Going to ground

Retailing that's harmless, friendly and kind. This is the image Sainsbury's wants to project. Why else would wind turbines and photovoltaics be used solely to illuminate a firm's corporate colours? "This is really us," it's saying. "It's where we're at." Of course it's easy to be cynical about an industry that has long pandered to the shopper as motorist. Out-of-town supermarkets might indeed be energy efficient, but only because it helps the profit margin. Sustainable they ain't.

It's also easy to argue that the Greenwich Millennium store, arguably the most energy efficient any supermarket chain has yet built, would have languished on the drawing board but for the government's master plan for the Greenwich Peninsula. It was, at its conception, going to be just one small part of the Millennium Village, an ultra-green development intended to demonstrate the practicalities of sustainable urban living.

That plan is both running late and being dumbed down, leaving Sainsbury's effort rather out on a limb. The 1150 car parking spaces on the entire retail site will be Sainsbury's life-blood, at least for the present.

That said, political wrangling should not detract too much from the project's very laudable objectives: to reduce emissions of carbon dioxide, reduce energy running usage of a typical superstore by up to 50%, and get a maximum BREEAM rating of 31 credits. To do this Sainsbury's tore up its conventional store specification and started afresh, re-engineering everything along passive principles.

Out went air conditioning in favour of a very novel natural ventilation strategy. Electric lighting, too, has been trimmed back, the extensive use of daylighting reducing the daytime ambient lighting load to a remarkable 3·5 W/m².

You won't find hcfc chillers and boilers here either – instead there's combined heat and power, groundwater cooling and propane-based refrigeration.

The site

Thanks to the benignly-powered signage, finding the building is easier at night than it is during the day. Although the building rises to 30 m, the profiled earth retaining walls make a good job of shrouding the structure – a back-to-the-future, bronze age barrow.

From the front, though, the store proclaims its identity. Curved (European sourced) timber-clad walls grow out of the earth to channel the shopper through a welcoming clear-glazed entrance.

Once inside, the average consumer will notice little difference over a conventional store. Unless they look up, that is. Instead of the usual suspended ceiling with its scattering of security cameras, fire detectors, and boring rows of fluorescent lamps, the Greenwich store rises 6·5 m to a curved, steel-framed sawtooth roof. This is punctuated by tiers of north-facing sealed windows through which daylight streams in.

An alloy-clad roof decays gently to the east and west, finishing at a surprisingly crude stepped detail at either side of the sales area. Behind the sales area, the roof covers the goods stores, bakery and the store's unloading bay.

Viewed from the rear, three symmetrically placed "eyebrow" features on the roof betray the locations of mezzanine plantrooms (largely for process plant). Conventional outbuildings house a transformer, borehole circuitry, and two combined heat and power engines.

The natural ventilation strategy

To the east and west the building is partially encircled by two mesh-covered systems. Under an alarm, the dampers fitted to the leading edge of the underfloor supply plenum will motor closed.

In general terms, the building is ventilated naturally by a displacement system aided by wind-assisted stack effect. The system makes use of the considerable thermal capacity of the building's concrete foundations.

Supply air is drawn by siphon effect from the plantroom into a clear 600 mm plenum under the sales area. After being thermally modified by the concrete, the air rises up through short duct spigots positioned strategically beneath product gondolas. The supply air enters the occupied space through simple perimeter grilles that form the gondola kickplates.

Warm, stale air rises by stack effect to be exhausted at the apex of the roof ridges. Here, flap dampers on the north and south sides of the ridge open alternately depending on wind direction.

Oscar Faber spent considerable time working with computational fluid dynamics to test the seasonal effectiveness of this strategy. Wind modelling also identified that both the location and opening strategy for the roof vents were crucial to maintaining extract rates without short-circuiting back into the building. Ultimately, the vents had to be sited right on the ridge of the roof.

On top of potential short-circuiting, designers had to consider the inherent fragility of a low velocity natural displacement system – particularly because of the close proximity of open freezer cabinets. The 18-28°C/30-70% rh comfort conditions also kept the thermal simulation specialists fully employed.

In winter, simple convectors located at the base of the gondolas can be supplied with lphw, to assist air movement and warm the supply air. As external ambient temperature climbs, the trench heater fans can be switched off.

The mid-season operation will rely wholly on natural ventilation, the effectiveness of which is dependent on the correct operation of the ridge ventilation. Again, Faber's spent many hours running cfd iterations on the position and design of the roof outlets, which proved highly sensitive to wind direction and pressure.

For high summer, there were a number of factors to consider, not least the relationship between displacement ventilation, occupant comfort and the relative humidity in the cold produce areas. Food fridge cabinets are basically large, space dehumidifiers, and the more moisture they remove, the more frost builds up on the coils. Among other harmful effects, this increases the number of defrost cycles.

Faber's canny response has been to reverse the supply air flow by retrieving cold, dehumidified air from the cold aisles through the gondola kickplates, into ductwork, and thence into a small air handling unit in the plenum plantroom. Here it is mixed with the incoming supply air, heated if necessary, and injected back into the dry goods areas.

This nifty solution led the cfd plots to predict that conditions in the store, even at maximum occupancy, should not climb above 26°C. Although the designers can claim these conditions are met without air conditioning, well, it is on the sly.

Comfort conditions will also be maintained by a Wirsbo underfloor heating and cooling system. This is split into two networks, one for the cold aisles sized at 150 W/m², and one for the dry goods areas, sized at 100 W/m². Again, this involved a great many cfd iterations to ensure that convection currents could be maintained in the cold aisles while still enabling cold air to be retrieved from the freezer cabinets.

There was a lot riding on the cfd modelling, and Oscar Faber did further analysis to check that underfloor heating would not drive up air from the refrigerated cabinets at times when it needed to be retrieved for the dry goods areas. To arrive at the right temperatures, the designers reached for cfd analysis to identify the floor temperatures necessary to achieve comfort conditions without disturbing the air flow.

Borehole services

Two 75 m deep boreholes are being used to extract water for process and comfort conditioning purposes. Sunk in the service yard, the boreholes have an extract licence for 31 litres/s. Unlike elsewhere in the UK, ground water levels in London are rising at an alarming rate, so as far as the Environment Agency is concerned, the more the merrier.

The water extracted is largely being used for heat rejection from the propane compressors serving the refrigerated storage areas. A small percentage is also used in the air handling unit cooling coils and the underfloor heating/cooling system.

The latter comprises two separate circuits to allow simultaneous heating and cooling in the dry goods areas (100 kW/m²) and the refrigerated areas (150 W/m²).

The convectors in the floor void can also call for cooling off the borehole circuit when temperatures allow. All secondary circuits are connected to the ground water circuit by conventional heat exchangers.

Rather than simply dispose of used borehole water directly to drain, Sainsbury's is using it to flush toilets and urinals, and to feed a small lagoon at the rear of the store. This is part of the 'Sainsbury's for the Millennium' project: an area set aside to demonstrate the company's care for the environment. The lagoon is surrounded by native trees and shrubs. The company hopes that the effort will attract dragonflies, butterflies "and small mammals", which sounds a bit alarming.

Building services

Given the steady electrical baseload, the Millennium superstore was an ideal candidate for combined heat and power. Two engines have been installed – at 210 kVA and 300 kVA – to cope with diversified loads. This provides a maximum heat output of 754 kW, which should be more than enough.

The electrical power is needed to meet the 24 h refrigeration baseload, with the remainder – synchronised with the incoming supply – used to power building services and lighting.

The building's estimated 625 kW maximum heat load is covered by the chp waste water. This supplies the east and west air handling units, the underfloor heating system and various process requirements such as the in-store bakery and cafeteria. As a result, there are no point-of-use water heaters.

As mentioned earlier, Sainsbury's has eschewed traditional hcfc refrigeration in favour of a natural refrigerant: propane. There are two circuits, a high temperature circuit operating at -6·5°C for the chilled cabinets, and a low temperature circuit at -28°C for the frozen cabinets.

As one might expect, the propane chillers were a packaged subcontract, and are housed in their own plantroom, with considerable provision for leak detection and venting to atmosphere. A beneficial use of the high temperature refrigeration circuit is to provide extra cooling to the food preparation areas to maintain 16°C when the borehole water (12°C) cannot cope.

Lighting strategy

The Millennium supermarket is the first foodstore to be extensively daylit. A series of high-angled north-facing windows has enabled the lighting engineer, Pinniger & Partners, to achieve 3·5 W/m2 (around 200 lux) for the sales floor. The designers originally aimed for a minimum 5% daylight factor, but it is likely to be closer to 8%.

To achieve an even illumination and reduce daylight asymmetry, the roof beneath the windows is of white painted steel planks and floor finishes were chosen for high reflectivity. However, to protect against glare on very bright days, external motorised Colt louvres have been installed to modulate under the command of sensors measuring external daylight and floor illuminance.

These simple, aluminium blades will also motor fully closed at night to prevent light spillage and marginally improve the roof heat loss characteristics. Although there is a Trend bems controlling the building services and electric lighting, the daylight louvres were a Colt subcontract package and have independent controls.

The electric ambient lighting is installed in the steel roof trusses immediately below the glazing. This is effectively high bay lighting, with T5 lamps in Urbis fittings equipped with perforated diffusers to add some sparkle.

The approach to daylight and ambient lighting was aided by what is becoming Sainsbury's de facto approach to product lighting. Again, this involves T5 fluorescent lamps, but in bespoke extrusions suspended around the perimeter of the merchandise gondolas. Output is around 1000 lux.

More details on the lighting design are available in the October issue of Building Services Journal's sister magazine, Light & Lighting.

Overall assessment

The Millennium superstore opened on the 14 September to general public acclaim. However, does the building really represent a new design narrative for Sainsbury's, or is it just a one-off marketing ploy? Well, the supermarket chain is keen to promote its full BREEAM rating, but is rather more coy on its energy targets. Partly, this is understandable: a lot of the technology is unique, and therefore the usual benchmarks don't apply. The hours of use and diversity will also be atypical.

Despite its integrated design, one can't help thinking that the store is tokenism writ large, rather than being the beginning of a new standard design for foodstores – the building cost 25% more than Sainsbury's own benchmark. Still, economies of scale could bring that down.

Being secretive about construction costs is one thing, but let's hope the supermarket chain shares the energy consumption data it is promising to gather. After all, Sainsbury's can't save the planet by itself.