Wireless Ion-Sensitive Field Effect Transistor PH Sensor Network for Offshore Microalgae Cultivation

Abstract

One of the preeminent challenges facing scientists and engineers in the 21st century has been and will likely continue to be the development of economically and technically feasible renewable energy technologies. While many of these efforts, such as wind, solar, and geothermal, address electricity generation, there are relatively few options to consider for transportation fuels which account for over 1/3 of the US energy needs. Biofuels, from corn or other plant products, have tremendous promise as they can serve as a drop-in replacement for use in our existing infrastructure. However, there is real concern over whether “conventional” bio-feedstock can be viable replacements for fossil fuels due to their need for arable land, high water usage, and relatively long growth cycle. Microalgae, on the other hand, does not suffer from these same limitations and many researchers world-wide have started to explore the cultivation of microalgae for biofuel production. These efforts vary from open ponds to closed photo bioreactors. For example, one technology under development is the NASA OMEGA project. With roots in developing life support systems for long spaceflights, NASA has developed the OMEGA system. This system uses the nutrient content in treated wastewater and waste carbon dioxide from flue gas sources to cultivate microalgae in floating photo bioreactors. The resulting biomass can then be harvested for biofuels and other algae products .Regardless of the specific cultivation process, it is critical to monitor several operating parameters to ensure the health and maximum yield of the algae culture. Measurement of pH is especially important as it can be used a proxy for measuring the amount of carbon dioxide available to growing algae culture. As carbon dioxide is added to the system, carbonic acid is formed, and the pH decreases. Since the algae can only photosynthesize in daylight, during night the algae respire, using O2 and creating CO2. In light of these rapidly changing growth editions, it is critical that the pH in the system is monitored accurately and continuously for correct operation of the control systems responsible for maintaining the health of the algae culture. The sensor systems must be robust enough to maintain reliable operation in a harsh marine environment, and be well tolerant of biofueling both inside and outside the system. The accessibility challenges inherent in any large scale cultivation system also require that the system be easily and infrequently maintained. A wireless system provides additional advantages, such as reducing the cost and maintenance work associated with numerous sensor cables running throughout a large-scale cultivation system. The fragile and costly sensor cables are replaced with radio links. For example, damaged sensor cables needed frequent maintenance and/or replacement at the OMEGA system. A wireless system also allows greater flexibility in choosing the point of attachment to the system, without a need to consider the cable length or it’s physical accommodation. This work proposes the development of a wireless ISFETbased (Ion-Sensitive Field Effect Transistor) pH sensor network for use in an offshore microalgae cultivation system.