Solar Team Twente wins Solar Challenge Morocco
01 September 2022
Working at the leading edge of solar power technology for the automotive sector, the teams which compete in events for pure solar-powered race cars are also single-minded competitors. The flagship race is the Bridgestone World Solar Challenge, a 3,000km route from Darwin in northern Australia to Adelaide in the south, but other races elsewhere in the world also provide teams with opportunities to test new cars, technologies & equipment against the world’s best solar racers.
This case study was originally featured in the September 2022 issue of EPDT magazine [read the digital issue]. And sign up to receive your own copy each month.
Here, Colin van Laar, Solar Team Twente Team Manager & Industrial Design Engineering student at University of Twente tells us how his Solar Team Twente used power measurement technology from T&M instrument firm, Yokogawa to help it squeeze every last watt out of its car’s solar power system to win the first ever Solar Challenge Morocco.
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. Excess energy is 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 solar photovoltaic (PV) power generation system; the motor; and the traction system. Race strategy also plays an important part: the driver must move as quickly 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 competing on the solar car racing circuit is Solar Team Twente. It first took part in the World Solar Challenge in 2005, with its car ‘SolUtra’, finishing in ninth place overall, the highest-placed new entrant. It has taken part in every World Solar Challenge since then, and in the 2019 race, was leading at the halfway point, only for its car to be blown off the road by a freak weather event, dramatically ending its participation. Undaunted, in 2021, the team entered the inaugural Solar Challenge Morocco, after the cancellation of the 2021 World Solar Challenge.
Solar Team Twente’s technical accomplishments are impressive, especially 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 students. This young 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.
As Solar Team Twente prepared its car to race in Morocco, its engineers faced the need to improve the motor, the battery and the energy generation system if it was to edge ahead of its rivals on the demanding African course. A crucial part in this endeavour was played by the precise power measurements made by the team’s electrical engineers, and by the insights they provided into the operation of the battery, the motor’s inverter and the array of PV cells which generate the energy the car uses.
To win a 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: the Solar Team Twente’s car is so aerodynamic that total wind resistance is the equivalent of a conventional car’s wing mirror.
The efficiency and power output of the car’s various electrical systems are also of vital importance. There are four important electrical systems in a solar race car:
• The array of solar panel generators
• The battery and its battery management system (BMS)
• 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
Even small optimisations can make the difference between winning and losing the race. So the team’s electrical engineers continually work to squeeze ever higher performance out of these electrical systems. This has included increasing the amount of energy generated by the array of PV cells on the body of the car, and the amount of solar energy which the car’s battery could store. The team has also produced a more robust and efficient inverter to its own design, replacing the commercial off-the-shelf (COTS) inverters used by competitors.
The team’s race strategy also calls for precise regulation of the battery’s state of charge. Every team’s goal is to finish the race with zero energy left in the battery to maximise total energy usage, and thus achieve the highest possible speed for the longest possible distance without running out of power. The more accurate the state of charge measurement, the more confidently the race team can set the optimum speed of the car’s cruise control, taking into account the weather, the capacity of the battery, the performance of competitors and other factors.
“If all the components are just 0.01% better than the competition’s, then you already have an advantage before you reach the start line in a race. So precision measurement is a crucial element of our success,” explains Camiel Lemmens, Solar Team Twente Marketing & Events Coordinator & Industrial Design Engineering student at University of Twente.
Solar Team Twente’s engineering design team needed to repeatedly perform power (concurrent voltage/current) measurements of the various electrical systems. The team was looking for minute differences: for instance, when selecting the best PV cells for the car’s solar array, engineers were analysing differences of just a few millivolts or milliamps between one cell’s output and another’s. When the differences are so small, the error in a test instrument’s measurements must be even smaller, otherwise the user is seeing only noise.
As electrical engineer, Rob Kräwinkel explains: “We were looking for that extra 1% efficiency to give us an edge over other race cars. When you are already at better than 95% efficiency, eliminating any remaining losses is really hard to do. You have to be able to look at a detailed level at tiny deviations in voltage or current. That means you need a really accurate and sensitive power measurement system.”
Solar Team Twente had four main uses for its power measurement system:
• To analyse the efficiency of solar PV cells, to enable the team to select the cells which produce the highest energy output, and to discard less efficient cells. The engineers needed to measure with very high precision the voltage and current of each cell’s output under a known photonic input.
• To measure the energy capacity of battery cells and select those which stored the greatest amount of electrical energy. This required measurement of voltage and current at a high sampling rate, as energy was stored in and drawn from a cell.
• To validate the accuracy of the fuel gauge, the car’s on-board sensor measuring the battery’s state of charge. This system measures current flowing into the battery (from the solar panels) and flowing out of the battery (to the motor). By subtracting output from input, it can calculate the residual charge in the battery. This required extremely accurate continuous measurement of current flows.
• To measure the power output of the motor system, including the inverter and the motor itself, at a range of power input values, to enable the team to refine the design and incrementally improve its efficiency. This called for extremely accurate power analysis at high sampling frequency.
To achieve the accuracy and precision required for system design and validation, the Solar Team Twente engineers chose a WT5000 Precision Power Analyzer from Yokogawa – the world’s most accurate precision power analyser. Measurement accuracy is rated at ±0.03%, and measurement bandwidth is 10MHz for voltage and 5MHz for current: these capabilities enabled the team to precisely track the voltage and current values of PV cells when exposed to sunlight.
The maximum sampling rate of 10 mega-samples per second exceeds the data refresh rate required to validate the fuel gauge system. WT5000 users can make simultaneous measurements on up to seven inputs, and view them on its high-resolution 10.1” touchscreen.
Solar Team Twente’s Rob Kräwinkel says that when the electrical engineers first installed the WT5000-based test set-up, they discovered that the current sensors in the fuel-gauge circuit had a previously undiscovered offset, which had been making the state of charge measurements inherently inaccurate.
“The current sensing circuit we had previously was already better than 99% accurate, but we were looking for better than that – it was only when we analysed the sensor with the WT5000 that we were able to compensate for the offset in the current sensor, and so configure the measurement outputs to achieve optimal accuracy. That crucial extra confidence in our state of charge measurements can give the driver a vital extra 1km or 2km of range at a given speed that we would not otherwise have been sure of getting from the battery”, says Kräwinkel.
“We wanted to put the most efficient solar cells on the car, and that’s why we needed the measurements taken by the WT5000,” said Remco Hoen, Solar Team Twente Account Manager & Biomedical Engineering student at University of Twente.
Solar Team Twente wins first edition of Solar Challenge Morocco
In the 2019 World Solar Challenge, Solar Team Twente had already demonstrated the superior performance of its technology, even though it did not finish the race.
This potential was turned into real achievement when the team next competed, in 2021’s Solar Challenge Morocco. Its RED Horizon car navigated the 2,500km course through desert and over the high Atlas mountains, pushing its closest rival, the winner of the 2019 World Solar Challenge, into second place.
The team’s development of its own custom powertrain and components – an engineering effort which relied heavily on the accuracy and precision of the measurements taken with the WT5000 – proved to be decisive.
And already the team is thinking ahead to the 2023 World Solar Challenge, and beyond. Remco Hoen says: “We will most certainly need the WT5000 again for exact, precise measurements of the solar cells, the battery pack and the powertrain.”
WT5000 Precision Power Analyzer
Offering the best in isolation, noise immunity, current sensing and filtering in a modular architecture, the WT5000 is an extensible measurement platform that provides precision power analysis, backed by the world’s leading in-house calibration facility for power analysers, in Amersfoort, The Netherlands.
Users can make simultaneous measurements on up to seven inputs and compare them in split-screen mode on the high-resolution 10.1” touchscreen. The modular architecture of the WT5000 provides seven slots supporting various types of input modules, giving the user a flexible measurement system. The WT5000 also offers advanced filtering options, including:
• Synchronisation source filter
• Enhanced frequency filter
• Digital parallel path filters
Yokogawa WT5000 Precision Power Analyzer
Operable by a touchscreen and hardware controls, the WT5000 offers an intuitive measurement experience. As Rob Kräwinkel says: “The WT5000 is a very nice instrument to use: it’s intuitive, and it’s easy to find your way around the controls. It’s also easy to tweak the display so that it shows exactly the measurement outputs you are interested in, like when it clearly demonstrated that our new inverter design outperformed the equivalent off-the-shelf model by a significant margin.”
About Yokogawa Test & Measurement
Yokogawa has been developing measurement solutions for over 100 years, consistently finding new ways to give R&D teams the tools they need to gain the best insights from their measurement strategies. The company has pioneered accurate power measurement throughout its history and is a market leader in digital power analysers. Its instruments are renowned for maintaining high levels of precision, continuing to deliver value for far longer than the typical shelf-life of such equipment. Yokogawa believes that precise and effective measurement lies at the heart of successful innovation and has focused its own R&D on providing the tools that researchers and engineers need to address challenges great and small.
The guaranteed accuracy and precision of Yokogawa instruments results from the fact that Yokogawa has its own European standards laboratory at its European headquarters in The Netherlands. This facility is the only industrial (non-government or national) organisation in the world to offer accredited power calibration at frequencies up to 100 kHz. ISO/IEC17025 accreditation (RvA K164) demonstrates the international competence of the laboratory.
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