Ilanga II: Under the hood of a sun chaser

September 27th, 2014, Published in Articles: EE Publishers, Articles: Vector, Featured: EE Publishers

 

The engineering challenge of designing and building a solar-powered racing car for a strenuous eight-day international endurance race brings together research, theory and development while promoting alternative energy and technological innovation.

The Ilanga II is a prototype solar racing car developed by the University of Johannesburg (UJ) Solar Team to compete in this year’s biennial Sasol Solar Challenge.

The 25-member UJ Solar Team developed the Ilanga II in collaboration with 30 industry partners. It will be one of 13 national and international teams competing in the challenge. The team’s original project in 2008 was born out of the engineering school’s curriculum requirements for practical work, making the Ilanga II the team’s third-generation solar car.

The race

The race, which starts in Pretoria and finishes in Cape Town eight days and 2000 km away, is more than a test of speed; it considers factors such as distance travelled, the amount of solar energy generated and energy efficiency and usage. The race is on between 08:00 and 16:00 each day and contestants may complete extra “loops” to increase mileage travelled. Team Ilanga II hopes to win the Olympia class (for distance travelled) and aims to cover 5000 km in the race.

The Sasol Solar Challenge is regulated by the Federation Internationale de l’Automobile (FIA) and, as the race is held on national roads, standard road and speed regulations apply. The teams must also comply with strict safety regulations set out by the FIA.

The South African track is not flat and is particularly challenging compared to the Australian track where the World Solar Challenge takes place. Road conditions vary drastically over the 2000 km trip and there are potholes to contend with. Sufficient sunlight is not guaranteed and the coastal regions’ weather is notoriously unpredictable. Taking on these challenges armed with a battery pack limited to only 30 kg calls for strategy, planning and judicious energy spend.

(Read the Sasol Solar Challenge 2014 highlights here.)

The UJ team says its competitive edge is the car’s weight (170 kg excluding the 65 kg driver) and its aerodynamics, made possible by highly-efficient photovoltaic (PV) cells developed for space missions and which are more compact than traditional silicon cells.

Electrical systems

The high-power electronics are designed for solar car racing by various manufacturers while most of the low-power electronics were designed in-house. The brushless DC hub motor is powered by solar energy from eight panel arrays, which also charge the backup battery pack.

Fig. 1 (a): Fitting the solar panel arrays – a time consuming and precise process. Marked out and with special 3M tape prepared, the panels’ connection points have to be lined up with predefined points.

Fig. 1 (a): Fitting the solar panel arrays – a time consuming and precise process. Marked out and with special 3M tape prepared, the panels’ connection points have to be lined up with predefined points.

 

Solar arrays

Gochermann triple junction Gallium Arsenide (GaAs) solar cells imported from Germany halve the space required by capacitive silicon panels and are 35% more efficient. Comprising over 400 business card-sized cells, the arrays cover 3 m2 of Ilanga II’s surface (Fig. 1).

PV panels are, ironically, vulnerable to intense heat and some have caught fire in previous solar races. The Ilanga II‘s panels will be cooled with distilled water at every stop, even though Gallium Arsenide cells are more heat resistant than capacitive silicon panels.

Fig. 2: The brushless DC hub motor fitted onto a custom lightweight tire, before it was mounted to chassis.

Fig. 2: The brushless DC hub motor fitted onto a custom lightweight wheel, before it was mounted to chassis.

The vehicle is powered from the right-hand rear wheel by a single axial flux brushless DC hub motor designed for solar racing and built to order (Fig. 2). It produces 10 kW maximum and 1,8 kW continuously, and has an efficiency of 98%. As the car has no gearbox (the motor is built into the wheel), the motorised wheel is angled outwards at 5° to compensate for side steering.

The vehicle runs primarily off the solar panels while batteries serve mostly as a backup for inclement weather. The panels generate 1000 W/m2 in battery power or, theoretically, 1080 W in full sunshine at sea level. The UJ team works on estimates of between 800 and 900 W per day.

The highly sensitive solar cells are encapsulated in an array of 66 cells. The layout of the solar arrays was custom-designed to the car’s shape for maximum power generation.

Local sunshine provides some 2 kWh/m2 or 8 kW per day. The batteries receive a full charge each morning before the race, but no charging is permitted during the race, other than from the PV panels.

The solar power goes through an MPPT converter which boosts the voltage to 160 V and controls the charge. This power is then available to drive the motor while the excess power charges the battery pack.

Batteries

The teams have a choice between standard batteries and it is up to them to optimise these batteries. Ilanga II’s battery pack consists of 38 blocks with 17 lithium-ion cells each (Fig. 3 (a)). UJ chose batteries with high energy density as these weigh less. They decided on Panasonic’s 245 Wh/kg (watt-hour per kg) cells. Together, the 17 cells in each unit produce a 2,9 Ah rate with an alternate voltage of 3,6 Ah on lithuim cells. The team charges the cells to 4,2 Ah, a safe overcharge. This, however, decreases the battery’s lifetime.

Fig. 3: Battery pack arrangement and wiring, showing how four blocks are grouped.

Fig. 3: Battery pack arrangement and wiring, showing how four blocks are grouped.

The battery pack is connected to the motor in parallel so that the batteries supply the required energy deficit, should the solar panels not produce enough energy. With these high energy-density cells, which charge at 5 kW/h, the team can cover 300 – 400 km without any sunlight.

Each of the cells was balanced individually before being placed in the pack as not all the cells are loaded equally. Balancing the batteries is done by placing all the cells in a unit in series beforehand. While most teams use nickel as the conductor on their batteries, the UJ team uses copper, a better conductor. The copper conductor points were machined to fit the cells and then spot-welded onto the cells.

These battery blocks are arranged into groups of four and are connected in series, with the groups in turn be connected to one another in parallel (Fig. 3 (b)). Connecting the blocks in groups with series provides a higher voltage, and by connecting the groups in parallel builds capacity. This affects both the battery charge and the current for speed and acceleration of the car.

A battery management system built into the battery pack monitors the battery thresholds of all the cells, and balances the individual cells.

The battery pack is cooled by means of small fans and by exploiting the airflow from the left-hand front wheel as inlet. This keeps the batteries at its 40°C optimal temperature, and the hot air exits at the right-hand rear wheel.

Mechanical systems

Contrary to convention, Ilanga II’s design started with its aerodynamics (which took 13 months to perfect), since wind drag is the biggest cause of energy loss. Complex simulations such as of trucks passing the vehicle took nine days to reproduce virtually on four computer system clusters.

The same software used in Formula 1 design, PTC Creo Parametric 3D CAD, was used in this car’s design (Fig. 4). In the end, it took 40 designs to produce the 3,5 x 1,6 x 1,2 m Ilanga II with its 130 km/h maximum speed and average target speed of 75 km/h when powered by the batteries only. Technological advancements in terms of efficiency and new materials also allowed a drastically different design from previous models.

The car’s shape takes advantage of the smaller top area to reduce drag. The closed fairing design of all four wheels is based on an observation at the previous World Solar Challenge that all the cars which flipped had open back fairings. Closed fairings also provide larger surface areas and skin friction and so reduce the overall drag due to decreased turbulence at the car’s rear.

The driver and the motor are situated in the rear, right-hand side of the vehicle, next to the battery pack on the left. The decision to move the driver away from the centre of the car was dictated by aerodynamics, and it reduced drag by 40%.

Ilanga II - Fig 4

Fig. 4: A CAD drawing of the Ilanga II solar race car. [Credit: UJ Solar Team]

The mechanical systems are custom designed using lightweight composite materials. One example is the specially-designed suspension which compensates for the car’s odd balance. The suspension system is made from carbon fibre and chromoly, a chromium and molybdenum alloy, and are lighter than the aluminium most other teams are likely to use (Fig. 5). The car’s gravity point is also lower because the vehicle is so low to the ground, leaving more flexibility in the balancing process.

Infusion moulding and vacuum bagging was used to compact and smooth the fibreglass moulds, and to rid them of excess epoxy, which adds weight to the car.

Fig. 5: The right rear suspension with the hub motor fitted to it as seen from above.

Fig. 5: The right rear suspension with the hub motor fitted to it as seen from above.

The steering system is a lightweight rack and pinion system and, apart from mineral oil on some of the joints, only sealed ceramic bearings are used.

The body is constructed from an Airex foam core sandwiched between carbon fibre sheets and has a good strength-to-weight ratio. The cockpit is isolated from the rest of the car in accordance with safety regulations, and the solar panels are designed to fracture or deflect away from the driver in case of an accident.

During previous races, vibration caused cables to snap, so wiring layout is one of the most important improvements on the Ilanga II. Using a single 4-core main cable throughout the car reduced the amount of wiring.

Weight was key to the design and every item in the assembly was considered carefully before being added. The holders in the battery packs, for example, are 3D-printed in light, durable plastic. Each holder (roughly 3 x 8 x 15 mm) weighs only 20 g as opposed to the 80 g holders used in previous designs.

Communication and control

The ergonomic, 3D-printed steering wheel is the main control interface in the car. With aluminium plate rib enforcement, it not only steers the car but also provides an interface for most of the electronics, including the accelerator, brakes, lights and radio communication (Fig. 6).

The entire system is connected with CAN Bus infrastructure and the speed is controlled by an electronic throttle controller over the CAN Bus (Fig. 7). Speed is regulated and measured electronically by means of pre-programmed algorithms.

Live data and radio transmissions are submitted over the telemetry system to a support vehicle following 3 m behind the solar car. Telemetry systems traditionally require a lot of power, and a 30 W telemetry system can halve the life of a 5 kWh battery. The UJ team, however, developed a telemetry system to function at below 10 W.

They did this by dividing the data flow into fractions of seconds so that each messages is sent in 0,02 s while the system “sleeps” the rest of the time. There makes no noticeable difference to the human ear or eye though, and the process saves energy.

Fig. 7: The Ilanga II’s steering wheel.

Fig. 6: The Ilanga II’s steering wheel.

Data communication takes place over a 2,4 GHz serial bridge connection, through an interface programmed by the strategist in C#. Not only is it based on open and common standards but, unlike Wi-Fi, it doesn’t have to re-establish broken connections.

Strategy

Once the car is technically ready, winning the race depends on strategy. The race strategy includes calculating the driving speed based on battery life and energy spend, while compensating for factors such as weather conditions and route (inclinations).

During the race, the strategist travels in a support vehicle following the solar car. The strategist and core engineering team capture and analyse live data feeds streamed from the car. They analyse the data as it comes in, factor in road and weather conditions and calculate energy spend. A strategy is based on this data and communicated to the driver. The race is not only about speed, but about energy management too.

Fig. 7: The motor controller (a 3-phase inverter), into which the solar power and battery packs plugs, and which controls the speed of the car over a CAN bus.

Fig. 7: The motor controller (a 3-phase inverter), into which the solar power and battery packs plugs, and which controls the speed of the car over a CAN bus.

Being so lightweight, the Ilanga II does not have a lot of momentum for negotiating the long inclines, for example, but it has the advantages in terms of acceleration and speed. The time of day will also determine the degree at which the panels are angled for optimum light exposure.

Cloudy weather, especially in the coastal regions, has its own advantage in the form of “sunspots”. These are intensified patches of light as the sun breaks through the cloud cover, producing immense power amplified from being reflected internally in the clouds.

If all goes according to plan, Ilanga II will challenge the previous Sasol Solar Challenge champions, the Netherlands team. The plan is also to compete in the 2015 World Solar Challenge. The current world record is 88 km/h running purely on solar.

Conclusion

The race, which started this morning, is the culmination of two years’ full-time research and development. The project has seen at least one provisional patent filed – thermo-sensitive plastic dye in the 3D-printed plastic battery holders, allowing the crew to tell the temperature of the battery pack at one glance.

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