# An Approach to Predicting Sediment Microbial Fuel Cell Performance in Shallow and Deep Water
## 3 Results
### Shallow Water Predictions (<100 m)
The data are sparser than in the Texas example, so only broad interpolated contours are assumed. Figure 9 shows bottom temperatures in February and August, derived from the NOAA CTD and XBT casts. Nearshore temperatures are again seen to increase approximately 10 \({}^{\circ}\)C from February to August. Figure 10 shows the estimated power density from a 1 m\({}^{2}\) anode, varying from approximately 1 to 8 mW/m\({}^{2}\). Power density again increases about 1 to 2 mW/m\({}^{2}\) in the summer when the water warms. Figure 6: Calculated MFC power density in February (**left**) and August (**right**) on the Texas shelf in the Gulf of Mexico. Figure 7: Bathymetry in the Yellow Sea. approximately 18 months, triplicate fuel cells, composed of small carbon cloth anodes and a large common carbon cloth cathode, were immersed in bubbled seawater at temperatures from \(-1\)\({}^{\circ}\)C to 12 \({}^{\circ}\)C. Total organic carbon, the nominative electron supplier to the anaerobic bacteria living in the sediment and the providers of the microbial fuel cell power, ranged from 1% by weight in the 1228 m samples, 2.1% in the 2002 m samples, and 3.6% in the 1000 m and 1188 m samples. MFC performance results from the deep sediment experiments are shown in Figures 12 and 13. Figure 12 (previously presented in [19]) shows the effect of temperature and TOC on power density from the deep sediments, measured in the laboratory at atmospheric pressure. Figure 13 compares power density from the deep-water sediments at 5 \({}^{\circ}\)C to that from shallow water samples which were scaled to expected values at 5 \({}^{\circ}\)C. The linear scaling is:  
Figure 2:0  
where temp C is the sediment temperature when power density was measured. This is the linear power density to temperature relationship shown in Figure 1 (right panel). The extrapolation to 5 \({}^{\circ}\)C causes some San Diego Bay power density predictions (blue dots and line) to be near zero or negative. These extrapolations were not used to create the shallow water power density estimates, but rather, only for comparison to power output from deeper sediments at the same temperature. Earlier SPAWAR laboratory data showed that pressure (or lack thereof) did not appear to have much of an effect on power output from MFCs in deep water sediments [20]. Figure 14 (previously published in [20]) shows relatively short-term (\(\sim\)1 month) MFC experiments on sediment collected at 1000 m directly west off the coast of San Diego, conducted in a pressure vessel with flowing oxygenated seawater. The sediment was not kept pressurized during retrieval from the bottom. This experiment was conducted at ambient laboratory temperature conditions in 2013. The power output shows daily fluctuations, most likely in response to changing room temperatures. These data and similar experiments suggest that pressure is not a significant determinant in MFC power output, particularly in light of the experimental difficulties imposed (see also [21]). An MFC deployed in situ at 1000 m near where the sediment was retrieved for the laboratory pressure experiment yielded power densities of only 0.5 mW/m\({}^{2}\). The MFC was deployed for the month of July 2013 in sediment with 3% TOC at a temperature of 8 \({}^{\circ}\)C (see fig. 6 in [20]). Figure 2:1 Field sample sites for laboratory measurements (Google Earth). Figure 2:2 MFC power density versus TOC and bath temperature. Figure 2:3 MFC power density from deep (red line) and shallow (blue and green lines) sediments normalized to 5 \({}^{\circ}\)C.  
Figure 15 emphasizes the relatively flat response of the power density to temperature in deep sediments as compared to shallow water sediments. Laboratory experiments were performed by slowly raising and lowering the temperature bath in 1 \({}^{\circ}\)C increments over a test period of 18 months. Temperatures were held at constant levels for at least one week. The percent TOC of sediment samples varied, as indicated in the legend. The linear fit equations for each sample are indicated on the figure.