# Design of a Modular Energy Production-Storage System for a Sustainable Bicycle
## 4 Discussion  
The comparison between the final design's features versus the initial design premises and the best features currently available in the market models are presented in a summarized way in Table 3.  
As Table 3 shows, most of the design premises fixed in previous phases were met in the final charging-storage system design. The final design candidate works above 9 km/h, can be fed from two sources (pedaling and solar power), the design is modular, and parts are easily replaceable and interchangeable, the storage capacity doubles one of the best models available, the fixing is made inside the frame--which turns it close to invisible--and the estimated cost is way below the cheapest option on the market. In addition, the drag force remains stable from 10 to 25 km/h, which makes its use much more comfortable than any other models available on the market.  
Figure 9: Module interconnection cable plugs detail and cable distribution for battery, solar panel, and USB charger connection.  
However, some of the features demanded were not possible to be met. Due to the extended capacity, the battery weight was doubled, and, therefore, the total weight of the system went above 200 g, which is still quite an acceptable value. Also, no Bluetooth connection capability or managing App was developed, although this could be easily performed in a later prototyping phase.  
Table 4 shows the marks obtained by the final designed model applying the same benchmark criteria previously used for commercial models.  
As can be seen in Table 4, the final score widely surpasses the highest scores of existing models.  
\begin{table}
\begin{tabular}{l l c c c} \hline \hline  & **Best Comparable Market Model** & **User Desirable** & **Designed Product** \\ \hline \multicolumn{2}{l}{**USB charger**} \\ \hline \(\bullet\) & Working speed & Working above 12 km/h & Working above 10 km/h & Working above 9 km/h \\ \hline \(\bullet\) & Design & As provided by manufacturer & Modular and interchangeable & Modular and interchangeable \\ \hline \(\bullet\) & Housing & Fixed outside the frame & Fixed inside the frame & Fixed inside the frame \\ \hline \(\bullet\) & Generation sources & 1 (hub dynamo) & 2 (hub dynamo/solar) & 2 (hub dynamo/solar) \\ \hline \(\bullet\) & Drag force & Variable & Stable at any speed & Stable from 10 to 25 km per h \\ \hline \multicolumn{2}{l}{**Battery pack**} \\ \hline \(\bullet\) & Design & As provided by manufacturer & Modular with replaceable parts & Modular with replaceable parts \\ \hline \(\bullet\) & Replacement & Must dismount carcass/not sold separately & Easy to replace by user/sold separately & Easy to replace by user/sold separately \\ \hline \(\bullet\) & Housing & Fixed outside the frame & Fixed inside the frame & Fixed inside the shank’s tube \\ \hline \(\bullet\) & Connection sources & \(1\) (USB charger OR solar panel) & 2 (USB charger AND solar panel) & 2 (USB charger AND solar panel) \\ \hline \(\bullet\) & Life cycle/weight & 300 cycles/150 g & Above 300 cycles/less than 150 g & 500 cycles/around 200 g \\ \hline \(\bullet\) & Design capacity & Around 5000 mAh & Around 8000 mAh & Around 10,500 mAh \\ \hline \multicolumn{2}{l}{**Solar charging system**} \\ \hline \(\bullet\) & Output ports & Single USB-A output & Double USB-A output & Double USB-A output \\ \hline \(\bullet\) & Output power & Above 28 W & Above 20 W & Above 28 W \\ \hline \(\bullet\) & Efficiency & 21.5–23.5\% & Above 20\% & 21.5–23.5\% \\ \hline \(\bullet\) & Size and weight & Small and low & Small and low & Small and low \\ \hline \multicolumn{2}{l}{**Interconnect wiring**} \\ \hline \(\bullet\) & Features & Does not exist & Universal and sturdy & USB (universal) and MATE-AX \\ \multicolumn{2}{l}{**N**o tools needed to connect & (sturdy)} \\ \hline \multicolumn{2}{l}{**Pricing**} \\ \hline \(\bullet\) & Pricing & EUR 80–260 & Around EUR 80 & Around EUR 45 (100 units) \\ \hline \hline \end{tabular}
\end{table}
Table 3: Comparison between features available in best comparable market models, user desirable features, and final design.