High powered

As the need to save energy becomes an integral part of building design wind can be an indispensable factor, helping to drive ventilation and heating or generating power via turbines. Stuart Nathan reports

The film is scratched and flickery and looks like it’s running in reverse. Men in frock coats and women in full skirts are clutching their hats and seem to be walking slowly backwards. But on second glance they are just struggling against a strong wind.

‘This was filmed in 1903 outside the Flatiron Building in New York, which had just been completed,’ said Graeme Flynn, senior project manager at BMT Fluid Dynamics. ‘It was known as the world’s windiest corner. Clearly they hadn’t accounted for the effect the building had on the wind.’

Flynn’s work uses wind tunnels to design today’s skyscrapers, modelling how the surrounding landscape affects tall structures and how their shapes can be changed to cope with the buffeting of the wind at high altitude. A stroll around London shows these problems are not still fully understood — one notorious corner of the Canary Wharf tower has handrails for those caught in gusts.

More challenges face skyscraper architects and engineers above street level. Buildings that sway in the wind are uncomfortable for their occupants and the constant motion puts a strain on the building materials, which could lead to failure or even collapse. But as the need to save energy becomes an integral part of building design, the wind can be a useful, even indispensable factor, helping to drive ventilation or heating, or generating power via turbines.

Flynn’s work centres around a wind tunnel with a 10m2 area, on which models of buildings, placed within their neighbourhoods, can be tested, generally at 1:200-1:400 scales.

Before the airflow reaches the models it is ‘conditioned’ to mimic the wind profile of the area. It is passed over a series of regularly spaced hard foam ‘fences’ of different heights. These can produce the effect of wind that has blown over the coast, woodlands, mountains or plains.

The wind tunnel is used, often with computational fluid dynamics (CFD), to tailor the aerodynamic shape of skyscrapers. The idea is that a shape that cuts through the prevailing wind will be less likely to sway backwards and forwards.

However, said Gavin Davies, director of building physics at engineering practice Ove Arup, fluid dynamics is a highly complex field, and mitigating one factor can exacerbate another.

If a building is given an aerodynamic profile to reduce swaying, another play, known as vortex shedding, comes into effect. As the air flows past the leeward side of the structure, it is whipped into swirling vortices, which cause the structure to vibrate perpendicular to the wind — the effect can be seen with a flag snapping in the wind off a flagpole. The two must be balanced carefully, said Davies, and a compromise reached.

If the building is designed to use natural ventilation to reduce the energy demand for air conditioning, the design cannot be too aerodynamic. For air to flow through the building, the air pressure on the windward side has to be significantly higher than that on the leeward side — and aerodynamic profiling works by evening out the pressure difference.

The Swiss Re building in London — known as the Gherkin — has suffered problems with its natural ventilation mechanisms, which some engineers believe is because its round, slippery shape equalises the pressure differential too much to allow an air current-driving pressure difference.

New laws stipulate that new buildings must be able to generate 10 per cent of their power requirement so attention is focusing on wind energy as a generation method. Wind turbines on buildings are now commonly used.

However, building-mounted turbines have proved disappointing, as urban wind is harder to harness than rural or offshore wind. Many architects and engineers believe the addition of wind turbines is more cosmetic than practical — worthwhile for grants and regulatory compliance but unlikely to generate much useful energy.

The type of turbine used is crucial because the characteristics of wind in cities are very different from those in the countryside. In open landscape, the prevailing winds tend to be fairly constant in speed and direction, guided by the shape of hills and valleys. Changes in wind direction are slow and gusts are relatively rare, so the classic horizontal-axis turbine, with its three huge blades facing into the wind, is a good design.

In the city, the wind is fickle and variable, changes direction suddenly and often, and builds up and dies down again equally quickly. A horizontal axis turbine, which works at optimum level when facing into the wind, cannot track the direction of air flow fast enough.

Vertical-axis turbines are proving a better option. The orientation of their blades means they will spin in the same direction regardless of where the wind is coming from although they do not suit gusty wind. Holger Babinsky, an aerodynamicist at Cambridge University’s engineering department, has been working on these problems and believes he has found a way to overcome them.

Babinsky is working with the urban wind turbine specialist Quietrevolution, whose turbines have been mooted for an ambitious project to install generation units across London (see The Engineer, 21 May).

The turbine has three swept-back blades that can capture the wind from any direction, but Babinsky noticed that the power output from the turbine was well below its rated value.

The key to the problem turned out to be the relationship between the generating performance and the tip speed ratio, which is the blade speed divided by the wind speed. There is a sharp maximum performance point, he found, when the tip speed is almost four times as fast as the wind speed. If the turbine spins faster, the performance falls away steadily. But if it spins slower, the fall is dramatic. ‘It’s like the performance drops off a cliff,’ said Babinsky.

And in a gust, the wind speed increases, so the tip speed becomes closer to the wind speed, so the tip speed ratio falls, and the turbine operates in that steep drop-off region. On a gusty day — and most days are gusty, Babinsky stressed — the turbine is well below its peak efficiency most of the time.

However, he also noticed that gusts that last less than a second do not seem to have much effect, because the turbine does not respond that fast to changes in wind speed. Also, he noted, the rotational speed of the turbine can be controlled.

To take advantage of this, he has designed a controller that links in to an anemometer that constantly monitors wind speed. The controller looks at the wind speeds over the last 300 seconds, and from that data, works out the probability of strong gusts lasting longer than a second over the next five minutes.

From that, it determines the optimum rotational speed for the turbine that will keep the tip speed ratio close to the operational maximum without dropping below it.

‘We’re always operating to the right of the peak power, so we aren’t at the maximum, but we don’t fall off the cliff,’ he said.

The view among many building engineers, however, is that it will always be easier to reduce the carbon emissions impact from a skyscraper by energy-saving methods, rather than by using renewable electricity generation.

The answer might be to incorporate wind turbines into the fabric of the building itself, rather than just to use the building as a mounting point; but this approach will require a synthesis of all the wind design techniques.

One project that is tackling this problem head-on is taking shape, almost inevitably in China. The Pearl River Tower in Guangzhou (formerly Canton), scheduled for completion in 2009, will be a 71-storey, 303m-high skyscraper — but it will look unlike any other skyscraper in the world.

Designed by Chicago architecture firm Skidmore, Owings and Merrill (SOM), the tower’s slab shape is perfectly flat on its short faces but is carved into billowing curves on the long faces, which sweep into deep horizontal slots around a third and two-thirds of the way up the height of the building. In each of these slots are two holes straight through the building from front to back, each housing a Quietrevolution vertical-axis turbine that helps to generate the power needed to run the building.

Pearl River Tower — perhaps ironically, destined to be the corporate headquarters for the China National Tobacco Company — is designed to be a net zero-energy building, said Roger Frechette, director of mechanical, electrical and plumbing for SOM and lead engineer on the project.

‘Putting it into Guangzhou won’t force the city to produce any more energy,’ he said. The wind turbines are just one of 32 technology strategies employed towards this goal, he said. The building also incorporates photovoltaic panels, water cooling rather than air cooling, a passive method to remove humidity from air entering the building and many other sustainable engineering techniques.

The environmental aspect was the driving force for the design. ‘Form is no longer good enough; you have to have performance,’ said Frechette. ‘Every turn, every curve, every angle on the building has a purpose.’

Guangzhou is hot and humid, with predictable prevailing winds: 10 months of the year, they blow from due south, and for the other two, from due north.

Conventional wisdom would design the building so that an aerodynamic edge faced into the wind, but SOM took the opposite tack: the building faces the wind head-on, with its long axis running east-west. The sweeping curves, designed using both wind tunnels and CFD, capture the wind and guide it up from street level and down from the top of the building, into the turbine openings.

The height of the openings above the ground was one important factor, as wind speeds increase with altitude, explained Frechette. Moreover, he added, the energy potential of the wind is proportional to the cube of its velocity, so doubling the wind speed leads to an eight-fold increase in the potential amount of energy generated.

During testing, the designers discovered that as long as the wind was not blowing directly into the side of the building — which only happens during typhoons, according to Frechette — the wind velocity into the turbine openings is not affected.

‘If the wind hits the building perpendicular to the turbine openings, the velocity across the turbines fall to zero, or close to it. But it only takes a small deviation — less than 10° — before the velocity picks up again,’ said Frechette.

What they weren’t expecting was the enormous acceleration of wind speed as the air rushes through the openings. ‘It isn’t the wind speed that affects the velocity through the hole, it’s the pressure differential,’ he added.

As the air is pushed into the curved slots, its speed increases by a factor of 2.5-3 — and the power potential increases as the cube of the velocity. ‘Four turbines in the building is equivalent to 32 turbines sited outside the building.’ he said.

The turbine openings are located at the building’s mechanical levels, which house air conditioning equipment and other machinery essential for building services; this maximises the amount of floor space available for offices.

This is made possible because the water-cooling used throughout the building, based around chilled concrete slabs in the ceilings and floors, reduces the amount of air that needs to be circulated around the building by 80 per cent. The equipment needed to circulate the air is therefore smaller, freeing space for the turbines.

The building also uses a passive method to cope with the muggy Guangzhou weather. The outer skin of the tower is an actively ventilated double wall, and air within the cavity is heated by the sun. The energy from the air is then drawn back into the building.

‘It’s very hot and humid in Guangzhou, and when you take that air and cool it down enough to wring all the moisture out of it, it becomes too cold to supply the building,’ said Frechette. ‘So normally you’d use electric or gas heaters to back up to an acceptable temperature. We use the sun’s energy for that reheating process.’

In fact, 65 per cent of Pearl River Tower’s zero net energy properties come from energy conservation strategies, with the remaining 35 per cent coming from the photovoltaics and wind turbines. ‘The original design did include microturbine technology, but that was eliminated from the project because of problems with power metering in Guangzhou,’said Frechette.

‘We suggested to the client that he might want to remove the wind turbines from the building, because they were performing a bit less than we’d hoped,’ he added. ‘The client said “Absolutely not.” One thing that was important to him was that the building communicate what it was doing to the outside world.

‘The turbines will give the building motion and animation; as you drive past, you will clearly see that there is something different and unusual happening, We’re hoping this will make people want to look at the other 31 sustainable strategies.’

So, although it may be true that wind energy is mainly a cosmetic factor in today’s buildings, it could be an integral part of the array of techniques needed to make large buildings more sustainable.

And it proves, as the windblown pedestrians of 1900s New York knew very well, that urban planners cannot ignore natural forces.