Sheltering Effect from Floating Photovoltaics over the Waterbody-atmosphere Interface

Floating Photovoltaics (FPV) technology benefits from a remarkable support worldwide for two main reasons: it produces energy for a reasonable carbon budget and it has a lower land-use footprint compared to similar renewable installations. With increasing concerns about freshwater availability, a third asset is likely to boost the momentum of FPV: the potential water savings of reservoirs. As shown in Figure 1, the FPV array is made up of buoys and photovoltaic modules that are prone to reduce the energy input and the action of vapour removal on the surface of the water basin. However, giving a precise assessment of how much water would be saved is complicated, as it relies on the technology of floaters (water surface openings) and the modified physics of the water-atmosphere interface. In this case, looking at the whole system as a canopy that acts on the water-atmosphere interface seems relevant to study the evaporative levels.

 

This contribution proposes a new modelling approach based on Computational Fluid Dynamics (CFD) calculations to assess the amount of water vanishing into the atmosphere when a reservoir is covered by a half-open structure. A first computational domain is built in which the PV module is explicitly represented as if it were standing in the PV array, considering modules as grid-aligned obstacles (Figure 2). The airflow located below the modules is assimilated to the canopy airflow, and modifying the module geometry has an impact on the advection-diffusion processes of the vapour at the bottom of the canopy. Evaporative rates are computed and a numerical function is created to link the rates to the velocity and direction of the wind. In order to obtain the rate at the reservoir level, a second simulation is setup using a microscale domain that encompasses a reservoir partially covered by an FPV array and the surrounding lands. The numerical function is plugged into the model so that the actions of the FPV array on the atmosphere and canopy flows are conserved during the upscaling process. The methodology is supported by a case study that includes a nominal FPV module geometry. A specific reservoir is analysed, the real elements of geographic information are digitised for this purpose, and a micrometeorological station is installed in the real reservoir. Preliminary measurements show good agreement with the humidity level predicted in the atmosphere, so spatially extrapolated results are proposed to estimate reservoir-level evaporation, and a modified advection-diffusion law related to wind velocity is proposed.

By linking local-scale interactions driven by structure effects (geometries of the floating setup) and the microclimate at the reservoir level, the contribution opens the door to floating structure optimisation with respect to water savings. Moreover, it allows one to predict how the reservoir system will be altered by the half-covered situation using lake modelling (e.g., Global Lake Modelling). This aspect is critical to better predict the evolution of physical parameters below the interface that may have a strong retroaction on the interface and the atmosphere.