RVCE solar car team from RV College of Engineering, Bengaluru has developed electric vehicle ARKA. The interdisciplinary team comprises more than 30 engineering students from electronics and communication, computer science, mechanical, aerospace and chemical engineering streams. It was formed in September 2013 with the motive to promote the use of solar energy in transportation sector, to drive research and innovation into solar technology and to spread awareness regarding the plausible benefits of renewable energy in our society.
The car body design
The team aims to achieve high performance and build India’s most efficient car. A brief description of its electronic sub-systems is given below.
Battery system (energy storage). The first step in designing a battery pack is to map out the requirements. These are decided on the basis of optimum voltage required for operating the motor and various other loads. These also aim to maximise the number of cells used, since an increase in the number of cell units in parallel increases the current given out. In ARKA electric vehicle, the RVCE solar car team used a configuration of 29S14P, which gives an output voltage of 104V and has a rating of 47Ah.
One of the major innovations they carried out in this sub-system is the battery box. As batteries form the heart of an electric car, these need to be sturdy enough to sustain vibrations and impact. The team made the battery box out of fibre-reinforced plastic (FRP), which was CNC-machined so that all modules could fit into it perfectly.
A modularised design allows them to replace any module, if damaged, which is not possible in conventional electric cars. Also, they use a battery management system (BMS) from Tritium, Australia. The BMS makes sure that the batteries are operating within safe limits.
In-house developed battery box
The battery pack used to store energy from the solar collector must have high energy density, high number of charge-discharge cycles and must deliver high power output to drive the vehicle, and yet be lightweight. Many types of batteries can be used to store energy, but the preferred ones are lithium-ion and lithium-polymer, as these offer high energy densities for lower weight. In ARKA, Panasonic l8650B cells are used. These are also used in Tesla and other modern-day electric vehicles.
Solar array. A solar array consists of solar modules that cover the top of a solar car. It is the only source of external energy in the car that charges the batteries. Most modern-day rooftop solar panels use cells that have about 15 to 17 per cent efficiency. These use 260 SunPower Maxeon Gen 2 monocrystalline silicon cells that have an efficiency of 22.4 per cent.
Solar array
Manufacturing a solar array. The manufacturing of RVCE’s solar module was done at HHV Solar, Bengaluru. The encapsulation process involves sandwiching the solar cells between two layers of ethylene tetrafluoroethylene (ETFE) and joining using organic adhesive. The baseplate is made of hard glass epoxy to give mechanical strength to the array, while the top sheet is made of clear plastic to ensure that maximum light can permeate to the solar cells. Due to this innovative technique, the team manages to reduce the drop-in efficiency by only one per cent. This also makes the solar module flexible, so that it can be placed on the curved top shell of the solar car.
The area of the solar array is restricted to 4 sq.m. in accordance with race regulations. Therefore they use 260 cells of 125mm×125mm dimensions. These cells are arranged serially into two strings that are connected in parallel to a set of maximum power point trackers (MPPTs).
Solar array maximum power point trackers
Motor. ARKA is driven by Mitsuba M-2096 D III brushless DC in-hub motor; 2096 signifies that it consumes 2.0kW and works best at 96V. The motor is the main consumer of the power generated by solar panels. If the power required by the motor is more than what the solar array can produce at a moment, the battery works as a backup and supplies the remaining required power to it. As the solar car has a single-wheel drive system, this motor generates enough power to drive the car beyond 90kmph by only taking 2kW power, which is equivalent to the power needed for a hair dryer.
Telemetry system. ARKA is a connected car—a vehicle that can optimise its own operation and maintenance as well as the convenience and comfort of passengers using onboard sensors and Internet connectivity.
The team is completely on board with the idea of a connected car and believes in its potential as our future. Arka embodies this philosophy, being a race car made for the most gruelling and competitive events in the solar car racing circuit.
Data acquisition occurs from the system with respect to various electrical and mechanical parameters. The state of charge of the battery is measured from the BMS. The vehicle’s speed (rpm) is measured from a pulse-width signal from the motor controller, and the distance travelled is duly calculated. Temperature of the battery pack, BMS and photovoltaic solar cells is also measured. Incident solar irradiance is measured with the help of an external pyranometer, which will eventually be implemented onboard.
Description of primary telemetry tasks
Data that is sensed in real time cannot be logged onboard due to constraints in the form of compute, memory and speed. This compels the team to log the data and store it in the cloud where it can be accessed by anyone, anywhere and processed directly online. The heart of the sensing and data acquisition is BeagleBone Black, which also functions as the processing unit for the dashboard display.
All data is sent to a chase vehicle, which follows the prototype, and is then uploaded onto the cloud. IBM helps with data analytics and storage. Team RVCE uses IBM’s proprietary IBM Cloud (Bluemix) platform. Data is stored in Cloudant NoSql databases to handle the large amounts of data that are constantly uploaded. Analysis of the received data is done using another IBM service (data science experience), which uses Apache Spark and Python to parse through the data and implement machine learning techniques to come up with an optimal speed at which the car should be driven to maximise battery efficiency and minimise race time.
