## 3 Experiments and Results
### Experimental Results
#### 3.2.1 Cooling Effect Produced by Fuel Cells  
At first, the results showing the cooling effect of the solid fuel stick when it supplies the fuel cell stack are presented. This is shown in Figure 8 where the fuel cell stick is seen to decrease from its initial temperature of 33 \({}^{\circ}\)C to 23.14 \({}^{\circ}\)C in around 9 min. Several such tests have been conducted in the absence of any external source of heat which can raise the temperature of the fuel stick. In every case, the result observed is that regardless of the initial temperature within a range of 20-35 \({}^{\circ}\)C, the final temperature of the fuel stick is about 7-10 \({}^{\circ}\)C lower than its initial temperature, when measured around 9-10 min from the initial time at which the fuel stick started supplying the stack and when hydrogen is being supplied at a constant rate (as needed by the H-30 fuel cell stack used in this paper). Figure 9 shows another set of results, where the fuel stick is seen to cool from an initial temperature of around 26 \({}^{\circ}\)C, to around 6 \({}^{\circ}\)C. It must be noted that such a large decline in temperature was observed when the fuel stick supplied the fuel cell for a longer duration (about 20 min or more) without any external heat source being in contact with the fuel stick, and also when the purge connections were initially left open, thus resulting in a large initial burst of hydrogen being released. The results in Figure 9a,b are provided to show that metal hydride fuel sticks have substantial potential to be used in applications which need cooling. The temperature decrease of metal hydride fuel sticks (cartridges) has been well documented [24], and it is due to the fact that the process of 'charging' or absorption of hydrogen in a metal hydride fuel stick is exothermic (releases heat), where as the reverse process, i.e., 'discharging' or release of hydrogen is endothermic (absorbs heat). Thus, motivated by the above observation, we present the results of the use of the fuel stick to cool a Li-ion battery onboard a drone while in flight. Please note that all temperature measurements were verified by at least one of two  
Figure 7: **Left: A prototype battery cooling assembly via contact with a solid fuel stick. Right: A view of the prototype cooling assembly mounted on the bottom of the drone. ** \begin{table}
\begin{tabular}{c c c c} \hline \hline \multirow{2}{*}{**Item**} & **Drone** & **Fuel Cell Holder Box with all** & **Prototype** \\  & **Chassis with** & **Required Components (Including** & **Aluminium Foil** \\  & **Battery** & **Fuel Stick and Pressure Regulator)** & **Cooling Jacket** \\ \hline Weight (g) & 1200 & 525.46 & 24.552 \\ \hline \hline \end{tabular}
\end{table} 
Table 3: Weights of drone components. thermal imaging devices and an infrared thermometer. Thermal imaging device 1 automatically plotted plus '+' signs at the center of the image and at the points corresponding to the lowest and highest temperatures. The '+' corresponding to the highest temperature is shown in red, but the color of the plus sign is only used to signify that it is the hottest point; the temperature scale on the right-hand-side of each of the figures provides a better idea of the actual temperature at a point. Thermal imaging device 2 produces an image of a circular mark with crosshairs superimposed on the thermal image. All devices used have an accuracy of \(\pm 2\%\) of the readings, so it was possible to reliably measure the temperatures. Also, experiments were conducted over a series of days, and some results from thermal imaging device 2 acquired towards the end of the series of experiments are shown, because it was observed that the visual quality of the images produced by thermal imaging device 2 was better than those obtained by thermal imaging device 1. Figure 8: (**a**) Temperature of the fuel stick before connecting to fuel cell stack; (**b**) temperature of fuel stick after being connected to fuel cell stack for about ten minutes.