CODE FOR SUSTAINABLE HOMES ’ZERO CARBON’ BIAS
Figure 1: Occupancy by floor area, as assumed by building regulations. 5
4 3 2 1
0 0 200 400 600 Dwelling size, m2
Figure 2: Energy for internal appliances by floor area, as required for ‘zero carbon’ status 40 35 30 25 20 15 10 5 0
0 200 400 600 Dwelling size, m2 800 1000
Occupancy
value of two people, 200 m2 people, but 1000 m2
gives three gives only four people.
The average dwelling size in the UK is around 80 m2
and the average number of
people per household is 2.4, so the SAP assumption holds for the average case. For large dwellings, however, the assumed occupancies seem unrealistically low. The average family has two children, so four occupants, but the average family house is not 1,000 m2
. The effect of this assumption is that Energy for internal appliances 800 1000
large dwellings have substantially lower occupancy-related energy uses than small dwellings when expressed by floor area, as shown in Figure 2: Energy for internal appliances by floor area, as required for ‘zero carbon’ status. This means that small dwellings have a much harder target for renewable energy generation to achieve ‘zero carbon’ status. ‘zero carbon’ status is difficult to achieve, since renewable electricity must be generated on-site. Biomass boilers, combined heat and power plants and heat pumps cannot achieve zero emissions alone, since they all have positive emissions. Energy-efficient fabric and passive design measures can reduce heating energy requirements to zero, but electricity will still be needed for pumps and fans, lighting, and internal appliances. Renewable electricity must therefore be generated, and for most buildings this means photovoltaic (PV) panels, since other renewables like wind turbines and hydroelectric turbines are highly site-specific. On small dwellings and multi-storey apartment blocks, there may be too little south-facing roof area to incorporate enough PV to generate the higher per-square-metre requirement. To generate
Case studies illustrating ‘zero carbon’ calculations for small and large dwellings Floor area, m2
Number of occupants Appliance energy use
Cooking energy use Total
CO2 emissions target (DER) PV needed (125kWp/m2
) Proportion of roof 28 CIBSE Journal June 2013 kWh/a
kWh/a/m2 kWh/a
kWh/a/m2 kWh/a
kWh/a/m2
kg CO2/a/m2 m2
Small 50 1.7
1,685 34
175 3
1,860 37.2
-20.9 22
44%
Large 1,000 4
10,444 10
236 0.2
10,680 10.4 -5.6 125
13%
factor 20
2.4 6.2 0.3 1.3 0.1 5.7 0.3 0.3 5.7 0.3
www.cibsejournal.com
kWh/m2
/a
Occupancy, no. of people
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