Power Efficiency Guide

New office buildings are often hyped up as being amazingly environment-friendly. Let's look at some numbers.

The William Gates building at Cambridge University holds computer science researchers, administrators, and a small cafe. Its area is 11110 m2, and its energy consumption is 2392MWh/y. That's a power per unit area of 215 kWh/m2/y, or 25 W/m2. This building won a RIBA award in 2001 for its predicted energy consumption. "The architects have incorporated many environmentally friendly features into the building." [5dhups]

But are these buildings impressive? Next door, the Rutherford building, built in the 1970s without any fancy eco-claims - indeed without even double glazing - has a floor area of 4998 m2 and consumes 1557MWh per year; that's 0.85kWh/d/m2, or 36W/m2. So the award-winning building is just 30% better, in terms of power per unit area, than its simple 1970s cousin. Figure E.12 compares these buildings and another new building, the Law Faculty, with the Old Schools, which are ancient offices built pre-

Energy rating bands

Homes

European average my house, before my house, after the Heatkeeper house g Cß

Figure E.12. Building benchmarks. Power used per unit area in various homes and offices.

Offices

UK service sector DFID Home Office NEF office Elizabeth Fry bldg, UEA Cambridge University | Old Schools | Rutherford building Law Faculty H Gates Building |

W/m2

112 kWh/y/m2 benchmark for offices

33 W/m2

167 kWh/y/m2 university arts benchmark

167 kWh/y/m2 university arts benchmark

7.4 W/m2 265 kWh/y/m2 science lab benchmark ---W/m2

10 15 20 25 30 35 Power per unit floor area (W/m2)

Old Schools

Rutherford building

Law faculty

Gates building

Old Schools

Rutherford building

Law faculty

Gates building

Cooling

Heating electrical energy electrical energy

required per unit of cooling required per unit of heating

7 ideal 6 coefficient of 5 performance 4 3 2 1 0

7 ideal 6 coefficient of 5 performance 4 3 2 1 0

ideal 7 coefficient of 6 performance 5 4 3 2 1 0

ideal 7 coefficient of 6 performance 5 4 3 2 1 0

1890. For all the fanfare, the difference between the new and the old is really quite disappointing!

Notice that the building power consumptions, per unit floor area, are in just the same units (W/m2) as the renewable powers per unit area that we discussed on pages 43, 47, and 177. Comparing these consumption and production numbers helps us realize how difficult it is to power modern buildings entirely from on-site renewables. The power per unit area of biofuels (figure 6.11, p43) is 0.5 W/m2; of wind farms, 2 W/m2; of solar photovoltaics, 20 W/m2 (figure 6.18, p47); only solar hot-water panels come in at the right sort of power per unit area, 53 W/m2 (figure 6.3, p39).

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