A review of the substrates used in microbial fuel cells (MFCs) for sustainable energy production.

Microbial fuel cells: From fundamentals to applications. A review

Electrons can be transferred from microorganisms to multivalent metal ions that are associated with minerals and vice versa. As the microbial cell envelope is neither physically permeable to minerals nor electrically conductive, microorganisms have evolved strategies to exchange electrons with extracellular minerals. In this Review, we discuss the molecular mechanisms that underlie the ability of microorganisms to exchange electrons, such as c-type cytochromes and microbial nanowires, with extracellular minerals and with microorganisms of the same or different species. Microorganisms that have extracellular electron transfer capability can be used for biotechnological applications, including bioremediation, biomining and the production of biofuels and nanomaterials.

Extracellular electron transfer mechanisms between microorganisms and minerals

The Microbe Electric: Conversion of Organic Matter to Electricity

Scientific research has advanced on different microbial fuel cell (MFC) technologies in the laboratory at an amazing pace, with power densities having reached over 1 kW/m3 (reactor volume) and to 6.9 W/m2 (anode area) under optimal conditions. The main challenge is to bring these technologies out of the laboratory and engineer practical systems for bioenergy production at larger scales. Recent advances in new types of electrodes, a better understanding of the impact of membranes and separators on performance of these systems, and results from several new pilot-scale tests are all good indicators that commercialization of the technology could be possible within a few years. Some of the newest advances and future challenges are reviewed here with respect to practical applications of these MFCs for renewable energy production and other applications.

Scaling up microbial fuel cells and other bioelectrochemical systems

The first demonstration of a microbial fuel cell as a viable power supply: Powering a meteorological buoy

Faculty and students at the URI Dept. of Ocean Engineering have conceived of a new class of AUV for deep sea profiling and sampling called a Mini Ocean Elevator (or MiniOE). The MiniOE is intended to operate as both a vertical and a horizontal AUV on the same mission, beginning its mission as a vertical profiler and then converting to a horizontal AUV for sampling prior to homing on an acoustic beacon at the surface. Such a vehicle has tremendous potential for ocean research while presenting unique challenges for stability and control. Horizontal AUVs typically achieve stability against roll and pitch through significant mass below the center of buoyancy along with tail fins. For a vertical vehicle this mass-ballast displacement must be in an orthogonal direction along the longitudinal axis to achieve stability in pitch and yaw, forsaking roll stability. Designing a cigar shaped AUV capable of both horizontal and vertical stability represents an unusual challenge. This paper describes the design work and testing involved in developing a stable vertical AUV that will later be equipped to convert itself to a stable horizontal AUV. This design effort used a balanced combination of physical experimentation and computer simulation to provide detailed information about how different configurations of an autonomous underwater vehicle (AUV) might affect its stability, and thus the controllability, while using conventional hardware and software resources. Passive vertical ocean projectiles like XBTs typically use maximum displacement between mass and buoyancy centers along with rotation and / or tail drag to achieve stability. But for a vertical AUV to guide its descent and maintain sensor orientation means rotation is undesirable, and thus must be stabilized or controlled. Of course vertical and horizontal vehicles need this stability in different directions. As a result converting from one to the other is particularly challenging. Descent rate control and orientation stability can be influenced by using tail drag to balance vehicle weight. For the purposes of designing a stable vertical descent profile for the Mini O.E., a computer model was constructed to test parameters that affect control and stability of the vehicle through hydrodynamics. The model was verified by measurements on a physical test-bed of the vehicle in shallow depths and with varying initial conditions. The test-bed vehicle included a Tattletale microcontroller/logger with rapid data acquisition from 3-axis magnetometer, 3-axis accelerometer, rotation and precision pressure sensors. The vehicle was dropped vertically first into a 5 meter deep tank, then into 20 meter deep water in Narragansett Bay. Experimental results served as "ground truth" for the vehicle's dynamic characteristics, showing the stabilizing tendencies in descent inherent with different configurations. The test-bed observations were reconciled with a custom Matlab simulation model which provided a cost effective method of demonstrating the influence of changing physical parameters. The model was able to show how the vehicle would act with active controls and under a wide variety of configurations and conditions, which would otherwise be costly to physically build and test.

Designing a Vertical / Horizontal AUV for Deep Ocean Sampling

The purpose is to use acoustic scattering theory and Sea Beam measurements to estimate seafloor roughness parameters. The Sea Beam backscatter data are from a test area in the Laurentian fan, a relatively flat region. Sidescan sonarlike images were reconstructed from the multibeam data. These images in the test area show two distinctly different types of areas (A) and (B). The backscatter model uses the Helmholtz–Kirchhoff formulation of scattering theory and correlation function C(r)=exp[−‖r/l‖n], where r is the displacement, l is the correlation length, and n is the exponent. A single rough interface model fits the backscatter data in (A). The root‐mean‐square roughness was 6–8 cm and the correlation lengths were 140–270 cm. The exponent n ranged from 0.95 to 1.5. The type (B) areas required a two‐layer model: the interfaces in the first type of areas (A) is covered by a sediment having a smoother surface. The rms roughness of the covering sediments were about 3 cm, the exponent n was nearly 2 and corre...

Interpretation of Sea Beam backscatter data collected at the Laurentian fan off Nova Scotia using acoustic backscatter theory

A prototype Shallow Water Expendable Environmental Profiler (SWEEP) for rapid environment assessment has been developed and tested by SACLANT Undersea Research Centre in collaboration with the University of Rhode Island Department of Ocean Engineering. The SWEEP system is intended for autonomous vertical profiling of acoustic, optical and physical properties of the water column and seafloor in depths down to 100 m. It is designed for data return and control via cellular phone or worldwide satellite packet communication while on the surface. The first prototype is an anchored profiler that uses an integral buoyant winch and sensor package to profile from the bottom up to the surface. The system is modular in design, allowing modules to be reused in new deployment alternatives. Already being planned is a Shallow water Environmental Profiler, in Trawl safe, Real time configuration (SEPTR). SEPTR is a bottom-up profiler with added ADCP, tide/wave gage and extended duration battery packages in a recoverable trawler safe housing. The design duration for the SWEEP is one month or 100 profiles in 100 m water depth. The design duration for SEPTR is 3 months or 360 profiles in 100 m. The profilers make use of new low cost, high performance sensors that include the following: 3-axis acceleration and magnetic field sensors; conductivity, temperature and pressure sensors; ambient light/ optical attenuation sensors; ambient noise and current sensors; seafloor acoustic sensors; DGPS navigation sensors. Two way communication of data, position and control allows profile results to be returned in real time, and operational commands, profile schedules, and DGPS correctors to be sent to multiple profiler instruments. Complete profiles will also be stored on board for recoverable units.

Shallow water expendable and trawler safe environmental profilers

Histograms of echo peaks from three different topographic regions in the outer beams of a Sea Beam sonar show distinct variation as the topography changes from the North Cleft ridge to the Cascadia abyssal plain in the Pacific Ocean. Parameter estimation of probability density functions (pdfs) shows significant departure from Rayleigh pdf for the nonabyssal plain sites. The spectral character of the sea-floor roughness from an acoustic backscatter model suggests that the ridge area is dominated by large-scale roughness and the abyssal plain by small scale. It appears that change in roughness magnitude effects the observed histogram characteristics.

Robert Tyce

Papers

The first demonstration of a microbial fuel cell as a viable power supply: Powering a meteorological buoy

Designing a Vertical / Horizontal AUV for Deep Ocean Sampling

Interpretation of Sea Beam backscatter data collected at the Laurentian fan off Nova Scotia using acoustic backscatter theory

Shallow water expendable and trawler safe environmental profilers

Relation of Sea Beam echo peak statistics to the character of bottom topography