INTRO DUCTION

Carbon is an essential constituent of all life on Earth and continuously cycles between the atmosphere, oceans, land and the Earth's interior through a range of processes and timescales (the ‘carbon cycle'). Throughout history, humans have relied upon the natural carbon cycle to provide food, fuel and construction materials from plantbased materials, which fix carbon dioxide (CO₂) from the atmosphere by photosynthesis and store the carbon as biomass (the stored carbon being released back to the atmosphere, mostly as CO₂, only when the biomass decays or is consumed or burned). The natural carbon cycle is now being perturbed by anthropogenic activities, particularly the burning of fossil fuels and deforestation, resulting in carbon being released into the atmosphere much faster than it can be naturally removed. This has led to increasing amounts of CO₂ being present in the atmosphere. Under standard atmospheric conditions, CO₂ is a thermodynamically and chemically stable molecule, which means that it persists within the atmosphere^[1].

The general scientific consensus is that the Earth's climate is changing due to anthropogenic emissions of CO₂ and other greenhouse gases (GHGs). Of the primary GHGs, CO₂ is the most important as it is emitted in much larger quantities than the other GHGs, predominantly from the burning of fossil fuels for transportation, heating and electricity^[2]. In an effort to mitigate climate change, numerous international and national policies and initiatives are now in place that aim to reduce GHG emissions (eg UN Kyoto Protocol[3], EU Emissions Trading System^[4], UK Climate Change Act 2008^[5]). The UK is committed to cutting GHG emissions by at least 80% compared with 1990 levels by 2050^[6].

The building sector is responsible for a large proportion of the UK's total energy consumption and associated CO2 emissions, both from operational energy and from energy consumed by all of the processes involved in the production of construction materials (embodied energy/ embodied carbon). In 2009, heating and powering homes and buildings accounted for 37% of the total UK CO2 emissions[6]. It has been estimated that 10% of the UK's total energy consumption is embodied in construction materials[7]. Identifying opportunities to reduce energy consumption and the associated CO2 emissions in buildings is one of the main priorities in the effort to mitigate climate change.

The principles of the ‘energy hierarchy' (Figure 1) are now widely used when assessing how to reduce a building's energy demand and CO2 emissions. The first step is to reduce the need to use any energy at all. This can be achieved through design measures (eg passive heating and cooling, thermal mass/insulation, airtightness, natural ventilation, use of daylighting) and through user behaviour (eg good energy management, turning off equipment and appliances). The second step is to use energy more efficiently through the selection of efficient fittings and products (eg lighting, heating, ventilation and cooling systems, heat recovery, electrical appliances). When the first two steps have been implemented, consideration can be made to sourcing energy from a low-carbon or renewable supply (eg solar, wind, biomass, hydro). Finally, if the use of energy from fossil fuel sources is still required, this should be from efficient and clean technologies[8]. To work effectively, it is important that the energy hierarchy takes account of the lifetime CO2 emissions associated with the manufacture, installation and maintenance of the measure(s) to be implemented.

Despite efforts to reduce CO₂ emissions from buildings, the vast majority of buildings in the UK continue to rely on energy from fossil fuel sources to operate, including electricity sourced from the national grid. In reality, use of grid electricity is likely to remain the most viable option in many cases and is arguably the most flexible solution for powering buildings in the future. As such, decarbonising the national power supply will play a key role in reducing building-related CO₂ emissions. Over the last decade, the use of low-carbon sources (renewables and nuclear) has increased through greater provision of large-scale renewables to now provide around one-quarter of the UK's electricity-generating capacity^[6]. While this growth is likely to continue, the need to meet the nation's energy demands means that, in the medium term at least, fossil fuels will continue to be required for electricity production. These circumstances, combined with the need to reduce CO₂ emissions to meet GHG emission targets, have led to increasing interest in using carbon capture techniques as a method to mitigate the CO₂ emissions from continued fossil fuel use^[9].