Boosting Biomass Production of Microalgae at Low Environmental Temperatures Using Sunlight-Driven Photothermal Photobioreactors

Temperature is a critical factor in optimizing microalgae growth and maximizing biomass productivity in closed photobioreactors during outdoor cultivation. However, maintaining effective temperature control remains a significant challenge, especially in cold climates or regions with low ambient temperatures and limited sunlight as current methods are often inefficient or energy-intensive. Here, we present a simple yet highly efficient method for passive temperature control of closed microalgal photobioreactors in low-temperature outdoor environments by integrating an existing photobioreactor with a newly developed solar-driven photothermal conversion film. The flexible nanocomposite film, composed of poly(methyl methacrylate) and photothermal converters (Cs_0.33WO₃), shows high light transmittance (>75%) in the visible light wavelength range and, more importantly, excellent near-infrared absorption capability (>90%). Under simulated solar radiation (∼650 W/m²), the nanocomposite film experiences a temperature elevation of 15 °C above ambient. In practical outdoor tests, the sunlight-driven photothermal photobioreactor exhibited an average temperature increase of 5 °C during daylight, compared to the photobioreactor without nanocomposite films. During a 10 day outdoor cultivation period, the sunlight-driven photothermal photobioreactor achieves an average microalgae (Chlorella pyrenoidosa) biomass productivity of approximately 0.069 g/L/day, which is 43.8% higher than 0.048 g/L/day observed in the photobioreactor without nanocomposite films. This work thus offers a promising approach for sustainable microalgae production in cold climates or regions with low environmental temperatures requiring no additional energy consumption.