The energy harvesting system for microbial fuel cell

With the strong support of HP, dry experiment, and wet experimental groups, Nanjing-China concluded that in order to apply microbial fuel cells to manned space systems, there must be components that can convert the current generated by microbial fuel cells into a stable, standard current that can be used directly by electronic components. So this year, the Hardware department of the team at Nanjing-China designed a special circuit that converts the power of microbial fuel cells into electricity that can be fed to electronic components.

Our design is also applicable to other new batteries, and we have disclosed the entire design process, which will provide useful references for other iGEM teams. We encourage iGEM teams in need to actively adapt and improve our designs.

The design attempts to fully meet aerospace requirements, including the use of highly reliable devices, super small size, and as simple as possible.

1. Abstract

According to our wet experiment needs, the hardware part designed an electric energy harvesting circuit for microbial fuel cells. Considering that the output power of the microbial fuel cell is small, which is insufficient to directly drive the load and boost circuit, the charge pump is used to collect the power of the battery as the starting voltage of the subsequent DC conversion circuit. The supercapacitor is used as the energy storage element, and finally the DC is converted into the voltage required by the load.

2. Circuit design background

2.1 Working principle of microbial fuel cell (MFC)

As shown in Figure 1^[1], the MFC is composed of an anode, a cathode, and a proton exchange membrane. At the anode of the battery, the organic matter is catalyzed by microorganisms to decompose electrons and hydrogen ions, the electrons are transferred to the cathode through the external wires, and the hydrogen ions are transferred to the cathode through the proton exchange membrane in the middle. At the cathode, electrons, hydrogen ions, and electron acceptor oxygen react to form water.

2.2 MFC output indicator-circuit input parameter

According to Figure 2 Figure 3 Figure 4, the REDOX activity of engineered strains was higher. The engineered strain of MFC cell has lower internal resistance. The maximum output power of the engineered strain was 243.77±25.2mW/m2. Although the strain used in our experiment has a good performance in the output power density, we still need to use a charge pump to collect battery power to start the subsequent DC conversion circuit.

3. Circuit design process

3.1 Circuit module diagram description

As shown in Figure 5^[2], the MFC uses a charge pump to charge the supercapacitor. Once the voltage of the capacitor reaches a threshold level, the charge pump sends a signal that triggers the switch to close, allowing the supercapacitor to start discharging and convert the power to the voltage level required for the load through the DC converter. When the voltage of the capacitor drops to the preset lower limit threshold, the switch is automatically disconnected, so as to realize the automatic cycle charging and discharging process of the supercapacitor.

3.2 Charge pump S-882Z24

The S-882Z Series is a charge pump IC for step-up DC-DC converter startup. Its features include:^[3]

• Operating input voltage 0.3 to 3.0 V
• Current consumption During operation : 0.5 mA max. (at VIN = 0.3 V) During shutdown : 0.6 μA max. (at VIN = 0.3 V)
• Discharge start voltage 1.8 to 2.0 V
• External component Startup capacitor (CCPOUT ), 1 unit*1

As shown in Figure 6 and Figure 7,

1. When power of 0.3 V or higher is input to the VIN pin, the oscillation circuit starts operation with that power, and the CLK signal is output from the oscillation circuit to drive the charge pump circuit.
2. The stepped up electric power output from the charge pump circuit is gradually charged to the CPOUT and the voltage of the CPOUT gradually rises.
3. When the CPOUT pin voltage reaches or exceeds the discharge start voltage, the output signal of the comparator (COMP1) changes from high level to low. As a result, the discharge control switch(M1), which was off, turns on.
4. The step up electric power charged to CCPOUT is discharged from the OUT pin.
5. When VCPOUT declines to the level of the discharge stop voltage (V CPOUT2) as the result of the discharge, M1 switches off, and the discharge is stopped.
6. When the VM pin voltage reaches or exceeds the shutdown voltage (V OFF), the output signal (EN-) of the comparator (COMP2) changes from low level to high. As a result, the oscillation circuit stops operation and the shutdown state is entered.
7. When the VM pin voltage does not reach V-OFF or more, the stepped up electric power from the charge pump circuit is recharged to C-CPOUT . (Return to the operation specified in (3).)

3.3 The DC-DC converter

Through DC-DC converter, the input voltage is from 1.0V to 3.0V and the output voltage is 3.3V, which makes the LED light on. Figure 8 shows the details of the circuit The chip we chose is TPS61023.

3.4 Other modules

• The energy storage element is a 1000uF supercapacitor.
• The switch uses a combination of PMOS and NMOS, thereby reducing static power consumption.

When the capacitor voltage is charged to 2.4V, the OUT pin of the charge pump outputs a high level of 2.4V, making the N-type FET on, and the drain pole of the N-type is low, that is, the gate pole of the P-type is low, so that the P-type is turned on, which is equivalent to the switch in Figure 5.

3.5 Overall circuit final design

The circuit schematic diagram is shown in Figure 9. The PCB layout is shown in Figure 10. The 3D model is shown in Figure 11.

4. Circuit test

The physical circuit after welding is shown in Figure 12.

When the open circuit output voltage of the microbial battery is above 0.3V, our circuit start to work. When the battery stable output voltage is 0.5V, it is connected to the circuit, and the LED light can be observed shining once about every fifteen seconds. Figure 13, Figure 14 and Video 1 illustrate this result for us. Although our circuits are already working very well.

However, due to the series LED resistance in the load circuit, the effective output efficiency of the circuit is greatly reduced.

If the load is replaced by other appliances replaced by other appliances with lower power consumption, such as the stm32u0, the efficiency of the circuit can be significantly improved.

Since the effect is not very intuitive, we will not show it.

5. Conclusion

MFC stores energy for supercapacitors through charge pumps, and the electronic switches are closed and disconnected between 2.4~2.0 V, so as to realize the automatic cyclic charging and discharging of energy storage capacitors, the charging speed is proportional to the input voltage, and inversely proportional to the size of the energy storage capacitors, and the final voltage DC is converted to 3.3V to provide electric energy for the LED. This circuit is suitable for the collection of electrical energy from microbial fuel cells, which can collect the energy generated by microorganisms for subsequent use.

6. Contribution

For the convenience of other teams in the iGEM community working on microbial fuel cells, we have chosen to make the circuit design document public. Any iGEM team can refer to our design.

Click here to download

References

[1]: LIU Yuan-feng, ZHANG Xiu-ling, LI Cong-ju. Advances in carbon-based anode materials for microbial fuel cells[J]. Chinese Journal of Engineering, 2020, 42(3): 270-277. DOI: 10.13374/j.issn2095-9389.2019.09.27.008

[2]: Electric energy harvester for microbial fuel cells[J]. Editorial Office of Optics and Precision Engineering, 2013,21(7): 1707-1712 DOI: 10.3788/OPE.20132107.1707.

[3]: Seiko Instruments,” ULTRA-LOW VOLTAGE OPERATION CHARGE PUMP IC FOR STEP-UP DC-DC CONVERTER STARTUP, ” S-882Z Series datasheet, 2005-2010 [Rev.2.0_00]