Urban Microclimate Analysis for High Performance Office Buildings

Abstract

This research explored whether urban microclimate analysis has significant impacts on high-performance office buildings. It studied the effects of detailed three-dimensional urban microclimate modelling on building performance simulation. The feasibility and necessity of developing an urban microclimate simulation system were explored. Currently, individual parameters of urban microclimate are modelled by individual programs. However, there was no individual software that could model airflow, Urban Heat Island (UHI) effects and building energy performance at the same time. A simulation system made it possible to model these features of urban microclimate together. Apart from the reliability of programs, accessibility and compatibility were also important for building a simulation system. The goal of this research was to determine the relative scale of the likely microclimate impacts on energy performance, not to present a system that made a precise estimate of these effects in combination. The scale of the variations of results due to changes of urban microclimate parameters were more significant than the values of the results themselves. This is because the focus of the research was on determining to what degree each parameter made a difference in the building performance. The goal was to determine whether it is necessary to model every urban microclimate parameter when their individual effects are combined. The parameters of urban microclimates included horizontal parameters like urban wind and UHI, and vertical parameters like lapse rate, urban boundary layer. In this research, the urban microclimate was modelled in three dimensions, but the process of urban microclimate modelling was time-consuming. This leaded to one of the central questions of the thesis: is there value in the time spent? How big is the scale of the influence of urban microclimate detailed modelling on the prediction of building performance? Is it worthwhile to model three-dimensional urban microclimates? When there is not enough time to calibrate all parameters, what are the parameters’ priorities? A prototypical high-rise office building was modelled based on the data about high-rise office buildings in London. Firstly, the effects of the horizontal parameters were explored. The UHI has larger effects than urban wind. Secondly, the significance of vertical parameters was also explored. At a lower floor, the influence of the wind speed exponent and the boundary layer thickness on building performance simulation is bigger than that of the air temperature gradient coefficient. However, at a higher floor, the influence of the air temperature gradient coefficient is bigger. Finally, a multilayer modelling method was developed to explore the inconsistent vertical variations. The multilayer model consists of the portion in the Urban Canopy Layer (UCL) and the portion in the Urban Boundary Layer (UBL). The effects of vertical variations increase with the distance between the studied height and the UCL height. The feasibility and necessity of developing the simulation system of urban microclimate detailed modelling were demonstrated in the climate of London. In different climates, is it still necessary? The effects of urban microclimate detailed modelling on windy, continental, and tropical climates were also studied. The necessity of urban microclimate detailed modelling has been demonstrated because the combined effects produced around -25% change in London’s climate and Wellington’ climate at most. In Beijing’s climate the change was around -6% and in Singapore’s climate was 2.2% at most. The UHI has a big impact in moderate and continental climates. In a continental climate, there is a big difference in the monthly thermal load prediction. It helps engineers optimize the design of heating in winter and cooling in summer. The effects of urban wind in a windy climate are bigger than those in other cities. The precision of vertical variations has very limited influence, especially in the tropical climate. The air temperature gradient in a tropical climate changed thermal load prediction a lot. The parameters’ priorities in different climates are different. There is no consistent pattern of one factor being less important than the others across all these climates. Therefore, to model the thermal performance of tall buildings in dense urban environments it is necessary to develop a simulation system that can model the Urban Heat Island, and the differences in 3D of variations of temperature, sun and wind within and above the Urban Canopy Layer. Finally, from the one case study examined, modelling urban microclimate in detail is more important for natural ventilation systems than for HVAC systems. Overall, the simulation system of urban microclimate modelling was developed gradually. It is necessary to develop the simulation system to approach a real urban circumstance. The accuracy of the detailed urban microclimate model depends on the engineers’ requirements. The priority of urban microclimate parameters depends on climatic features.