Design of a novel flat-plate photobioreactor system for green algal hydrogen production

https://doi.org/10.1016/j.ijhydene.2011.02.091Get rights and content

Abstract

Some green microalgae have the ability to harness sunlight to photosynthetically produce molecular hydrogen from water. This renewable, carbon-neutral process has the additional benefit of sequestering carbon dioxide and accumulating biomass during the algal growth phase. We document the details of a novel one-litre vertical flat-plate photobioreactor that has been designed to facilitate green algal hydrogen production at the laboratory scale. Coherent, non-heating illumination is provided by a panel of cool-white light-emitting diodes. The reactor body consists of two compartments constructed from transparent polymethyl methacrylate sheets. The primary compartment holds the algal culture, which is agitated by means of a recirculating gas-lift. The secondary compartment is used to control the temperature of the system and the wavelength of radiation. The reactor is fitted with probe sensors that monitor the pH, dissolved oxygen, temperature and optical thickness of the algal culture. A membrane-inlet mass spectrometry system has been developed and incorporated into the reactor for dissolved hydrogen measurement and collection. The reactor is hydrogen-tight, modular and fully autoclaveable.

Introduction

There is urgent global demand for an energy carrier that is clean, renewable and available. Molecular hydrogen (H2) is a fuel that has the potential to provide the clean energy required for transport, heating and electricity. The aim of the Solar Hydrogen Project at Imperial College London is to produce H2 by a carbon-neutral, sustainable process that uses unlimited natural resources – sunlight and water [1]. The green alga Chlamydomonas reinhardtii (C. reinhardtii) has the ability to photosynthetically produce H2 under anaerobic conditions [2]. Initially, photons are absorbed within the chloroplast of C. reinhardtii. This solar energy facilitates the photochemical oxidation of water into protons and molecular oxygen (O2) by the photosystem II (PSII) photoactive cluster centre. Electrons generated in this process are transferred to the iron-hydrogenase enzyme, which catalyses proton-electron recombination to produce H2. The process may be summarised by the equations below.2H2OO2+4H+4ewater oxidation4H++4e2H2proton-electron recombination

Iron-hydrogenase activity is inhibited in the presence of O2, which implies that this direct biophotolysis process is self-limiting. In order to maintain continuous H2 production, it is necessary to remove O2 as it is being produced. Sulphur deprivation of C. reinhardtii diminishes its ability to repair PSII proteins, thus reducing photosynthetic O2 production below the level of respiratory O2 consumption so that overall, O2 is being used up [3]. The algal metabolism is therefore responsible for creating an anaerobic environment that leads to sustained H2 production. This H2 production can be sustained over a period of approximately five days because the algal cells deplete resources under anaerobic conditions.

Biohydrogen production is both renewable and sustainable. The microalgal feedstock is inexpensive and readily available, and the light intensity requirements for algal growth and H2 production are modest, typically in the range of 40 W m−2 [4]. There is also the additional benefit of cultivating algal biomass lipids for subsequent biodiesel production, as well as the potential for temporary CO2 sequestration within the algal cells. The main barriers to the commercialisation of green algal H2 production are the low photochemical conversion efficiencies of the process and the prohibitive photobioreactor (PBR) costs [5].

Commercial growth of algal biomass (e.g. Chlorella and Dunaliella, grown for the pigmenting agent astaxanthin and β-carotene respectively) is normally restricted to inexpensive open systems such as natural ponds, circular ponds with a rotating arm for stirring, or raceway ponds [5], [6]. During photobiological H2 production it is necessary to efficiently harvest a highly mobile and diffusive gaseous molecule – an enclosed PBR is required. Enclosed PBRs also provide reproducible cultivation conditions, good heat transfer control, better biomass yield, better product quality and the opportunity for flexible technical design [7], [8]. A typical PBR is essentially a four phase system consisting of the solid algal cells, the liquid growth medium, the gaseous H2 product, and the superimposed light radiation field [9]. An understanding of the complex interaction between the biohydrogen production reaction and the associated environmental parameters such as the fluid dynamics and light transfer within the reactor is therefore required. The productivity of a closed PBR is limited by various design features, but most importantly the reactor needs to operate under favourable illumination conditions, with an optimised surface-to-volume ratio and light–dark cycle, and with adequate mass transfer properties [10]. The light intensity and wavelength incident on the PBR are both important, as are factors that determine the light dilution, light attenuation and light mixing throughout the system [11]. The PBR geometries regularly considered in the literature are the stirred-tank reactor [12], [13], the vertical-column reactor [14], [15], the horizontal tubular reactor [16], [17] and the flat-plate reactor. A focused comparison of these PBR geometries is given in Table 1.

Flat-plate reactors are characterised by a high surface-to-volume ratio, which leads to the best photosynthetic efficiencies observed for any PBR [4]. Artificially illuminated flat-plate reactors are often vertical, with the light source incident on the reactor from one side. Outdoor flat-plate reactors are typically tilted at an angle that allows optimal exposure to solar irradiation [5]. The region immediately adjacent to the illuminated reactor surface is a photic zone where light saturation, and consequently the photoinhibition of algal growth and H2 production processes, repeatedly occurs. In addition, the light energy available to algal cells decreases exponentially away from this photic zone – it has been estimated that for a fully grown culture of C. reinhardtii, effective light penetration is limited to a depth of 0.8 mm [15]. It is therefore important to minimise these light gradients and control the light-dark cycles of the algal cells by means of an effective agitation system [18]. Flat-plate reactors are subject to relatively low mass transfer rates because the space between panels, known as the light path, is restricted and this reduces the clearance efficiency of the dissolved O2 produced by photosynthesis [19]. Good O2 diffusion rates through the reactor are required to achieve optimal algal biomass growth. Flat-plate reactors provide operational flexibility as they may be run in both batch and continuous modes [6]. The height and width of flat-plate reactors are the two dimensions available for scale-up, but only up to a practical limit of 2–3 m [9]. Typical limitations of flat-plate reactors are the scaling-up requirements (the need for many compartments and support materials), the difficulty in controlling culture temperature, the possibility of algal cell clustering on the reactor wall, and the incompatibility with certain algal strains [19]. Algal cell clustering is prevalent near the top of vertical reactors with a fixed liquid level, such as vertical-column and flat-plate reactors featuring a gas-lift agitation system. Certain C. reinhardtii strains have a tendency to pellet to the bottom of the reactor and require energy-intensive mechanical agitation (such as shaking) to survive.

Section snippets

Design specification

The motivation for designing this PBR was to create a simple system that would enable quick measurement of the key parameters in the biohydrogen production process under carefully controlled conditions. As part of the design specification, the parameters that should be controlled and/or measured were selected. These parameters were used to drive the reactor design, and the process was completed by developing the ancillary systems and equipment required to service the reactor. The critical

Light intensity distribution

Overall, C. reinhardtii cells have modest light intensity requirements – the growth, sulphur deprivation and H2 production processes need no more than 200 μE s−1 m−2 of photosynthetically active radiation (PAR) – about 40 W m−2 [2]. Algal cells that remain in the dark for extended periods (dark zone) or that become overexposed to illumination (photic zone) will not grow or produce H2 efficiently. The photic zone corresponds to an illumination exceeding 100 W m−2, although this value is algal

Conclusion

A flat-plate vertical one-litre photobioreactor that facilitates the biophotolytic H2 production process has been designed and constructed. A comprehensive literature review has identified the advantages and limitations of various reactor geometries. The flat-plate reactor geometry was subsequently chosen due to its superior surface-to-volume ratio, which results in the highest observed photochemical efficiencies for H2 production. The reactor enables rapid and accurate measurements of the key

Acknowledgements

The Solar Hydrogen project is funded by the UK Engineering and Physical Sciences Research Council (EPSRC).

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