Location: Crop Production Systems ResearchTitle: Improvement of an optical density sensor for algae pond monitoring and process control Author
Submitted to: ASABE Annual International Meeting
Publication Type: Proceedings
Publication Acceptance Date: 9/14/2012
Publication Date: 9/15/2012
Citation: Yao, Y., Thomasson, A., Yufeng, G., Sui, R. 2012. Improvement of an optical density sensor for algae pond monitoring and process control. ASABE Annual International Meeting, Paper No.:12-1338095, 14 pp. Interpretive Summary: Algae are the most productive plant to convert sunlight into biomass. It has been considered as one of the most promising biomass resources for biodiesel because its advantages in growth rate, yield, and oil content compared to conventional biodiesel crops. In cultivation of algae, it is critical to monitor its growth status such as biomass density as feedback in a system to control the growth conditions required for optimizing the yield and quality. However, no such sensor commercially available for use to automatically control algae growth conditions such as fresh medium, nutrients, light and temperature in real-time in-situ. A scientist at Crop Production Systems Research Unit at Stoneville, MS, collaborated with Professors and graduate student in Texas A&M University to develop a multispectral optical sensor for real-time measurement of algal biomass density. The sensor prototype was fabricated and evaluated in an algae-cultivation lab. Results showed that the sensor was capable of accurately determining algal biomass density. Sensor-measured optical density of algae agreed with the spectroscopically-determined (R2=0.98). The sensor was very stable with the change of operational temperature. In addition to the long-term growth trend, the sensor also responded well to all algal cultivation events such as adding water and media. The sensor could be used as a standing alone device or connected to a control system on algal production site so that algal growth status can be measured in real time in-situ.
Technical Abstract: The objectives of this study are to (1) improve a previously developed optical density (OD) sensor for the measurement of biomass concentration in algae cultures, and (2) test the performance of the improved sensor. The sensor was improved in the following several aspects. First, the sensor housing was redesign to accommodate new optical measurement configuration and a reference cell. Second, a constant current LED driver circuit was built and included. Third, a feedback temperature control mechanism (including thermistors and thermo-electrical cooling modules) was built to control the temperature of LEDs. Finally, a logarithmic IC chip was used for processing of raw outputs from photodiodes. The new sensor was tested with a 40 L algae cultivation raceway for seven days. The sensor showed high accuracy (R2 = 0.98 and root mean squared error = 0.034 absorbance unit between sensor-predicted and spectroscopically-determined OD). The sensor responded to the long-term growth of algae (i.e., the alternate growth patterns in light-on and light-off cycles) very well. The sensor also responded to cultivation events including water and media addition and culture transfer very well. The temperature dependency of the new sensor was 0.0033 and 0.0039 V/°C for the NIR and red channel, respectively, compared to 0.0137 and 0.08 V/°C for the old sensor.