Experimental Investigation on the Flow Boiling of Two Microchannel Heat Sinks Connected in Parallel and Series for Cooling of Multiple Heat Sources.

Cooling methods for multiple heat sources with high heat flux have rarely been reported, but such situations threaten the stable operation of electronic devices. Therefore, in this paper, the use of two microchannel heat sinks is proposed, with and without grooves, labeled Type A and Type B, respectively. Experimental investigations on the flow boiling of two microchannel heat sinks connected in parallel and in series are carried out under different mass fluxes. In addition, a high-speed camera is used to observe flow patterns in the microchannels. The cold plate wall temperature (Tw), heat transfer coefficient (HTC), and pressure drop (PD) are obtained with the use of two microchannel heat sinks. The flow patterns of the bubbly flow and elongated bubbles in the microchannels are observed. The results of the analysis indicated that the Tw, HTC, and PD of the two microchannel heat sinks connected in parallel were degraded, especially when using the Type A-B parallel connection. Compared to the use of a single heat sink, the maximum decrease in HTC was 9.44 kW/(m2K) for Type A heat sinks connected in parallel, which represents a decrease of 45.95%. The influence of the series connection on the Tw, HTC, and PD of the two heat sinks is obvious. The Type A-A series connection exerted the greatest positive effect on the performance of the two heat sinks, especially in the case of the postposition heat sink. The maximum increase in HTC was 12.77 kW/(m2K) for the postposition Type A heat sink, representing an increase of 72.88%. These results could provide a reference for a two-phase flow-cooling complex for multiple heat sources with high heat flux.

Limitation of voltage reversal in the degradation of azo dye by a stacked double-anode microbial fuel cell and characterization of the microbial community structure.

In this study, two double-anode microbial fuel cells (MFCs) were connected in series for degradation of the azo dye reactive brilliant red X-3B. After the series connection, the electricity generation of one of the MFCs decreased, and the other was not affected too much. Due to the special structure in the double-anode MFC reduced the imbalanced performance between the MFC units, the occurrence of voltage reversal was limited. The removal efficiencies in two MFC reactors were not consistent after the series connection, the results showed that the MFC with the reduced electricity generation had the higher removal efficiencies, it was 12.90, 11.66, and 40.05% higher than in the MFC in which the power generation capacity was not affected after the series connection, the MFC without serial connection, and the control group, respectively. Meanwhile, the microbial communities related to the degradation of refractory organic compounds increased and related to electricity generation decreased in the MFC with the reduced electricity generation, the changes of the microbial communities were consistent with its electricity generation and the removal efficiencies. The degradation products in the effluent from two MFC units showed that had the products generated from the MFC with the reduced electricity generation had simpler structures comparing the other MFC unit.

Wearable high-powered biofuel cells using enzyme/carbon nanotube composite fibers on textile cloth.

Wearable biofuel cells with flexible enzyme/carbon nanotube (CNT) fibers were designed on a cotton textile cloth by integrating two components: bioanode fibers for glucose oxidation and O2-diffusion biocathode fibers for oxygen reduction. The anode and cathode fibers were prepared through modification with glucose dehydrogenase and bilirubin oxidase, respectively, on multi-walled carbon nanotube-coated carbon fibers. Both biofibers woven on the cloth generated a power density of 48 µW/cm2 at 0.24 V from 0.1 mM glucose (human sweat amount), and of 216 µW/cm2 at 0.36 V, when glucose was supplied from a hydrogel tank containing 200 mM glucose. Our fiber-based biofuel cell deformed to an S-shape without a significant loss in cell performance. Furthermore, we demonstrated a series-connection involving the tying of biofibers on a cloth with batik-based ionic isolation. The booster four cells generate power at 1.9 V that illuminated an LED on the cloth.

Elimination of voltage reversal in multiple membrane electrode assembly installed microbial fuel cells (mMEA-MFCs) stacking system by resistor control.

Voltage reversal (VR) in series connection of multiple membrane electrode assembly installed microbial fuel cells (mMEA-MFC) is eliminated by manipulating the resistor control. Discharge test results collected from two mMEA-MFCs initially operated (designated as P1 and P2) confirm that the performance of P2 exceeds that of P1. Thus, driving P1 and P2 as serially stacked MFCs generate the VR in P1. Controlling the inserted resistor adjust the current production of P2 to maintain balance with P1, and the VR in P1 is eliminated in the operation of stacking mode. Thus, manipulating the internal resistance provide an applicable approach to suppress VR in the stacking of mMEA-MFCs system.

Self-recoverable voltage reversal in stacked microbial fuel cells due to biofilm capacitance.

In order to assess the effects of biofilm capacitance on self-recovering voltage reversals, the restored current is determined and compared with the measured biofilm capacitance by analyzing the results of electrochemical impedance spectroscopy. This comparison demonstrates that self-recovering voltage reversals are caused by temporary damage to, and the recovery of, biofilm capacitance which arises due to the ability of redox enzymes in the electron transfer system to temporarily store electrons. Thus, the development of procedures for voltage reversal control and for the maintenance of serially connected microbial fuel cells (MFCs) should take into account such temporary voltage reversal phenomenon. This discovery and characterization of self-recovering voltage reversals is expected to be practically useful to enhance the reliability of MFCs to be scaled up and implemented in practical systems.

Electricity generation and microbial community in response to short-term changes in stack connection of self-stacked submersible microbial fuel cell powered by glycerol.

Stack connection (i.e., in series or parallel) of microbial fuel cell (MFC) is an efficient way to boost the power output for practical application. However, there is little information available on short-term changes in stack connection and its effect on the electricity generation and microbial community. In this study, a self-stacked submersible microbial fuel cell (SSMFC) powered by glycerol was tested to elucidate this important issue. In series connection, the maximum voltage output reached to 1.15 V, while maximum current density was 5.73 mA in parallel. In both connections, the maximum power density increased with the initial glycerol concentration. However, the glycerol degradation was even faster in parallel connection. When the SSMFC was shifted from series to parallel connection, the reactor reached to a stable power output without any lag phase. Meanwhile, the anodic microbial community compositions were nearly stable. Comparatively, after changing parallel to series connection, there was a lag period for the system to get stable again and the microbial community compositions became greatly different. This study is the first attempt to elucidate the influence of short-term changes in connection on the performance of MFC stack, and could provide insight to the practical utilization of MFC.

Understanding energy loss in parallelly connected microbial fuel cells: Non-Faradaic current.

In this work, the mechanisms of energy loss in parallel connection of microbial fuel cells (MFCs) is explored using two MFC units producing different open circuit voltage (OCV) and current. In open circuit mode, non-Faradaic current flows in low OCV unit, implying energy loss caused by different OCVs in parallelly stacked MFCs. In a stacked MFC in parallel under close circuit mode, it is confirmed that energy loss occurs until the working voltage in high OCV unit becomes identical to the other unit having low OCV. This result indicates that different voltage between individual MFC units can cause energy loss due to both non-Faradic and Faradaic current that flow from high voltage unit to low voltage unit even in parallelly stacked MFCs.

New architecture for modulization of membraneless and single-chambered microbial fuel cell using a bipolar plate-electrode assembly (BEA).

A new architecture for a membraneless and single-chambered microbial fuel cell (MFC) which has a unique bipolar plate-electrode assembly (BEA) design was demonstrated. The maximum power of MFC units connected in series (denoted as a stacked MFC) was up to 22.8±0.13 mW/m(2) for 0.946±0.003 V working voltage, which is 2.5 times higher than the averaged maximum power density of the non-stacked MFC units. The power density in the stacked MFC using BEA was comparable to the stacked MFC using electric wire. These results demonstrate that BEAs having air-exposed cathodes can potentially be used in the stacking of membraneless single-chambered MFCs. In addition, we confirmed that the current in the stacked mode flowed faster than the non-stacked mode due to voltage increase by series connection, and the poorest of the stacked units quickly faced current depletion at higher external resistance than the non-stacked mode, leading to voltage reversal. These results imply that stacked MFC units require a relatively large current capacity in order to prevent high voltage reversal at high current region. To increase total current capacity and prevent voltage reversal of stacked MFC units, we suggested series/parallel-integrated MFC module system for scaling-up. This new concept could likely allow the application of MFC technology to be extended to various wastewater treatment processes or plants.