An experimental study of the impact of cool roof on solar PV electricity generations on building rooftops in Sharjah, UAE

Abstract

Cool roof is the technique that delivers higher solar reflectance and higher thermal emittance than standard roofing products. Its fastest developing applications in the built environment comes with various benefits, such as reducing the urban heat island effect by decreasing surface temperatures, which reduces the flow of heat into the atmosphere, and offsetting warming caused by greenhouse gases. In this study, a Cool Coating Paint has been experimented while the impact was measured in terms of an integration with the Solar PV technology. This research investigated cool roof applications on building’s rooftop together with PV panels for the Middle East climatic conditions, and its impact on the electricity generation. The preliminary findings of the experimental study indicated that there is a likely impact of 5–10% improvement of electricity generation with the cool roof applications.

1 INTRODUCTION

Urban areas usually experience higher temperatures when compared to rural surroundings. Several studies show that specific urban conditions are strictly connected with the Urban Heat Island (UHI) phenomenon, which consists in the environmental overheating related anthropic activities [1, 2]. Many causes bring higher temperatures in urbanised environments, such as anthropogenic heat, excess of heat stored by buildings, roads, and other constructions materials that absorb heat during the day and re-emit after sunset, decreased long-wave radiation losses from urban areas, lack of vegetation and reduced evaporate-transpiration processes, reduction of wind speed and consequent reduction of convective heat removal from urban surfaces to the atmosphere [3].

Several studies demonstrated that urban microclimate effects on building energy consumption. According to Santamouris et al. [4], the cooling load of urban building may increase two-fold and the peak cooling load may be tripled by the UHI effect, which was noticed in Athens. Kikegawa et al. [5] developed a simulation model that can describe relations between summer weather conditions in urban contexts and building energy needs. They found out that the peak-time cooling load in Tokyo could decrease by up to 6% with a reduction of the outdoor air temperature by more than 1°C.

Therefore, several strategies have been studied on the UHI effect in summer in order to reduce cooling energy demand and to improve indoor thermal comfort. Santamouris et al. [6] has reviewed of several advanced cool materials systems usable to reduce the UHI. Such materials could be implemented on roofs in order to reflect more heat to the sky (high albedo, high emissivity) or to delay the heat transfer toward the inside the building (thermal mass, phase-change materials). Gagoetal [7] has also reviewed several research works analysing strategies to mitigate the UHI effect, including changes in green spaces, trees, albedo pavement surfaces, vegetation, and building types and materials. In the United Arab Emirates (UAE), the higher urban temperature caused an increase usage in air conditioning. Around 70% of Dubai’s peak demand is for cooling energy load. Most of the country’s air-conditioners run on electricity that is generated by natural gas-fired power plants, which emit heat-trapping greenhouse gases into the atmosphere. Reducing the cooling load would help to decrease the country’s carbon footprint [8]. Additionally, the UAE vision 2021 aims for a low carbon, sustainable and resource efficient future, and to ensure sustainable development while preserving the environment and to achieve a good balance between economic and social development. By providing sustainable housing to improve the quality of life of UAE’s citizens can be achieved through mitigating the UHI effect. It can be decreased by increasing the cooling systems’ efficiency, adding vegetation to buildings (like green roofs and walls), and cooling paved surfaces with highly reflective paint [9].

Based on some researchers’ [10] definition of what constitutes a cool roof, the technique is considered a passive solution and a building typology that assists in reducing the cooling loads and energy demands on a building’s envelope. Cool roofs can be surfaces that reflect sunlight and emit heat more efficiently than that other dark roofs, which is also known as the ‘albedo effect’. Using cool roofs give various level of benefits. On the urban level, it can contribute in reducing urban air temperatures by decreasing the quantity of heat transferred from roofs to the urban environment [11, 12]. This can be done by implementing retro-reflective materials and reflective coatings [13]. On the building level, cool roof improves indoor thermal comfort and it reduces energy bills by reducing the usage of mechanical air conditioning systems [14]. A typical application for a cool roof will achieve a reduction of ~10–40% in air conditioning energy [15–17]. Long term benefit is a lower roof temperature reduces maintenance and, hence, extends the life of the roof [10]. In terms of solar panels, it may help improving the efficiency of the solar cells used in a Photovoltaic (PV) system for generating electricity, which is what the study investigated.

Since most dark roofs absorbs 90% or more of the solar energy, the roof can reach temperatures higher than 66°C when the weather is warm and sunny. In summertime, higher roof temperatures increase the heat flow into the building, causing the air conditioning system to use more energy. However, cool barrier technologies can create surfaces that absorb less than 10% of the solar energy, which results in reducing the roof temperature and the energy usage of air conditioning [18].

Some researchers [17] have done a study to estimate the effect of using cool coatings on energy loads and thermal comfort in residential buildings. They performed simulations for 27 cities worldwide representing different climatic conditions, including Mediterranean, humid continental, subtropical arid, desert conditions. They used a single story house with a roof area of 100 m² as a base case building in the simulation. They found that the cool coating application resulted in increasing the roof solar reflectance by 0.65, and reducing cooling loads by 8–48 kWh/m². Moreover, another study was conducted by the University of Melbourne (UniMelb) about the cool roof paint [19]. They installed three single room buildings at the University of Melbourne’s Burnley campus in Melbourne, Australia. Two buildings were painted with cool roof paint, and the third one was left unpainted. They also did a computer simulation by TRNSYS software package. Their results show that cool roof paint products are effective at reflecting solar radiation and decreasing roof surface temperatures, when compared with a standard metal roof material. The paint reduces the indoor temperatures of the test buildings during day and night. For a typical residential building, the attic space was 18.5 degrees cooler with the use of cool paint. In terms of cooling energy reductions, a commercial building in Melbourne could benefit by ~3%. The study [20] showed that changes in agricultural practices in the vicinity of Paris and the use of cool materials for roofs and pavements would decrease the UHI effect by 2 K and 1 K, respectively. The European Cool Roofs Council compared several case studies about the cool roof impact in Italy, Greece and UK [21]. In Trapani, Italy, a test was carried out on 800 m² office/laboratory block. After the cool roof application, the net cooling demand was reduced by 54%. In Athens, Greece, a white elastomeric waterproof coating was applied on 410 m² school block. Their results show a 20% reduction in the cooling load. In London, UK, pink elastomeric coating was applied on an office building with 137 m². The heating and cooling demand was reduced by 3.5%. Another study [22] shows that solar panels, by shading of the roof, slightly increase the need for domestic heating in the winter by 3%. In summer, however, the solar panels reduce the energy needed for air-conditioning by 12% due to the shading of the roof. They also lead to a reduction of the UHI effect. During summer daytime, the deployment of solar panels can reduce the surrounding temperatures by 0.2 K. At night, the presence of solar panels leads to a mitigation of surrounding temperatures up to by 0.3 K.

Most studies, as previously mentioned, focused on the impact of cool roof paint on the indoor comfort in buildings, which is an important factor for building environments, however, equally, it is important to quantify the other benefits such as the benefits through other active systems, i.e. solar technologies. For this reason, the aim of this research is to test the effectiveness of the cool coating paint on PV panels through investigating cool roof applications on building’s rooftop together with PV panels for the Middle East climatic conditions, and its impact on the electricity generation.

2 METHODOLOGY

Experimental method was considered in this study where a test conducted on the laboratory rooftop of the University of Sharjah (UOS), in the UAE, which was at the same time integrated within the housing competition project of the Solar Decathlon Middle East (SDME) 2018 Dubai. Moreover, to record the generation of electricity and to compile the analysis report for PV modules, PV-Analysator and PROFITEST PV were used. The preliminary findings of the experimental study indicated that there is a likely impact of 5–10% improvement with the cool roof applications. Such findings could help with significant impact on the longer-term benefits. Overall, the domestic building sector could improve and benefit through the electricity generation.

Abolin Cool Barrier – High Solar Reflective (SRI 113) Acrylic Elastomeric One Component Water Based Top Coating accordingly with ASTM D 6083 [18]. Table 1 shows the characteristic of the cool coating paint used in this study.

In October 2017, an experiment conducted to test the potential improvement of PV performance by increasing the diffused radiation onto the PV surface. The location of the experiment is the rooftop of the solar PV lab, W12 building – the Central Laboratories of the University of Sharjah, as shown in Figure 1. The red box represents the location of the experiment setting, which is oriented in parallel with the PV lab. That means that the PV panels are oriented 35 degrees from South direction towards West direction (Figure 1). The PV system layout and performance was tested in real-life scenario. A tailored panel’s rack was designed and fabricated; a nylon sheet will be coated with a special reflective paint (cool coating) and combined to the PV panels support rack. The increased solar radiation onto the PV surface was measured with sensors and digitally stored with data-logger and workstation.

The solar radiation potential can be utilised in several applications such as the desalination, solar thermal collectors, heating of buildings and PV cells. One of the main concern of the researchers is maximising the amount of useful energy that can be extracted from the incoming solar radiations. The performance of these devices is believed to show remarkable changes based on the proper installations of these solar systems. Mainly climatology, latitude, orientation, tilt angle, azimuth angles, and usage over a period in a particular geographical region affect the performance of the systems above [24, 25] – higher solar radiations in general means the better performance of the PV. In general, cases, setting the tilt angle or inclination angles of the PV to the latitude of the region is applied which is accepted globally [26]. Therefore, the systems with lower tilt angle have the higher performance during summer, and the systems with higher tilt angle have the higher performance during winter season [25, 27].

2.1 Setting up the experiment

This experiment consists of two scenarios; each scenario is with two cases. The first scenario is about comparing two cases, one case is with the cool coating paint and the other case is without the cool coating paint. The second scenario is similar to the first one but the second case is with a black carpet. The required area for each case is 5 m × 5 m (25 m²), for both cases an area of 50 m² is required. Table 2 presents the equipment list of the experiment.

As part of the experiment, two identical PV panels were selected; on 31st October 2017, four (98 cm*195 cm) BlueSolar Monocrystalline PV panels [23] were tested on the rooftop of W12 building. All panels were connected to data loggers and computer workstations. By using PV-Analysator software, the data of each panel was collected and then analysed using Excel spreadsheets. Table 3 shows the comparison of each panel results. Panels #2 and #3 were chosen due to the similarity of their generated power.

Two wooden racks were prepared in M12 building – Civil workshop – University of Sharjah. The duration of this activity was from 9 November 2017 to 21 November 2017. The material of the racks is Plywood; each rack required 4 ft × 8 ft Plywood panel to hold two PV panels where the type of both PV panels is monocrystalline with dimensions of 98 cm × 194 cm.

2.2 Painting with cool coating paint

On 26 November 2017, the Nylon carpet and Rack #1 were painted with the cool coating paint at W12 – Rooftop – UOS as shown in Figure 2. Eighteen litres were used to paint 25 m² of the Nylon carpet. The painting activity was carried out in 2 days, half of the bucket was used in each day.

2.3 Connection for recordings

The set of the experiment was located on the solar PV lab rooftop. Two workstations inside the solar PV lab were connected to two data loggers as shown in Figure 3. The data loggers were connected to the solar panels, which were placed on the rooftop. Connecting the data loggers to the PV panels required 8 m cables, connected to the lab below through two small openings. The cables were not shielded from heat although the original covering is weather resistant. Through two small openings within the solar PV lab, the connection was solved.

2.4 Data recordings

Throughout the test period, which was from 18 March until 12 April 2018, the researchers agreed on running the data logger from 7:30 AM to 4:45 PM, and collecting the readings every 15 min. The total readings for each case is 38. Moreover, four tilt angles were chosen for testing based on previously completed research works by [24–28]: 45°, 35°, 25°, and 15°, giving one day for each tilt angle. Ideally, it is meant to be seasonal monitoring. Figure 4 shows angle 35 under testing in the second scenario with two cases, case 1 with the cool carpet, and case 2 with the black carpet.

3 RESULTS AND ANALYSIS

By using Excel, the readings of each angle was analysed. The collected data consist of four parameters: Power in watts, Irradiance in W/m², and Temperature in °C. To understand the results clearly, a comparative analysis has been completed. There are seven parameters to compare the readings; Power difference in %, Irradiance difference in W/m², Energy production difference assuming 16% efficiency, Power difference, Energy in WH with cool painted carpet, Energy in WH without cool painted carpet (or with black carpet), and Energy difference in WH.

Scenario 1 is analysed in Figures 5–7 where the graphs shows the comparison between ‘with cool carpet’ case and ‘without carpet/normal roof’ case, in terms of the generated power and the irradiance for angle 45, 35 and 15. The maximum generated power in ‘with cool carpet’ case reached 251 W at angle 45 at 1:15 PM, yet it reached 244 W in ‘without carpet/normal roof’ case. Moreover, the maximum generated power in ‘with cool carpet’ at angle 35 reached 257 W, but it dropped to 236 W at angle 15.

‘With cool carpet’ case clearly performs better at angle 45 and 35 as shown in the power difference between the cases. At angle 45, the average of power difference is 2.9%, and at angle 35, the average of power difference is 4.0%. On the other hand, at angle 15, the difference is lower as you can see in Figure 7; the curved lines that represent the cases are almost united. The average of power difference at angle 15 is 0.5%.

The maximum value of the irradiance in ‘with cool carpet’ case at angle 45 is 1053 W/m² at 1:15 PM, yet it reached 967 W/m² in ‘without carpet/normal roof’ case as shown in Figure 5. Whereas it increased at angle 35 to reach 1096 W/m² as shown in Figure 6. However, at angle 15, the irradiance decreased, the maximum value of the irradiance reached 1011 W/m² as shown in Figure 7. Moreover, the summation of the irradiance difference between both cases at angle 45 is 1654 W/m², yet it increased at angle 35 and 15. At angle 35, the summation of the irradiance difference reached 1810 W/m², and it reached 1871 W/m² at angle 15.

Scenario 2 is analysed in Figures 8–10 where the graphs show the comparison between ‘with cool carpet’ case and ‘with black carpet’ case, in terms of the generated power and the irradiance for angle 45, 35 and 15. The maximum generated power in ‘with cool carpet’ case reached 238 W at angle 45 at 2:00 PM, yet it reached 233 W in ‘with black carpet’ case. Moreover, the maximum generated power in ‘with cool carpet’ at angle 35 reached 241 W, but it dropped to 218 W at angle 15. Regarding the average of power difference in this scenario, you can see that it is easier to notice the difference at all angles compared to Scenario 1. At angle 45, the average of power difference is 5.6%. However, at angle 35 and 15, the difference decreased as shown in Figure 9 and Figure 10. At angle 35, the average of power difference is 3.1%, and at angle 15 is 1.6%.

Scenario 2 shows the difference in the generated power and the irradiance clearly. The maximum value of the irradiance in ‘with cool carpet’ case at angle 45 is 1069 W/m² at 1:15 PM, yet it reached 966 W/m² in ‘with black carpet’ case as shown in Figure 8; whereas it increased at angle 35 to reach 1082 W/m² as shown in Figure 9. However, at angle 15, the irradiance decreased, the maximum value of the irradiance reached 992 W/m² as shown in Figure 10. Moreover, the summation of the irradiance difference between both cases at angle 45 is 2434 W/m², yet it decreased at angle 35 reaching 2255 W/m², and it reached 1905 W/m² at angle 15.

Figures 11–13 show the comparison between Scenario 1 and Scenario 2 in terms of the percentage of power difference and the irradiance difference. Scenario 2 that represents the comparison between ‘with cool carpet’ case and ‘with black carpet’ case, did not show higher differences at all times at all angles compared with Scenario 1 that represents the comparison between ‘with cool carpet’ case and ‘without carpet/normal roof’ case.

In Figure 11, which is related to the previous Figure 5 and Figure 8, it is obvious that Scenario 2 generated a higher difference compared to Scenario 1 from 7:00 AM to 3:00 PM with two exceptions; at 11:30 AM and 1:15 PM; while the irradiance difference in Scenario 2 was higher than that of Scenario 1 from 7:00 AM to 3:00 PM with no exceptions. In both scenarios, the power difference reached the maximum value in the morning, while the irradiance difference reached the maximum value in the afternoon. The maximum value of the irradiance difference in Scenario 2 was at 12:30 PM, while in Scenario 1; it reached the maximum value at 1:45 PM.

In Figure 12, which is related to the previous Figure 6 and Figure 9, Scenario 1 generated a higher power difference compared to Scenario 2 in general, while the irradiance difference in Scenario 2 was higher than that of Scenario 1 from 7:00 AM to 1:15 PM only. From 1:15 PM to 3:00 PM, the irradiance difference in Scenario 2 was lower than that of Scenario 1. In both scenarios, generally, the power differences reached the maximum value in the morning. The maximum value of the irradiance difference in Scenario 2 was at 12:00 PM, while in Scenario 1; it reached the maximum value at 1:45 PM.

In Figure 13, which is related to the previous Figure 7 and Figure 10, Scenario 1 generated a higher power difference from 7:30 AM to 9:15 AM compared to Scenario 2. After that, Scenario 2 generated a higher power difference from 11:00 AM to 3:00 PM compared to scenario 1. While the irradiance difference in Scenario 2 was higher than Scenario 1 from 7:00 AM to 11:15 PM only. From 11:15 PM to 3:00 PM, the irradiance difference in Scenario 2 was lower than that of Scenario 1. In both scenarios generally, the power difference reached the maximum value in the morning. The maximum value of the irradiance difference in Scenario 2 was at 11:45 PM, while in Scenario 1; it reached the maximum value at 12:45 PM.

4 DISCUSSION

In Scenario 2, ‘with black carpet’ case, at angle 15, the temperature was very high, yet the PV panel got a low amount of diffused light in ‘with cool carpet’ case. In Scenario 2, ‘with black carpet’ case, at angle 45, the temperatures were lower, yet the PV panel got higher amount of diffused light in ‘with cool carpet’ case.

In Scenario 2, angle 45 and 35 generated almost the same amount of power, but angle 15 generated less amount of power, which is unusual because in this particular region, the closer the PV panel to be positioned flat the better the PV panel performs. Therefore, this means that the compensation of the cool roof paint is actually changing the general understanding of the tilt angle of PV panels in this region.

In both scenarios, the PV panel that was adjusted on the cool carpet generated more power at angle 45, due to the higher amount of reflection and solar radiation that the cool coating caused, particularly at the timeframe of the experiment. This means that the higher the angle the higher the irradiance levels as shown in Figure 5–13. These figures showed how clearly the generated power, i.e. electricity, is related to the irradiation; higher irradiation leads to higher amount of power. In addition, it is understandable that the power generation drops when the angle is closer to be flat (on zero). It is important to mention that all the readings used in all graphs where generated in March 2018, because all the results are related to the weather factors that keep changing mainly every season, e.g. temperature, wind speed, humidity, drought weather, rainy days, dust and low/high sun. It is safe to say that when the PV panels are set on cool coated carpet and tilted 45 degrees, they would generate higher amounts of power than that of the zero tilted PV panels (in the case of an experimental study conducted in March).

Since most studies until now have focused on the impact of the cool coating paint on building indoor comfort and environments, it is not possible to have direct comparisons with other studies and the experimental study conducted in this paper. It is however worthwhile to discuss the potential benefits of the study and testing the effectiveness of the cool coating paint on PV panels through investigating cool roof applications on building rooftops together with the impact on electricity generation through PV panels for the climatic conditions of the Middle Eastern countries, such as the UAE. Furthermore, it may be worth noting that the cool roof (or building envelope as a whole) can also contribute to reducing the building’s cooling load due to the sunlight reflection. This study did not measure the temperature surrounding the PV tested and it also examined only mono-crystallised PV cells, and therefore, further studies will concentrate on different PV types as well as other parameters from surrounding environments.

Looking at the preliminary results from this experimental study, the applied cool coating paint demonstrated benefits with a likely impact of 5–10% improvement with the selected cool roof application. The findings could help with significant impact on the longer-term benefits, and the building sector could improve and benefit from the increasing electricity generation.

5 CONCLUSIONS

Case studies are significant to better understand how cool roofs work in practice and through different applications. In this paper, the experimental study helped in understanding further benefits of cool roof applications by extending knowledge to solar applications, in this case, PV panels and their electricity generation. Although the study is only presenting preliminary results, the findings have been justified through robust measurements and methodology, and the paper demonstrated somewhat significant impact of cool coating paint on the generated electrical power of the solar PV utilised panels. Further studies through experiments will be made showing seasonal variations of the results as well as different angle usage of the PV panel applications on building rooftops.

Most case studies performed in the European Union (EU) demonstrated that the technique is relevant to reduce the cooling demand in cooled buildings, which is also applicable for the Middle East regions, and to improve the comfort conditions. With this study, the quantified benefits have been extended to other applications creating innovative solutions for increasing energy demand in buildings. In this case, the production of energy through solar applications can be further boost with the cool coating roof paint. Moreover, the UAE Vision 2021 would welcome such innovation and technology benefitting application in rapidly growing built environment in the country and also throughout the wider region – Middle East and the North Africa (MENA).

ACKNOWLEDGEMENT

The authors would like to acknowledge Mr. Francesco Favarò, CEO of Watergy International Group, for their invaluable input towards this research and also supplying the cool coating paint for the experimental study. The project was conducted as part of an ongoing bilateral agreement by the two organisations.

REFERENCES