Improving Algae Photobioreactor Energy Efficiency Through Active Irradiance Control for Dynamic Carbon Dioxide Fixation

We have created a 40-liter Chlorella vulgaris algae photobioreactor testbed for carbon dioxide removal that responds to elevated levels of carbon dioxide, while reducing energy consumption during nominal operation. Energy efficiency will be an important consideration for the bioregenerative life support systems (BLSS) integrated into future space missions that will provide a crew with breathable air and other resources. A reduction in energy expenditure has the potential to reduce the overall mass, power, and volume (MPV) for a mission. However, though it is important to reduce the energy cost of BLSS components, this should not come at the expense of mission safety. The life support subsystems must retain the ability to adapt to the dynamic crewed cabin environment. In our photobioreactor, the LED lighting and circulation pump account for nearly all of the system energy consumption, at 75% and 23% respectively. In this paper we characterize energy reduction through active irradiance control. Using the Blackman relationship between photosynthetic rate and the three factors of irradiance, temperature, and environmental carbon dioxide, we designed a feedback controller to react to the sensed dynamic cabin environment by varying the photobioreactor�s irradiance levels. Thus, the carbon fixation rate of the algae is matched to the cabin carbon dioxide concentration. We tested our photobioreactor in controlled and uncontrolled scenarios. Results indicated that the controlled photobioreactor responds effectively to a step increase in CO2, while using up to 57% less energy on lighting than an uncontrolled photobioreactor over the course of the response. The development of a photobioreactor that utilizes active irradiance control to respond to carbon dioxide, as described in this paper, is an important step towards reducing BLSS MPV. Future work should focus on further optimizing LED- and pump-related energy savings as well as other BLSS subsystems through similar active control of high-power components.