Understanding And Modelling Thermal Energy Demand And Emissions In Urban Environments

Refrigeration, air conditioning and heat pump (RACHP) systems currently account for nearly 20% of UK grid electricity use and over 7% of all UK greenhouse gas emissions. This research project has investigated the sources and levels of emissions from RACHP systems and how the cooling (and heating) energy and emissions from buildings might be reduced by optimizing the building’s design, construction and operation. Analysis of data from site surveys and maintenance logs confirmed that leakage of refrigerant can be a significant contributor to total RACHP emissions. TEWI (total equivalent warming impact) analyses showed that for RACHP systems with high GWP (global warming potential) refrigerants and annual leak rates of 10% or more, direct emissions from refrigerant leakage can exceed the indirect emissions associated with energy use. However, for heat pump and air conditioning systems, with typical leak rates of below 3%, using low GWP refrigerants (GWP = 500 or less), the direct emissions do not make a significant contribution to building emissions. A new dynamic energy balance model and Excel based tool were developed to help improve the understanding of building energy use and emissions. The tool can be used to predict the sensitivity to different building design concepts, features and operation and the parameters of the installed RACHP plant. Results for an office building suggest that the building fabric (with the exception of the glazing) is not necessarily a key factor influencing the total energy use and emissions. However, relatively simple measures to reduce electricity use and to reduce solar gain could each reduce building emissions by 10% or more. Results for a dwelling built to 2006 Building Regulations demonstrated an overheating risk in summer, even with mechanical ventilation, but adding a 2 kW air conditioning unit could prevent overheating, with lower energy use and emissions than a similar dwelling incorporating mechanical ventilation. Climate change simulations for the year 2080 predicted a net increase in energy demand and emissions of about 5% for the office building (mainly associated with the use of grid electricity), implying that the grid carbon factor is likely to be a key determinant of future emissions from such buildings. For dwellings without mechanical ventilation or air conditioning, internal temperatures might rise as high as 40°C in summer months, but a small air conditioning unit could maintain temperatures below 25°C with no increase in total energy use and emissions compared with the present day. For a grid electricity carbon factor reduction of 80%, total emissions for the simulated office building would fall by about 70% and for the dwelling by about 50%