# Design of a Modular Energy Production-Storage System for a Sustainable Bicycle
## 1 Introduction  
In recent decades, the problems of mobility, congestion, parking, and pollution from fossil fuels have become key points for the development of sustainable transport strategies [1]. According to the European Green Deal [2], a 90% reduction in greenhouse gas emissions should be reached by 2050 [3]. Within this panorama, bicycles stand as the least noisy, least polluting, and cheapest urban vehicles as they can run without any fuel and are not affected by traffic [4, 5]. Many municipalities of medium-sized and big cities promote their use by assigning dedicated paths for them separated from vehicle traffic [6]. They are sustainable means of transport, capable of generating clean energy through a simple alternator that transforms mechanical energy into electrical energy, which can be also stored in a portable battery if needed. This energy can be later used for a wide range of applications, such as charging a mobile phone [7], feeding and managing a GPS system [8], or even helping with pedaling, turning conventional bikes into electrically assisted bikes (if pedaling is needed to move the wheels) or e-bikes (if propulsion can occur without pedaling) [9]. Both are heavier and more expensive than conventional bikes and are sold as a full set of integrated elements, making it difficult to customize or replace damaged parts with parts from a different manufacturer. Photovoltaic energy is another clean source of energy that can be used on a bicycle [10]. Solar energy is free, available worldwide, and is usually utilized in e-bikes to help charge the batteries [11, 12]. Standard bike users also use photovoltaic panels fixed on the saddle bag zones of the bike to help recharge phones or GPS navigators when they face long-distance rides. However, although some examples of the conjoint integration of energygeneration-storage systems based on pedaling or solar panels exist [13], they are scarce and are mainly focused on assisting pedaling and not on charging user devices. Therefore, providing a system that allows for the storage of energy generated while pedaling, as well as the energy received from the sun at the same time--and to use it later to charge user's digital devices such as a mobile phone or GPS navigator--would contribute to the concept of environmentally friendly and sustainable cities, decreasing the annual energy expenditure at home used for charging these devices, and therefore turning conventional bikes into more sustainable ones. Until a short time ago, research into the battery charging process for bikes focused on the acceleration of the transient response, while the efficiency of the power conversion was considered a secondary objective [14; 15; 16]. Nowadays, reaching a high power conversion efficiency is of paramount importance in power supply system design in order to accomplish modern trends in energy saving [17; 18; 19; 20], and to improve the state-of-charge balancing control that allows for the optimization of battery charge utilization [21]. DC-DC converters are used to stabilize the output of the AC-DC converter that receives the power from pedaling, as well as to minimize the variability in DC voltage values generated by solar panels. The most commonly used converters in solar panel applications are buck- and boost-type ones [22] but other alternatives such as SEPIC [23], boost [24], hybrid boost [25], or Cuk converters [26] have been successfully used for different energy storage or electric vehicle charging applications. Several design approaches can be used to reach a product design that successfully integrates all elements of the modular system.