Photovoltaic and green ROOF – PROOF

The project is based on a comparison of experimental and simulation results. Cerema's green roof platform was used to install the study plots. Each was equipped with an instrumented photovoltaic panel (measurements of PV surface temperatures, electricity production and other variables linked to the nature of the soil (including surface temperatures and water content in the substrate). The PV/surface exchanges were modelled using the Monte Carlo technique, which allows rapid simulations in complex scenarios. At the same time, biodiversity surveys (Flora) were carried out between May and September and over the duration of the project, to monitor changes in the vegetation under the panel and compare it with a control area (without PV). The results of the modelling and experimentation were used to feed the COMETH tool for calculating a building's heating, comfort and self-consumption requirements. At this scale, the environmental assessment by LCA took into account the energy contribution. A design assistance tool has been developed to assess the ecological functions and ecosystem services provided. This tool, intended for decision-makers, includes descriptive variables for the building and roof.
A methodological approach has been developed at neighbourhood level in order to integrate multidisciplinary environmental benefits (climatic, hydrological, ecosystemic) into a quantitative assessment process, using the UrbanPrint tool.

The experiments carried out showed that for an extensive green roof, the presence of a photovoltaic panel altered the development of the vegetation. During the course of the project, the various surveys showed that the photovoltaic panels modified the structure of the plant communities that were initially dominated by sedum, giving way to species that are less able to withstand the extreme environmental conditions of green roofs. The presence of the panels creates a new ecological niche, allowing the development of species already present on the roofs. We were also able to demonstrate a beneficial effect of the presence of the panels on the functioning of Sedum floriferum, with a significant increase in photosynthesis and nitrogen uptake by the plants and a concomitant significant reduction in their stress levels. Contrary to the hypotheses formulated, the presence of vegetation under the PV did not lead to a significant increase in electricity production. This is due to the virtual disappearance of the sedums. On the contrary, the coolroof coating showed better PV performance. These two results can be explained by the different surface temperatures: those of the coolroof cladding were lower than those of the green roof, thus reducing radiative exchanges between the ground and the back of the PV. The impact is even more marked at building level. In fact, the energy simulations of production with biosolar roofs are slightly lower than for other types of roof (~1%). Even if the simulations were able to show differences in production between roofs, these were too limited to produce significant differences in the results and therefore in a preferential choice.
The LCA study of construction scenarios confirmed that biosolar roofs have a higher carbon impact than other scenarios: between 6 and 65% at roof level, between 2 and 39% at macro-component level and no real benefit at building level.
However, biosolar roofs perform three functions: covering the building, producing energy and supporting biodiversity, which is not possible with a green roof or PV alone.

Several issues have been identified for the PROOF project. The heat exchange model would benefit from better integration of water transfers in the substrate. This work could be carried out as part of a new thesis in collaboration between Cerema and LEMTA.
It would seem appropriate to take a closer look at one or more real cases, either at the scale of the biosolar complex or at the scale of the building. This would make it possible to observe the evolution of flora in a multi-PV configuration and to test the robustness of the thermal model by instrumenting several PVs with the same sensors as those used in the PROOF project. We could also better integrate the multi-functionality of biosolar roofs into the carbon balances in order to put the calculations into perspective. At neighbourhood level, the methodology developed to assess the impacts and benefits of biosolar roofs in development scenarios could be extended to include urban climate issues, for example.
Lastly, the design assistance tool could be developed in collaboration with green roof professionals to be optimised and made more user-friendly.

L. Garcia-Gutierrez, M. Aillerie, J.P. Sawicki, Z. Zheng, and R. Claverie. Evaluation of solar photovoltaic efficiency on green and flat roofs : Experimental and comprehensive numerical analysis. Solar Energy, 278 :112750, 2024

Thomas Villemin, Rémy Claverie, Jean-Paul Sawicki, and Gilles Parent. Thermal characterization of a photovoltaic panel under controlled conditions. Renewable Energy, 198 :28–40, 2022

Thomas Villemin, Olivier Farges, Gilles Parent, and R?y Claverie. Monte Carlo prediction of the energy performance of a photovoltaic panel using detailed meteorological input data. International Journal of Thermal Sciences, 2024

Thomas Villemin, Olivier Farges, Gilles Parent, and Rémy Claverie. Simulation of a rooftop photovoltaic system : a focus on the energy performance of the building. onférence IHTC-17, Cape Town, South Africa, August 2023

Thomas Villemin, Julien Bouyer, Rémy Claverie, Gilles Parent, and Maeva Sabre. Experimental study and numerical simulations of the airflow around a photovoltaic panel in urban configuration. ICUC11, Sydney (Australia), «August 28-Sept. 1st, 2023«

Thomas Villemin, Olivier Farges, Gilles Parent, and Rémy Claverie. Simulation of a rooftop photovoltaic system : a focus on the energy performance of the building. Conférence IHTC-17, Cape Town, South Africa, August 2023

Thomas Villemin, Julien Bouyer, Rémy Claverie, Gilles Parent, and Maeva Sabre. Experimental study and numerical simulations of the airflow around a photovolic panel in urban configuration. ICUC11, Sydney (Australia), «August 28-Sept. 1st, 2023«

R. Claverie, M. Aillerie, S. Boddaert, J. Bouyer, A. Brachet, M. Colombert, M. Dufournet, O. Farges, L. Garcia, D. Marzougui, G. Parent, E. Pichenot, L. Reynier, M. Sabre, J.-P Sawicki, C. Sirguey, J. Solano, T. Villemin, N. Schiopu, G. Séré, Z. Zheng, and K. Zibouche. PROOF : A collaborative project to assess the multi-scale benefits of biosolar roofs. ICUC11, Sydney (Australia), August 28-Sept. 1st, 2023

The PROOF project (Photovoltaic and Green ROOF) aims to compare roofing systems and their energy-environment impacts and performance with contrasting urban development scenarios, linked to the associated territorial challenges. It is particularly interesting to study an innovative combined system, combining an extensive green roof and a photovoltaic panel. To address this problem, PROOF brings together a consortium composed of Cerema, LEMTA, LMOPS, LSE, CSTB and Efficacity. It bases its scientific approach on four hypotheses that it intends to verify during the project: 1) Incident solar energy in summer dissipated by a green roof mainly in the form of latent heat fluxes, creates a decrease in the localized air temperature providing conditions favourable to the increase in electrical efficiency of a photovoltaic panel; 2) an extensive green roof with a structure capable of storing rainwater, promotes evapotranspiration flows and can therefore further improve the panel's efficiency; 3) at the building level, we assume that the overall energy balance (production/energy consumption per use + grey energy) is more advantageous for a combined system than for a standard bare or green flat roof; 4) compared to a conventional roof configuration, a combined system provides additional ecosystem services that can be assessed and valued at the neighbourhood level. To address these different hypotheses, PROOF is divided into four scientific tasks. The first is to provide all the data and characterizations needed for modelling heat exchange between the panel and the green roof, as well as for modelling heat transfer in the panel and its impact on performance. This task also provides comparison data for other roof configurations (standard, cool-roof and extensive green roof with rainwater storage). Both models are studied in detail in Task 2: contribution of radiative, convective and latent heat fluxes; evaluation of the temperature at the rear of the panel on the delivered power. The transition from system scale to building scale is addressed by Task 3, which assesses the thermal performance of different configurations at the building scale, but also the energy-environmental and ecological performance at both scales (devices and building) under different climatic conditions. The aim is to highlight the savings on consumption at the handset scale, improved efficiency, increased service life and at the building scale. Finally, Task 4 seeks to identify and evaluate the impacts and benefits associated with the types of devices tested, which are to be compared with the local challenges of the neighbourhoods, settings and urban areas in which they will be located.