SOLAR ELECTRIC VEHICLE: A SUSTAINABLE MODE OF TRANSPORT

Transcription

1 SOLAR ELECTRIC VEHICLE: A SUSTAINABLE MODE OF TRANSPORT Team Solaris 1, Dr. Samsher Gautam 2 1 Team Solaris: Mohd Bilal, Parth Desai, Prateek Jain, Sagar Biyani, Shashank Tayal, Vaibhav Arora Delhi Technological University, New Delhi Abstract The Solar Electric Vehicle, popularly known as SEV, is an important innovation to meet the future demands of sustainable modes of transport. The present study investigates the need for development of SEV, and the strategy of the Solar Electric Vehicle development at Delhi Technological University over the years. The competition race strategy that is based on the maximization of the operational efficiency of the batteries to 90% is also discussed. The flow of energy in Avenir, the latest version of the DTU-SEV is also presented. Further, the design parameters of the chassis used in Avenir have been discussed, and stress analysis of the same on SolidWorks 2010 is done to reflect the feasibility of the design on the basis of factor of safety, calculated to be 2.3 or higher. Keywords: Solar Car, Solar Electric Vehicle, SEV, Renewable Energy, Sustainable modes of transport, Solar photovoltaic cells, MPPT 1. Introduction The automotive industry currently relies on non-renewable sources of energy like petroleum oil. At the current rate of consumption, these conventional sources of energy may be exhausted in the near future. In addition, consumption of petroleum and other non-renewable sources of energy by the automotive industry contributes highly to the ever increasing pollution levels, that has deleterious effects on biodiversity. Consequently, there have been innovations that utilize cleaner and sustainable sources of energy, especially in the automotive industry, and extensive research is being carried out in this field. In this regard, solar energy is of particular interest as it can be utilized very efficiently as a substitute for fuel in the automotive as well as other engineering sectors. The amalgamation of the automotive and the solar energy sector gave rise to the development of Solar Electric Vehicles, popularly known as SEVs, which derive the input energy from the Sun. The solar energy obtained from the sun, transformed into electrical energy by the cells, placed on the surface of the vehicle, charges the batteries. The power from the battery pack drives the motor and eventually powers the shafts, thus transforming the electrical energy to mechanical energy. With the efficiency of commercially available Solar Cells increasing to around 30% [1], highly efficient solar electric vehicles have been developed in countries across the globe to fully utilise the solar energy. India especially has a huge potential in this field as the solar intensity is high at all times of the year. Hence, Solar Electric Vehicles hold immense significance in India in near future because of the abundant solar energy available. In India, an initiative in the field of development of SEV was first taken by the faculty and students of Delhi Technological University. The first vehicle under the project SOLARIS was presented in Thereafter, two other vehicles with modifications and alterations in the design have been engendered, viz. Rogue in 2008 and Avenir in 2011.Rogue participated in the 2008 South African Solar Challenge while Avenir participated in the World Solar Challenge in Australia in October There have been improvements in design and structure of the vehicle with each new version and the project continues to serve its aim of providing an alternative to fuel in the automotive industry in future. The fourth vehicle is under development, with an objective to culminate into a commercial Solar Electric Vehicle, ready to compete with the cars available in the market, running on fuel or gas. This paper is an overview of the development made by Delhi Technological University students in the SEV over the years. The structure, components and efficiency of the three models have been compared based on data obtained from testing and that collected at the competitions. A strategy of maximizing the battery life was followed at the 2011 World Solar Challenge. The paper showcases the race strategy in detail. Also, the energy management systems including design of vehicle, analysis of race data are discussed in detail. 2. Comparative study of Rogue (2008) and Avenir (2011) The electrical and mechanical components as well as the design of the two versions of the Solar Electric Vehicle- Rogue( 08) and Avenir( 11) differ in type, configuration, and efficiency. Technical modifications were made to improve efficiency, durability and to satisfy the event regulations of World Solar Challenge, which are different from those in South African Solar Challenge. The differences in the geographical conditions of Australia and 365

2 South Africa were also considered while making the changes to the previous model. A detailed comparison is made in the following sections. 2.1 Electrical System Rogue The battery pack consisting 8 number of Lead acid batteries (pure lead tin type) each of 12V, 42 Ah rating connected in series was used in Rogue( 08). Each battery weighed 13 kg, thus giving a total weight of 104 kg. Solar panels of monocrystalline PM84 series by MoserBaer, with efficiency of 16% and rated voltage and current as 0.6V and 5.5A respectively, were employed. Eight panels with 36 cells each were used on top of the vehicle giving a total output power of 674 watts Avenir 4 batteries of conventional lead acid type with a configuration of 12V, 33 Ah were used in order to make the battery pack with 48V rated voltage. Each battery with a weight of 20kg made a total weight of 80 kg of the battery pack, thus decreasing the total weight of the vehicle. Solar panels of polycrystalline cells I6MU series by IndoSolar with an efficiency of 16.66% and rated voltage and current as 0.6V and 7.7A respectively were used. It had a rated power of 4 watts. Four panels of 60 cells each were used on top of the vehicle giving a total power of 960 Watts. The frame was also eliminated from the panels and a rubber coating was used to absorb the shocks on edges and corners, thus reducing the weight of the panels. A brushless DC hub motor was used instead of a separately excited motor connected through chain transmission thus reducing the losses. 2.2 Mechanical Systems Rogue The chassis of the SEV was made of Aluminium 6063 alloy extrudes after stress analysis on ProE software. The analysis showed that the chassis could bear 1.5 times the normal estimated weight of the car. The total weight of the chassis was measured to be around 54kg.Front suspension of MacPherson strut type had been used, with a trailing arm suspension on the rear. Single reduction chain and sprocket type system with gear reduction ratio of 6.5:1 transmitted the power from the motor to the rear wheel Avenir The chassis made of HINDALCO-aluminium alloy weighed around 50kg. Stress analysis of the chassis on SolidWorks 2010 revealed that a total load equal to 2.3 times the estimated weight of the car could be borne by the chassis. Double A-Arm type suspension was used on the front as it provides better control. The suspension was designed so that it improved the ride quality and reduced tire wear considerably. Fig.1. Energy Flow Diagram of Avenir

3 3. Energy Flow in the SEV The power which drives the SEV is obtained from the batteries which are in turn charged by the solar energy obtained from the solar cells arranged on the surface of the vehicle. The power from the battery pack drives the motor and eventually powers the shafts, and hence transforming the electrical energy to mechanical energy. The flow of energy in the 2011 version of the SEV is shown in Fig Solar Energy The area covered by the cells on the surface was 6 m 2 complying with the regulations of the World Solar Challenge [2]. The amount of electrical power generated by the cells from solar energy depends on various factors like the weather conditions, solar isolation at a particular location, time of day and the angle of placing the solar cells on the surface. The efficiency of the polycrystalline silicon solar cells used was 16.66%and the efficiency of a single module 13%, as tested in the lab [3]. Taking into account the above mentioned facts and factors, the overall power generated by the cells over a 6 m 2 area can be calculated as: P s = I.A.η (1) I = intensity of sunlight per unit area= 1000 W/m 2 (assuming clear sky during day) [6] A = Surface area covered by cells = 6 m 2 η = overall efficiency of the module = 13% We get the value of power from solar cells at clear sky P s = 780 Watts The cut off voltage of a single cell is 0.5V and so a total voltage of 60V is obtained as 120 cells are arranged in series. Considering this constraint, a 48V lead acid battery pack was used. The power from the solar cells charges the battery every moment until it is charged to its maximum capacity and battery voltage is maintained at 48V by the charge controller. In the mobile state of the car, the power from the batteries is continuously used by the motor and hence it gets discharged. 3.2 Mechanical Power The force required at any moment for driving the SEV, assuming constant velocity is equal to the aerodynamic drag and the rolling frictional force acting on the car. Motorcycle tires of the SEV were of dimensions 100/80 R17 and had a rolling resistance co-efficient of 0.02 [4]. The drag coefficient C d for a particular aerofoil profile can be calculated using empirical formulae provided in the literature, and was evaluated to be [1]. Hence, the power required at any moment can be calculated as follows: P m = P a + P r (2) P a = Aerodynamic loss = 1/2.ρ.C d.a. v 3 Where ρ = air density = 1.44 kg/m 3 C d = co efficient of drag = [6] v = velocity of car A = Frontal area of car = 1 m 2 P r = Rolling resistance = µnv where µ= co-efficient of rolling friction= 0.02 [4] ; N = Normal Reaction on the tires =500 kgf 3.3 Battery Efficiency The batteries used in an SEV should be robust, reliable and electrically isolated from the chassis of the car in order to prevent any catastrophe. Lead Acid batteries are the most conventional option and four 12V, 33 Ah batteries connected in series make a 48V, 33Ah battery pack. The efficiency of a battery can be defined as the ratio of the energy used in charging the battery to the energy supplied (in this case, by the solar cells). The efficiency of a battery increases with the decrease in charge. As mentioned by the manufacturers, the efficiency of the battery is minimum(about 60%) when it is more than 80% charged is maximum (about 90%) when it is less than 50% charged. 4. Race Strategy for Competitions Considering the variations in the efficiency of the battery, the speed and the time during which the car should be run was strategized in order to get the optimal usage of batteries and charge them at maximum efficiency for the longest time possible during the race. For this reason, the usage of batteries was divided into 3 different sections on the basis of the amount of charge in the batteries: Fully charged to 80% - at 60% charging efficiency 80% to 50% - at 75% efficiency (assuming average efficiency) 50% to 20% - at 90% efficiency 367

4 The energy equation for any section mentioned above can be given as: (P m - P s ) t = D s (3) D s = battery discharge T = time On substituting the values of velocity of the car, the amount of time in any particular section of battery discharge can be obtained. Hence, on substituting the values of speeds from 25 to 60 kph on increments of 5 kph, the corresponding time and hence the distance that could be travelled is obtained. The mechanical power required to propel the car different velocities is listed in Table 1. Table 1. Power consumption at different speeds Sr. No. Velocity(kph) P a (Watts) P r (watts) P m (watts) The corresponding mechanical power for the velocities from 60 to 25 kph is obtained. Using these values, the time (using equation (3)) and hence the distance that could be travelled for two different sections are calculated. The observations are as follows: Table 2. Distance covered by the SEV at different battery charge percentage Battery discharge Battery discharge Sr. V(kph) 80%-50% 50%-20% No. Time(min) Distance(km) Time(min) Distance(km) The efficiency of the battery is least(60%) while the battery discharges up to 80% of its capacity. Hence, for this range the car is driven at its maximum speed, i.e. is 60 kph in order to cover maximum distance in least possible time. For the battery capacity range of 80%-50%, maximum distance is covered at a speed of 30 kph, but the time taken is much higher than that at 40 kph, as inferred from Table 2. While the time taken for discharge at 50 kph is lesser, there is a considerable loss of distance that can be covered. Hence, 40 kph is the optimum speed to drive at in this range. Also, for the battery discharge of 50%-20%, where the efficiency of the battery is about 90%, the car is driven at 30 kph, thus covering maximum distance and allowing the battery to be operated at the highest efficiency. 5.Chassis Design Chassis is the main frame of the vehicle that provides strength and rigidity to the vehicle and houses the driver, thus providing protection. Other components like batteries and electronic system are mounted to the chassis. Table 3. Weight of the Components of the SEV Component of the SEV Weight(kg) Chassis 50 Upper Body and Solar Panels 142 Lower Body 41 Batteries 82 Motor 15 MPPT 4 Wheels and other mechanical systems 40 Driver Weight

5 Total 464 Thus, it is imperative to calculate the amount and type of load exerted by these components on the chassis. Hence, the weight of the components was measured accurately, and is listed in Table Design considerations Chassis must have high torsional rigidity, and must be able to withstand bump and droop forces, and support the passenger and body loads. In case of overturning, the roll cage must be strong enough to protect the driver. It is difficult to conduct real life experiments to determine the torsional rigidity and stiffness of the chassis. Hence, a model of the chassis was developed on SolidWorks 2010, and stress analysis was done for various load cases. The resultant stresses were then compared with the yielding stresses of HINDALCO Aluminium of which the chassis was originally fabricated. 5.2 Design implementation To increase the rigidity of the structure a combination of The Pratt Truss and The Warren truss was used in 3 dimensions, leading to a Space-frame type chassis. For proper stress distribution and to provide support to the solar panels, the top rear of the chassis had an additional Warren Truss as shown in Fig.2. The mounting points of suspension and the driver s seat were strengthened in a similar manner. Extrudes of HINDALCO Aluminium alloy of 14 gauge and 1inch outer diameter were used. The alloy has an elastic modulus equal to 69GPa, Poisson s ratio 0.33 and yield strength of 80MPa [5]. Fig.2.Chassis design of Avenir SEV showing load carrying family of members 5.3 Static Structural Analysis of Chassis The stress analysis of chassis was done on SolidWorks Geometry of the chassis design was created using SolidWorks GUI as per the design considerations mentioned above. Proper weldments were provided at the joints of the 3 dimensional structures to simulate the actual fabricated chassis. The structure was meshed using the Beam element, and load was applied on different members as listed in Table 4. The model was then solved for Von Misses stress distribution using Direct Sparse Solver. The report of the analysis was generated by SolidWorks In the present analysis, the distribution of stress and the factor of safety for various members of the structure are studied. 369

6 Table 4. Type and Distribution of load for static analysis Load No. Load type Corresponding Member Family No. Load value(n) Distribution 1. Weight of Solar Panels and Upper Body Universally 2. Battery weight Universally 3. Driver s weight Universally 4. Lower Body Weight Universally Total 3500 Load Direction 6. Results &Discussions The stress analysis results are conducted to study the strength and stiffness of the chassis. The maximum and minimum values of Von Misses stress in different members are observed for the worst case analysis. Fig. 3 depicts the distribution of load across various members as well as the variation of Von Misses stress. It is observed that the maximum value of stress generated for any member is MPa, which is much lower than the yield stress (80MPa) of HINDALCO Aluminium Further the factor of safety is also computed and its minimum value is found to be 2.5 as shown in Fig.4. Thus, the analysis shows that the structure is safe in static load conditions. Fig.3. Von Misses Stress distribution in structural members 370

7 Fig.4. Factor of Safety (FOS) of the Structural Members 7. Conclusions Three versions of the Solar Electric Vehicle have been developed at Delhi Technological University since A steady improvement in the technical design of the SEV has been made, as shown in the comparative study. Analysis of the data, collected during the race at the World Solar Challenge, shows that the car operated in the high efficiency range of the batteries, thereby increasing the life of the batteries. The race strategy as hereby proposed allowed the team to obtain a high efficiency of the system, as well as in maximising the distance travelled in minimum possible time. Appraising the data in table 2, the speeds at which the car should be driven for optimum usage of the battery are finalized as 60, 40 and 30 kph for the three discharge periods with corresponding efficiencies of 60%, 75% and 90% respectively. Moreover, the analysis of the chassis shows that the minimum factor of safety for any member is 2.3, which is an optimum value for static loading in case of ductile materials. Hence, the chassis structure is safe. The chassis design and optimum utilisation of energy by the proposed method can be applied to develop a highly efficient Solar Electric Vehicle. The development of the SEVand successful participation at various international competitions viz. South African Solar Challenge in 2008 and The World Solar Challenge in 2011 underlines the feasibility of a cost effective and efficient Solar Electric Vehicle that can meet the demands of the automotive industry for a sustainable source of energy. References [1] [2] [3] - product range [4] Cossalter, Vittore (2006). Motorcycle Dynamics (Second Edition ed.).lulu.com. pp ISBN [5] [6] Carroll, Douglas R. (2003). The winning solar car: A design guide for solar race car teams. SAE International. ISBN