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Heat fluxes through roofs and their relevance to estimates of urban heat storage Meyn, Stephanie Katrin

Abstract

The storage heat flux (ΔQs) constitutes a large term in the heat balance of cities. This flux is difficult to measure but can be parameterized using relations between the net radiation (Q*) and the heat flux conducted into and out of the typical materials that form the surface of cities (QG)- The urban heat storage parameterization could be improved if there were more and better estimates of the net radiation vs. storage relation for typical urban and suburban roofs. This thesis presents the results of a study of the heat storage characteristics of 5 different roof assemblies (typical of many North American commercial/industrial and residential buildings) in Vancouver, B.C. These observations are used to verify the Simplified Transient Analysis of Roofs (STAR) model, which is then used to estimate the heat storage parameterization for other roof types, thereby extending the usefulness of the scheme to a wider range of cities. Distinct differences in heat storage characteristics are observed between roofs with gravel surfaces and those with modified bitumen or asphalt shingles. The surface conductive heat flux (QG) ranges from 22% to 30% of the net radiation (Q*) for gravel roofs under high wind (between 1.5 and 4 m s-1) and low wind (less than 1 m s-1) conditions respectively. For wet roof conditions, QG is approximately 25% of Q* irrespective of wind speed. For modified bitumen surfaces and asphalt shingles, it is considerably less (~ 10%), indicating that these roof surface types are more energy efficient. It is concluded that the STAR model gives good agreement between measured and modelled surface heat fluxes, and is a useful tool for modelling a wide range of roof types. The model works very well for the case of dry roof conditions, but is less accurate under wet or mixed conditions because latent heat (QE) is not taken into account.

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