EcoHouse Research

Anna Clements (MEng, 2013)

 

The built environment accounts for a significant proportion of UK energy consumption and environmentally-harmful CO2 emissions, and as such provides the potential for significant savings in energy and emissions. It is important to consider the lifetime energy of a building once a long term horizon is adopted, and how this energy can be minimized. This requires consideration of the two constituents of lifetime energy - embodied energy of the building fabric and the use energy. Energy to maintain a comfortable internal temperature is a large proportion of the use energy in a building, and this heating energy can be further broken down into overall heat loss due to differences between the mean internal and external temperatures, and comfort heating due to the transient response of the building system. This project considers both these aspects of heating energy and focuses on an investigation of the use of laminates to reduce the overall heating energy. Laminates offer the potential for improved property trade-offs relative to traditional homogeneous materials.

 

The first part of this research extends work on minimising the lifetime energy of a building by focusing on the steady-state heat transfer properties of laminates. Previous work on the optimal insulation thickness necessary to minimise lifetime energy is extended to the optimum thickness of laminates using the Hybrid Synthesiser tool in CES Selector 2013. This analysis shows that laminates involving materials with low embodied energy, such as straw, for example, would have to be used at impractical thicknesses in order to minimise lifetime energy. It also shows that the majority of building materials currently used in the UK, and the thicknesses at which they are typically used, are not well suited to the climate, and at least in terms of minimising the lifetime energy are not appropriate. Consequently, use energy dominates the non-optimal lifetime energy for many of our buildings.

 

The second part focuses on the transient thermal response of laminates, and how their properties might be exploited to affect the transient thermal response of the entire building. To this aim the effects of timescales, thicknesses, materials and architectures of a panel on the transient thermal response were investigated using the finite element analysis software program Abaqus. A brick wall was used to provide a reference. Analysis of this led to the definition of two new graphical methods portraying the transient thermal response of the wall, providing useful insights. The aim was to investigate the possibility of characterising the transient thermal response of laminates, in the same way that diffusivity characterises the transient response of homogeneous materials.

 

Two sandwich panels with different architectures, but the same average thermal properties as the brick wall, were analysed and their responses considered in the new graphical portrayals. The transient thermal response of each wall structure was different, despite having the same average thermal properties, and consequently possessing the same steady-state heat transfer characteristics. This means that the thermal properties of the materials that comprise a laminate cannot be combined to give an effective transient heat transfer characteristic, such as an effective diffusivity. It is not possible to characterise the transient response of a laminate for one scenario and use this same measure in a different set-up – in other words, the diffusivity of a sandwich panel is non-unique.

Two cases of real-world laminates were used as case studies throughout the analysis. The large insulating panels of the Equinox House in Illinois and a straw-bale and lime render panel praised for its low embodied energy were found to be mostly insulating. They did not give a particularly significant transient response.

 

The research showed that the architecture of a laminate influences the transient response, giving rise to the possibility of exploiting the order and properties of the materials to enhance the thermal comfort of a building. Thermal comfort depends on the thermal mass of a building envelope: the degree of inertia with which the building responds to a change in external temperature. The results of this research indicate what the optimum order of insulating and conducting layers in asymmetric panels might be. It was surmised that having the conducting layers towards the interior and the insulating layers towards the exterior would provide a greater transient response. This could then lead to a more thermally massive building, permitting a saving in comfort heating energy.