Food for thought

No comments

BDP and Tesco have made sure nothing goes to waste in an innovative mixed-use scheme destined for Kingston. Andy Pearson reveals how the supermarket giant is finding new ways to expand into tricky areas

Alongside the traffic-choked Kingston bypass, an experiment in urban sustainability is about to get under way. Supermarket giant Tesco is behind the development of a pioneering mixed-use project that will provide over 800 new apartments and a big, new Tesco store on a four-hectare site at Tolworth. What’s different about this scheme is that organic waste from the supermarket will be used to provide heat and power for the site. Tesco claims it will be the first mixed-use scheme in London of such a high sustainable calibre.

The grocer knew that getting permission to build a new supermarket close to Tolworth's existing shops was not going to be easy. The site, originally occupied by the Ministry for Agriculture Farming and Fisheries, was allocated for housing development. Unperturbed by this, Tesco snapped it up and approached Kingston’s planners with an aggressive development philosophy: "We'll give you all your housing, and more, if you'll let us build a supermarket on the site." And so the Bridge at Tolworth concept was born.

The task of developing a scheme to squeeze more than 800 apartments and a supermarket on the site fell to the Building Design Partnership (BDP). The firm's engineers and architects had their work cut out: not only did they have to contend with the Kingston bypass, but also an existing bowling alley and a railway line complete with down-at-heel station.

At the heart of the design team’s solution is an elevated green park, which forms the roof of the 6000 m2 supermarket, its car park and the residents’ car park. Two blocks of affordable housing and a block of private housing will rise up from this verdant plateau, too.

South of the park, the architects have placed three multi-storey blocks of private apartments. These face south to benefit from views over the North Downs and exploit the opportunity for passive solar space heating in winter. A semi-circle of studio flats around the bowling alley serves to squeeze in even more housing. The final part of the scheme will be a new pedestrian bridge spanning the busy bypass which, in addition to giving the scheme its name, will link the new community to the existing shopping area and provide a new high-level route to the railway station (see site map, below).

BDP's sustainability arm has been fundamental in both initiating and developing the scheme's green credentials. There was no planning requirement for the scheme to use renewable energy, but that didn’t stop Trevor Butler, director of sustainability at BDP, who convinced Tesco to embrace this approach.

“This was too good an opportunity to miss: a supermarket open 24 hours a day, 365 days a year, with bags of waste – and a residential development with a demand for heating and hot water 24 hours a day, 365 days a year," says Butler, the brains behind the sustainable concept. A water and energy service company (a WESCO) – not Tesco – will be responsible for delivering power, heat and water to the scheme.

The decision as to whether this, one of the most exciting urban sustainability schemes, will finally go ahead lies with Kingston's planners.

For a detailed explanation of the scheme's sustainable features, see overleaf.

Water

The scheme uses three water sources: rainwater, groundwater
and treated mains water.

Rainwater run-off from the green roof and from the roofs of each of the apartment blocks is stored in a tank beneath each block – this will be used to flush toilets and irrigate the green roof.

A borehole, which taps into the chalk aquifer deep beneath the south end of the site, will supplement the rainwater for “non-potable” uses. In addition, BDP has recommended that the design of the apartment blocks incorporate low-water-use appliances to keep water demand to a minimum. According to Trevor Butler, director of sustainability at BDP, this system will reduce the water demand from utility supplier Thames Water by 40%. Potable water will come from the utility main.

Organic waste

Non-organic waste can be disposed of in the supermarket's recycling area, but what of the organic waste from the supermarket and residents? Normally, organic waste is disposed of off-site, but on this scheme the organic waste will provide fuel for a combined heat and power system after being fed into an anaerobic bio-digester, where it is used to create “biogas” (or methane, as it is more commonly known). Sewage from the apartment blocks will also feed the bio-digester, and the resulting gas will be mixed
with natural gas to power the combined heat and power engine. Nothing is wasted: inert soil compost and the treated liquid effluent from the bio-digester is intended to fertilise “energy crops” as it is not suitable for fertilising food crops. As technologies develop, the CHP unit could be replaced by a fuel cell in the future.

Combined heat and power

A mixture of biogas and mains gas fuels the CHP unit, which is sized to provide sufficient power to meet the store's base electrical load. Meanwhile, waste heat from the CHP is piped to a buffer unit where it will provide the domestic hot water. Depending on the apartments’ design, waste heat from the CHP may also be used to provide heating. Additional power will come from photovoltaic roof panels, which generate enough power to meet the domestic lighting load. Additional evacuated tubes could generate hot water if the housebuilders shun the CHP option.

Should it all go wrong, the scheme has been designed with 100% back-up using gas-fired boilers to provide heating and hot water when the CHP is out of action. Likewise, the electricity supply has been sized to cope with load when the CHP unit is off-line.

Absorption chiller

In addition to providing domestic heating and hot water, waste heat from the CHP unit is also used to provide cooling through an absorption chiller. Unfortunately, the chilled water produced by the absorption chiller is not cold enough for use in the store's freezers. Instead, it will be used to remove waste heat from the condensing units attached to the store's freezer cabinets.

The advantage of this arrangement is that water-cooled condensers, which are smaller and quieter than the more conventional air-cooled units, can be used.

However, Butler does admit that this is not the most efficient arrangement, and explains that this aspect of the development is still under review while the designers try to find a way to use the waste heat directly in the scheme.

Getting to 10%: a practical guide

On Wednesday 25 April, BSj will be hosting a conference entitled “Getting to 10% – a practical guide to designing for and managing renewable energy”. The event will take place at the Institute of Mechanical Engineers, Birdcage Walk, London – for more details, go to bsjonline.co.uk/events or call Marilyn Dent on 020 7560 4193.