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How green roof saving the energy

 green roof save energy

2.3 Urban Heat Island Effect and Indirect
Energy Savings
‘Summers by 2050 will be 1.5-3.5°C hotter...in central London the urban heat island currently adds 5-6°C to summer night time temperatures and will intensify in
the future. [Ref: 3.10]
Urban areas can have a higher average temperature than surrounding rural areas; this difference in temperatures is called the UHIE and is caused by the reduction in green space through urbanisation and the large amount of hard surfaces that provide high thermal mass. The dark surfaces of roofs exacerbate the UHIE by absorbing summer heat and radiating it back to the atmosphere during the night. As summer temperatures rise due to climate change and more intense UHIE episodes are experienced, demand for air-conditioning will increase, increasing the energy demand. Heatwave periods often coincide with poor air quality episodes and intense urban heat island episodes which collectively exacerbate health problems, especially in the old, young and vulnerable.
The evaporation and evapotranspiration from a green roof cools the air. Furthermore by providing a cooler surface at roof level the green roof and better thermal insulation reduces the need for air conditioning during periods of higher than normal temperatures. The combined effect
is to reduce the UHIE.
One study [Ref:3.11] concludes that ‘Sustainable Urban Futures’ demand the use of vegetation, with the greatest benefit to be had when both green roofs and green walls are utilised, enveloping the whole building fabric in vegetation. Such an approach could lead to an 84 per cent reduction in cooling demands.
A further study considers the appropriateness of using green roofs and green walls as a mitigation technique
in various European cities. The report concludes that:
‘For all climates examined, green walls have a stronger effect than green roofs... Nonetheless, green roofs have a greater effect at roof level and, consequently, at the
urban scale...if applied to the whole city scale, they
could mitigate raised urban temperatures, which can
lead to significant energy savings, more “human friendly” urban spaces, ensuring a viable future, from
a thermal point of view, for urban dwellers.’ [Ref: 3.12]
A modelling scenario undertaken in New York by the New York Heat Island Initiative determined that providing 50 per cent green roof cover within the metropolitan area would lead to an average 0.1-0.8°C reduction in surface temperatures. It noted that for every degree reduction in the UHIE roughly 495 million kWh of energy would be saved. The same study also looked at various mitigation strategies other than green roofs, including urban forestry and cool roofs, and noted that green roofs provided greater benefits than white or ‘cool roofs’. It was clear from the study that a combination of various mitigation strategies for UHIE, including green roofs, should be considered by the city. [Ref: 3.13]
A study in Toronto estimated that the city comprised 50 million m2 of potential roof space that could be greened. Overall it was estimated that the effect of greening the rooftops would lead to 0.5-2°C decrease in the UHIE. The study estimated that a reduction of this magnitude would lead to indirect energy savings citywide from reduced energy for cooling of $12 million, equivalent to 2.37 kWh/m2 per year and that this would reduce peak demand at a rate of 0.0023 kWh/m2. [Ref: 3.7]
As a result of this work the city of Toronto has developed a green roof policy in order to encourage green roof uptake within the city. [Ref: 3.7]. [See Appendix 1]
The US city of Chicago has been promoting green roofs
at a city level for a number of years. A study undertaken on a hot summer’s day in 2001 noted that the temperature
on a conventional roof was 28°C higher than that on a green roof. Chicago has a number of policy initiatives to encourage and provide incentives for the use of green roofs specifically as a means of mitigating against the negative impacts of UHIE. [Ref: 3.8] [See Appendix 3]
Green Roof, Chicago City Hall One of first to be installed in programme to reduce UHIE. Photo: Mathew Frith
In Japan, many cities suffer from the severe effects of the UHIE. The average annual temperature in Tokyo has increased by 3°C in the last century. This is four times higher than what could be explained by to the effects
of global warming. [Ref: 3.14]
‘The Tokyo based Organisation for Landscape and
Urban Greenery Technology Development estimates that if half of the roofs in the city were planted with gardens, daytime temperatures in summer would fall by 0.84°C, which would save 110 million Yen on air conditioning costs.’ [Ref: 3.15]
The city has introduced policies that require green roofs to be installed on 20 per cent of all new flat surfaces
on government buildings and 10 per cent of all flat
roofs on private dwellings. [Ref: 3.1] [See Appendix 1]
Green Roofs and Photovoltaic Solar Panels
There is a perception that a building can either have green roofs or solar production at roof level but not both. However, it is possible to take a more pluralistic approach and use both technologies in tandem. In fact there is substantial evidence from Germany that the
use of both solar/photovoltaics and green roofs
provides dual benefits in terms of energy production
and energy saved.
Solar/Photovoltaic (PV) A-Frame panels at roof level
are known to work more efficiently when installed on
a green roof rather than a on a conventional surface.
The green roof element not only saves energy during
the summer time (see above) but can also increase efficiency of PV by reducing fluctuation of temperatures at roof level and by maintaining a more efficient microclimate around the PV Panels. Crystalline silicon photovoltaic panels, as a rule of thumb, lose 0.5 per cent/°C in efficiency above 25°C. The green roof serves as a natural cooling mechanism, thereby maintaining

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