Defining a zero-carbon building including embodied energy of materials

Abstract

A drastic reduction in carbon emissions is needed in the coming decades if the internationally ratified 2°C limit on rising global temperatures is to be met. As the construction and operation of buildings is responsible for nearly one fifth of all carbon emissions globally, improvements in the performance of buildings in this regard could yield significant benefits. There are many compulsory and voluntary environmentally aligned building frameworks that have been developed to bring about such improvements in performance, all with the same goal in mind; to limit (ideally to zero) the impact of buildings on climate change. However, the approaches taken to achieve this and the precise framework definitions vary, resulting in a complex landscape in which compliance with one zero-climate-change-driven building framework does not guarantee automatic compliance with any others. This raises the question of whether all of these building frameworks are equally beneficial with respect to climate change avoidance. Such building frameworks are collectively referred to in this thesis as zero climate change (ZeroCC) frameworks.
Environmental building performance has traditionally been defined in terms of operational energy demand and/or associated carbon emissions, usually measured with reference to a building’s floor area. However, many ZeroCC frameworks are criticised for not including in their requirements limits on carbon emissions or energy demand associated with the plug in appliances that are part of a building’s operational life. Less criticised is the fact that most ZeroCC frameworks ignore the carbon emissions or energy demand tied to the construction of the building in the first place. The issues surrounding such embodied carbon and energy, and the definition of appropriate metrics to capture this element of a building’s performance, are widely discussed at present and a consensus on how to approach these issues is yet to be reached. However, it is clear that embodied carbon and energy are an important part of the performance of a building system as a whole, and are likely to become proportionally more so as operational building performance improves.
Interestingly, there is general agreement within ZeroCC frameworks that energy demand, rather than carbon emissions, should be the metric used to measure building performance. This is despite the fact that it is carbon emissions that drive climate change. It is widely assumed that energy demand is a good proxy for carbon emissions, and therefore many ZeroCC frameworks are designed with a view to improving performance by requiring reduced energy demand per unit floor area. With the development of renewable energy technology, the drive for reduced energy demand has evolved into ideas around net zero energy, and net zero carbon buildings. This necessarily requires building designs to include the often energy and carbon intensive technology needed to generate the renewable energy for offsetting against operational energy demand. As embodied energy and carbon are usually excluded from measures of building performance, renewably generated energy is seen as being free from carbon emissions, and/or free in general.
The inclusion of renewable energy in the assessment of building performance brings further complications. Net zero carbon or energy buildings are often criticised for allowing vast seasonal mismatches in energy demand and renewable energy generation (although an annual balance may be achieved), with little regard for how energy can be usefully stored. In addition, ZeroCC frameworks tend to take the view that grid generated energy (particularly electricity) is carbon intensive, so offsetting even a small amount of grid energy with renewable energy is always beneficial. However, it is clear that energy grids are becoming less carbon intensive in response to climate change concerns, meaning that the climate change mitigation value of renewably generated energy is not static, and is likely to decrease. This suggests that the optimised designs that ZeroCC building frameworks seek to identify inhabit a landscape that is only partially mapped and is in constant flux.
In this research an integrated building carbon and energy model was created to explore this landscape. For the first time, building system performance was measured on the basis of both carbon emissions and energy demand, and included renewable energy generation (via roof-mounted photovoltaics) and embodied carbon and energy measurements. A conceptual building framework was varied element by element and applied to a variety of domestic building designs. The result was a design space matrix consisting of over 24 million building design-conceptual framework cases each producing an assessed outcome measured in terms of annualised net carbon emissions and net energy demand. A classification tree approach was used to interrogate the design space. The analysis did not seek to identify optimised design choices on the basis of individual building performance. Instead, the analysis made comparisons across the total population of buildings in the design space on the basis of binary building classifications (zero or non-zero energy or carbon). The results show that the zero carbon building design space is almost twice the size of the zero energy design space, and that, while these two spaces currently overlap to some extent, the overlap will shrink in future as energy grids are increasingly decarbonised.

Date of Award	20 Nov 2019
Original language	English
Awarding Institution	University of Bath
Supervisor	David Coley (Supervisor) & Sukumar Natarajan (Supervisor)