Insect ingenuity and the power of the earth have inspired low-energy ventilation
Clever little creatures termites. As nature’s master-builders they possess an intuitive grasp of architectural engineering that enables them to construct highly elaborate homes out of mud. Termite mounds are not only self-supporting, they also provide a self-regulating, living environment that responds naturally to changing conditions.
It works like this: wind over the mound draws air up from an underground chamber and over a large, radial earth fin so that air is heated or cooled as required. When the central core becomes too hot and stale air needs to be expelled, the termites open vents in the core to draw fresh air up through the porous earth. When it gets too cold, they simply plug up the holes with mud. No fans, no pumps, no mechanical parts at all.
Architects and engineers are becoming interested in very similar ways of heating, cooling and ventilating buildings, which is also known as ground-coupled cooling. One such termite-inspired system is the thermal labyrinth, which will be used at Kew Gardens’ new Alpine House designed by Wilkinson Eyre and environmental engineer Atelier Ten.
The thermal labyrinth system comprises a serpentine arrangement of galleries constructed beneath a building, rather like a medieval garden maze made of concrete. Supply air to the building is drawn through the galleries and brought into contact with as much of the heavyweight concrete as possible.
Patrick Bellew principal of Atelier Ten has long believed in thermal labyrinths. “They de-couple the ventilation element of the building load from the room,” he says.
“Even in a conventional heavyweight building, with lots of exposed concrete and natural ventilation, the first thing the outside air does when it comes in the window is to add a thermal load to the room. This adds to the solar gains and heat from office equipment and lighting. All that does is warm up the space.
“A thermal labyrinth is different,” he continues. “It acts like a battery pack sitting between the air intake and the building’s air handling plant. That battery pack can be flushed with colder night air during the summer, so it can cool incoming air during the day. We can either reduce the cooling loads or heating loads depending on the time of year.”
Atelier Ten has enjoyed considerable success with thermal labyrinths on some prestigious projects, such as the huge Federation Square project in Melbourne, Australia; a business school designed by Cesar Pelli Architects for the University of Illinois; and the $50 million (£27 million) Grand Rapids Art Museum in Michigan.
Designed by Munkenbeck & Marshall Architects, the Michigan museum will sit on top of a thermal labyrinth. In summer, air will be drawn over a fountain for pre-cooling and then into the labyrinth before being injected into the building’s systems. In winter, supply will come from air warmed in a solar pre-heat wall in the south facade. Power for the systems will come from 350sq m of photovoltaics.
Back home, Atelier Ten is concentrating on applying ground-coupled cooling to more modest projects. Wilkinson Eyre’s new Alpine House in the Royal Botanic Gardens in Kew, Richmond, is a good example.
“Alpine plants need cool roots and a permanent breeze of cool air over their leaves,” says Bellew. “They also need lots of light, but the problem is a glass building can make them overheat.”
Atelier Ten and Wilkinson Eyre Architects have devised an ingenious solution: a 110sq m, 15m-long single-glazed structure formed by two parabolas of glass that form an arched pathway. Under the floor will be a compact version of a thermal labyrinth, which will be used to mimic alpine ground conditions.
Instead of Alpine plants being in refrigerated trays as they are in the old building, outside air drawn through the thermal labyrinth will keep the roots below 24°C. The cooled air will then be injected into the greenhouse by outlets designed to flow the cool air over the plants’ leaves (see In Detail, page 20).
Solar-powered photovoltaic panels on the south-west elevation will power the fans used to pump the air through the labyrinth and over the plants. Work has already begun on Kew’s Alpine House, which is due to open to the public next summer.
While Bellew calls labyrinths “profoundly logical”, he admits they are not perfect.
“One of the problems with labyrinths is they are not really coupled to the ground. You have to flush them at night to discharge the heat built up during the day” he says. “As a result you are not making much use of the passive energy of the earth. They are also perceived as being quite expensive, and you have to build a basement for them if you haven’t got space already.”
Bellew’s answer to the labyrinth’s shortcomings is something even more basic: the aptly described earth duct. This is another method of ground-coupled cooling and is essentially a concrete- or steel-lined ventilation tube, buried about a metre underground, and used to bring a building’s incoming air into contact with the thermal mass of the earth. Unlike a thermal labyrinth, the earth duct uses the massive thermal capacity of the ground to maintain a steady supply air temperature to the building.
Property developer Easter Developments has opted to use the earth duct for the first phase of its Butterfield Office development in Luton. This is a collection of two-storey, 400-800sq m blocks designed by architects Hamilton Associates and Atelier Ten. When work starts on site in early 2005, the scheme will be the first office project in the UK to use earth ducts as its ventilation and cooling mechanism.
Each office block will be served by approximately 80m- long, 600mm-diameter earth ducts made from standard concrete drainage pipes running under the landscaped grounds. Air will be drawn in from raised intake grilles, pre-conditioned in the earth ducts, and then filtered, heated and pumped into each building’s under floor displacement ventilation system. In summer, the earth ducts will maintain the incoming air temperature at around 18°C and in winter reduce the amount of energy needed for heating.
The claimed energy savings over conventional air conditioning are startling. A typical fan-coil air-conditioning system consumes around 225kWh per sq m per year for heating, ventilation and mechanical cooling. Using the earth duct approach, Bellew believes the Luton offices will consume no more than 45kWh/sq m/year. And if a high efficiency heat recovery system is installed, energy consumption could fall as far as 28kWh/sq m/year.
Ground-coupled cooling is also capable of reducing the risk of summertime overheating, a problem often experienced by more complicated designs of natural ventilation that rely on myriad ventilators and actuators.
Atelier Ten took the comfort guidelines as laid down in the British Council for Offices Guide 2000 for naturally ventilated buildings and demonstrated that earth ducts can reduce the annual number of hours exceeding 25°C during occupied periods to .67% compared to the BCO recommendations of 5%.
“With earth ducts we think the tenant is more likely to be satisfied and much less likely to think that the building might be uncomfortable during the peak months and end up installing air conditioning,” says Bellew.
So how much do thermal labyrinths and earth ducts cost? Labyrinths tend to be slightly more expensive than conventional air conditioning systems but generally pay for themselves within 10 years. Earth ducts are generally cheaper. Luton’s Butterfield project is costing less than conventional cooling. Traditional ventilation or equipment serving the building will account for around 20-30% of the total building cost, and is usually refurbished if not totally replaced every 15 years. As well as the cost, this generates waste materials, only a small percentage of which are likely to be recycled.
Given the opportunity, which system would Bellew choose?
“It depends on whether the labyrinth can use existing space beneath the building or whether it has to be constructed,” he says.
“The latter obviously adds to the cost, but even then the client can expect the system to pay for itself well within the structural life of the building. Earth tubes, on the other hand, are much easier and cheaper to construct.”
Despite being commonplace in Germany, earth ducts and labyrinths are yet to prove popular in the conservative UK property market.
“Here the letting agents are still God,” says Bellew. “At BCO’s conference this year the talk is still about slightly more efficient air conditioning. But if we are trying to make a serious reduction in carbon emissions, we are not looking at a 10% reduction but several orders of magnitude better than that.
“That’s what sustainable design is all about, thinking beyond the first iteration in the life of a building,” says Bellew. “The real strength of ground-coupled cooling is longevity — the savings you are making you are making forever.”
Roderic Bunn is a technical writer working with the Building Services Information & Research Association (BSIRA).
Options for earth duct systems
The best method of achieving ground-coupled cooling in the most sustainable way is to use off-the-shelf products. For the earth duct method, there are two drain sections available: ribbed metal tube or concrete drain sections. By virtue of the ribs, the steel tube will give high surface area and encourage more turbulent airflow, thus bringing more air into contact with the ground and improving heat transfer. The downside is that steel is more prone to corrosion and water leakage.
Concrete drain sections, are easier to make watertight with a gasketed joint. As long as the concrete is left untreated, its porous nature helps to control the humidity of the supply air.
All types of pipe are easy to monitor with a standard drainage-type camera system. Insect and vermin screens installed at the air intake will prevent creatures from making the duct their home.
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