COOLING GUIDE TARGETS THE LONG, HOT SUMMER

Transcription

1 COOLING GUIDE TARGETS THE LONG, HOT SUMMER By Dr Paul Osmond Towards the end of this unseasonably warm Sydney winter the Cooperative Research Centre for Low Carbon Living launched its Guide between ground, buildings and atmosphere; and by generation of heat within the city itself (Earl et al, 2016). The nature of urban materials and to Urban Cooling Strategies. This document is designed to provide practical project-based guidance on moderating urban microclimates and mitigating urban heat island effects. The audience comprises built surfaces (hard, paved), land cover (lacking vegetation) and metabolism (waste heat from Cities are significantly hotter than the surrounding countryside the urban heat island effect environment professionals and regulatory agencies, the scope covers Australian cities across climate zones from Darwin to Melbourne, and the focus is on the public realm. The Guide uses three dimensions to contextualise its urban cooling strategies: urban form, climate type and the nature of the design intervention. Urban form refers to the intensity of development, from dense CBD to suburban sprawl; interventions refer to a suite of design options including tree canopy, shading, cool materials and water features. So what is it about cities that calls for these kinds of interventions? Urban climates are distinguished by the balance between solar gain and heat transport, industry and air conditioning) creates a significant temperature discrepancy between city and country the abovementioned urban heat island (UHI) effect (Akbari et al, 2008). Quality of urban life at risk The combination of a warming climate, UHI and longer, hotter and more frequent heatwaves is having a growing impact on the quality of urban life, from simple low-grade misery to increased risk of death. Heatwaves kill more Australians than any other natural disaster it is estimated that heatwaves currently contribute to the deaths of over 1000 people aged over 65 each year across the country (Nairn and Fawcett, 2013). lost from walls, roofs and ground; by heat exchange via air movement Cooling guide targets the long, hot summer Dr Paul Osmond 1

2 If we can cool our outdoor environments we also decrease the demand for air-conditioning, which in turn results in less waste heat production and prevents the feedback loop between Air conditioning generates waste heat, which means a hotter outdoor environment which encourages yet more air conditioning outdoor heat stress and energy consumption during heatwaves. So UHI mitigation represents an important way to reduce carbon emissions in our cities. As noted above, existing UHI mitigation techniques recommend urban vegetation, cool materials, water and shading as potential solutions to moderate temperatures and increase the adaptive capacity of cities to the warming climate. The effectiveness of each technique varies according to the location, urban context (density, scale) and climate zone. Before exploring vegetation-based solutions, it is worth briefly reviewing other design responses. Many ways to cool the city Cool pavement and facade materials store and/or reflect less heat compared with conventional products. Urban surface materials can also be cooled via increased permeability. Permeable paving allows water to drain and evaporate though the urban surfaces. In dry climates water Traditional parks provide an abundance of greenery bodies can remove ambient heat through evaporation; fountains and to support urban cooling, like the classic Himeji running water over urban surfaces (pavements, walls) provide an active Japanese garden in Adelaide s Parklands. approach to water-based cooling. Evaporative spray cooling systems offer thermal relief on hot days, even in a subtropical climate. Exposure Silver Maple (Photo Source MkWebber2016 Cooling guide targets the long, hot summer Dr Paul Osmond 2

3 to solar radiation is the obvious driver of heat storage in urban materials, so casting shade on surfaces can be a logical way to decrease heat accumulation and subsequent emission to the environment. This can Living architecture, such as the Patrick Blanc include shade from buildings, temporary or permanent artificial shade designed vertical garden, Central Park, Sydney structures and of course tree canopy discussed below. can help to reduce building cooling energy Lack of vegetation cover is a defining feature of built-up urban areas. consumption. It is also a major contributor to the UHI effect through decreased evapotranspiration in cities. Vegetation facilitates UHI mitigation via evapotranspiration, shading and providing cooler surfaces to reduce mean radiant temperature (the averaged effect of heat radiating from surrounding surfaces). Suitable species selection and planting design with taller vegetation shrubs and trees can also help channel cooling breezes to where they are needed. In addition, urban vegetation supports more effective stormwater management, improved air quality, biodiversity, urban ambience and energy saving, often referred to as co-benefits. The role of vegetation urban nature in human health and wellbeing is increasingly well documented (see for example the research roundup on the Journalist s Resource website, studies/environment/cities/health-benefits-urban-green-space-researchroundup) as well as increasingly crucial to our future as an urban species. Let s harness the park cool island to beat the urban heat island Maintaining and enhancing green open space such as urban parks represents another clear win-win approach to city cooling. Radiant Cooling guide targets a long hot summer Dr Paul Osmond 3

4 temperatures in urban parks with adequate irrigation are typically 2-4 C cooler compared with adjacent built-up areas, while air temperature reduction varies from 1-2 C depending on the park s extent and the proportion of trees (Block et al, 2012). The relative coolness of urban green space is known as the park cool island (PCI) effect. The extent of cooling in PCIs varies with vegetation type and irrigation. Parks with moderate tree canopies which rely on natural precipitation in dry climates tend to achieve their highest cooling capacity several hours after sunset, when the UHI effect is at its peak. Parks with dense tree canopies and those with significant water supply, reach their maximum cooling capacity during the afternoon. Natural turfs and ground covers use similar principles for surface cooling through evapotranspiration, which of course is highly dependent on availability of water for irrigation. Landscaping at ground level is the traditional method of vegetating cities, but when there is a shortage of space for parks and street trees, living architecture or landscapes on structure may provide a solution. Green roofs are typically classified into extensive As our cities densify, living architecture can both cool and beautify (with shallower growing medium and lighter vegetation cover) and intensive (full rooftop gardens). Green roofs of both types require adequate loadbearing structure to support the extra load of soil and plants, an extra insulation layer, waterproofing membrane, specialized drainage layer, root barrier, engineered growing medium (with or without soil) and appropriate plant selection. A comparative study of green roofs and cool roofs Urban greening can include high-tech interventions the Singapore Gardens by the Bay project continues to draw tourists from the island state and from across the world. Cooling guide targets a long hot summer Dr Paul Osmond 4

5 (which reflect heat) in a Mediterranean climate indicates that both wellirrigated green roofs and high emissivity cool roofs perform well in summer and hot climates (Zinzi and Agnoli, 2012). The plant choice for green wall systems is more limited compared with green roofs. They are often categorised into green facade and living wall systems: the former refers to climbing plants growing on a mesh or cable framework fixed to the building; the latter involves plants growing in containers fixed to the building. In both systems, the green wall provides additional thermal insulation and passive energy savings (Perez et al, 2011) as well as the range of co-benefits offered by vegetation in general, as outlined above. Green roofs and walls are especially recommended for urban transformation projects, high density developments where ground space is limited, and when green roof/wall co-benefits are a design objective. References: Akbari, H., Bell, R., Brazel, T., Cole, D., Estes, M., Heisler, G., Hitchcock, D., Lewis, M., McPherson, G., Oke, T. and Parker, D., Urban heat island basics. Reducing Urban Heat Islands: Compendium of Strategies. Block, A.H., Livesley, S.J. and Williams, N.S., Responding to the urban heat island: a review of the potential of green infrastructure. Victorian Centre for Climate Change Adaptation Research, Melbourne. Earl, N., Simmonds, I. and Tapper, N., Weekly cycles in peak time temperatures and urban heat island intensity. Environmental Research Letters, 11(7). Jacobs, B., Mikhailovich, N. and Delaney, C., Benchmarking Australia s Urban Tree Canopy: An i-tree Assessment, prepared for Horticulture Australia Limited by the Institute for Sustainable Futures. University of Technology Sydney: com. au/ media/7141/final-report_ pdf. Nairn, J.R. and Fawcett, R.G., Defining heatwaves: heatwave defined as a heat-impact event servicing all community and business sectors in Australia. Centre for Australian Weather and Climate Research. Perez, G., Rincon, L., Vila, A., Gonzalez, J.M. and Cabeza, L.F., Green vertical systems for buildings as passive systems for energy savings. Applied energy, 88(12), pp Wong, E., Akbari, H., Bell, R. and Cole, D., Reducing urban heat islands: compendium of strategies. US Environmental Protection Agency. Wong, N.H. and Jusuf, S.K., Study on the microclimate condition along a green pedestrian canyon in Singapore. Architectural Science Review, 53(2), pp Zinzi, M. and Agnoli, S., Cool and green roofs. An energy and comfort comparison between passive cooling and mitigation urban heat island techniques for residential buildings in the Mediterranean region. Energy and Buildings, 55, pp Cooling guide targets the long, hot summer Dr Paul Osmond 5

6 Dr Paul Osmond is the Director of the Sustainable Built Environment program at the University of New South Wales. His research interests cross a range of topics within the broad ambit of built environment sustainability, including urban metabolism, green infrastructure and ecosystem services. Before joining the Faculty of Built Environment in 2010 Paul managed the University s operational Environment Unit. His previous work experience includes landscape and urban design practice in local government and consultancy. Global Offices Citygreen Australasia 821 Pacific Highway, Sydney, NSW 2067 Phone: (+61) info@citygreen.com Website: Citygreen USA 515 S. Flower Street, 36th Floor, Los Angeles, California Phone: (+1) info@citygreen.com Cooling guide targets a long hot summer Dr Paul Osmond 6