Urban form influence on microclimate and building cooling demand: An analytical framework and its application on the Rotterdam case

An analytical framework and its application on the Rotterdam case

Abstract

As a determinant of urban microclimate and building energy performance, urban form plays a critical role when planning city transitions toward decarbonization. Even though energy use for cooling has tripled between 1990 and 2016 globally, and global increase in temperature is reinforcing this trend, the complex relationship between urban form and cooling demand remains understudied. Additionally, in urban climate conditions where the Urban Heat Island (UHI) further contributes to a temperature increase, a comprehensive quantification of form-dependent microclimate impacts on building cooling demand is limited by the methodological approaches employed.

The thesis aims at providing conceptual and methodological instruments to better understand the nature and the magnitude of the urban form-energy link by addressing the question ‘How does urban form influence building cooling demand in urban microclimate conditions, and how can the magnitude of the relationship be assessed?’.

By answering the main research question, the thesis contributes to the conceptualization and understanding of both the intrinsic and the extrinsic energy role of urban form. Furthermore, it proposes a novel methodological framework for increasing the accuracy of the numerical assessment of urban form-related climate and energy performance. The application of this framework on the city of Rotterdam provides an understanding of how and to what extent building and context form influence building cooling demand, illustrating the magnitude of UHI impacts in temperate climates and proving the relevance of informing planning and design practice.

In a first instance a transdisciplinary literature review (Chapter 2) identifies nine energy-related form attributes and over 54 descriptive parameters related to building, street canyon and urban fabric units. The analysis of the associated thermal processes highlights a twofold role of urban form in determining the cooling demand of buildings. The intrinsic role of form lies in building characteristics, which directly influence energy loads by impacting thermal gains and losses. The extrinsic role of form lies in the indirect effect of canyon and urban fabric on microclimate (e.g. by altering wind flows, radiation, sensible heat fluxes) which determine the conditions of the context in which a building energy system operates.

Following, the thesis addresses the limitations in assessing the magnitude of urban form influence on building cooling demand for analysing the direct effects of building characteristics and the indirect, microclimate effects of context characteristics . Informed by existing morphological approaches in the climatology and energy domains, the thesis concludes that to assess urban microclimate conditions from a morphological perspective, four conditions should be met. The morphological approach should i) allow for multi-variables, ii) allow for a multi-scalar description, iii) use analytical units of proximity, and iv) acknowledge heterogeneity of the fabric in the selection of representative form patterns. Additionally, the thesis suggests that building demand calculations should include the use of microclimate boundary conditions for a more precise cooling demand estimation. Based on this list of requirements, the framework developed enables the identification of Local Climate Types (LCTs), the assessment of the microclimate influence of buildings and context types within these LCTs (Chapter 3), and the simulation of building cooling demand within local microclimate conditions (Chapter 5). The latter method for coupling microclimate and energy demand models is initially tested on a case study in Zurich, Switzerland (Chapter 4).

The application of the framework on the case study of Rotterdam helps to identify five building types and five context types, which together make combined 25 LCTs. The microclimate assessment during two representative hot days shows that urban form variables result in a change in urban air temperature up to 2.5°C, 3m/s change in wind speed and 5% change in relative humidity. As a consequence, daily cooling demand is on average 23-32% higher in urban microclimate conditions compared to rural conditions, and among the analysed buildings the increase in cooling loads varies between 3.6% and 100%. A sensitivity analysis showed which building and context form parameters determine the variations.

Finally, the general outcomes of the study are discussed, interlinked and placed within the context of the existing body of knowledge (Chapter 6). Conclusions are presented, providing recommendations for future research and applications of the thesis results in planning and design (Chapter 7).