# Towards Bio-Hybrid Energy Harvesting in the Real-World:
## 3 Research Progress in Energy Harvesting from Microorganisms
### Applications of MFCs
#### 3.4.1 Electricity Generation  
Lab-scale MFCs, which are in the order of milliliters in volume [99] generate electricity in the range of mW/m\({}^{2}\) power density [11; 102; 136] and around mA/m\({}^{2}\) in current density [199]. Up until now, most of the dedicated PMSs have been developed for marine remote sensors with low power requirements [200]. MFCs are widely used for powering marine sensors which mostly measure temperature [11; 23; 28], levels of the phreatic aquifers [154] or pH level [201]. These applications require low power and still providing power output of this order with MFC systems is challenging. This is related to the fact that voltage reversal occur while stacking MFC cells and every MFC-based system require customized power management units. [PERSON] et al. [202] reported cascading 18 MFCs into an array into a new type of distributed benthic MFC with dedicated PMS for powering up 2 sub-sea temperature sensors. A similar task for powering up 2 temperature sensors was performed by [PERSON] et al. [203] where they used a power management unit (PMU) with a fly-back converter to ensure continuous power  
Figure 9: A hierarchical presentation of MFC applications. supply. [PERSON] et al. [204] reported application of an economical off the shelf power management unit to generate micro-watt range power in both continuous and burst mode. The popular choices for MFC PMSs are capacitors [205], charge pumps [23], boost [206] and flyback converters [207, 208]. When using capacitors, there are two modes of operation: intermittent energy harvesting (IEH) [23] where energy is first stored and then released in bursts and continuous energy harvesting (CEH) mode [205] where a continuous output power is maintained at the load side. IEH mode has been proven to be more suitable for MFC-based power generation systems [112]. The most simple form of an MFC PMS is an IEH mode capacitor circuit that charges one or more capacitors until enough energy is accumulated for powering up the load. While its operation is simple and straightforward, the output voltage is limited by the open circuit potential of the capacitor used [200]. Structurally, charge pumps are slightly more complex than capacitors and yet found at low cost and are widely used in MFC PMS [23], which is basically an inductor-less direct current/direct current (DC/DC) converter with capacitors. The S-882Z series charge pump from Seiko Instruments are very popular for MFC applications [200]. A DC/DC boost converter is a customized electric circuit that converts direct current power from one voltage level to a higher level [200]. An example of popular ones for MFCs include L6920 DB from STMicroelectronics [200] and LTC3108 from Texas Instruments [206, 209]. Flyback converters [208] are more complex in the sense they use a coupled inductor instead. Theoretically their voltage gain can be infinite but are only applicable for below 100 W power options [210]. A recent addition to MFC PMS development is application of maximum power point tracking technology [211]. Many other components including voltage balancing circuits [122], super-capacitors [11; 28; 212] and semiconductor devices [109] have also been used. For more information regarding a summarized version of recent progresses on MFC based energy harvesting systems and their dedicated PMSs, readers are referred to the reviews by [PERSON] et al. [200]. For the recent progress on dedicated power management cracking systems, please refer to the works of [PERSON] et al. [211]. Many efforts have been made to customize MFC-provided electricity for remote communication purposes. In one work by [PERSON] et al. [28] the authors powered up a wireless temperature sensor for consistent data transmission to a PC from the sediment of Lake Michigan. Another wireless-sensor network powering by sediment-MFC was reported by [PERSON] et al. [209] where the implemented system provided a stable performance in sending high rate a signal from the source. [PERSON] et al.