Abstract:
An electronic circuit to increase voltages from one or more energy sources. The electronic circuit can include a first set of capacitors and a second set of capacitors, and a first set of switches associated with the first set of capacitors and a second set of switches associated with the second set of capacitors. Also included is at least one energy source and an external load. The first and second set of capacitors, first and second set of switches, the at least one energy source, and the external load are arranged and connected such that the first set of capacitors is connected to the at least one energy source in parallel while the second set of capacitors is connected to the external load in series, and vice versa.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
       [0001]    This application claims priority of U.S. Provisional Application 61/538,356, filed Sep. 23, 2011, the contents of which are incorporated herein by reference. 
     
    
     FIELD OF THE INVENTION 
       [0002]    The present invention is related to a capacitor circuit, and in particular a capacitor circuit for arrays of power sources such as microbial fuel cells that can provide a constant energy supply with avoidance of voltage reversal. 
       BACKGROUND OF THE INVENTION 
       [0003]    In-series application of reaction-based energy systems [e.g., batteries, fuel cells, and microbial fuel cells (MFCs)] can be limited by a phenomenon called voltage reversal. Voltage reversal occurs, for instance, when multiple batteries are arranged in series and one battery malfunctions due to reduced power capability while the other batteries are actively providing electric potentials. Under such a circumstance, electrode reactions in the malfunctioning battery are governed by the electric potentials of the adjacent batteries. As such, the anode of the malfunctioning battery is governed by a cathode potential from an adjacent battery and thus becomes more positive than the cathode of the malfunctioning battery which is dominated by an active anode of an adjacent battery. As a result, reduction occurs at the malfunctioning anode and battery materials are oxidized at the malfunctioning cathode. This “voltage reversal” not only prevents stable operation of the multiple battery system, but can also damage electrode systems of the malfunctioning battery. 
         [0004]    Voltage generation from commercial batteries or fuel cells is generally less vulnerable against voltage reversal problems because their electrode reactions are chemically driven and therefore very stable. However, if electrode reactions are driven biologically by microorganisms, an in-series system can be more affected by voltage reversal as reported for MFCs (e.g. see Aelterman et al. 2006, Oh and Logan 2007). An MFC is a bioelectrochemical system for harvesting electric energy from aqueous organic matter. Exoelectrogenic bacteria at an MFC anode oxidize organic matter and simultaneously transfer electrons to the anode. A typical counter reaction at an MFC cathode is reduction of oxygen. This red-ox couple (i.e., oxygen reduction and oxidation of organic matter) creates a maximum theoretic voltage of 1.1 V (12-mM acetate as organic matter; p O2 =0.2 atm; pH=7) (Logan 2008), however open circuit potentials of a well-controlled laboratory MFC usually do not exceed 0.8 V. 
         [0005]    While MFCs can be used to convert organic contaminants in wastewater into electric energy, the low voltage problems have been one limiting factor for practical applications in large scale wastewater treatment plants. Attempts have been made to overcome the low voltage problems, but results have been unsuccessful. For example, multiple MFCs were arranged in series to increase output voltages; however, voltage reversal was easily induced by an imbalance in organic matter concentration (Oh and Logan 2007) and by a high current condition (Aelterman et al. 2006). Thus, in-series operation of MFCs has heretofore not been an option for practical applications of MFCs. 
       SUMMARY OF THE INVENTION 
       [0006]    An electronic circuit to increase voltages from an energy source is provided. The electronic circuit can include a first set of capacitors and a second set of capacitors, and a first set of switches associated with the first set of capacitors and a second set of switches associated with the second set of capacitors. Also included is at least one energy source and an external load. The first and second set of capacitors, first and second set of switches, at least one energy source, and external load are arranged and connected such that the first set of capacitors is connected to the at least one energy source in parallel while the second set of capacitors is connected to the external load in series, and vice versa. Such an arrangement provides for alternately charging the first and second set of capacitors when connected in parallel with the at least one energy source while the second and first set of capacitors in series with the external load, respectively, are discharging. In some instances, two or more energy or power sources are used and voltage reversal is avoided between the energy sources since they are not connected in series and operation of one energy source does affect energy or power generation of another energy source. 
         [0007]    The first and second set of capacitors can be switched from the parallel arrangement to the series arrangement, and vice versa, by a pair of electronic switches connected to a positive end or terminal and a negative end/terminal of each capacitor. In addition, the electronic switches can be operated by an automated program and thus provide a generally steady energy output to the external load. 
         [0008]    In some instances, the energy source can be an array of multiple fuel cells, e.g. microbial fuel cells (MFCs), arranged in parallel to charge the capacitors and thereby avoid voltage reversal, while in other instances the energy source can be an array of multiple rechargeable batteries arranged in parallel. In still other instances, the energy source can be an array of multiple primary batteries arranged in parallel to avoid voltage reversal and in still yet another instance, the at least one energy source can be an array of multiple microbial fuel cells arranged in parallel. It is appreciated that an array of multiple microbial fuel cells can enhance oxidation of organic matter and may or may not contain a microbial electrolysis cell that produces hydrogen. Finally, the first set or second set of capacitors can provide power back to at least one energy source in order to improve performance thereof. 
         [0009]    A process for providing increased voltages from a MFC is also disclosed. The process can include providing the first set of capacitors, second set of capacitors, first set of switches associated with the first set of capacitors, and the second set of switches associated with the second set of capacitors. In addition, the MFC can be the at least one energy source and an external load is also included. The components are connected in a manner such that the first and second set of capacitors are connected to the MFC in parallel when the first and second set of switches are in a first position while the first and second set of capacitors are connected to the external load in series when the first and second set of switches are in a second position. The process then includes alternately charging the first and second set of capacitors with the MFC by placing the first set of switches and the second set of switches, respectively, in the first position and alternately discharging the first and second set of capacitors to the external load by placing the first set of switches and the second set of switches, respectively, in the second position. It is appreciated that the first set of capacitors can be charged by the MFC while the second set of capacitors is being discharged to the external load, and vice versa. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0010]      FIG. 1  is a schematic diagram of an electronic circuit according to an embodiment of the present invention (A) as a whole circuit and (B) as a simplified circuit showing only closed circuit lines; 
           [0011]      FIG. 2  is a pair of graphs illustrating (A) output voltage and (B) power from a single microbial fuel cell (MFC) with and without connection to an inventive capacitor circuit according to an embodiment of the present invention; 
           [0012]      FIG. 3  is a pair of graphs illustrating (A) voltage and (B) power generation from four MFCs with and without connection to an eight-capacitor inventive circuit according to an embodiment of the present invention or a simple in-series arrangement; 
           [0013]      FIG. 4  is a graph illustrating induced voltage reversal for an inventive capacitor circuit according to an embodiment of the present invention; 
           [0014]      FIG. 5  is a graph illustrating additive maximum power from individual MFCs and comparison to maximum power when using the same MFCs with an inventive capacitor circuit according to an embodiment of the present invention; and 
           [0015]      FIG. 6  is a pair of graphs illustrating (A) voltage and (B) power variation from a single MFC with elongated charging and discharging time intervals (DT) connected to an inventive capacitor circuit according to an embodiment of the present invention. 
       
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
       [0016]    The present invention provides a method and/or system to capture energy at high voltages from multiple power sources, such as an array of microbial fuel cells (MFCs), without voltage reversal problems. In the inventive system, voltages can be effectively combined from multiple power sources without voltage reversal by using an electronic circuit built with capacitors. One or more power sources charge one or more capacitors in parallel, and then the capacitors are discharged in series, achieving an increase in the voltage in proportion to the number of capacitors charged and their individual voltages. 
         [0017]    An example of two MFCs  100  charging four capacitors C 1 -C 4  according to an embodiment of the present invention is shown for illustrative purposes in  FIG. 1A  at reference numeral  10 . A simplified circuit diagram is shown in  FIG. 1B  at reference numeral  20  where the circuit  10  is shown as isolated circuits  21 ,  22  to better illustrate the charging and discharging patterns. In  FIG. 1B , capacitors C 1  and C 2 , which are charged from the MFCs  100 , are arranged in parallel, while capacitors C 3  and C 4 , which discharge to an external load (R), are arranged in series. In addition, the MFCs  100  are connected in parallel as they charge C 1  and C 2 . Thus, this in-parallel arrangement  21  for both the MFCs and charging capacitors (C 1  and C 2 ) prevents voltage reversal in the MFCs, while the in-series arrangement  22  for the discharging capacitors (C 3  and C 4 ) increases output voltages to the external load. 
         [0018]    Switches are used to control which capacitors are charged or discharged. In the illustrative example, a SPDT (single pole double throw) type switch  110  is connected to each end of a capacitor ( FIG. 1 ), so that the switch controls whether the capacitor is under the charging or discharging condition. By using the switches, two capacitor sets (e.g., one set of C 1  and C 2  and the other set of C 3  and C 4 ) alternate the charging and discharging roles to stably deliver electric energy from the MFCs to the external load. For instance, in an example based on  FIG. 1  (i.e., C 1  and C 2  for charging and C 3  and C 4  for discharging), the given charging and discharging system is maintained for one second, and for the following one second, C 1  and C 2  discharges to the external load and C 3  and C 4  are charged from the MFCs. These two distinct steps can be repeated so that the MFC power is continuously and stably delivered to the external load (R). The alternating time interval can be varied, and depends for example, on the number of capacitors, the total capacitance of the capacitors, and the response time of employed switches. 
         [0019]    This inventive system can be used for multiple additional purposes. One MFC can be used to provide power for another MFC in order to increase current in the second MFC. Power can sometimes decrease in an MFC, for example, when substrate concentrations are low and thus the current is low. By adding the voltage from the capacitors in the circuit to another MFC, the current density can be increased and thereby afford faster removal of substrate in the second MFC. This arrangement can be done with individual electrodes, multiple electrodes, or combinations therein to better control current and voltages from arrays of electrodes and MFCs. The inventive system, which uses energy to charge a resistor, can also be used to power a microbial electrolysis cell (MEC). The systems can be hooked up in such as way as to power individual electrodes or completely separate reactors. For example two MFCs could charge a capacitor that is then discharged to the MEC. By alternately charging two sets of capacitors as described above, nearly constant current can be delivered to the MEC for hydrogen gas production or methane production. 
         [0020]    In order to better teach the invention and yet not limit its scope in any way, one or more examples are provided below. 
       EXAMPLE 1 
     Proof of Voltage Increase 
       [0021]    In the lab, an electronic circuit system was built with 8 capacitors (1-F capacitance, M-series, Cooper Bussmann, MO). Switches (5 VDC/1 A SPDT Micro Relay, RadioShack, TX) were controlled by a programmable microcontroller (Mega2560, Arduino, Italy) to alternate the charging and discharging conditions every one second. As an example, a single MFC was connected to an external load via an electronic circuit that had 8 capacitors ( FIG. 2 ). Resulting voltages from a single MFC were successfully increased. For instance, the open circuit voltage of the MFC (0.7 V) was increased up to 2.5 V via the capacitor circuit ( FIG. 2A ). The maximum power of the MFC was ˜0.8 mW, and this maximum power was maintained with the circuit, indicating there were negligible energy losses in the circuit ( FIG. 2B ). 
       EXAMPLE 2 
     Comparison with In-Series System 
       [0022]    The capacitor circuit of 8 capacitors was further examined with 4 MFCs, and its performance was compared with that from the same 4 MFCs arranged in series without the capacitor circuit. The voltage and power performance was almost identical if the voltage reversal failure is ignored for the in-series results ( FIG. 3A ). In addition, the identical maximum power at ˜2.9 mW confirms negligible energy losses in the circuit under maximum power conditions ( FIG. 3B ). 
       EXAMPLE 3 
     Proof of Eliminated Voltage Reversal 
       [0023]    Another set of experiments was performed with 4 MFCs to prove that voltage reversal was eliminated with the capacitor circuit. Voltage reversal was intentionally induced as one MFC (MFC- 2 ) was initially fed with low substrate concentration (0.5 g/L sodium acetate) while the other MFCs (MFCs- 1 ,  3 , and  4 ) were fed with 1 g/L sodium acetate. Due to the imbalanced organic matter concentrations, voltage reversal started in MFC- 2  at ˜12 hours as the 4 MFCs were operated in series without the capacitor circuit ( FIG. 4 ). As a result of the voltage reversal, the total output voltage dropped quickly from 1.5 to 0.7 V. As soon as the 4 MFCs were connected to the 8 capacitor circuit, the voltage reversal disappeared from MFC- 2  and the total output voltage was restored from 0.7 to 1.4 V. 
         [0024]    The maximum power produced by the MFCs was shown not to be altered by external factors. In particular, experiments were conducted with two MFCs containing different phosphate buffer concentrations (50 mM and 5 mM) in order to produce different maximum power densities. The 50 mM cm −1  solution had a conductivity of 7.4 mS cm −1  and the 5 mM solution had a conductivity 0.9 mS cm −1 . In addition, and due to the different solution conductivities, the maximum power from the 5 mM MFC was only 0.22 mW, which was approximately one fourth of the maximum power of 0.85 mW from the 50 mM MFC. Also, the sum of the individual maximum powers was 1.07 mW, which is almost identical to 1.09 mW produced from the two MFCs connected to 8 capacitors in the electronic circuit as illustrated in  FIG. 5 . As such, the comparison verifies that in a practical power system with numerous MFCs, power generation with the capacitor circuit will not be limited by a few malfunctioning MFCs. 
         [0025]    The inventive circuit disclosed herein can also be used to further increase maximum power over a short time period by increasing the charging and discharging cycle time. For example, a single MFC was connected to a 1000 ohm external resistance through the electronic circuit having 8 capacitors and as the time period for charging and discharging (dt) increased from 1 to 4 min, the magnitude of the voltage oscillation increased from 30 to 110% of the average voltage (0.85 V) as illustrated in  FIG. 6A . It is appreciated that even with substantial voltage oscillation, the average voltage was stable and varied by only ±3% over the various time intervals. The peak voltage reached 1.39 V and the peak power was 1.95 mW for a 4-minute time interval ( FIGS. 6A and 6B ) whereas the peak power was 2.5 times the maximum power with the one-second time interval (0.78 mW as shown in  FIG. 4B ). As such, the data illustrate that the inventive capacitor circuit can be used as a power management system to harvest lower MFC power densities over a relatively longer time and then release high power over a shorter time interval to operate electronic devices. Considering the relatively simple design and negligible power losses, compared to previous circuits built with a DC-DC converter, the capacitor circuit design has a significant potential for such different applications. 
         [0026]    In summary, the inventive circuit disclosed herein can eliminate voltage reversal by using arrangements of multiple capacitors. While a DC-DC converter requires a minimum input voltage that is typically larger than the working voltage of an MFC (0.5 V), even a very small voltage at the end of a fed-batch cycle (e.g., &lt;0.01 V) can be boosted using the capacitor-based circuit disclosed herein. In addition, based on maximum power densities produced using the capacitors in the circuit, there were minimal energy losses. In comparison, a power management system with a DC-DC converter can lose 40% of the produced power under similar current conditions. Therefore, the circuit design described here can be used to boost MFC voltages in a stable and efficient way. The inventive circuit can also produce higher power densities over shorter time intervals by controlling the time interval for charging and discharging the capacitors. 
         [0027]    In view of the teaching presented herein, it is to be understood that numerous modifications and variations of the present invention will be readily apparent to those of skill in the art. As such, the foregoing is illustrative of specific embodiments of the invention, but is not meant to be a limitation upon the practice thereof and the specification should be given a broad interpretation. 
       REFERENCES 
       [0000]    
       
         Aelterman, P., Rabaey, K., Pham, H. T., Boon, N. and Verstraete, W. (2006) Continuous electricity generation at high voltages and currents using stacked microbial fuel cells. Environmental Science and Technology 40, 3388-3394. 
         Oh, S.-E. and Logan, B. E. (2007) Voltage reversal during microbial fuel cell stack operation. Journal of Power Sources 167(1), 11-17. 
         Logan, B. E. (2008) Microbial fuel cells, John Wiley &amp; Sons, Inc., Hoboken, N.J.