EARTH STORAGE SYSTEMS
There is a group of renaissance villas in Costozza, Italy, that are served by a remarkable airconditioning system. Each dwelling is connected by a shaft to a series of caves that run beneath the mountains. The caves remain cool throughout the summer months by virtue of their thermal coupling to the earth and their lack of exposure to solar gain. When a breeze blows against the hillside it drives air through the tunnel system which cools the air before it is delivered to the internal rooms through marble floor grilles - simple but effective passive cooling!
There are similar examples in nature. The Barossa termite has perfected building its 800storey-high structures (relative to its body size) with fully integrated passive temperature control that maintains the temperature in the queen's chamber at the heart of the nest to within 1°C throughout the year. The main mechanism for this precise temperature control is the heat exchange with the surrounding ground.
Air is drawn through a huge subterranean chamber, supplemented in the most extreme conditions by evaporative cooling achieved by the termites traipsing down tunnels to the water table to collect minute quantities of water to put into the 'system'.
This may seem a far cry from the needs of a contemporary building but the basic physical principals are readily transferable; in particular, the opportunities that exist for heat exchange with the ground through earth ducts and with large heat stores (sometimes called labyrinths).
These technologies are still a rarity in the UK, despite having been proven in other parts of the world. They can be classed as 'decoupled' thermal storage systems (as opposed to 'room coupled' systems), because the thermal mass is not thermally coupled to the space that it is serving - it may be quite remote.
EXPOSING CONCRETE The technique of exposing internal concrete surfaces within buildings to exploit their thermal storage capacity and control internal temperature fluctuations has become commonplace during the past decade. This is room-coupled thermal mass. The idea is that by ventilating the buildings through the night, the mass of the building can be cooled down and the 'coolth' held in storage to absorb heat gains the following day. The most significant issue is that there is a limit to the amount of cooling that can be stored in this way and buildings using this technique can be prone to overheating. This is because the spaces containing the heat storage cannot be cooled below comfort temperatures overnight or they will require heating in the morning.
The energy required to heat or cool the outdoor air before bringing it into contact with the occupants of buildings also represents a significant energy load. In a naturally ventilated building on a hot summer's day, the incoming hot air from outside will absorb a large proportion of any stored coolth within the room and will tend to exacerbate overheating problems.
GROUND CHILL EFFECT Since 1996, Atelier Ten has been developing ideas for remote thermal storage, in the form of labyrinths and earth ducts, in an effort to exploit the passive heating and cooling effect available from the ground.
The idea is simple: air is exposed to a large surface area of concrete on its way into the building by passing at low velocity down a tunnel of between 80m and 300m in length. In the case of a labyrinth, the walls of the tunnel have been cooled by night-time air. With an earth duct, cooling is a combination of the same night cooling effect and the innate thermal mass of the ground.
The first labyrinth that Atelier Ten completed was at the Earth Centre in Doncaster and it served to completely eliminate the need for any cooling. We then built a much larger labyrinth to condition air serving the atrium - a huge galleria-like structure - at Federation Square in Melbourne. The labyrinth is quite simply made from rippled concrete walls forming long air paths for the air to flow down.
The air is always cool at night in Melbourne and the heat of the day can be flushed out of the concrete, along with some of the moisture, so that the mass is cool the following day.
The flow of air can be controlled through the various chambers to provide some responsiveness to external conditions. In addition to the thermal storage, evaporation cooling is achieved using stored rainwater on the hottest days, just like the termites after their trek to the water table.
The labyrinth here also eliminated the need for cooling, and the space has been kept at comfortable temperatures, even when it is 40°C outside. The annual energy consumption for the fans is a tenth of the amount that would have been required for the conventional overhead cooling system.
After three full summers of operation, the scheme has been a great success. When the cooling is not needed in the autumn months, the air is diverted to the adjacent galleries of the museum to reduce their energy demand.
The most recent evolution of the idea can be found below the new Alpine House, a glasshouse at the Royal Botanical Gardens in Kew, designed by Wilkinson Eyre Architects. The alpine plants like to be kept in a cool breeze because it helps to maintain compact foliage. Historically, alpine plants are kept on chilled shelves with big opening windows behind to maintain the requisite airflow. In order to eliminate cooling, we developed a labyrinth solution combined with perimeter natural ventilation and shading to provide the ideal environment for the plants.
BURIED PIPES Labyrinths work well for some situations but they have the disadvantage that it is necessary effectively to construct all of the thermal mass in the form of concrete. We started to experiment with the idea of buried pipes to ventilate schools, first at Notley Green Primary School (with AHMM) and then at Tower House School (with Glas Architects).
Further research led to the realisation that this type of earth storage system is very common in German and other northern European office buildings, where the trend is very much to avoid air conditioning.
We located dozens of schemes in Germany in particular - for clients such as Audi and Mercedes-Benz - where the earth ducts were the principle source of air temperature control.These may be concrete or steel pipes buried beneath the ground, which run for anything up to 100m long to pre-condition the incoming air.
PUMPED UP Earth cooling can be viewed as a renewable resource and may catch the attention of developers and designers as they start to wrestle with the 10 per cent renewable energy component required by the new GLA guidelines and by the new Part L requirements that will come into effect from January 2006.
These new regulations will almost certainly lead to the increase in use of one other environmental technology - ground source heat pumps.
These come in a variety of permutations but with one thing in common: they use the ground as a heat sink in the summer and as a heat source in winter. This can be achieved either by deep piles (up to 250m) with a closed-loop pipe circuit, by multiple shallower piles, or by an open-loop circuit connecting to the water table and extracting ground water for use as a heat source or heat sink. There are 900 such pile installations in the UK, including the one we did at Keble College in Oxford with Rick Mather.
There are 30,000 new installations of this type per year in Sweden and 900,000 exist in the US, so it is not exactly an untried technology.
This is a system that is well suited to urban applications and is likely to find increasing application in the UK as the new Building Regulations start to bite.
Patrick Bellew is a founding director of Atelier Ten and teaches environmental design at Yale University School of Architecture