Power to the people

Indeed PowerGen has amassed an almost indecent number of gongs from bodies as diverse as the RIBA, the Royal Society for Prevention of Accidents and the British Institute for Facilities Management. All bear witness to the building's ground-breaking passive architecture and low energy engineering, and the diligence and care lavished upon its occupants by a people-oriented organisation.

Nearly five years on, the building can rightfully be regarded as seminal in many of its engineering and architectural principles. The tapered exposed concrete coffer devised by Bennetts Associates has been much copied, while the complementary mixed-mode services, so novel for their time, have become mainstream.

Not so much though to tempt the property market to revise its view of what is marketable and lettable. The mutually reinforcing relationship between agents, funders and investors is extremely resistant to non-financial arguments, largely operating on the basis that what lets today will let tomorrow. The arrant world of property speculation is fearful or dismissive of longer timeframes and broader horizons.

All of which hints at why PowerGen's extension to its headquarters building, the phase 2 annexe, is almost diametrically and fundamentally the opposite of its lauded parent. Indeed its sealed envelope, fan coil air conditioning and minimal exposed thermal mass is almost a throwback to the 1980s.

True, the layout is similar, the envelope detailing is the same, and the architect BDP has chosen from the same palette of materials rather than attempt a dramatic counterpoint. But in substance it is still a bit of a reversal. Awards for passive engineering are likely to be rather thin on the ground.

In PowerGen's defence, it wasn't after medals, just guaranteed lettability. The company needed office space for around 330 people if it was to acquire new businesses like East Midlands Electricity. However, the vagaries of the power generation market created enough corporate uncertainty for PowerGen to play it safe – irrespective of how much the firm might like its headquarters building.

But how safe is safe? Was the services consultant able to repeat any of the design principles so successfully pioneered on phase 1? Or did the institutional specification snooker any development? The institutional requirements certainly affected the way the building was procured and planned. PowerGen's agents Francis Graves developed an outline specification to which architects were invited to submit suitable designs.

Although BDP won the commission ahead of phase 1 architects Bennetts Associates, Ernest Griffiths & Son was reappointed by PowerGen as the client's m&e consulting engineer. John Laing was appointed as the design & build contractor.

Access to the annexe is via a covered walkway attached to the existing reception in phase 1 (see map). This connects to a central full-height foyer which, being glazed on both its north and south elevations, would be easy to re-engineer as a separate point of entry.

Currently the foyer acts as the social hub of the building, containing a ground level cafe, an open staircase and three glazed, side-acting hydraulic lifts. Bridges at each level connect the two, three-storey office wings.

The 15 m-wide wings have a six and nine metre column split which allows the offices to be split into three zones, while the use of a 1·5 m planning module gives high flexibility for partitioning.

A lower ground floor occupies the space created by the sloping site. This floor forms the plinth to the building and contains the main communications room and the various m&e plantrooms – banished from the rooftop to avoid visual conflicts with the building's illustrious neighbour. While the first and second floors are traditional open plan, the ground floor of one wing is given over to a kitchen and 14 meeting rooms of various sizes.

Externally the concrete-framed building is very similar in appearance to phase 1, with external brise-soleil and rectangular windows, albeit non-openable. Interestingly the building is on a north-south axis, (phase 1 is east-west) so the offices are subject to direct solar gain in the morning and afternoon.

BDP was keen to incorporate as many low energy features as it could within the constraints of accepted institutional standards. Insulation levels are better than Building Regulations, with the Velfac double glazing achieving a reasonable U-value of 1·6 W/m²K.

To combat glare the client has fitted a variety of manually controlled internal solar shading devices, from opaque and semi-opaque roller blinds on east and west faces, interstitial Venetian blinds in clerestory windows and full length Venetian blinds for the ground floor meeting rooms.

On the day of the visit many roller blinds were extended to control glare on vdu screens. The perimeter fluorescent lamps are individually occupancy controlled via internal passive infrared detectors, creating conditions for the classic blinds down, lights on situation. Many staff have their backs to the windows, so laziness as well as direct sun on vdu screens will make them less likely to raise the blinds.

M&E engineering

In services terms, the engineers were limited to efficient control of the fan coil air conditioning system, with C0₂ indexed recirculation to control the fresh air volumes, run-around coils, a free cooling cycle on the packaged chillers, and inverter controlled pump sets.

This was all covered by a very detailed specification, with explicit requirements on design of system and preferred choice of supplier, plus sample coordination drawings for a typical floor. Most laudably the designers have got the supply fan power down to around 1·5 W/litre/s, largely by limiting the permitted size of fan motor.

Not surprisingly the building had to be all-electric, so the designers were stuck with multi-stage electric heater batteries in the air handling units and storage heaters for basement storerooms and circulation corridors.

Despite this political correctness, PowerGen clearly had both eyes on the gas market when the phase 2 annexe was conceived: all the appropriate electrical services have been designed to be converted to lphw heating. The fan coils came equipped with heating coils, and the designers also ensured that the chilled water pipework was laid out to allow a through route for flow and return heating pipes. The main ahu heater batteries are cartridge types which can be easily replaced with lphw coils.

The reception area is heated by a 50°C lphw circuit buried in the screed, the source of the heat being an electric water flow heater. Again, if PowerGen wanted to revert to heat pumps rather than chillers, the system can accommodate it.

Ernest Griffiths & Son is a fan of two-port control on fan coils, which tends to work best on a ring main circulation system. Here, the shape of the building militated against a ring main, but the engineers persevered with a variable speed two-port system, with an interesting enhancement on the controls.

PowerGen clearly had both eyes on the gas market when the phase 2 annexe was conceived

The problem with fan coils – or indeed any God-will-provide building service – is that malfunctions are only discovered when people complain. For the small cost of a thyristor and a bems-controlled test routine the engineers believe they have provided a preventative maintenance system that should catch failing fan coils before they cause discomfort.

The bems is set up to regularly poll the fan coils, checking the effect fully open valves have on the supply air temperature and reporting any defaulters. "On a similar job we also shut both heating and cooling valves and check that the fan coil is not doing anything," said consultant Nigel Griffiths. "It's a good way to check for sticking valves."

Elsewhere in the building three-port control valves and bypasses are used for larger and more critical loads. Also, the fan coil units serving offices in the corner of each floor have four-port control valves in order to maintain chilled water flow through all pipework.

The building's ventilation system is fairly conventional, with a single supply ahu catering for the 4340 m² open plan offices, and two smaller ahus for toilet ventilation and the 14 meeting rooms. Six vertical ahus cater for the large communications room (plus one standby) with another two units equipped with dx coils and humidifiers for rh control. Electric steam humidifiers are used on the main office plant.

Supply to the offices is via extruded linear swirl diffusers around the perimeter and conventional 600 mm square swirl diffusers elsewhere. Extract is through the light fittings and selected perforated ceiling tiles.

Return air is down the builderswork duct to a plenum under the basement corridor, which connects to the service shafts located either side of the foyer. The extract ahu draws air from this void, but if the supply system calls for recirculation, a bypass damper connecting the extract plenum to the supply plantroom, enables extract air to mix with fresh air. The variable speed extract fan will throttle back accordingly.

Refrigeration is again relatively conventional, with two packaged R22 chillers supplying chilled water to the fan coils and downflow air handling units serving the communications room. The usual duty and standby (variable speed) pumps handle the chilled water circuit. The smaller of the two chillers is nominated as the "essential" chiller, which operates from the diesel generator in the event of mains failure.

The generator itself is backed up by a static ups with its battery pack sized to give the generator three attempts to fire up before the controls initiate a five minute shutdown procedure for the communications equipment.

Electrical services and lighting

Again, the electrics in the phase 2 annexe are fairly conventional with low voltage systems like security and fire alarms extended from the existing building. Although power is taken from the 11 kV ring main, the annex has its own 1000 kVA oil-filled transformer.

The building facilities managers are very keen on the Ackermann Spider Cable management system, which comprises a hub unit feeding power, voice and data services through up to 12 flexible umbilical tubes to simple grommetted terminals in floor tiles. This means 12 outlet points can be moved without having to move the underfloor box itself. The only significant drawback, says Nigel Griffiths, is the need to keep clear access through the floor tiles above each underfloor box.

BDP's lighting designer Barrie Wilde handled the design of the office lighting, which relies on high efficiency triphosphor fluorescent lamps in bespoke Thorn fittings. These incorporate a concave diffuser into which a proportion of light is directed to give a modicum of indirect illumination.

The integral luminaire controls can dim the addressable light fittings in accordance with prevailing daylight, although as already mentioned the north-south orientation of the building can cause the internal blinds to be fully extended to overcome the shortcomings of the brise-soleil in cutting out direct sun.

All fittings are equipped with Thorn's Sensa 2 occupancy detection system. This is apparently working well, certainly better than the telephone keypad lighting control system that initially caused problems for PowerGen in the first building. Such difficulties are by no means unusual², and the key factors for success seem to be sensible dead bands, time controls configured to conform to occupancy periods, and local switches for manual override outside those periods.

Compared to the exposed concrete coffers of the first building, the suspended ceilings of the annexe are rather less exciting. In fact, the uplit component of the fluorescent fittings seems to accentuate the lack of modelling elsewhere, particularly as there are no wall washers or uplighters on columns.

Workplace design

Fortunately there are other factors in the office space design to raise the spirits of the occupants, such as the business centres on each office floor where heat and ozone-producing office machinery has been corralled.

This was first used in the main building to control floor equipment loads, but in the air conditioned annexe this is far less critical. However, the centres in the first building worked so well as "parish pumps" that the occupants asked to have them in the annexe. It's interesting to witness the serendipity of an engineering need being turned into a business advantage – and by humble occupants.

There are also coffee-making facilities in posh alcoves on each floor. The policy seems to be to provide china cups and saucers rather than plastics cups. At once ecologically sound and a corporate demonstration of quality.

So even with its institutional specification, has the annexe retained enough character of its award winning parent? No amount of quality finishes can really compete with the serenity and spatial qualities of the original building. Even after four years the headquarters building looks and feels fresh, and the acoustics seem particularly good given the amount of exposed mass.

BDP has done well within a tight budget to incorporate the style of the original building in the annexe. The finishes are high quality even though m&e specification is a throwback to 1980s norms. Sadly there are no energy targets for the new building, and currently no energy data is available for benchmarking either building.

The truth is PowerGen wanted an instantly marketable building. That has created something which is compliant with the prevailing norms set by letting agents, or rather the things that letting agents think people want.