In July, the Government announced its intentions for the Zero Carbon Definition.
Dave Cheshire explains what it actually means in practice for housing development projects

The consultation on the definition of zero carbon proposed three steps towards compliance. The first stage sets a minimum level of energy efficiency, the second sets a minimum target contribution from low and zero carbon technologies called 'carbon compliance' and a third stage uses on and off-site 'allowable solutions'. The last stage is there to make up the difference in CO2 reductions between the first two stages and full zero carbon and includes solutions such as exporting heat to surrounding buildings or contributing towards community heat infrastructure and plant.

July's written statement by John Healey MP confirmed the definition of zero carbon. This included setting the minimum level of 'carbon compliance' for dwellings at 70% compared with 2006 Part L - while the level for nondomestic buildings has yet to be set. It also proposed a guideline maximum price that they expect the industry to bear to implement 'allowable solutions'. This has been set at £100 per tonne of CO2. Assuming this is the same for non-domestic buildings and capitalising this figure over a 30-year period gives an equivalent capital expenditure of £3,000/tonne CO2 emitted a year. So, how does this compare to energy efficiency and on-site renewable technologies?

There are some potentially low-cost passive energy efficiency measures that require fundamental architectural design changes. Optimising window sizes to balance daylight with heat gain/loss; providing rooflights and light wells; changing the internal configuration of spaces to ensure areas that have high internal heat gains are positioned towards the north elevations. If these changes are implemented as part of the design concept, then they could have little or no cost implications. In fact, measures such as optimising window sizes may even reduce the glazed areas and so provide potential savings.

Then there are low-cost active measures, such as lighting controls and improved air tightness which could cost around £2-3,000/tonne of CO2. Then it starts to get more expensive with presence detectors on lighting costing around £5-6,000/tonne. Costs could rise to £10-20,000/tonne of CO2 for advanced insulation standards. So, assuming energy efficiency compliance is set at an appropriate level, then meeting the requirements through low and zero cost measures could cost around £2-3,000/tonne.

On-site renewable energy costs ('carbon compliance') can vary widely depending on the technology applied. Typically, costs are around £5-10,000/tonne CO2, which makes renewables less attractive when compared with the lower cost energy efficiency measures or allowable solutions. And on many sites there will be a physical limit to just how much energy can be generated on-site. On a constrained inner-city site, biomass and wind may be ruled out, and if the building is overshadowed, there would be limited yield from solar technologies. Ground
area may limit the contribution from ground source heat pumps. On a constrained site it makes sense to move quickly to allowable solutions, but on a site with potential for on-site generation, it may well prove to be cost-effective to exceed the minimum levels of carbon compliance.

The allowable solutions for housing include both further enhancements on-site (for example, advanced building control systems which reduce the level of energy use in the home) and off-site solutions to reduce the overall carbon emissions (for example, investments in low and zero-carbon community heating infrastructure).

This is likely to be offered as a choice between implementing further measures on-site or paying into a local community fund. In most situations, it is likely to be more cost effective to pay into a local community fund.

For some projects it may be beneficial to incorporate on-site technologies, for example, a CHP plant that can export heat to the local community. Contributions to off-site wind turbines, for example, could be cheaper than the £3,000/tonne of CO2 maximum for allowable solutions and so could make it even more likely that developers would only invest the minimum necessary to achieve energy efficiency and carbon compliance.

For housing, the allowable solutions have to cover all the remaining emissions from the site, including those not covered by the Building Regulations (unregulated emissions). These emissions are related to appliances and other equipment installed by the occupants of the building. In fact, one of the allowable solutions for housing refers to the installation of energy efficient appliances as a way to reduce carbon emissions. It is not yet clear whether unregulated emissions will also be included in the definition of zero carbon for non-domestic buildings,
or whether it will be sufficient just to address all of the regulated emissions. For some nondomestic buildings, the unregulated loads can effectively double the energy use of the building and they can't be controlled by the designers. It is difficult to see how allowable solution could be applied to speculative buildings.

There are other reasons why on-site renewables may be preferable to jumping straight to an off-site allowable solution - current planning policy and the newly proposed tariffs for generating heating and power both provide potential incentives for on-site generation.

The Feed-In Tariff for electricity and the Renewable Heat Incentive were proposed by the Government in the Low Carbon Transition Plan and they are intended to provide money for electricity and heat generated on site. The Feed-In Tariff will considerably reduce the paybacks on renewable technologies such as photovoltaics, wind turbines and electricity generated by biomass or anaerobic digestion. A rough calculation shows that the paybacks on photovoltaics would be reduced from 70-80 years down to 15 years. This is still a long payback, but is much more palatable. This will mainly be of interest to owner-occupiers as it is unlikely to translate into much of an increase in property value for speculative developments.

Current planning policies also often promote on-site renewables. The proposals for the zero carbon definition will be implemented through Building Regulations, but zero carbon starts to cross over into planning when it comes to importing heat (carbon compliance) or exporting heat and power (allowable solutions). This is mainly because Building Regulations really only applies to individual buildings whereas planning can deal with decentralised energy generation at the community scale.

The new Planning Policy Statement (PPS1) supplement on planning and Climate Change is starting to provide a more logical approach to community energy generation by requiring all local authorities to identify suitable locations and opportunities for decentralised energy generation based on an evidence base. This requires an in-depth understanding of the potential for energy technologies and carbon reductions for both new and existing buildings in the area.

This co-ordinated approach should mean that sites with potential to generate energy on-site will be expected to install more renewables onsite and constrained sites will be able to use the evidence base to identify opportunities to connect to local energy infrastructure.

Local authorities will need to co-ordinate and enable the delivery of energy projects that link new and existing buildings, including: town centre district heating systems, waste gasification and energy infrastructure and micro-generation installations.

It will be interesting to see what happens as projects start competing for the same off-site solutions, particularly in an area where a lot of new development is happening. Presumably, there would come a point in time where there would be no cost-effective local off-site solutions left in an area, which would mean developers being forced to revert back to on-site solutions.

Ultimately, decisions on achieving the different levels proposed in the zero carbon consultation are going to be based on cost-effectiveness, which will require more work on costing out energy efficiency measures and low/zero carbon technologies on projects to determine the most cost-effective routes to achieving the targets.

The Government is proposing to consult further on aspects the definition of non-domestic zero carbon buildings that are not yet fixed later this year.

David Cheshire is a regional director in AECOM's Sustainable Development Group. AECOM is working on various zero carbon projects including Target Zero (, a project sponsored by the BCSA and Corus, and the recently completed zero carbon Defra headquarters building in Alnwick which achieved the UK's first A+ Energy Performance Certificate rating



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