Tree canopy shade impacts on solar irradiance received by building walls and their surface temperature
Introduction
The world's population is rapidly urbanising, and countries such as Australia now have more than 80% of their population in cities or towns. Urban centres are hot-spots of energy consumption and associated greenhouse gas emissions, much being related to thermal regulation of building internal space. Urban centres are also hot-spots of human stress and mortality during heatwaves, as the urban heat island exacerbates high temperature events but also the most vulnerable; low socio-economic elderly or people with existing health problems, are often housed in the poorest quality building stock. Building thermal regulation is a managed conflict between building properties and the urban climate [1]. Constructing more thermally efficient and insulated buildings is of prime importance, but in lieu of that the addition of simple shading mechanisms can greatly reduce building solar load, surface temperature increase and internal heat transfer. Vegetation shade is a simple and effective means of reducing building solar loads and thermal regulation requirements in summer. Several empirical and modelling studies have demonstrated that shade trees can provide significant summer energy savings when strategically located and maintained near buildings [2], [3], [4], [5], [6], [7], [8]. Improved building thermal regulation from vegetation shade can provide significant economic gain during warm summers but especially heat wave events when air conditioner energy use is the prime contributor to power supply overload and power cuts [9]. Similarly, improved building thermal regulation can go some way to protecting vulnerable sections of society from heat stress, stroke or mortality.
The net radiation received by building walls and roof surfaces is a primary driver of building microclimates [10]. Tree canopies absorb and reflect large proportions of both solar and terrestrial radiation received by a building, which in summer can reduce the difference between internal and external building temperatures and directly reduce energy use for internal space cooling [11], [12], [13]. Shade trees can also influence building microclimates through evapotranspiration and wind control [12], [14], however, a reduction in solar irradiance received is the most effective way to reduce building external wall surface temperatures in summer [15], [16], [17].
Climate, building envelope, surface albedo, tree canopy characteristics and placement all affect the amount a shade tree can reduce building surface temperatures, with the greatest perceived energy savings achieved in buildings with low albedo surfaces and poor insulation [18], [19]. In the southern hemisphere, it is expected that summer energy savings will be greatest when trees shade both the north and west facing walls of buildings. The use of deciduous trees, rather than evergreens, reduces the likelihood that summer energy savings may be offset by higher winter heating costs [12], [20], [21].
Long term field studies on building surface temperatures and microclimates are expensive and infrequent [22], [23], [24]. Instead, several simulation approaches and mechanistic models have been developed and applied to simulate the effect of trees on building thermal balances and/or outdoor microclimates [7], [8], [25], [26], [27]. Some of the more mechanistic models comprehensively predict microclimate, wind turbulence, short-wave and long-wave radiation exchange and reflection, transmittance and stomatal evapotranspiration. However, these models also have exhaustive data input requirements [28]. Pandit and Laband [29] noted there were few empirical field studies of tree shade effects upon building thermal balance, and that these often suffered from small sample size and adequate controls to provide useful calibration or validation data for mechanistic models.
In older housing stock, walls rather than roofs, often offer a greater potential for tree shade to reduce surface temperatures and thereby provide energy savings because walls are often less well insulated and contain windows that transmit solar radiation [23], [30]. Research suggests tree shade coverage and shade intensity both contribute to the reduction building wall surface temperatures, but qualitative estimates of shade intensity (e.g. light, moderate and dense) make it difficult to compare the quality of shade produced by different tree species in different locations [3], [8], [15], [31]. For these reasons, studies have called for practical methods to model shade patterns while others have limited their focus to external wall surface temperatures [8], [30]. Building external wall surface temperature is a good proxy for building wall heat exchange, and is far easier to measure and spatially replicate. Similarly, measuring levels of direct, diffuse and reflected irradiance received by a wall may provide a good indication of shade intensity.
The overall aim of this study was to establish a range of tree shade conditions and to develop methods of measurement to investigate the relationships between tree shade cover, solar irradiance received on the wall (vertical plane) and building external wall surface temperatures. Building wall surface temperature will be greatly influenced by both the ‘amount’ and the ‘quality’ (intensity) of the tree canopy shade. Percentage shade cover obviously provides a good indication of the ‘amount’ of shade, whilst solar irradiance received on the wall is an indication the ‘quality’ of shade. A shading coefficient goes some way towards an indication of both shade quality and amount. For the study, three research questions were posed:
- 1.
Can simple, reliable and reproducible measures of tree shade cover, solar irradiance received on the wall in a vertical plane and/or shading coefficients be used to predict the building wall surface temperatures?
- 2.
How do tree shading coefficients change seasonally and diurnally?
- 3.
What impact does canopy height and distance to building walls have upon wall surface temperature and microclimate?
The study was carried out on three identical small, wooden weatherboard dwellings. Along north and west walls of two buildings trees were placed, one with evergreen Angophora trees the other deciduous Fraxinus trees. Diurnal measurements of external wall surface temperature, percentage shade cover, solar irradiance received on the wall in a vertical plane and microclimate conditions were made on cloudless days through spring and summer, and predictive models developed and tested against a one day of validation data collected in late summer.
Section snippets
Methods
The study was conducted at the University of Melbourne Burnley Campus (37°49′47.16″S, 145°01′25.55″E) between August 2011 and April 2012. Melbourne has an oceanic climate (Köppen classification Cfb) characterised by hot summers that closely resemble a Mediterranean climate [32]. The mean winter (April to September) minimum and maximum in 2011 was 9.3 °C and 17.5 °C, respectively. The mean summer (October to March) minimum and maximum for 2011/12 was 14.9 °C and 24.7 °C, respectively. The mean
Diurnal patterns of shade, solar irradiance and surface temperature
Daily tree shade coverage and solar irradiance was related to building wall surface temperatures for data collected on 18 October and 22 December 2011, 17 January and 23 January 2012 (Fig. 3). Shade coverage and solar irradiance were inversely related to building wall surface temperatures. That is, for a given surface temperature, shade coverage increased as solar irradiance received on the wall decreased. Shade cover ranged between 0.0% and 97.4%, solar irradiance received on the wall ranged
Simplicity and reliability of measurement methods
Measuring external wall surface temperature using an infra-red ‘gun’ was quick and did not require contact with the wall, a distinct advantage in restricted access or high wall positions. However, comparative temperature measurements with a Type-K thermocouple were needed in shaded and unshaded conditions to calibrate for differences in emissivity. The small difference between wall surface temperatures (0.2–0.9 °C) measured on 15 December 2011 among the three buildings when trees had been
Conclusions
This study directly quantified and related tree shade coverage and solar irradiance to building external wall surface temperatures. This required the development of methods to rapidly and accurately quantify surface temperatures, shade coverage and solar irradiance on building walls. Differences in solar irradiance between walls with tree shade and those without were used to construct dynamic shading coefficients according to the time of day and season. Tree shade coverage and solar irradiance
Acknowledgements
This study was part of the ‘Energy Saving Benefits of Urban Trees’ collaborative research effort between the Melbourne School of Land & Environment, the Melbourne School of Engineering and the Faculty of Architecture, Building and Planning. This research was funded by Nursery and Garden Industry Australia (NGIA) and special thanks goes to Anthony Kachenko for his support and encouragement.
Thanks goes to: Dr Dominique Hes and Dr Nick Williams for field and equipment support and discussion;
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