Simulation of shading and algal growth in experimental raceways

Many algae research groups use elevated experimental raceways to characterize algal biomass productivity. Side walls and a relatively large paddlewheel shade the culture and lower productivity, particularly in winter and with low culture depth. This paper presents a four-step method to calculate shading for a given raceway shape: (1) develop a computational mesh of the inner surface geometry of the raceway; (2) offset the mesh horizontally in the x and y directions on the water surface as a function of wall height, solar zenith angle, and solar azimuth angle; (3) calculate the shaded area for each projected mesh with the shoelace algorithm; (4) use regression to develop a response surface for shaded area as a function of horizontal x and y offset. For each time step, the shaded area is calculated with the regression equation based on the x and y offset at that time step. The shading model was added to the Huesemann Algae Biomass Growth (HABG) model. In the model, productivity was sensitive to the assumed light distribution below the water surface. Shading and light distribution models were evaluated with biomass concentrations from three Regional Algal Feedstock Testbed (RAFT) experiments with three different species of algae. Averaging light in each layer resulted in higher calculated growth rate than calculating growth separately in shaded and unshaded areas. The Local Photon Flux Density Variable Light (LPFD_V) shading algorithm reduced the HABG estimate of productivity and improved the agreement with observed productivities. The LPFD_V RMSD values ranged between 0.01 and 0.07 g L⁻¹ and the bias factors ranged between 0.9 and 1.1. Shaded area was sensitive to raceway orientation, water depth, and time of year. A north-south raceway orientation had less average shading in winter but more average shading in summer than an east-west orientation.