An Optofluidic Photobioreactor Strategy for Bioenergy

A novel approach to cultivating cyanobacteria is presented which utilizes near-field evanescent waves on the surface of waveguides to excite the photocenters in the thylakoid membranes of cyanobacteria. This approach to light delivery when applied to photobioreactor design may provide an opportunity to significantly increase yield and make biofuels an economically competitive alternative to traditional fossil fuels. INTRODUCTION Growing concern over climate change and fossil fuel supplies has led to greater interest in alternative energy sources[1]. Of the options available solar energy is by far the most abundant. Over 2.7 billion peta joules (PJ = 10 15 J) of solar energy is incident on the earth’s surface per year, dwarfing global energy demand of 472 thousand PJ [2]. Significant advances have been made in capturing this energy both passively and actively through photovoltaic and solar thermal technologies. However, electricity as an energy carrier is difficult if not expensive to store, particularly in large volumes for sustained periods of time. Fuels have long been recognized for their high energy density and applicability to a unique, yet significant subset of our energy needs, including transportation and heating. As a consequence, our reliance on petrochemical based fuels has accelerated global changes to global temperatures currently being observed. Alternative liquid fuels that are competitive economically and energetically with fossil fuels that do not impact atmospheric carbon levels are necessary. Of the alternatives to petroleum based fuels, microalgae based biofuels show significant promise towards substantially displace our reliance on fossil fuels for transportation [3]. Microalgal biomass exhibits high growth rates and fuel densities that are orders of magnitude greater than any other crop plant [4]. Furthermore, their cultivation does not compete with food crops for arable land and can be grown harvested in regions and environments inhospitable to most other plants. Consequently, much attention is being given to increasing the efficiency of microorganisms to generate both biomass and direct evolution of usable fuel [5]. Cyanobacteria, a particular type of photosynthetic bacteria, are of particular interest as most require only CO2 and water to grow, with minimal amounts of other nutrients [6]. This has benefits from both a cost and emissions perspective as fuel produced from such organisms comes very close to being carbon neutral. Concurrently, advances in photobioreactor design and characterization of their yield potential have been pursued. However, as of yet, a commercial scale solution that can compete with the comparatively low price of fossil fuels has yet to be realized [7]. Here we demonstrate a novel method to improve light distribution in PBRs that may enable a paradigm shift in the way that PBRs are designed. This approach takes advantage of near-field evanescent waves on the surface of waveguides to stimulate the thylakoid membranes of cyanobacteria directly adjacent to the surface of the waveguide. When applied to photobioreactor design on a large scale, this technique would provide a means to excite bacteria optimally, and pack bacteria at high density within the reactor, addressing many of the key challenges posed by current photobioreactor architectures. Growth of photosynthetic organisms in response to an Proceedings of the ASME 2011 International Mechanical Engineering Congress & Exposition IMECE2011 November 11-17, 2011, Denver, Colorado, USA