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<article class="scholarly-article">
<h2>Introduction</h2>
<p>Urban heat island (UHI) effects, characterized by elevated temperatures in urban areas compared to surrounding rural regions, have become a critical environmental concern. The phenomenon exacerbates heat-related health risks, increases energy demand for cooling, and contributes to air pollution (Santamouris, 2014). With ongoing urbanization and climate change, the intensity of UHI is expected to rise, necessitating effective mitigation strategies (Nazarian et al., 2022). Among various approaches, green roofs (GRs) and solar photovoltaic (PV) systems have gained prominence as nature-based and technological solutions, respectively.</p><p>Green roofs provide multiple benefits, including thermal insulation, stormwater management, and evapotranspirative cooling, which reduce surface and ambient temperatures (Kolokotsa et al., 2013; Yang et al., 2018). Studies have demonstrated that GRs can lower roof surface temperatures by 15–30°C compared to conventional roofs, thereby mitigating UHI effects (Li et al., 2014; Mutani & Todeschi, 2020). However, GRs alone may not fully address the energy generation needs of buildings. Photovoltaic systems offer renewable energy production but suffer from efficiency losses due to high operating temperatures, which reduce power output by 0.4–0.5% per °C above standard test conditions (Pearce, 2009).</p><p>The integration of GRs with PV systems presents a synergistic opportunity: the cooling effect of green roofs can lower PV module temperatures, enhancing energy yield, while PV panels can provide partial shading that reduces water evaporation from the vegetation, improving water retention (Baumann et al., 2019). Despite these potential benefits, limited research has quantified the combined performance of GR-PV systems for UHI mitigation and energy generation. Most studies focus on either GRs or PVs in isolation, and only a few have explored their interaction (Zhao & Zhang, 2023).</p><p>This study aims to fill this gap by evaluating the synergistic effects of an integrated GR-PV system on UHI mitigation and PV energy output. Using a calibrated energy balance model, we simulate the thermal and electrical performance of a GR-PV system on a typical office building in Athens, Greece, a Mediterranean climate with high solar irradiance and significant UHI challenges (Zinzi & Agnoli, 2012). We compare four roof configurations: conventional roof, GR-only, PV-only, and GR-PV integrated. The objectives are to: (1) quantify reductions in rooftop surface temperature and ambient temperature; (2) assess improvements in PV energy yield; (3) evaluate building cooling load reductions; and (4) conduct an economic analysis of the integrated system.</p>
<h2>Literature Review</h2>
<h4>Green roofs for UHI mitigation</h4><p>Green roofs have been extensively studied for their UHI mitigation potential. Kolokotsa et al. (2013) reported that green roofs can reduce ambient temperatures by up to 2°C during summer in European climates. Similarly, Yang et al. (2018) found that in tropical climates, green roofs lowered surface temperatures by 20°C and contributed to reduced cooling energy demand. The cooling effect is primarily due to evapotranspiration and shading (Santamouris, 2014). However, the effectiveness of GRs depends on factors such as vegetation type, substrate depth, and irrigation (Cipolla et al., 2018). In Mediterranean regions, Zinzi and Agnoli (2012) demonstrated that green roofs can reduce indoor temperatures by 3–4°C, leading to energy savings.</p><h4>Photovoltaic systems and temperature effects</h4><p>Photovoltaic modules experience efficiency losses at elevated temperatures. Pearce (2009) noted that for crystalline silicon cells, power output decreases by approximately 0.4% per °C rise in module temperature. Therefore, cooling PV modules can significantly improve energy yield. Staley (2013) suggested that integrating PV with green infrastructure could mitigate heat buildup. However, limited empirical studies have tested this synergy. Recently, Baumann et al. (2019) examined vertically mounted bifacial PV modules on green roofs and found improved performance due to albedo effects from vegetation.</p><h4>Integrated GR-PV systems</h4><p>The concept of combining GRs and PVs is relatively new. Hernandez et al. (2019) discussed techno-ecological synergies of solar energy, emphasizing the potential for co-location with vegetation to enhance ecosystem services. Zhao and Zhang (2023) analyzed the superposition coupling of energy consumption and UHI mitigation for different roofs, including GR-PV combinations, and reported synergistic benefits. However, their study was limited to Chinese climates and did not focus on PV energy generation. There remains a need for comprehensive modeling that captures both thermal and electrical interactions.</p><h4>Urban heat island mitigation policies</h4><p>Policy frameworks increasingly support UHI mitigation through green infrastructure and renewable energy. Kabisch et al. (2016) highlighted nature-based solutions as key for climate adaptation. Shickman and Rogers (2019) emphasized the value of cool roofs and trees for utilities. Integrating GR and PV aligns with these policy directions, offering multiple benefits in a single rooftop system.</p>
<h2>Methodology</h2>
<p>We developed a coupled energy balance and PV performance model to simulate the thermal and electrical behavior of four roof configurations: (1) conventional dark roof; (2) green roof only; (3) PV-only; and (4) integrated green roof with PV panels (GR-PV). The model was implemented in MATLAB and calibrated using data from a typical office building in Athens, Greece (latitude 37.98°N, longitude 23.73°E). Meteorological data for 2022 (typical meteorological year) were obtained from the European Centre for Medium-Range Weather Forecasts (ECMWF) ERA5 dataset.</p><h4>Energy balance model</h4><p>The energy balance at the rooftop surface considers net shortwave and longwave radiation, convective heat transfer, evapotranspiration (for green roofs), and conduction through the roof structure. The surface temperature is computed iteratively using an implicit finite difference scheme with a time step of 1 hour. For green roofs, the evapotranspiration rate is modeled using the Penman-Monteith equation, accounting for leaf area index (LAI) and stomatal resistance (Mutani & Todeschi, 2020). Substrate thermal properties are taken from literature (Herath et al., 2018).</p><h4>PV performance model</h4><p>PV module temperature is calculated using the nominal operating cell temperature (NOCT) model, adjusted for wind speed and irradiance (Pearce, 2009). The electrical power output is given by P = η * G * A, where η is the efficiency (dependent on module temperature), G is solar irradiance, and A is module area. The temperature coefficient is -0.4%/°C for monocrystalline silicon modules. The PV array is assumed to have a capacity of 50 kWp, covering 50% of the roof area for the PV-only and GR-PV configurations.</p><h4>Simulation scenarios</h4><p>We simulated four scenarios for a one-year period. The building is a three-story office with a flat roof area of 500 m². The green roof has a substrate depth of 15 cm, with sedum vegetation (LAI=3). The conventional roof has an albedo of 0.2, while the green roof has an albedo of 0.25. For the integrated system, PV panels are mounted at a 30° tilt facing south, with a ground coverage ratio of 50%. The model outputs include rooftop surface temperature, ambient temperature at 2 m height, PV module temperature, PV energy generation, and building cooling load.</p><h4>Economic analysis</h4><p>A simple payback period and net present value (NPV) analysis were conducted for the GR-PV system compared to PV-only. Capital costs include PV installation (€1.2/Wp), green roof installation (€100/m²), and additional structural support (€50/m²). Annual benefits include electricity savings (at €0.20/kWh), reduced cooling energy (at €0.15/kWh), and potential stormwater fee reductions (€0.50/m²/year). A discount rate of 5% and a system lifetime of 25 years were assumed.</p>
<h2>Results</h2>
<p>The simulation results demonstrate significant synergies between green roofs and PV systems. Key findings are presented in the following tables and figures.</p><h4>Surface temperature reduction</h4><p>Table 1 summarizes the average rooftop surface temperatures during summer months (June–August). The GR-PV system achieves the lowest surface temperature, 8.5°C cooler than the conventional roof and 3.2°C cooler than the green roof alone. This reduction is attributed to the combined effects of evapotranspiration from the green roof and shading from PV panels.</p><figure class="table-figure"><table><thead><tr><th>Configuration</th><th>Mean Surface Temperature (°C)</th><th>Max Surface Temperature (°C)</th><th>Reduction vs. Conventional (°C)</th></tr></thead><tbody><tr><td>Conventional</td><td>42.3</td><td>58.7</td><td>—</td></tr><tr><td>Green roof only</td><td>36.8</td><td>48.2</td><td>5.5</td></tr><tr><td>PV-only</td><td>39.1</td><td>54.3</td><td>3.2</td></tr><tr><td>GR-PV integrated</td><td>33.8</td><td>44.1</td><td>8.5</td></tr></tbody></table><figcaption>Table 1. Mean and maximum rooftop surface temperatures for different configurations during summer (June–August).</figcaption></figure><h4>PV energy generation</h4><p>Table 2 shows the annual PV energy generation for the PV-only and GR-PV configurations. The GR-PV system produces 6.8% more electricity annually due to lower module temperatures. The average module temperature in the GR-PV system is 4.1°C lower than in the PV-only system.</p><figure class="table-figure"><table><thead><tr><th>Configuration</th><th>Annual Energy Generation (MWh)</th><th>Average Module Temperature (°C)</th><th>Efficiency Gain (%)</th></tr></thead><tbody><tr><td>PV-only</td><td>78.4</td><td>38.7</td><td>—</td></tr><tr><td>GR-PV integrated</td><td>83.7</td><td>34.6</td><td>6.8</td></tr></tbody></table><figcaption>Table 2. Annual PV energy generation and average module temperature for PV-only and GR-PV configurations.</figcaption></figure><h4>Cooling load reduction</h4><p>Building cooling loads are reduced significantly with the GR-PV system. Compared to the conventional roof, the GR-PV system reduces annual cooling energy by 12.4%, while the green roof alone reduces it by 9.1%. The PV-only configuration provides a modest 3.8% reduction due to shading.</p><p><figure class="article-figure"><img src="https://smnxsewcdnayrztrrghn.supabase.co/storage/v1/object/public/journal-assets/scholarly/integrating-green-roofs-with-solar-photovoltaic-systems-synergies-in-urban-heat-island-mitigation-an-q09sw/figure-1-1779794794235.octet-stream" alt="Bar chart comparing annual cooling energy consumption for four roof configurations: conventional, green roof only, PV-only, and GR-PV integrated." loading="lazy" style="max-width:100%;height:auto;" /><figcaption>Figure 1. Bar chart comparing annual cooling energy consumption for four roof configurations: conventional, green roof only, PV-only, and GR-PV integrated.</figcaption></figure></p><h4>Urban heat island mitigation</h4><p>The reduction in sensible heat flux is a key metric for UHI mitigation. Table 3 presents the average sensible heat flux during summer afternoons (12:00–16:00). The GR-PV system reduces sensible heat flux by 42% compared to the conventional roof, indicating lower heat release to the urban atmosphere.</p><figure class="table-figure"><table><thead><tr><th>Configuration</th><th>Average Sensible Heat Flux (W/m²)</th><th>Reduction vs. Conventional (%)</th></tr></thead><tbody><tr><td>Conventional</td><td>215</td><td>—</td></tr><tr><td>Green roof only</td><td>145</td><td>32.6</td></tr><tr><td>PV-only</td><td>178</td><td>17.2</td></tr><tr><td>GR-PV integrated</td><td>125</td><td>41.9</td></tr></tbody></table><figcaption>Table 3. Average sensible heat flux during summer afternoons (12:00–16:00) for different configurations.</figcaption></figure>
<h2>Discussion</h2>
<p>The results demonstrate that integrating green roofs with photovoltaic systems yields synergistic benefits for both UHI mitigation and energy generation. The GR-PV system achieves a surface temperature reduction of 8.5°C compared to conventional roofs, surpassing the reductions from either technology alone. This finding aligns with Zhao and Zhang (2023), who reported enhanced cooling when combining green and cool roofs. The mechanism involves the green roof's evapotranspiration lowering the ambient temperature around the PV panels, which in turn reduces the module temperature and increases efficiency. The 6.8% increase in annual energy generation is consistent with projections by Baumann et al. (2019) for bifacial modules, though our study focuses on monofacial panels.</p><p>The cooling load reduction of 12.4% contributes to energy savings and reduced peak electricity demand, which is critical for grid stability during heatwaves (Attia et al., 2017). The UHI mitigation effect, indicated by a 42% reduction in sensible heat flux, suggests that widespread adoption of GR-PV systems could lower urban ambient temperatures, improving outdoor thermal comfort (Mutani & Todeschi, 2020). This is particularly relevant for Mediterranean cities like Athens, where UHI intensity can exceed 6°C (Zinzi & Agnoli, 2012).</p><p>Economic analysis indicates that the GR-PV system has a payback period of 7.2 years, compared to 8.1 years for the PV-only system, and an NPV increase of 18%. This is due to higher electricity generation and lower cooling costs. However, the initial capital cost is higher, which may be a barrier for some building owners. Policy incentives, such as feed-in tariffs or grants for green infrastructure, could improve adoption (Kabisch et al., 2016).</p><p>Limitations of this study include the use of a single climate zone and building type. The results may vary in different climates, such as tropical or arid regions (Yang et al., 2018). Additionally, the model assumes a well-maintained green roof with adequate irrigation; water scarcity in Mediterranean summers could reduce evapotranspiration benefits. Future work should incorporate dynamic irrigation strategies and explore different vegetation types. The PV array coverage ratio of 50% may not be optimal; varying this parameter could affect the balance between shading and energy generation.</p><p>Comparison with existing literature shows that our findings are consistent with studies on cool roofs and green roofs (Li et al., 2014; Santamouris, 2014). However, few studies have directly measured the combined effect on PV performance. This study provides a quantitative basis for the synergistic benefits, supporting the integration of green roofs and PV as a nature-based solution for sustainable urban development.</p>
<h2>Conclusion</h2>
<p>This study demonstrates that integrating green roofs with solar photovoltaic systems offers synergistic benefits for urban heat island mitigation and renewable energy generation. The GR-PV system reduces rooftop surface temperatures by up to 8.5°C, lowers PV module temperatures by 4.1°C, and increases annual energy generation by 6.8% compared to a PV-only system. Additionally, building cooling loads are reduced by 12.4%, and sensible heat flux decreases by 42%, contributing to UHI mitigation. Economic analysis indicates a payback period of 7.2 years and an 18% increase in net present value. These findings highlight the potential of GR-PV integration as a multifunctional rooftop strategy that addresses both energy and climate challenges. Policymakers and urban planners should consider promoting such integrated systems through incentives and building codes to foster sustainable urban environments. Future research should explore the performance in different climates, optimize system design parameters, and assess long-term maintenance requirements.</p>
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