Multi-bandgap Solar Energy Conversion via Combination of Microalgal Photosynthesis and Spectrally Selective Photovoltaic Cell

Abstract

Microalgal photosynthesis is a promising solar energy conversion process to produce high concentration biomass, which can be utilized in the various fields including bioenergy, food resources, and medicine. In this research, we study the optical design rule for microalgal cultivation systems, to efficiently utilize the solar energy and improve the photosynthesis efficiency. First, an organic luminescent dye of 3,6-Bis(4'-(diphenylamino)-1,1'-biphenyl-4-yl)-2,5-dihexyl-2,5-dihydropyrrolo3,4-c pyrrole -1,4-dione (D1) was coated on a photobioreactor (PBR) for microalgal cultivation. Unlike previous reports, there was no enhancement in the biomass productivities under artificial solar illuminations of 0.2 and 0.6 sun. We analyze the limitations and future design principles of the PBRs using photoluminescence under strong illumination. Second, as a multiple-bandgaps-scheme to maximize the conversion efficiency of solar energy, we propose a dual-energy generator that combines microalgal cultivation with spectrally selective photovoltaic cells (PVs). In the proposed system, the blue and green photons, of which high energy is not efficiently utilized in photosynthesis, are absorbed by a large-bandgap PV, generating electricity with a high open-circuit voltage (V_oc) in reward for narrowing the absorption spectrum. Then, the unabsorbed red photons are guided into PBR and utilized for photosynthesis with high efficiency. Under an illumination of 7.2 kWh m^-2 d^-1, we experimentally verified that our dual-energy generator with C₆₀-based PV can simultaneously produce 20.3 g m^-2 d^-1 of biomass and 220 Wh m^-2 d^-1 of electricity by utilizing multiple bandgaps in a single system.

Figure 1

Theoretical efficiency. (a) Theoretical maximum power conversion efficiency (temperature: 300 K) and electron flux of semiconductors with various bandgaps under AM 1.5 G illumination. (b) Spectra of AM 1.5 G and chlorophyll absorption (top) and simplified photosynthesis model (bottom).

Figure 2

Spectral conversion. (a) Number of photons per wavelength (lines in A.U.) and their relative portion (dots) for the real AM 1.5 G (black) and artificial solar-simulating light source (red) used in this study. UV, visible, and IR regions are shown in blue, green, and pink colors, respectively. (b) Outdoor-simulating experimental set-up with solar-simulating light source, defined illumination area, periodic boundary condition, and back reflector. (c) Measured absorbance and photoluminescence spectrum of D1 in the film state. (d) The growth curves of microalgae in volume concentrations with and without D1 coating. (e) The schematic illustration of optical loss mechanism for dye-integrated bioreactors.

Figure 3

Dual-energy generator. (a) Dual-energy generator integrating PVs with microalgal photosynthesis. (b) Energy band diagram of the C₆₀-based PV and its current density (J)-voltage (V) curve. (c) Separately measured EQE and absorption of a C₆₀-based PV and the optical distance of microalgae cells with arbitrary concentrations. (d) Simulated EQE (red) and absorption of the C₆₀-based PV (grey) and microalgae cells (green) in the dual generator (inset: photosynthetic rate profile inside the configuration with R_max = 0.30 W g⁻¹). (e) Measured growth curve of microalgae cells in the growth phase for the flat PBR (gray) and dual-energy generator (red). (Inset: Blueprint and photograph of the dual-energy generator used in our experiment).

Figure 4

Potential of dual-energy generator. Theoretical maximum PCE (PV) and biomass productivity (containing a bioenergy of 4.2 kcal g⁻¹ or 9.45 kcal g⁻¹) of the dual-energy generators as a function of the PV band gap under 7.2 kWh m⁻² d⁻¹, when light first impinges on (a) PV (red region in the insets) or (b) microalgae (green region in the insets). The absorptions of the PV and algae were assumed to be perfect for the photons below their bandgaps.