Effects of photovoltaic panels on roof heat transfer (2023)

This study investigated the effects of chilled roofs, green roofs, and roofs with solar panels on near-surface temperature and cooling energy demand through regional modeling in the Greater Chicago Area (CMA). The new parameterization of green roofs and solar panel roofs based on model physics has recently been developed, updated and coupled with a multi-layered building energy model that is fully integrated with the weather research and forecasting model. We evaluate the model performance using observational measurements to show that our model can be a suitable tool to simulate the heat wave event. Next, we investigate the effects by characterizing the near-surface air temperature and its daily variation from experiments with and without the different roofs. We also estimate the influence of the roof on urban island intensity (UHII), surface heat flux and boundary layer. Finally, we measure the influence of the different roofs on the city-wide air conditioning consumption. The results show that the use of the cooling roof can lower the near-surface temperature the most over urban areas, followed by green roofs and solar panel roofs. The chilled roof experiment was the only one in which the near-surface temperature tended to decrease as the urban fraction increased, suggesting that the chilled roof is the most effective mitigation strategy among these three roof options. Cooling energy consumption can be reduced by 16.6%, 14.0% and 7.6% respectively when cooling roofs, green roofs and roofs with solar panels are used. Although solar panel roofs show the smallest reduction in energy consumption, if we assume that all electricity production can be used for cooling needs, we can expect a saving of almost half (46.7%) in cooling energy needs.

The implementation of rooftop photovoltaic panels (RPVPs) is one of the most effective strategies to mitigate urban heat islands and reduce urban energy demand with renewable energy sources, which is especially needed during extreme heat waves. However, the impact of RPVPs on urban thermal environment cooling and energy saving has not been fully explored in terms of RPVPs coverage (CV) and conversion efficiency (CE). To answer this question, this study performed a numerical assessment of thermal environment, cooling energy consumption (EC) in buildings, and power production (EP) RPVPs during an extreme heat wave event in a semi-arid Chinese city using the coupled WRF model with building effect parameterization and building energy model. Twelve scenarios with four CVs and three CEs were simulated. The results indicated that the implementation of RPVPs could lower the 2m air temperature in general due to the higher effective albedo and higher emissivity of RPVPs. Increasing the CV and CE of the RPVPs resulted in stronger cooling effects and larger cooling surfaces, with the RPVPs with a CV of 100% and a CE of 0.3 being able to reduce the 2m air temperature by 0.4 – 0.7°C and a significant contributor to this was a 14.74% decrease in cooling EC. The EP could equal and even exceed the cooling EC with the EP/EC being 182.61%, 135.77% and 123.37% at 100% coverage (CE=0.3), 75% coverage (CE=0 ,3) and 100% coverage (CE) achieved =0,2) scenarios suggesting that RPVPs could potentially generate enough power to compensate for EC cooling in semi-arid cities. Our results have practical implications for the implementation of RPVPs and the need to improve conversion efficiency for better thermal and energetic benefits.

Despite the technical maturity and significant cost reduction potential of BIPV technologies, there are still challenges to scaling up BIPV applications and their wider adoption on a global scale. Among these, adapting the PV integration to specific climatic and environmental conditions of the local solar architecture is crucial. This will facilitate the transition to sustainable buildings and mitigate climate change. In this context, this study proposes for the first time a novel BIPV climate design framework for positioning and adapting PV buildings to the local climate to minimize energy consumption and resource use. With the review and analysis of a large number of BIPV studies worldwide for seventy parameters grouped into eight main categories of an open access database, the global horizontal irradiance (GHI) value is selected as an additional index to the Köppen-Geiger classification scheme. The expansion takes into account the suitability and vulnerability of urban space and prioritizes the building integration of photovoltaics. Four zones of cold (low GHI), temperate (intermediate GHI), warm (high GHI), and hot (very high GHI) climate regions are considered and applied to 127 cities worldwide. In this framework, the order of integration of PV building components is proposed according to the local climate of each zone, and the energy efficiency of buildings is maximized towards their positive energy contributions and their participation in local, district and city grids. Obstacles and limitations of larger-scale BIPV implementation are discussed and emerging research needs are identified.

Photovoltaic modules are usually used on the roof to generate electricity. However, the installation of PV on the roof also has a potential impact on the heating and cooling load of the building. This work investigated these indirect benefits of rooftop PV modules by conducting experiments in Raipur, India and comparing the results with the exposed roof. Furthermore, a mathematical model is presented to analyze the annual effect of PV shading in terms of thermal load saving and power generation. Annual variations in cooling/heating load, PV power generation and overall energy saving efficiency index are presented for different climate zones of India. The average annual reduction in roof and ceiling temperatures for different cities ranges from 6.05-10.96 °C and 3.94-7.15 °C, respectively. Reduce annual cooling load by 70.33-94.37%. Although shading of PV modules during the winter season has negative effects by increasing the building's heat load in all climate zones, the building's overall heat load can decrease by as much as 22.76–74.07%. In addition, the overall energy saving efficiency index varies from 21.27 to 25.12%, with the maximum efficiency observed for the city of Jodhpur, followed by Raipur. Overall, this study highlights the potential of rooftop PV application in buildings in different climate zones of India.

Renovating existing buildings to reduce energy consumption is a priority worldwide to reduce greenhouse gas emissions. Millions of buildings around the world have old roofs that are poorly insulated but with large roof areas that could potentially provide significant renewable energy production. Often, photovoltaic panels are simply added to existing buildings, regardless of thermal integrity. However, due to weathering, roofs often need to be repaired over the course of their useful life. We are evaluating a comprehensive methodology using EnergyPlus and TRNSYS simulation tools to assess how best to combine solar power generation and improved insulation to achieve cost reductions, improve efficiency and utilize renewable energy at the time of rooftop access to improve. The benefits of pooling efforts into a single intervention have not been fully explored in building energy research. We also consider important effects that PV standoff arrays have on roof thermal performance, from shading and long-wavelength irradiance to sky. These influences have important comfort implications for poorly insulated structures with increasing summer heatwaves accompanying climate-induced warming. They also have a strong interaction with installed roof insulation levels. Based on an example analysis in Milan, Italy, we looked at three typical residential building types (single-family house, multi-family house, multi-family house) with different geometries, insulation thicknesses and roof constructions. We evaluated two options: roof remediation (roof needs to be repaired/replaced) and remediation (roof remediation energy intervention). We have optimized the insulation values ​​of the roofs using state-of-the-art building energy simulations, taking energy and documented costs into account. PV power generation was quantified in detail, also examining how the roof shading of the PV array affects the thermal performance of the roof, an impact that had not previously been considered. This is particularly important for non-insulated buildings where the upper floors can experience excessive heating in summer. Both heating and cooling needs are considered to determine the optimal roof intervention and what savings and associated costs can be achieved depending on different parameters. We have found that combining proper insulation with PV can be a cost effective option to reduce net primary energy consumption in residential buildings. Savings from insulation alone ranged from 3% (condominium) to 17% (single family home). When adding PV systems with an initially uninsulated roof, the net savings range from 55% (condominium) to 80% (single family home). Freestanding PV shading reduced summer cooling loads by 17% in non-insulated apartment complexes and provided large projected comfort improvements in upper floor apartments adjacent to exposed roofs.

This paper evaluates the energy efficiency of an integrated adaptive envelope system (AES) when applied to single-family homes in four US climate zones. Three main technologies are part of the AES, including cooling roofs, moveable PV-integrated shading devices (MPVISDs), and switchable isolation systems (SISs). For this study, the AES is operated to minimize annual heating and cooling energy consumption. The analysis results clearly show that the integrated AES have a high potential for cooling energy savings for residential buildings. In particular, MPVISDs offer the highest contribution, followed by attic and wall-integrated SISs. Overall, the integrated AES enables on-site power generation and offers savings of between 234 kWh/year and 949 kWh/year in cooling energy, depending on the US climate. The use of AES alone enables US households to achieve near-net-zero energy designs, especially in mild and hot climates.

Solar Energy, Volume 103, 2014, pp. 541-549

The performance of a photovoltaic module under operating conditions essentially depends on two factors: solar radiation and cell temperature. These two factors are strongly influenced by assembly geometric parameters. This article examines the effects of mounting parameters on photovoltaic (PV) module performance using CFD and single diode models. The results show that the maximum power output occurs at the tilt angle where the incident solar radiation peaks. The relationship between efficiency and tilt angle works differently depending on the wind speed inside the gap. The air gap height, the distance between the module and the roof, determines the heat transfer mechanism within the air gap and then greatly affects the module performance. When the flow within the air gap is fully developed, both the module performance and the efficiency reach the highest value. The result of this study provides a theoretical basis for designing mounting parameters for the installation of PV modules and helps to maximize energy efficiency under practical operating conditions.

Energy, Volume 175, 2019, S. 978-985

In this study, the efficiency of back panel air velocity in cooling based on the temperature and insolation of the environment where the panels are located was examined. As the panels cool down, the backplane temperature drops and the open circuit voltage of the panels increases accordingly. Currently, the most important losses in modules are due to the increase in module temperature depending on solar radiation and outside air temperature. In this study, the temperature changes on the backside were observed at air velocities of 0-5 m/s and 10-40 °C. The calculations show that during winter weather conditions the temperature of the panels did not rise to a level that would require cooling. In this study, the heat transfer from the surface was examined as a function of the outside air temperature, the back wall air velocity and the changing back wall temperature. The influence of different outside air temperatures on the rear wall heat transfer is minimal. At an air speed of 5 m/s and an outside air temperature of 10-40 °C, the heat transfer in the Poly Crystal Solar module was calculated to be 11.6 W/m²K.

Solar Energy, Band 107, 2014, S. 510-522

Building-integrated photovoltaic (BIPV) panels are emerging as a useful technology to help achieve net-zero energy buildings. The main disadvantage of BIPV systems is currently the cost per kilowatt hour of electricity generated. In addition to cheaper production of photovoltaic modules, increases in efficiency can be achieved by reducing module temperatures. This is often accomplished by adding a cavity under the panels to allow ventilation of the back of the panel. However, the details of the airflow in the cavity and the effects on cooling have not been thoroughly studied. Extending lifespan against degradation is also an effective technique to reduce the cost of generated electricity. Moisture ingress and thermal stresses are among the main reasons for the deterioration of BIPVs; These processes are directly affected by air and moisture flows around the panels. The surface temperature thermographs and airflow observations performed in this work help to understand the transport mechanisms above and below the plates. For this purpose, a novel setup was developed consisting of a building model with a simulated BIPV panel and a solar simulator placed in an atmospheric wind tunnel. Particle image velocimetry (PIV) and infrared thermography were performed to simultaneously monitor surface temperature and airflow above and below the plate. The study clearly shows how the accelerated airflow inside the cavity increases the heat exchange between the PV and the airflow and consequently lowers the PV temperature. It is also shown that the tiered open layout of the panels is more effective in reducing temperature compared to a flat layout. This arrangement is also better resistant to air and moisture intrusion.

Energy and Buildings, Volume 108, 2015, pp. 195-204

The impact of a building-extended PV system on the building's energy demand is analyzed at different times of the year. A methodology based on internal and modified components of TRNSYS is developed through temperature measurements in a pilot university building in western Greece. Two roofs, one conventional and one under a 9.6 kW polycrystalline PV array system, are studied as part of a detailed building energy system, taking into account the microclimatic external flow patterns, canopy architecture geometry and power generation. The complex air flow under the canopy is also analyzed and its effect on heat transfer is shown. The vertical temperature distribution is validated by measurement data at a roof area of ​​0.55 absorption value. In addition to electricity generation, a seasonal increase in heating load of 6.7% and a decrease in cooling load of 17.8% are determined using the simulation results under typical energy management considerations of a residential building on the top floor. The results indicate that the impact of rooftop PV on building energy efficiency should be considered in seasonal strategies for efficient design and improved net-zero energy operation.

Renewable and Sustainable Energy Reviews, Band 43, 2015, S. 264-280

Photovoltaic (PV) green roofs combine PV with green roofs, are a new trend in the construction sector and offer additional advantages (compared to simple green roofs) such as:on sitepower generation. The present study is a critical review of several factors related to PV green roof systems. Representative studies from the literature are presented with critical comments. The studies show that the plant/PV interaction leads to an increase in PV output depending on parameters such as plant species, climatic conditions, evapotranspiration, albedo, etc. In addition, a PV green roof is compared with a PV gravel roof from an ecological point of view, showing that the PV green system compensates for its additional load in the long term through higher electricity production. In addition, a systematic classification of Mediterranean plant species with regard to their suitability for PV green roofs is also carried out as part of the present study. The results show that the increase in PV output achieved by PV green roofs depends on several factors and among the studied plant species,A nailed seatshows the best interaction with the PVs and the building. Experimental results and findings on the environmental profile of PV green roofs are also presented and critically discussed. In summary, PV green roof systems show promise, especially for warm climates.

Energy, Volume 137, 2017, S. 1152-1158

In addition to generating electricity, solar photovoltaic (PV) modules can also serve as external shading devices for buildings. Solar PV shades can effectively reduce solar heat gain through windows, but can have a negative impact on indoor daylight output. Therefore, it is worth investigating the optimal design of solar PV shades as heat, daylight and power generation performance are closely related. In this work, a numerical simulation model based on EnergyPlus was used to investigate the energy saving potential of PV shading with different tilt angles and orientations in Hong Kong. The results show that the optimal mounting position for solar PV shades is the south facade with 30° tilt angle to maximize power generation. However, considering the electricity savings from the air conditioning and the increased electricity consumption for the artificial lighting, it is recommended to install PV shading on a south-facing facade with a 20° tilt angle. In addition, the total annual power benefits of solar PV shades were compared to the widely used indoor blinds. The results show that the well-designed solar PV shades can achieve much more total annual electricity savings than indoor blinds.