Innovations to ensure energy savings in a hot, arid climate
There has been always a link between rapid urbanisation, continuous economic development and global warming, including the degradation of the microclimate in urban areas and increased ambient urban air temperatures, leading to what is known as an urban heat island (UHI).
According to the 2021 United Nations Climate Change Conference (COP26), resulting changes in the climate continue “to be perceived as the gravest threat to humanity” and, with the present lack of successful action on climate change, the greatest damage to date on a global scale could be incurred in the next 10 years (Global Risks Report 2022).
In response, the United Arab Emirates (UAE) Green Agenda 2015-2030 was very keen to put the UAE at the frontline of global efforts to prove that climate action can go hand in hand with continuous economic development.
In a collaborative project between the United Arab Emirates University (UAEU) and London South Bank University (LSBU) in the United Kingdom, research was conducted to tackle climate change and provide feasible solutions for the urban heat island effect in the Gulf Cooperation Council (GCC) region.
The project investigated the feasibility of applying green roofs and reflective coatings known as ‘cool roof’ on indoor energy saving and outdoor cooling impact at pedestrian level in hot climates. ‘Cool roof’ is an expression used to define roofing material with a high solar reflectance and albedo, leading to a significant degree of reflection of solar radiation.
The investigations show that, by applying high-albedo materials and adding more green spaces in urban areas, there is the potential to mitigate UHI effects. This is achieved through a reduction in sensible heat flux and through different mechanisms whereby the green space converts solar radiation into latent heat, while the high-albedo materials reflect most of the solar radiation back into the sky.
As such, the thermal properties of this approach may represent a paradigm shift in heat mitigation infrastructure, as the materials in the coating allow the roof to remain cooler than the ambient air temperature during periods of high solar radiation. Adding to this is the ease of installation, since adding reflective coatings or paint on buildings is very simple compared to other passive measures as it can be applied as ordinary paint, whereas the initial and maintenance costs of a green roof are much higher.
This might be an optimal solution for the GCC to mitigate the UHI effect and the higher energy loads for cooling, and to overcome the high rate of heat stress and other heat-related illnesses due to the very high temperatures.
In this context, the study aims to explore the potential of different roof types including conventional, green and cool roofs on cooling the ambient air temperature when applied at the micro-urban level and to quantify building energy savings for a typical residential house.
The research method was designed based on three phases. Phase one included the identification of a typical residential building with all the required data and characteristics through a literature review, followed by the creation of a building energy simulation for validating the base case.
Phase two involved the development, at the building level, of a parametric energy analysis to compare different roof retrofit technologies: a green roof and a high-albedo coated cool roof, with a typical concrete roof, for annual cooling load energy savings.
Phase three focuses on the analysis and comparison, at the urban microscale, of the effect of the same roof retrofit technologies in simultaneously mitigating the UHI effect and pedestrian-level air temperature.
Figure 1. The ENVI-met 3D model of the neighbourhood
When it comes to the experiment findings, all the tested scenarios achieved better cooling load reduction compared to the base case, the typical concrete roof.
The cool roof achieved the greatest load reduction, recording 25,312 KWh and a 10% reduction compared to the base case; this was 1.9% less than the green roof, which recorded 25,785 kWh and a total saving of 7.5% compared to the base case. The most interesting outcome was the no insulation roof, which had a predicted annual increase of up to 6% in energy usage for cooling, highlighting the crucial role of thermal insulation in energy savings.
In terms of the surface temperatures of the tested roof and ambient air temperature, all roof surface temperatures followed the shortwave radiation pattern with a delay. The cool and green roofs shared almost the same drift but with lower values compared to the conventional roof.
During the daytime, although the external layer of the cool and green roofs had a temperature close to the ambient air temperature, the green roof surface temperature remained below the average ambient air temperature across the simulation, while the cool roof exceeded the ambient air temperature only once, by 1.4°C at noon. The base case or typical roof had a higher surface temperature during the day when solar irradiation is more intense.
At night, the surface temperature of every roof dropped significantly due to the absence of solar irradiation and the sky vault radiative exchange cooling effect. However, the base case roof took longer to cool and had almost the same temperature as the ambient temperature, while the cool roof was cooler than the green roof.
In conclusion, the cool and green roofs were found to have very small daily fluctuations in temperature compared to the conventional roof.
Furthermore, the cooling effects, of the ambient air temperature at pedestrian level, can be seen for all the tested scenarios during the day, where the reduction values in the air temperature seem to be related to albedo value; that is, the higher the albedo, the more reflection of incoming shortwave radiation, leading to decreased air temperature.
The same cooling effect was noticed at night, again possibly due to the material properties, as the high albedo caused a reduction in heat storage during the day; this compares to a conventional roof, which normally absorbs heat during the day before releasing it back into the atmosphere at night, causing a slight elevation in air temperature.
Accordingly, the cooling effect of the tested roof scenarios was always more compared to the base case of the typical roof. The maximum reduction in air temperature reached -0.95°C for the cool roof at 24.00, against -0.72°C for the green roof compared to the base case.
Between 08.00-15.00, the average reduction in air temperature was -0.8°C for the cool roof, and -0.6°C for the green roof. After 15.00, the average cooling effect of the green roof was -0.6°C, compared to -0.4°C for the cool roof. This might be explained by the different mechanisms of these roofs in dealing with solar radiation.
While the high albedo of the cool roof reduces the amount of stored energy during the day, the green roof employs the evapotranspiration effect. In the latter, after sunset there is no photosynthesis, which lowers the latent heat flux as only evaporation of the substrate layer can occur. In comparison, the conventional roof absorbs much more heat during the day and releases it back at night.
In conclusion, building energy modelling indicated a 10% reduction on the cooling load for the cool roof compared to a 7.5% for the green roof, compared to the conventional one. It was demonstrated that using cool coatings as a rooftop tactic with proper insulation, in an arid climate, can provide a more thermally comfortable environment for naturally ventilated or unairconditioned indoor spaces.
From the micro-urban modelling, it was found that the surface temperatures of the cool and green roofs were very close to the ambient air temperature; the green roof’s surface temperature remained below the average ambient air temperature during the whole simulation.
The potential benefits of building energy savings and a cooler indoor environment, as well as improved cooling effect on air temperature at pedestrian level, strongly indicate that a cool roof with high solar reflectance and albedo is a promising strategy for buildings in regions with hot, arid climates.
The approach is economically and technically very feasible for application as an urgent action to combat climate change and its impact, and in response to Sustainable Development Goal 13 on climate action, which is concerned with improving education, awareness and human and institutional capacity on climate change mitigation, adaptation, impact reduction and early warning, and which is intrinsically linked to all 16 of the other goals of the 2030 Agenda for Sustainable Development.
This is the fourth in a series of articles promoted by the United Arab Emirates University.