Optimal Design of a Hybrid Solar Vehicle

In the last years, increasing attention has been spent toward the applications of solar energy to cars. Various prototypes of solar cars have been built and tested, mainly for racing and demonstrative purposes [1] [2]. Despite of a significant technological effort and some spectacular outcomes, the limitations due to low density and unpredictable availability of solar source, the weight associated to energy storage systems, the need of minimizing weight, friction and aerodynamic losses make these vehicles quite different from the current idea of a car. But, while cars powered only by the sun seems still unfeasible for practical uses, the concept of an electric hybrid car assisted by solar cells appears more realistic [3][4][5][6][15]. In fact, in the last decades Hybrid Electric Vehicles (HEV) have evolved to industrial maturity, after a relevant research effort [7]. These vehicles now represent a realistic solution to the reduction of gaseous pollution in urban drive and to energy saving. Nevertheless, the need of mounting onboard both thermal and electrical machines and a battery of significant capacity makes these vehicles heavier than the conventional ones, at the same power, while solar cars are characterized by very limited power and weight. On the other hand, there is a large number of users that utilizes daily their car for short trips with limited power. Some recent studies of the UK government report that about 71% of UK users reaches their office by car, and 46% of them have trips shorter than 20 min., mostly with only one person on board [8]. In spite of their potential interest, solar hybrid cars have received relatively little attention in literature [15]. An innovative prototype has been developed at Western Washington University [5][6] in the 90’s, adopting advanced solutions for materials, aerodynamic drag reduction and PV power maximization with peak power tracking. Other studies and prototypes on solar hybrid vehicles have been presented by Japanese researchers [3][4] and at the Queensland University [9]. Although these works demonstrate the general feasibility of this idea, a detailed presentation of results and performance and a systematic approach to the design of a solar hybrid vehicle seems still missing in literature. Such a model is particularly necessary since the technological scenario is rapidly changing, and new components and solutions are becoming available or will be available in the next future. A specific difficulty in developing a HSV model is due to the many mutual interactions between energy flows, propulsion system component sizing, vehicle dimension, performance, weight and costs, whose connections are much more critical than in conventional and also in hybrid cars. Preliminary studies on energy flows in a HSV has been recently developed by the authors [10][11]. In the paper, a more detailed study on the optimal sizing of a solar hybrid car, based on a longitudinal vehicle dynamic model and considering energy flows, control strategies, weight and costs, is presented.