What is 'zero carbon'?

Since the Chancellor Gordon Brown's remarks in the pre-Budget speech before christmas about his aspiration to achieve zero-carbon buildings, there has been (not surprisingly) a plethora of comment in the press about how this is to be delivered.

Phrases such as 'zero net carbon', 'carbon neutral' and 'zero carbon' have become interchangeable, and different writers are using conflicting descriptions to describe the same thing.

This is, to the say the least, unhelpful and will only add to the confusion in what is already a very involved and difficult debate. Unless we are all clear in our understanding of the terminology we use and agree the definitions, it will be impossible to reach either an understanding of what it is we want to achieve, or indeed how we will get there.

We have therefore made an attempt to clarify our understanding of the definitions being used in the area of low- and zero-carbon technologies and buildings. However, we are not attempting to apply - nor should it be seen as applying - any judgement as to which is better than any other.

In any discussion it is vital to agree upon and state the context in which one is using a phrase such as 'zero carbon'. For instance, is it being used to describe a generating system or technology such as a wind turbine, an individual dwelling or a complete development of 500 houses?

Without all parties defining and agreeing on the context, discussion is meaningless.

Zero carbon defined
Literally, this is defined as absolutely no emissions of CO2. For energy generation this means solar (passive, hot water and photovoltaics), wind and hydro-electric. Most people would also include bio-mass (wood, straw and manure) even though there is some cost in transporting or processing this in terms of energy input.

If these sources are used to produce energy carriers such as electricity, hydrogen or bio-fuels, then these carriers can be classed as zero carbon. Generation can be at the dwelling level (eg. wood pellet stove and photovoltaic panels on each roof), or at the development level (eg. large windmill and central bio-mass combined heat and power system), or even offsite (eg. offshore windfarm). However, offsite generation has issues of linking with the development both physically and in perception.

For a building or development to be zero carbon, only these sources can be used to operate the building. It is also important to define what is included in terms of building operation. For example, building regulations include heating/cooling, hot water and lighting only, but appliances not included, such as fridges, cookers, TVs and patio heaters, can emit more CO2 than the regulated areas. For the sake of brevity these should be described as 'regulated energy' (heating, water, lighting) and 'total energy' (regulated plus all other uses).

Zero-carbon buildings or developments for total energy should not be connected to either the mains electricity or gas networks. However, if they were, they would then fall into the next category - zero net carbon or carbon neutral.

Zero net carbon or carbon neutral
Many buildings or developments generate zero-carbon energy, but also use carbon-containing energy such as gas, coal or an energy carrier that has been produced by similar sources (eg hydrogen from coal). These buildings cannot therefore be defined as zero carbon. At best, they can be zero net carbon or carbon neutral, which can be achieved by the building or development generating and exporting sufficient zero-carbon energy to offset the carbon emitted from its non-zero energy sources.

For example, take a house that emits 1t of CO2 from its gas boiler, but generates and exports 2,500kW/h of electricity from photovoltaic panels on its roof. Each kW/h of electricity supplied via the National Grid causes 400g of CO2 in its generation, so by substituting 2,500kW/h of zero-carbon electricity, there is a saving of 1t of CO2 - so the house is carbon neutral. Of course, if the house imports any grid electricity, it has to generate the same amount of zero-carbon electricity for export, as well as sufficient to offset the gas for its boiler.

Carbon neutrality at development level is the same in principle. In practice, it can be easier to generate the zero-carbon energy for export at development level due to economies of scale (one big wind turbine may be cheaper and more effective than a hundred small ones) and applicability of technologies (bio-mass CHP is not a single-dwelling technology). However, there are serious questions as to how small sites (the typical Housing Corporation development consists of 12 dwellings) or individual dwellings can become carbon neutral cost-effectively.

Another way of achieving carbon neutrality is to reduce existing CO2 emissions through reducing overall energy use, rather than substituting zero-carbon energy for an existing carbon-rich energy.

For the new house emitting 1t of CO2 from its gas boiler, rather than generating and exporting the 2,500kW/h of zero-carbon electricity, another dwelling from the existing stock could have its cavity walls insulated and hot water cylinder lagging topped up. This would save 1t of CO2 from existing stock emissions, thus offsetting the new house's emissions.

There is no technical reason why mix-and-match solutions could not be used, particularly with mixed-use regeneration schemes where some zero-carbon generation coupled with upgrading existing stock could be envisaged.

Relationship to Code for Sustainable Homes (CSH)
The CSH has six levels, of which just two are of concern here -namely Level 5 and Level 6.

Level 5 is a 100% reduction of CO2 from regulated energy use. In terms of explaining definitions, such a home would be zero net carbon or carbon neutral for regulated energy uses. The CSH does not, however, define the context (i.e. whether this is at dwelling or development level).

For Level 6, the situation is even less well defined, with the requirement covering total energy use, but not stating whether it is zero carbon or zero net carbon. There are also significant differences in the technical solutions and the costs for the two options.

Clarity from the government on both the CSH and the definitions is needed to allow the industry to progress.

John Tebbitt is industry affairs director for the Construction Products Association.

[Contract Journal, 17 January 2007, p35]