Self-Biased Solar-Microbial Device for Sustainable Hydrogen Generation

by Allison Kerley (Photo above of Hanyu Wang, first author of this paper at the University of California, Santa Cruz.)

Most hydrogen generating devices require an external addition of a 0.2 to 1.0 V electric potential in order to sustain the hydrogen generation. Wang et al. (2013) explored the feasibility of a self-powering photoelectrochemical-microbial fuel cell (PEC-MFC) hybrid device to generate hydrogen. The PEC-MFC was a combination of a photoelectrochemical fuel cell and a microbial fuel cell. The Hydrogen production of the device was tested when powered by a ferricyanide solution inoculated with a pure strain of Shewanellla oneidensis MR-1 and when powered by microorganisms found naturally occurring in the municipal wastewater. In both scenarios, given replenishments of fuel, the device produced enough voltage to be self-sustaining. However, when the device was powered by wastewater it produced both a lower current and a smaller hydrogen production than when powered by ferricyanide solution. Continue reading

Undersea Ocean Renewable Energy Storage

Ocean Energy Storage

by Allison Kerley

Slocum et al. (2013) propose a new design for an energy storage and generation unit composed of underwater concrete spheres and offshore wind turbines. The proposed design utilizes pumped storage hydraulics (PSH). During times of low energy demand from the grid, the cylinder would contain water at equal pressure with the surrounding ocean. In the proposed design, the floating wind turbines generate energy and the excess energy is used to pump water out of the storage sphere, creating a vacuum. When energy is needed from the sphere, the turbine would open, allowing water to pass through into the sphere. The proposed sphere design would have an inside diameter of 25 m, and would retain a 1/20th-atm environment when fully discharged. The proposed design could be used without alteration in depths between 200 and 700 m, and would continue to be economically feasible to a depth of approximately 1500 m. The authors tested a small-scale dry version of the proposed design, with the test sphere having an inner chamber diameter of 75 cm, with a ten meter height difference from the top of the pump and wind turbine to the top of the sphere. The test unit was found to have a low round-trip efficiency of 11%, which the authors attribute to their inability to use the most efficient pump and turbine technology due to the small size of their test model. They calculated that in a full scale model, the lowest round-trip efficiency would be 70%. Continue reading

Membrane-Free Lithium/Polysulfide Semi-Liquid Battery for Large-Scale Energy Storage

by Allison Kerley

Yang et al. (2013) discussed their new proof-of-concept lithium/polysulfide semi-liquid battery as a potential solution to large-scale energy storage. The lithium/polysulfide (Li/PS) battery uses a simplified version flow battery system, with one pump system instead of the traditional two. The Li/PS battery was found to have a higher energy density than traditional redox flow batteries, with the 5 M polysulfide solution catholyte cell reaching an energy density of 149 W h L–1 (133 W h kg –1), about five times that of traditional vanadium redox battery. The Li/PS battery cells were also found to have a high coulomb efficiency peak around 99% before stabilizing at around 95%, even after 500 cycles. The authors conclude that the Li/PS cells maintain a steady rate of performance after 2000 cycles, and Continue reading

High Power Density from Extremely Thin Solar Panels

by Allison Kerley

Bernardi et al. (2013) investigated the absorbance of graphene and three different monolayer transition metal dichalcogenides (TMDs)—MoS2, MoSe2, and WS2—alone and in various combinations as the active layer in ultrathin photovoltaic (PV) devices. In calculating the upper limits of the electrical current density (measured in mA/cm2), each material can contribute to the total absorption of a device. The authors found that subnanometer thick graphene and TMD monolayers can absorb the equivalent short-circuit currents of 2–4.25 mA/cm2, while 1 nm thick Si, GaAs, and P3HT (commonly used materials in current PV devices) were found to generate currents between 0.1–0.3 mA/cm2. Further testing suggested that the high absorption of the monolayer MoS2 is due Continue reading

Efficient Tandem and Triple-Junction Polymer Solar Cells

by Allison Kerley

In response to current interest in organic solar cells, W. Li et al. investigated the effectiveness of a new small band gap semiconducting polymer, PMDPP3T, in single, tandem and triple junction solar cells. In single-junction cells PMDPP3T was found to have an external quantum efficiency (EQE) of up to 7.0% when mixed with [70]PCBM. In single-junction solar cells, PMDPP3T was found to absorb radiation closer to the infrared light spectrum than current polymer, absorbing up to 960nm. In multi-junction solar cells, it was found that all solar cells utilizing PMDPP3 in conjunction with PCDTBT, reached efficiencies between 8.4% and 8.9%. In addition, W. Li et al. also found that tandem and triple-junction solar cells, compared to single-junction solar cells of the same layer thickness, were 50-60% more efficient.

Li, W., Furlan, A., Hendriks, K.H., Wienk, M.M., Janssen, R.A., 2013. Efficient tandem and triple-junction polymer solar cells. Journal of the American Chemical Society 135, 5529-5532.

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Potential of Wastewater Grown Algae for Biodiesel Production and CO2 Sequestration

Chris Weichert

Algae, photo by Chris Weichert

In response to a growing fear surrounding increasing levels of CO2 in the atmosphere and rapidly dwindling supplies of traditional oil as a source of energy, A. Fulke et al. investigated the CO2 sequestration rate (as a source of CO2 mitigation), the biomass creation (as a source of biofuel), and lipid composition of algae used in the wastewater stabilization ponds of industrial wastewater treatment plants. The green algae species of the algae they found naturally occurring in the wastewater stabilization ponds have a lipid structure equivalent to vegetable oil currently used to produce biodiesel. In the two most dominant algal classes Chlorophyceae and Cyanophyceae, they found four distinct species (Scenedesmus dimorphus, Scenedesmus incrassatulus, Chroococcus sp. and Chlorella sp.) currently being globally explored as sources of biodiesel. They isolated and cultured samples of these four species and examined the biomass concentration, lipid content, and CO2 fixation rates, finding that the samples where all four of these species were present (as opposed to each species cultured alone) had a biomass concentration (g L-1) and lipid content (g g-1) nearly twice as high as any alone, and a CO2 fixation rate (g L-1d-1) at least double individual species cultivations. They concluded that industrial wastewater could support a diverse culture of algal species capable of being used as a source of biodiesel. —Allison Kerley

Fulke, A., Chambhare, K., Sangolkar, L., Giripunje, M., Krishnamurthi, K., Juwarkar, A., Chakrabarti, T., 2013. Potential of wastewater grown algae for biodiesel production and CO2 sequestration. African Journal of Biotechnology 12, 2939–2948.

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Hydrophobic Amino Acids as a New Class of Kinetic Inhibitors for Gas Hydrate Formation

Sa et al. investigated the effects of amino acids as Kinetic Hydrate Inhibiters (KHIs) on the initial formation, the continued growth, and the structure of hydrate blockages in natural gas and oil pipelines. They found that hydrophobic amino acids were more effective KHIs than the more commonly used polyvinyl pyrrolidone (PVP). It was also observed that in general, amino acids with shorter alkyl chains were more effective KHIs than those with longer alkyl chains, with glycine and L-alanine being the most effective KHIs. Examining how various KHI’s impacted the rate of growth of hydrate blockages, it was found that both PVP and glycine as KHIs caused a decrease in the rate of formation of the hydrogen blockage. When comparing amino acids with varying alkyl chain lengths, it was observed that as the length of the alkyl chain increased, its ability to act as an effective KHI decreased. It was found that the crystal structure of hydrates formed did not change in the presence of the amino acid KHIs. However, it was found that all amino acids, regardless of their hydrophobicity, were effective in inhibiting hydrate blockages once the blockages had begun to form, as seen by the increased number of ice crystals in the hydrate in the presence of glycine.—Allison Kerley

Sa, J., Kwak, G., Lee, B., Park, D., Han, K., Lee, K., 2013. Hydrophobic amino acids as a new class of kinetic inhibitors for gas hydrate formation. Scientific Reports 3, 2428.

                  Using nucleation kinetics measurements to observe the onset of hydrate blockage formation, Sa et al. examined the effects of different amino acids and PVP on hydrate formation in fresh water and memory water. The “memory effect” of memory water is a phenomenon in which hydrates form more easily in gas and water that has formed hydrates in the past. While PVP did not display any effect on the inhibition of hydrates, glycine (at an increased concentration of 1.0% mol) slowed the formation of hydrates.
                  Synchrotron powder X-ray diffraction (PXRD) was used to identify the structure of the hydrate blockages, enabling Sa et al. to determine whether KHIs affected the structural makeup of hydrate blockages. It was found that in the presence of glycine, hydrate blockages displayed more ice crystals, which was attributed to water molecules freezing instead of forming hydrates.

Allison Kerley

Recyclable Organic Solar Cells on Cellulose Nanocrystal Substrates

Flexible recyclable solar cells are the most recent addition to solar cell research, and have the potential to lower the costs of solar cells while eliminating the need for the petroleum-based components of traditional solar cells. Zhou et al.(2013) studied the power conversion efficiency, the rectification in the absence of light, and the recyclability of polymer solar cells created with cellulose nanocrystal (CNC) substrates. CNC’s are extracted from plant fibers, and are more environmentally attractive due to their ability to be recycled than their petroleum-based counterparts. Zhou et al. found the power conversion efficiency (PCE) of solar cells on CNC substrates to be noticeably higher than previous attempts at solar cells on paper-like substrates. The CNC substrates and solar cells were also found to be separable when dispersed in distilled water at room temperature, allowing for the various components to recovered and recycled.
 —Allison Kerley
Zhou, Yinhua, Fuentes-Hernandez, Canek, Kahn, Talha M., Liu, Jen-Chieh, Hsu, James, Shim, Jae Won, Dindar, Amir, Youngblood, Jeffrey P., Moon, Robert J., Kippelen, Bernard. 2013. Recyclable organic solar cells on cellulose nanocrystal substrates. Scientific Reports 3, 1536

                  Zhou et al. found their CNC substrate-based solar cells to have a 2.7% PCE; they attribute the low PCE to both the limited transmittance of the thin Ag layer which served as the bottom electrode and to the uneven and random distribution of the CNC in the clear CNC film. The CNC film is desired to be as transparent as possible, while intensifying the light onto the detector. However, the inconsistencies in the spread of the CNC’s (which are only a few hundred nanometers long) cause scattering of the light, lowering the intensity of the light hitting the detector. The efficiency of the solar cell is determined by the intensity of the light hitting the detector, and the transmittances of CNC substrates were found to be lower than that of glass.
                  The other piece of the study conducted by Zhou et al. examined the recyclability potential of the CNC substrate solar cells they created. It was found that the CNC substrate solar cells could be separated into their major components (substrate, organic and inorganic materials) by immersing them in distilled water at room temperature and running the solution through several simple filtering processes. Immersing the solar cell in water disintegrates the CNC substrate, allowing the solid wastes to be filtered out with a simple paper filter. The photoactive layer could then be separated from the electrodes by rinsing the solid wastes over filter paper using chlorobenzene, leaving the materials which acted as electrodes behind.

                  Zhou et al. theorize that the CNC substrate could be tweaked and modified to decrease the scattering of the light, increasing the PCE of the solar cell. In addition, they propose that using a different or modified material for the bottom electrode would increase the transmittance.