(587aw) LED Technology for Energy Efficient Microalgae Growth

Submitted to the 2013 AIChE Annual Meeting, “Global Challenges for Engineering a Sustainable Future“, November 3-8, 2013, Hilton San Francisco Union Square, San Francisco, CA

LED Technology for Energy Efficient Microalgae Growth

Daniel Eltringham, University of New Hampshire, Durham, NH

Ihab H. Farag, University of New Hampshire, Durham, NH

Biography

Daniel Eltringham is a sophomore Chemical Engineering major at the University of New Hampshire (UNH). He joined the UNH Biodiesel Group as a freshman, and has completed several projects both during the school year and summer. His research focuses on utilizing microalgae in industrial production of fuel and chemical products.

Ihab Farag is Professor of Chemical Engineering at University of New Hampshire. In Sept 1976 he joined the faculty at UNH after receiving his doctorate of Science (Sc.D.) from the Massachusetts Institute of Technology (MIT) in Chemical Engineering. He has been active in Biofuels, Pollution Prevention, energy efficiency, and chemical hazard screening applications.

Abstract

Microalgae have excellent potential for industrial applications that require the production of lipids/oil. Algae oil can be used to produce biodiesel fuel and other valuable products such as omega-3 fatty acid¹. Significant research² has focused on improving algae growth and increasing lipid content. Photosynthetic growth of algae requires water, carbon dioxide, nutrients and photonic energy, e.g., sunlight. When algae are grown indoors, synthetic light is used, typically fluorescent light or high intensity discharge (HID) lights. These lights are well suited for general illumination but are not efficient for algal photosynthesis. Estimates are that more than 50% of the light energy from conventional illumination is not utilized during algae growth³.

The objective of this study was to develop a light-emitting diode (LED) array, which can supply only the wavelengths of light most useful for algae growth. A “smart” lighting prototype was developed to change lighting in response to culture turbidity. Green algae contain chlorophyll A and B in the ratio (3 chlorophyll A: 1 chlorophyll B). The best LED arrangement would have LED colors corresponding to the highest algae light absorption. Chlorophyll A has two absorption peaks, the first around 430 nm (blue/violet color) and the second at 660 nm (deep red). Chlorophyll B absorption peaks are 460 nm (blue) and 630 nm (red) ^4,5. In more mature cultures, red and blue light is absorbed by the algae cells closest to the LEDs source. If the tank is not well mixed, the outer layer of algae that absorbed the red and blue LED lights will prevent further light penetration resulting in “self-shading” or “light starved algae”. In these mature cultures, wavelengths of light that can penetrate the outer layer of algae, reaching the light starved algae farthest from the LED lights, are desirable.

These challenges were studied using Chlorella vulgaris microalgae, grown in fresh water using an 80 liter batch photobioreactor (PBR) with colored LEDs as the primary light source. The objective was to identify an LED lighting profile that maximized production of lipid rich biomass. Changing the LED light source mid-process allows a lighting profile to be tailored to each growth stage. Different algae growth stages have different lighting requirements due to changes in cell population, age, and turbidity within the PBR⁶.  Different colored LEDs were used; red, blue, etc. Biomass production was compared with a daylight fluorescent photobioreactor (PBR) of equivalent intensity. Results from the 80 liter PBR were compared with similar experiments using 2 liter PBRs with LED panels. A 3:1 red: blue LED light combination was found to produce the most daily biomass (178 mg dry algae/liter-day versus 88 in fluorescent) in the 2 liter PBR. The red-blue light also produced the highest lipid content (7g lipids/100 g dry biomass versus 1g/100g for fluorescent). LEDs require less than half the electric energy used by Fluorescent lights to produce the same light intensity⁷, and also do not waste energy producing undesirable wavelengths of light. Thus, LEDs are considered a greener, more cost-effective light source and hence more sustainable than fluorescent lights. The results of the ongoing study will be discussed.

Acknowledgements

            The authors would like to acknowledge the support from the UNH Hamel Center for Undergraduate Research. Thanks to Dr. Leland Jahnke, UNH Plant Biology Dept. for algae input; Mrs. Marian Elmoraghy for help and technical assistance, Dr. Nancy Whitehouse, UNH Dairy Research Center, for help with large-scale centrifugation and freeze drying; and Mr. Jon Newell, UNH Chemical Engineering Dept., for technical assistance.

REFERENCES

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