Cambridge University Solar Car Uses Vicor

November 22, 2011

Vicor’s MegaMod DC-DC converters provided a reliable, compact and lightweight power solution for Cambridge University Eco Racing’s World Solar Challenge entry car in 2009 – and they’ve done so again for the 2011 event.

The Veolia World Solar Challenge is a 3,000 km race across Australia, traversing the Stuart Highway from Darwin in the north to Adelaide in the south. The challenge, for solar-powered cars, is based on the original notion that a 1,000 W car would complete the journey in 50 hours, with a nominal 5 kWhs of stored energy. All other energy must come from the sun or be recovered from the kinetic energy of the vehicle.  The race is held every two years and attracts considerable global media interest. Cambridge University Eco Racing (CUER) competed for the first time in 2009, finishing fourteenth.

The original car, named Endeavour, was used again in the 2011 race, after making several significant changes to improve the battery and the reliability of the electrical system, in addition to making major aerodynamic improvements to the canopy and wheel fairings. They completed the race successfully, coming in at number 25 out of 37 contestants. Impressive given they were competing against teams with $multi-million budgets, compared with CUER’s £50,000 ($80,000) backing comprised of finance and equipment.

During the race they had to contend with adverse weather conditions: rain, thunder, lightning and cloudy skies were exacerbated by thick haze from a large bush fire. Driving a solar car under such conditions is impossible, so Endeavour had to be trailered onwards or rested, depending on circumstances. The conditions meant that only seven teams completed the entire race under solar power. In spite of all this, Endeavour completed 1,487 solar km out of the 2,998 possible. It also achieved an average speed of 80 kph driving in the midday sun over the final 120 km into Augusta.

Vicor supported CUER’s efforts in 2009 and again in 2011 by contributing MegaMod chassis-mounted DC -DC converters. For the 2011 event, two of these were installed into the car; one inside the battery pack and one external to it. Vicor supplied two more as spares.  Endeavour houses a number of interrelated electronic and electrical systems to handle the solar panel array, drive motor, steering, battery management, onboard computer, telemetry and wireless communications. All the electronics systems require 12 VDC, while the computer needs 5 VDC as well. The MegaMods meet this entire requirement by converting down from the battery’s nominal 130 VDC output.

The performance of the cells within the solar panel array depends on their loading.  Accordingly, the solar array electronics subsystem dynamically adjusts the load across each cell to maintain optimum efficiency. It also converts the cells’ output voltage to a level suitable for charging the battery. The battery management subsystem has a somewhat similar role. The battery array comprises Lithium Ion Phosphate cells in a series/parallel array. These are double the weight of equivalent Lithium Polymer types, but are robust and reliable, and can deliver high power levels instantly on demand. For the battery to operate at peak efficiency, each cell must be maintained at the same output voltage and same state of internal charge. The battery management system ensures this by switching load resistors as appropriate. Another part of the battery management system monitors the voltage level across each cell. If any become excessively high or low, the cell is disconnected by opening a contactor, to avoid a fire risk.

Another issue is regulatory compliance, which demands that the battery pack must be isolated by a contactor when the car is switched off. Therefore the battery management electronics must remain powered at all times to respond to the start switch and energise the battery contactor.

The car is driven by a hub motor mounted inside the single front wheel. The motor is powered directly from the battery. Its speed must be adjusted as prevalent conditions change, for example if the car starts to climb a hill. The motor control electronics look after this as part of a telemetry loop described below. Another electronics subsystem manages the driver’s steering wheel.

All the vehicle electronics systems communicate across a CANbus to a ruggedised onboard computer with a solid-state hard disk drive, which filters the CANbus I/O data and packages it for communication across a wireless telemetry link. This reaches a laptop computer being operated by a passenger in a support car following Endeavour. The laptop processes the telemetry data to calculate Endeavour’s optimum speed for current conditions; the operator then passes this result back to the solar car’s driver.

Vicor MegaMod DC-DC converters performed well in the 2009 car and again in 2011. “Above all, all the converters have been very reliable” commented Alisdair McClymont, CUER’s Project Manager for the event. “Apart from temperature extremes experienced in the Australian Outback, and shock and vibration due to the car’s stiff suspension, the converters have consistently operated without problem in an electrically noisy environment. This is created by a number of SMPS devices in the car’s electronics subsystems.” He continued: “We appreciated other aspects of the converters’ design as well – in particular its wide input voltage range, which accommodated our 92 V requirement in 2009 as well as our 130 – 140 V requirement in 2011. The modules are also lightweight and compact, and can be chassis mounted without onerous heatsinking requirements.”

CUER is already engaged in plans for 2013’s World Solar Challenge. They will be giving particular emphasis to developing a vehicle more relevant to practical everyday use – an ambition which fits in well with the ideals of the event.

You can see a YouTube video showing preparation of the car and its participation in the event here: CUER at World Solar Challenge.

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