Solar Power Squeeze

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Twente’s RED E electric car took victory in Solar Challenge Morocco

Power analyser helps Team Twente squeeze every last Watt of power in the Solar Challenge Morocco event

Yokogawa Test & Measurement’s WT5000 Precision Power Analyzer has been put into use by “Solar Team Twente” to win the first ever Solar Challenge Morocco event in Africa. The WT5000 instrument helped the engineers of Solar Team Twente to squeeze every last watt out of its car’s innovative solar power system and ensuring that it crossed the finish line of the 2,500 km course in first place to take the chequered flag.

Solar Team Twente is made up of a group of aerodynamic, electrical, mechanical and structural engineers from the University of Twente and the Saxion University of Applied Sciences in the Netherlands.

Their efforts to improve the efficiency of their car led to triumph in a race that pitted contestants against both each other and the harsh desert and mountain landscape of the north African country with its harsh and unforgiving terrain.

Slender design

The basic design of all of the cars in the race is similar. They all have an aerodynamic wing shape, which is covered in arrays of photovoltaic panels that are used for converting the sun’s light into electric power, which is then fed directly to a motor, which drives the wheels.

To win a race like the Solar Challenge Morocco event, these racing cars must generate as much solar energy as possible and convert the electricity they generate as efficiently as possible into mechanical power delivered to the wheels. At the same time, they must keep energy losses to a minimum. To help in achieving this level of efficiency, the race teams must pay minute attention to the aerodynamic design of the vehicle in order to keep wind resistance to a minimum.

In the case of RED E, the name given to Solar Team Twente’s racing car, the total wind resistance that the team achieved is the equivalent of just that of a conventional car’s wing mirror.

The challenge doesn’t end there though because the driver of the car must then race as fast as possible but not so fast that the car’s battery runs out of power when it is not in bright sunlight.

Electrical Systems

There are four important electrical systems in a solar racing car including the array of solar panels, the battery and its management system, the inverter ( or motor drive), which converts the solar panels’ direct current output to a three-phase alternating current supplied to the motor and the motor itself. The team was aiming for better than 99% efficiency for the electrical power conversion circuits throughout the power-train.

Preparing for the race, the team’s engineers needed to improve the motor, the battery and the energy generation system if it was to move ahead of its rivals on the demanding African course. In this respect, even small improvements can make a significant difference.

“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,” says the team’s Camiel Lemmens. “So, precision measurement is a crucial element of our success.”

The team’s electrical engineers therefore work continually to squeeze ever higher performance from the car’s electrical systems. Achievements have included increasing the amount of energy generated by the PV cells and the amount of energy that the battery can store. The team has even produced a more robust and efficient inverter of its own design, replacing the standard commercial inverters used by competing teams.

Power Analysis

An important part of ensuring the powertrain reached its maximum efficiency was the use of precision power analysis and to this end, the team made use of the previous experience it had had with the Yokogawa WT5000 Precision Power Analyzer. On this occasion, they needed to complete four crucial tasks.

Firstly, they needed to analyze the efficiency of solar PV cells to allow the team to select the cells with the best voltage and current output for a known photonic input. Another task was to measure the energy capacity of battery cells and select those which stored the greatest amount of electrical energy.

They also needed to validate the accuracy of the ‘fuel gauge’, the car’s on-board sensor measuring the battery’s state of charge and displaying it to the driver. 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.

The fourth task was 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 a high sampling frequency.

According to the team’s Rob Krawinkel, they discovered with the WT5000 that the current sensors in the fuel-gauge circuit had an offset, which were making the car’s state-of-charge measurements inherently inaccurate.

“By compensating for the offset, we could confidently provide the driver with a vital extra 1 or 2 km of range from the battery at a given speed,” he says.

The modular and extensible WT5000 is Yokogawa’s most accurate power analyzer, with a specified measurement accuracy of ±0.03%. Measurement bandwidth is 10MHz for voltage and 5MHz for current, and the maximum sampling rate of 10M samples/s exceeds the data refresh rate required to validate RED E’s fuel gauge system. With outstanding isolation, noise immunity and advanced filtering capabilities, WT5000 users can make simultaneous measurements on up to seven inputs and view them on its high-resolution touchscreen.

Krawinkel says that 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,” he continues.

The team’s development of its own custom powertrain and components – a feat which relied heavily on the accuracy and precision of the WT5000 – proved to be decisive in the victory and the team is looking forward to further success in the forthcoming World Solar Challenge of 2023.

Jonathan Newell
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