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Benthic Unattended Generator (BUG)
The Naval Research Laboratory's Center for Bio/Molecular Science and Engineering has deployed a weather buoy that is operating in the Potomac River, near the south of the end of the Laboratory's pier. The buoy is unique in that it is solely powered by a set of NRL-developed fuel cells, known as BUGs. These BUGs consist of electrodes imbedded in sediment in the bottom of the river that are electrically connected to electrodes in the overlying water. The buoy monitors air temperature and pressure, relative humidity, water temperature, and performance indicators of the BUGs, and sends data by a radio transmitter (also BUG-powered) to a receiver in a nearby NRL building.
Organic matter residing in many fresh-and salt-water marine environments constitutes a practically inexhaustible fuel. Furthermore, substantial oxygen residing in overlying water constitutes a practically inexhaustible oxidant. BUGs electrochemically react with this fuel and oxygen to generate electrical power that persists indefinitely. BUGs are being developed by NRL to persistently power a wide range remotely deployed marine instruments.
How it works
In many fresh and salt-water marine environments substantial organic matter resides in sediment, which sustains microbial activity that is limited by the flux of oxidants (such as oxygen and sulfate) into sediment from overlying water. Within the topmost layers (millimeters to centimeters) of such sediments, microorganisms preferentially deplete oxygen, causing microorganisms deeper in the sediment to use less potent oxidants (such as sulfate) and generate potent reductants (such as sulfide) as byproducts.
As a consequence, a natural redox gradient exists across the sediment/water interface in which porewater within such marine sediment (millimeters to centimeters) beneath the sediment surface, is enriched in reductants compared to overlying water.
Because of this redox gradient, an electrode imbedded in such marine sediment will equilibrate to a voltage that is often more than 0.7 volts negative that of an identical electrode positioned in overlying water at open circuit (i.e., when the electrodes are not electrically connected and therefore no electrical current flows between the electrodes).
Connection of the electrodes by an external circuit of appropriate resistance results in sustainable electron flow (electrical current) from the sediment-imbedded electrode (termed "anode" because of its relative negative voltage) to the electrode in overlying water (termed "cathode" because of its relative positive voltage). Current is sustained at the anode by continual oxidation of reductants in sediment porewater and at the cathode by continual reduction of oxidants in water.
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