to do with driving them, they would still be a tribute to human ingenuity and intelligence, providing cooling when and where it was needed.

* * *

The term ‘solar chimney’ threw up this image, from www.jeremylevine.com. I’m including it as an endnote because it seems to link the stack effect with recent concerns such as courtyards, gardens, biophilia, and SANAA.

Pocket Courtyard

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By 400BC, Persians had developed a system for making ice in winter and storing it throughout the summer and in a hot desert climate, in buildings they called yakhchal.

Most of what you’ll find written about yakhchal on the internet seems to come from wikipedia. Most diagrams come from one academic paper but images are from several private sources. Yakhchal and the idea of ice cream before refrigerators appeals to many people – some for its sheer ingenuity and inventiveness, some for reasons of cultural grandstanding, some for the genius of vernacular problem solving, and some for the low-energy aspects of energy storage that may have application today. Not least of all,

thermal energy storage using ice is practical because of the large heat of fusion of water. One metric ton of water, one cubic metre, can store 334 million joules (MJ) or 317,000 BTUs (93kWh or 26.4 ton-hours). In fact, ice was originally transported from mountains to cities for use as a coolant, and the original definition of a “ton” of cooling capacity (heat flow) was the heat to melt one ton of ice every 24 hours. [W]

The following is a summary of what’s involved.

1. Even cities in Persian deserts were serviced by a system of underground canals called qanat. Here’s how they work.
2. In winter, water from these qanat was led into channels and allowed to freeze overnight. High walls shaded these channels from the sun from the south and often from the east and west as well. The walls also protected the channels from the wind to facilitate freezing. Ice was made in layers over several evenings, and when it was about 50cm thick, was cut into blocks and stored in the domed yakhchal building. The door was sealed at a special ceremony and opened in summer at another.
   .
3. This all sounds very simple. However,if we have a piece of ice in shallow ponds with a determined height, and some water with a diameter equal to “s”, the following relationships can be expressed to determine the size of the ice made, where Eq 1 describes the cooling of water up to freezing point and Eq 2 describes the freezing of water in a layer of water at zero degrees.
   Eq 1: (Qr1+Qe1+Qc1-Qs1)∆t1= ρw*As*Cw*twi
   Eq 2: (Qr2+Qe2+Qc2-Qs2)∆t2= ρw*As*hif
   where:
   Qr1: transitive heat of radiation
   Qe1: transitive heat of evaporation
   Qc1: transitive heat of convection
   Qs1: transitive heat of water above ice surface
   ∆t1: Time to cool the water to zero degrees C
   ∆t2: Time to freeze the water
   ρw: Density of water
   As: Area of ice surface in cavity
   hif: Freezing enthalpy
   twi:  Primal temperature of the water
   Cw: Specific Heat of waterTherefore, the size of the ice the can be made can be calculated from Eq 3.
   S´=ρw * s/ρi* ∆t
   where ∆t=∆t1+∆t2 [thanks Mahdavniejad and Javanrudi] I only mention this to show that even just making the ice involves many factors that need to be understood empirically, if not theoretically. Just filling a hole with water on a winter’s night will not guarantee timely, sufficient and good quality ice.
4. The yakhchal storehouse itself was designed to keep the ice frozen for as long as possible and it did this by several means.
   1. Thermal mass: The section shows two parts – the dome and the pit beneath it. Warmer air rose and was vented from the top of the dome, whilst cool air remained in the underground portion that was already insulated by the ground. The pit could be as large as 5,000m³ and the dome span as much as 11m.
      
   2. Insulation: The walls of the dome were at least two metres thick at the base, and made of mud brick coated with a special waterproofing render composed of sand, clay, egg whites, lime, goat hair, and ash. This mortar had excellent insulating properties. I can’t find any information for how the optimum ingredients or mix for the mortar were discovered. I can imagine the goat hair may have functioned like the glass fibres do in fibreglass, but what properties do the egg whites add to the render? And how did anyone know they had those properties? There must be some lost science here because the idea of adding egg whites and seeing what happens is not random. Somebody thought about it and said “I think adding some egg whites might do the trick!”
   3. Evaporative cooling (?): Here, accounts differ. One of the most reblogged sentences about yakhchal states that the “continuous cooling waters that spiral down its side keep the ice frozen throughout the summer” but I could find no reference to these domes being shaped to allow water to spiral down the exterior, evaporating, and thus evaporatively cooling it as it “spirals down”. Presumably, somebody would have to climb onto the building and pour water over it. However, Hosseini and Namazian write that
      

      One of the advantages of these vaults was that they could be built step-like. They used stairs to help workers to cover the external crust of the vault with thatch to protect it from rain, snow, sun and atmospheric variations. They built smaller stairs between these stairs so workers could maintain or repair them easily.

      This seems more likely, especially since the domes don’t look especially spiral. (See also archnet.) However, it could just be that the stairs were used to cover the vault with straw during the day to prevent heat buildup, and the straw removed at night to facilitate night sky radiant cooling of the dome. We already know that the ancient (and not so ancient) Persians knew about night sky radiant cooling to make ice, so why not use it to cool the dome as well? Hosseini and Namazian say the straw is to “protect the dome from atmospheric variations” – and so it would during the day but, removing the straw at night would cause the dome to act in synergy with atmospheric variations.

Applications: The ancient Persians had already worked out how to passively cool  buildings. Their knowledge of orientation, sun angles and their invention of different types of wind towers for different uses is well documented. The most common type of wind tower draws air out of the building so it can be replaced by air cooled by those same underground qanat.  Such wind towers (called bagdir = Persian: بادگیر‎ bâdgir: bâd “wind” + gir “catcher”) were often positioned in fours around water cisterns. In the sense that yakhchal use the off-peak power to store energy in the form of ice, they are the forerunners of modern-day thermal energy storage systems. Since A/C was sorted, the manufactured ice was used in summer to cool drinks and make

Faloodeh, faludeh or fālūde (Persian: فالوده‎), which is a Persian cold dessert consisting of thin vermicelli noodles made from corn starch mixed in a semi-frozen syrup consisting of sugar and rose water and is often served with lime juice and sometimes ground pistachios. [W]

.

The recipe for this has not changed since 400BC. At that time, faloodeh, ice cream and other chilled desserts and drinks would have been only for the wealthy only but, by 1870

there were plenty of yakhchals in Isfahan; some of them were for private use. Nevertheless, the poor could also use the yakhchal to cool water. Sherbets and fruit were preserved with ice in all shops. Huge chunks of ice were carried by donkeys and sold all over the province. In Isfahan, people could buy ice either in the bazaar or straight from the yakhchal building. (Ernest Holster*)

A “yakhchal”, built near Kerman, Iran (Photo credit: Wikipedia)

Further reading:

(1) “An Overview of Iranian Ice Repositories, An Example of Traditional Indigenous Architecture”, Bahareh HOSSEINI, Ali NAMAZIAN 

(2) “Historical Ice Houses: Remarkable Example of Iranian Cultural Heritage”, Amirkhani ARYAN, Okhovat HANIE, Pourjafar Mohammad REZA, Zamani EHSAN

(3) “An Overview of Some Vernacular Techniques in Iranian Sustainable Architecture in Reference to Cisterns and Ice Houses”, Amir Ghayour KAZEMI & Amir Hossein SHIRVANI 

(4) “Assessment of Ancient Fridges: A Sustainable Method to Storage Ice in Hot-Arid Climates”,  M. MAHDAVINEJAD, Kavan JAVANRUDI

*Ernest Holster was a German photographer kicking around Persia at the time.

Many architects look to Nature for inspiration for how to make their buildings look more natural even though buildings are very unnatural objects. Instead of actually growing out of the ground or landscape like plants, buildings are constructed from putting lots of building materials together and it takes a lot of people, resources and money to do that. We can conclude two things from this.A building that looks as it it’s built out of lots of building materials will:

1. have more integrity than a building that, say, is trying to look like a rock or a tree or a cloud or a river and
2. will therefore cost less than an equivalent building encrusted with ornament or trying to look like something it’s not.

As you would expect, misfits is totally against biomimicry in the sense of making buildings look like the result of natural processes like, say, this building does

or the architecture of Greg Lynn

(and its brand extensions).

However, misfits is all for the use or adaption of natural world processes if better performing buildings in the artificial world are going to be the result. Biomimicry is passive design, basically. Biomimicry is to building performance what ‘organic’ was/wasn’t to building aesthetics.

The air conditioning system design for the Eastgate Building in Harare, Zimbabwe is famously mentioned as an exemplar of biomimicry for having been modelled on the configuration that Macrotermes michaelseni (termites, to us) use to maintain the temperature inside their nests. Dave Parr, over at biomimicron.wordpress.com, coins the word biomythology for the claim that these termites maintain a temperature of 87 ± 1°F inside their nests, and offers us the following graph from this interesting paper.

Parr,D., 2012, Biomimicry Lessons for Building Ventilation, Submitted for qualification of Environmental Design of Buildings MSc, at Cardiff University – Welsh School of Architecture. (Thanks Dave – good luck!)

The graph shows that the temperature is anything but constant and sometimes even greater than ambient temperature. From this I conclude that this entire system, ingenious as it is, is to encourage ventilation in order to prevent excessive heat buildup rather than regulate to temperature per-se. But let’s give the architect, Mick Pearce and those nice folks at Arup some credit for trying to learn something from termites.

Let’s also give the termites some credit because – unlike the Eastgate Centre – they don’t use any electricity to open and close the air intake vents and nor do they use fans to regulate the flow of air and the internal temperature. If ants can develop such a system perhaps we should at least try to do something similar for our own structures and maybe even improve upon it if we can. With these termite nests, all the action is really happening underground where there’s less diurnal variation in temperature.

For the ants, the towery bit adds to the convection effect. For the architects, juxtaposing photos of their own towers adds to the eco-cred effect.

Leafcutter ants and their nests are perhaps even more impressive but don’t get as much publicity since they don’t build towers but high-density lo-rise. Here’s what a leafcutter nest looks like above ground. The average population of a nest is about 5 million. The average size of a nest is about 3-4 metres across like this one.

Here’s a link to a larger one being excavated.

Again, most of the action is underground. Now these leafcutter ants are pretty amazing creatures.

• There are 3 types of leafcutter ant: the queen, soldier and worker and there are five types of worker ant: foragers, gardeners, those that chop up leaves, tiny ants that distribute leaf bits to the fungi and those called minimae that tend the fungus and dispose of waste produced by the fungus. Different types have different sizes.
  

  A forager ant (Photo credit: Wikipedia)

• The forager ants leave a chemical trail so they can always find their way back to the the nest. They collect leaves from all layers of the forest, from the floor to the upper canopy. They are capable of carrying over 50 times their own body weight. They will travel several hundred metres in search of the right kind of leaves.
• Leaves brought back to the nest are used as a fertiliser for a fungus ‘garden’ in the nest. This fungus is the big grey mass at the centre of the nest. This fungus is the only food source for the ants. Not only that, if the ants bring the wrong kind of leaves, the fungus will create a chemical to tell the ants. Wild ants collect leaves from all layers of the forest, from the floor to the upper canopy.

There are other amazing things about these ants but here’s a schematic of a nest.

• When the nutrients have been removed from the leaf material, the waste is transported to the peripheral dump chambers where dead ants and dead fungus are also placed.

Kleineidam C, Ernst R, Roces F have researched how these nests are ventilated. This is the abstract of their paper,

Wind-induced ventilation of the giant nests of the
leaf-cutting ant Atta vollenweideri

“Surface wind, drawing air from the central tunnels of the nest mound, was observed to be the main driving force for nest ventilation during summer. This mechanism of wind-induced ventilation has so far not been described for social insect colonies. Thermal convection, another possible force driving ventilation, contributed very little. According to their predominant airflow direction, two functionally distinct tunnel groups were identified: outflow tunnels in the upper, central region, and inflow tunnels in the lower, peripheral region of the nest mound. The function of the tunnels was independent of wind direction. Outflow of air through the central tunnels was followed by a delayed inflow through the peripheral tunnels. Leaf-cutting ants design the tunnel openings on the top of the nest with turrets which may reinforce wind-induced nest ventilation.” (Kleineidam et al. 2001:301, from asknature)

Biologist Bert Hoelldobler at Arizona State University says that ant colonies provide scientists with an invaluable way to gain empirical data around how living in societies developed. Compare the biological blueprint of an ant society with that of humans, he says, and you quickly see that much of human society is built on culture rather than genetics. The basic blueprints for society in our genes are much simpler than those coded within the ant’s.

Biologists Wilson and Hoelldobler have proposed a new class of life: the superorganism. “A superorganism is a closely knit group that divides labour among its members altruistically,” says Wilson. “There are individuals who reproduce in the group and are promoted to be reproducers, and those that do not reproduce and are workers. This allows the group to function as a giant organism.”

In their recent book, Superorganism, Wilson and Hoelldobler describe their idea by comparing each ant in a colony with a cell in, say, the human body, each one specialised for a task and working  for the good of the organism as a whole.

Hoelldobler says a superorganism has a sort of intelligence where an ant colony acts as a problem-solving unit (or even a simple brain). “When you look at the incredible nest structures of these leaf-cutter ants – 8 metres down, an area of 50 sq metres – no single ant could do that, or even has the concept of it, but the interaction, the behaviour of millions of individuals that react to particular stimuli that are created by other workers, leads to these fantastic structures. An ant colony is a problem-solving instrument, in a way.” [guardianonline]

The most impressive thing of all is that leafcutter ants have been around for about THREE MILLION YEARS doing exactly the same thing. If we’re talking about future generations and stuff, I think that’s as sustainable as sustainable needs to get. For most of those three million years, leafcutter ants have been the most complex and ecologically successful social systems on the planet. And probably still are.

About 60,000 years ago Homo sapiens began to spread out from Africa, were established in Europe 42,000 years ago and in Indonesia about the same time. About 20,000 years ago, they crossed the Bering Strait to Alaska and had colonised most of the Americas by 15,000 years ago and most of the remaining parts of the Pacific by 3,000 years ago. Check out this book for the relative achievements of ants and humanity.

Man learned how to use fire to catch animals and birds about 45,000 years ago and, by about 10,000 years ago, had worked out how to make tools, weapons, containers, and how to clear and cultivate land. The first known city in the world was probably Eridu from about 5,400BC and the first permanent settlements not that much earlier. The oldest pyramid is from about 4,900BC.

Humans have really only been in a position to control their environment for the past 60,000 years. But now we know how to use fire and make tools and telescopes, play the piano, understand the rules of perspective and some other stuff as well, it might be time to think about how to work together in groups to solve problems like how to grow enough food to feed ourselves, how to design our structures so their climate can be effortlessly regulated and how not to screw up the environment while we’re at it.

It’s still early days.

This is part of the village of Kandovan, on the Iran side of the border with Azerbaijan. Dating from the 13th century, the original inhabitants allegedly escaped to these strange mountains of volcanic ash, to get away from invading Mongols – as one does. You can find more pictures of Kandovan here.

Local residents say that the homes are not only strong but also unusually energy efficient – the homes require minimal supplemental heat during the long cold season and remain cool in the summer.  [Thanks environmentalgraffiti.]  

Something very similar happened in Cappadocia, in Anatolia in what is now Turkey. There are several towns like this one, Goreme. Their main purpose, allegedly, was to provide a place where the (very) early Christians could be secure against Romans out to get them. It goes back that far, apparently – although, to me, carving houses out of rock doesn’t convey any sense of urgency as far as finding safe refuge goes. I doubt those insecure Cappadocians gave much thought to the benefits of thermal mass, but I suppose the less time spent gathering firewood when the hills are alive with Romans, the better.

No account of the history of thermal mass would be complete without a mention of The Pyramids. The temperature of what little inside there is, is a cool 68°F (20°C) and does not change – ever. This is interesting  but not very useful. Of more practical use to people actually alive, is the town of Coober Pedy in South Australia. Opals are mined here.

Why people choose to carve their dwellings instead of construct them above ground seems to attract myth and speculation whatever the century.

Supposedly, many soldiers returning from the First World War moved onto the opal fields and easily adapted to living in the dug-outs they had made looking for opal. They found the temperature in their dug-outs was consistently in the low 20s; cooler than outside in hot weather and warmer than outside in the cold weather. (Thanks travellingaustralia.)

As you might expect, these houses aren’t much to look at on the outside but their construction cost is comparable to that of a ‘surface house’. Running costs are, of course, much less. Most countries have had some form of semi underground or cave house. Here’s some in Germany.

Here’s two views of a nice one in Tunisia. Some people may recognise it as the Tatoonie home of Luke Skywalker who was a character in a movie called Star Wars.

The cave dwelling is now the hotel Hotel Sidi Driss and you can stay there for just $12 a night. Nearby is the Dune Sea where R2-D2 and C-3PO crashed in Episode IV.

Sooner or later, Architecture was going to come along and mess with what is basically a nice idea. The problem of course, with building inside a mountain or underground is that you need to own a mountain or some ground to go under. Building in mountains or underground will never be mainstream. Ever. Most people don’t own land, let along mountains. 

Florida-based practice Oppenheim Architecture + Design have released these images of their proposals for 47 desert lodges at a resort in Wadi Rum, Jordan. [Thank you Dezeen, thank you Oppenheim Architecture + Design.]

If you think that’s scary …

… it won in WAF’s Future Commercial and Future Experimental categories. Thinking about it, where would an architectural competition be without those categories eh? There was and still is no green or sustainable category btw. If there had been, then Oppenheimer A+D would surely have won that as well because of this compelling justification you can see on their website.

If I was any of those critters, I’d be very worried about their Construction Waste Management Plan. Anyway. The climate in Wadi Rum, Jordan isn’t really that extreme – I think it looks rather pleasant. I suppose a bit of fancy glass would stop any thermal advantage from being totally wasted.

What we are seeing here is an architectural statement of course, the unnecessary proof that anything can be used to justify anything. The most recent example of architecture not driven by thermal mass is Villa Vals – you know the one, by SeARCH [!].

The planners were pleased that the proposal did not appear ‘residential’ or impose on the adjacent baths building. The scheme was not perceived as a typical structure but rather an example of pragmatic unobtrusive development in a sensitive location. [Thanks ArchDaily.]The villa is thermally insulated and features ground source heat pump, radiant floors, heat exchanger and uses only hydroelectric power generated by the nearby reservoir. [From the architects’ website.]

Once again, thermal mass is not mentioned. The only reason this house is underground is so that people in Zumthor’s famous baths nearby don’t have to look at it – as you can see from the following two images. The house lies between the baths and the building you can see in the background in the second photograph.

The desert people seem to get it right. Thermal mass works best when there are large day-night temperature variations.

The wall predominantly acts to retard heat transfer from the exterior to the interior during the day. The high volumetric heat capacity and thickness prevents thermal energy from reaching the inner surface. When temperatures fall at night, the walls re-radiate the thermal energy back into the night sky. In this application it is important for such walls to be massive to prevent heat transfer into the interior. [Thanks W.]

http://www.earthbagbuild.com/brief_history.htm

Plus, you don’t need to mess with your mountains. In the thousand-year old city of Shibham in Yemen, the walls are mud brick and thick. Many of them are shaded by other walls. The narrow streets channel what breeze there is and, importantly, helps dissipate heat stored in the walls. The uneven rooftops disturb the smooth flow of that breeze, extracting maximum value from it. I’m not saying that the old ways were better but only that people did extract quite a lot of performance from what they had, and with only observation and acquired experience to go on. Today we pay consultants.

A high-density artificial city such as Shibam with its various passive (read, cheap) ways of doing things is always going to be a winning prototype for modern commercial development so it’s no surprise to find these lessons applied in Transsolar’s conceptual guidelines for Masdar.