Daily Maximum Demand 140000 Current Demand 120000 100000 8000 Reserve 28% 6000 4000 2000 0
Conventional 83.5% Nuclear 15%
Renewable 1.5%
Between 17:30 and 18:00 on 7 December 2010, UK electricity demand peaked at just over 59 GW. At that point in time less than 17% of the demand was met from low carbon generation sources, 15% from nuclear power and 1.5% from genuine renewable sources
It is conceivable that current policies on zero carbon homes could actually lead to higher carbon emissions overall
on which they are installed. In an urban environment buildings change, and rooftops that might presently be an ideal location for photovoltaics can become shaded by adjacent new developments, negating their potential and wasting the investment. The fact is that, on the whole, it is better to develop renewable energy resources out of urban centres. We are also beginning to see the use of
biofuels in city centre locations in order to achieve high environmental ratings for what are essentially conventional, energy-guzzling air-conditioned buildings. However, there is no distribution infrastructure for biofuels, in the way that there is for gas or electricity. Thus, adopting biofuel as a low carbon
CITY EMISSIONS
In terms of transporting biomass into cities, there are concerns about the air pollution this may cause. For example, in the City of London, air pollution is such a significant issue that there is a dedicated campaign to raise awareness among business in the city. With transport accounting for 75% of city emissions, trucking biofuels into London will not help to reduce city pollution. See
www.cityoflondon.gov.uk/cityair for more information.
solution for buildings brings with it an increase in transport fuel consumption, city centre pollution and traffic congestion in order to deliver the fuel by road (see box, left). Furthermore, not every renewable fuel is unlimited in supply: we need to exercise judgement about the best use for biofuels. To use biodiesel for combined heat and
power to offset unnecessary consumption in buildings denies the use of that fuel for road transport, which is a much more valuable use for society overall. So, whilst the carbon performance of an individual building is improved, the social costs may negate any benefit gained. The present single-minded pursuit
of zero carbon homes may also have unintended consequences. Due to the subsidies available under feed-in tariffs and the Renewable Heat Incentive, zero carbon in housing is likely to be achieved by
60 CIBSE Journal October 2011
installing an air-source heat pump supplied with renewable energy from photovoltaics on the roof. This approach to zero carbon does not actually lead to energy self- sufficiency, but relies fundamentally on a form of carbon offsetting. Photovoltaics do not generate on a
winter’s night when the heat pump demand is highest. Without any means to store electricity, it is necessary to generate sufficient surplus during the summer to offset consumption by the heat pump during the winter. This works provided that zero carbon homes are in the minority, leaving sufficient consumers to absorb the surplus generation, displacing fossil fuel and allowing the zero carbon adherents to claim the carbon offset. Since new zero carbon homes represent such a small proportion of the total, then this strategy will work fine in the short term. However, across the UK economy we
need to achieve an 80% reduction in carbon emissions by 2050. New-build housing will only account for a small fraction of the stock in 2050, so if we are to achieve this target we will also have to make all existing housing near-zero carbon. With the subsidies available for the photovoltaic/heat pump approach, it is reasonable to expect that existing housing would follow suit. Now, if every household in the UK
were to install sufficient photovoltaics to offset their winter heating requirement, then, on a sunny July day, the generation from households alone would far exceed the current UK consumption – notwithstanding that this consumption will be reduced in the future through
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