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A rubbish concept?

A look at how using waste and other methods of passive design can provide energy savings and efficient buildings.

Valeriano Handumon/ITP Images.
Valeriano Handumon/ITP Images.

In a progressive sector, where attention is seemingly fixed on the sharp end of design, or the latest conceptual building or innovation, it is hard to imagine that rudimentary design, aimed at alleviating energy demands through complementary methods, could deliver equal or greater benefits to Dubai.

Recently, Construction Week documented the difficulties associated with sustainable design, highlighting that varying degrees of interpretation meant progress followed different strands and disciplines. But what was revered among all other facets of smart and efficient buildings was the implementation of passive design.

Ive heard talk about naturally ventilated buildings in Dubai where they don't use air conditioning. But I doubt they'd be able to function to the internal temperatures people call comfortable.

Engineering firm Whitby and Bird, which has an emphasis on coherent sustainable design and the life-cycle costs of building, is involved in two unique projects that demonstrate clearly the alternative approaches a company can take to conserve energy through the use of intelligent design and application of basic physical principles, coupled with knowledge of material behaviour.

According to Rob Dickie, the company's associate director, while current examples of sustainable design are visually impressive and iconic, they are the antithesis of the principles espoused by Whitby and Bird, as they derive minimal benefits.

"In regards to the actual sustainable engineering behind them, they open up more questions than they answer. For buildings that are so big and brash and bold, what is the actual amount of energy saving they are achieving? I would think ours are far more energy efficient."

Here, Dickie is referring to two regional projects that in their separate way, demonstrate how passive design can contribute to reducing annual cooling loads in buildings. Firstly, the company was responsible for building the British Embassy in Sana'a, Yemen, which as a result of the sustainable policies adopted secured the prestigious Royal Institute of British Architects (RIBA) International Award Winner 2007. The second project, a one- storey, 2000m2 site, located in Dubai creek for the British Council, is unique for the proposed cooling system Whitby and Bird is keen to implement, claiming it would reduce annual cooling loads by 34% and peak cooling loads by 25%, which translates into a significant cost saving.

On the building in Sana'a, Dickie says: "In line with the prevailing philosophy for British government buildings, including those outside of the UK, sustainability was a key part of the brief. Early concepts incorporated several energy and water saving features and most of these were retained in the final building."

The project itself had an extremely onerous security brief, which dictated the structural form, says Dickie. But the stand-off distances that the site allowed meant that the facades could be significantly punctured by windows. "This was essential in getting daylight to penetrate deep into the space and reduce electrical lighting loads. The challenge was keeping the solar gains out."

To do this, external shading elements in the form of brise-soleil and canopies were integrated into the façade. It was imperative to undertake this aspect of the design with the maximum amount of care, adds Dickie, as opposed to simply paying lip service incorporating generic rules of thumb or calculations to estimate overhand details.

"We did endless iterations on how the exclusion or inclusion of the solar gains would heat internal surfaces and what that meant to cooling load. We wanted to make the thick concrete security walls part of the environmental strategy. We knew that the large diurnal temperature swings could be harnessed to significantly reduce the cooling loads by delaying peak outside temperatures until the occupants had left for the day and retaining the cooling energy in the structure overnight for use during the day. This is a simple enough technique, but is seldom followed through."

Rain, which falls infrequently, but in heavy bursts, is collected and stored at the bottom of the site for use later in irrigation and feeding wash-down hoses. Sewage treatment also includes a clean water take-off stage, which uses clean, but non-potable water for irrigation. Sunlight, an abundant resource at 10,000ft above sea level, is used to heat water by solar hot water panels for washing.

Dickie adds that treating the building skin as a climatic modifier, rather than a climatic excluder, went a long way in reducing the required cooling energy to maintain comfortable temperatures. This idea was further compounded through the use of an underground thermal labyrinth to reduce the cooling load even more. The tubes absorb heat during the day in winter and release it as useful heat energy in the cooler evenings and early mornings. In summer, it is cooled at night to provide pre-cooling for fresh air during the day.

This method - if agreed upon - would be transferred to Dubai where the principles would be replicated, but with modified material. "The trick is to create a series of MEP systems that responded to the environment rather than excluded it, utilising what was available at site to reduce capital cost." This, Dickie points out, is the opposite of perceived wisdom that sustainable technologies add capital costs.

One immediate advantage for the creek site was that the existing building was being demolished, leaving plenty of rubble. This was then used to supplement the air conditioning load. Whereas the Sana'a project used long lengths of clay piping as the heat exchange surface, the rubble is to be used instead.

"The Sana'a experience taught us that a clay pipe labyrinth requires a large surface area as the air stream down the inside of the clay pipe doesn't have a very contact factor. The challenge with this project was to create a more compact heat exchanger."

Dubai has a large diurnal range during the summer months, explains Dickie. Although the night time temperatures are warm, they are not as hot as the daytime temperatures. Storing the cooler at night time temperatures and using them during the day means significant energy savings. Temperatures are expected to be reduced by eight or nine degrees centigrade through this rock store method.

To store this cooling energy requires materials with high heat capacities - the ability of the material to retain the cooling energy put into it. Concrete has a very good specific heat capacity and can retain cooling energy for long periods of time. The trick is to transfer the cooling energy from the outside air into the concrete rubble. What was learnt at Sana'a was that the clay labyrinth pipe requires a large surface area as the air stream doesn't have a very good contact factor. It was therefore imperative to create a more compact heat exchanger.

"Air passing over spherical shapes have excellent convective heat transfer characteristics. Placing spherical shapes in banks increases these characteristic, as we are increasing the degree of difficulty for the air passage ensuring that the air has maximum contact with the sphere surface," explains Dickie. Building rubble forms rough speheres and when placed in a pit form a perfect heat exchanger, with the ability to have maximum contact with the air stream, and the ability to absorb and retain night time air temperatures.

Dickie adds that the implementation of this system, which is unique to the Middle East in its application, is destined to pay for itself in four years and pay for the remainder of the air conditioning systems over the lifetime of the project. The only other example of such an approach is in Harare, Zimbabwe.

Dickie is quick to point out that such approaches do not represent a panacea to cure all energy demands in the emirate, and should be viewed for what they are - a complementary approach in efficient design. Bigger buildings by their very nature could not be supported by such technology, yet it serves to demonstrate what can be done.

"I've heard talk about naturally ventilated buildings in Dubai where they don't use air conditioning. But I seriously doubt they would be able to function to the internal temperatures people call comfortable." Furthermore, it illustrates the more important point that for such technology to be successful, it must be tailored to that specific building. "It only works if it is bespoke. If it is tailored to the building in the right context, then it will work. If it isn't, then there is no point in doing it." For larger, more technically challenging buildings, approaches such as peak lopping and phase change utectics are more suitable, adds Dickie.

The overriding point being made by Dickie is that implementing effective, energy efficient structures, doesn't have to be iconic, or sensational, it merely needs to demonstrate common sense and an understanding of what will work. For example, in Yemen, shading combined a common sense approach with practical application. The shading was provided by black water pipes, which act as solar water heaters.

"It's all about delivering a coherent sustainable strategy and not about dripping buildings in the latest renewable 'jewellery'," Dickie concludes.

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