Test & measurement

Boosting solar race cars

to the max T

he ultimate challenge for pure solar- powered race cars is the biennial Bridgestone World Solar Challenge, on a

3,000km route from Darwin in the north of Australia to Adelaide in the south. Teams of engineers and drivers compete to finish the race in the shortest possible time, in specially designed cars which have only one power source: the light of the sun. The basic design of all cars in the race is

similar: an aerodynamic wing shape covered in arrays of photovoltaic panels to convert the sun’s light into electric power, which is fed directly to a motor driving the wheels, with any excess stored in a small on-board battery. The most highly placed teams are those which can best optimise the various elements of the car’s design – the aerodynamics, the power generation system, the motor and the traction system. Race strategy also plays an important part: the driver must move as fast as possible, but not so fast that the car’s battery runs out of

power when the car is not in bright sunlight. One of the most experienced teams to take

part in the 2019 running of the race was Solar Team Twente. The team first took part in 2005 with its car ‘SolUtra’, finishing in ninth place overall, and the highest-placed new entrant. It has taken part in every World Solar Challenge since then. In 2019, it raced ‘RED E’, its most technologically advanced solar vehicle yet. Solar Team Twente’s technical accomplishments are impressive given that the team does not benefit from the know-how and resources of an established manufacturing company: in fact, it is led and run by 19 students. This team of aerodynamic, electrical, mechanical and structural engineers is drawn from the University of Twente and the Saxion University of Applied Sciences in The Netherlands.

One of its most important engineering challenges is to optimise the efficiency of the motor and battery-management systems. In doing so, it benefits from measurements made by a power analyzer

donated by Yokogawa. This is the story of the way the power measurements made by the RED E designers helped the team to achieve in the 2019 race the team’s highest ever average speed.

BACKGROUND To win the World Solar Challenge, a race car must generate as much solar energy as possible, and convert the electricity it generates as efficiently as possible into mechanical power delivered to the wheels. At the same time, it must keep energy losses to a minimum: race teams pay minute attention to aerodynamic design to keep wind resistance to a minimum. In the case of RED E, total wind resistance is the equivalent of a conventional car’s wing mirror. The cars’ designers also aim for better than 99 per cent efficiency in the various electrical power conversion circuits. There are four important electrical systems in a solar race car:

The array of solar panel generators

The battery and its battery management system

The inverter (motor drive), which converts the solar panels’ direct current output to a three- phase alternating current drawn by the motor

The motor itself Solar Team Twente has been using self-

developed motors in their cars since 2013, which have been surpassing the efficiency of off-the-shelf models. For the 2019 World Solar Challenge, the team decided to seek an additional performance advantage by also abandoning the use of commercial inverters, being used by most of their competitors, and to design a more robust and efficient version themselves from scratch.

52 June 2020 Instrumentation Monthly

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