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On the 21st October participants took off for the 20th edition of the famous Panasonic World Solar Challenge.

This race for solar-powered vehicles across Australia started in the city of Darwin and covered a staggering

3010 km across the Australian desert towards the finish in Adelaide. The objective of the World Solar Challenge

is to stimulate the research in the use of solar energy. The official website of this event is

http://www.wsc.org.au/

Picture 1: Solar Team Twente with scale model

Solar Challenge 2007 –

3010 km powered by the sun

Application note ►automobile- & motor vehicle industry ►mobile

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Solar Team Twente

The Solar Team Twente, a team of students of the Saxion Poly Technical College and of the University of

Twente (Enschede, The Netherlands) took part in this event. Together they constructed the 'Twente One' as

they called their solar vehicle. With new regulations for the 2007 event, they went for a completely new and

innovative design. Not only mechanically, but also electrically and electronically the Solar Team Twente uses

the newest technology.

Picture 2: ‘Twente One’, the solar car of the Solar team Twente

Catching the sun

In the WSC 2007 the active area of solar cells is limited to 6 m2. These photovoltaic cells come in different

qualities. And, as with many other things, the best quality is the most expensive. Since the budget of the team

was limited they decided to use a mix of both Silicon cells and Gallium Arsenide (GaAs) cells. The latter, which

are the most efficient cells, have an efficiency of around 30%. To use these cells to their full extend they added

an innovative idea: concentrators.

Concentrators

Linear Fresnel lenses are being used on two sides of the 'solar wing'. The goal is to position the focal line exactly

on the high efficient cells underneath. One of the difficulties here is the shifting of the focal line when the angle

of the incident radiation changes. To overcome this, the array of (GaAs) solar cells was mounted on a

translating surface. The whole surface can move slightly sideways by means of some small electro motors.

Picture 3: top-view of solar panel with concentrators on each side

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Picture 4: Fresnel lenses concentrating sun light onto the cells

Picture 5: Translating Solar Panel

Tilting panel

Understandably, when driving from north to south, the angle at which the solar rays reach the solar panel

change during the day. This results in a substantial lower efficiency in the morning and in the evening. Can we

just follow the sun and stay perpendicular to the rays, they must have thought. Well, that's exactly how

they overcame this problem. The tilting solar panel was born. Although the tilting panel solved the problem of

the changing angle of incedent radiation, it also introduced many other mechanical and aerodynamic problems.

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Picture 6: Solar car with tilting 'solar wing'

Controlling a solar car

The 'Twente One' is a high-tech vehicle, not only mechanically, but certainly electronically. The basic idea is

straight forward: Solar panel output is in parallel with the batteries and the motor controller. But, since in a

solar car one is constantly looking for an optimum, many parts are actively controlled.

Picture 7: Electrical schematic of the solar car

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Right from the early design stage of the car, the electrical students thought of CAN-bus as the main bus type for

linking the various electrical components. In the 'Twente One' it was decided for two separate CAN busses, one

for the critical processes (CAN_C) and the other (CAN) for more common tasks. Even the pedal box speeks

‘CAN’.

Solar cells optimum

The maximum energy you can get from photovoltaic cells depends both on the generated voltage and the cur-

rent drawn. Too high a current will make the voltage drop and hence the delivered power. A high voltage with

little current is also not very efficient. Here we need the help of some electronics. The solar cells are connected

to a total of six Maximum Power Point Trackers (MPPT). These controllers adjust voltage and current a few

time per second to make the solar cells operated at their maximum. The output voltage of the MPPTs is at 150

VDC. With the help of these MPPTs solar cells can also be switched off in case of an unsafe situation. All six

MPPTs have a CAN-bus interface to communicate with the vehicle. The power from the solar panel on the

‘Twente One’ depends of course on the amount of sunlight, but an output power of 1800 Watts is very well

achievable.

Picture 8: Maximum Power Point Trackers control the solar panel’s output

The output of solar cells is affected by their temperature. The higher the temperature the smaller the delivered

energy. In the testing phase the cell temperature was closely monitored with 32 thermocouples in the 'solar

wing'. These thermocouples are measured with two imc CANSAS modules that convert the thermocouple volt-

ages into themperature values on the CAN bus. The decreasing performance of the cells at higher temperature

was compared with possible aerodynamic measures taken. Making a hole to cool the cells has its effects on the

aerodynamic behavior.

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Battery

Since the incedent radiation and the energy consumption of the vehicle are not constant, batteries are used.

This holds especially for regenerative braking, all this energy is stored in the batteries. The total capacity of the

Lithium Polymer batteries is 5 kWh. The batteries are positioned in stiff boxes made of sandwich material with

a controlled airflow. It is important that these batteries do not get overheated. Therefore a sophisticated elec-

tronic Battery Management System (BMS) monitors the charging condition of the cells and takes action in case

of error. Via a shunt the BMS keeps track of the amount of energy going in and out of the batteries. This is

important to predict the amount of energy left in the battery (state of charge).

Motor

The 'Twente One' is equipped with a 6 kW CSIRO electromotor with permanent magnets. Such a motor gives an

almost constant torque over the complete rpm range. To make this three phase motor run it is connected to a

motor controller that converts the 150 VDC from the cells or battery to an electrical wave shape. Like all the

other electronic units in the vehicle, the motor controller is connected to a CAN-bus.

Monitoring and control

The hart of the electronic system (see picture: 9) is the imc CS7008 Intelligent Data Acquisition System. This

unit serves more than one purpose:

• Communication with the trailing car via WLAN

• Link to the GPS antenna, gathering GPS data

• •Controller for two CAN-buses,

• Acquisition unit with internal storage

• Connection to various analog sensors

• Controller for dashboard display

Picture 10: imc CS7008, Data Acquisition System

With all these functions it is possible to monitor all measurement data pre-processed, real-time, in the car

following the solar vehicle. When cruising along, around 200 data channels are continuously transmitted from

the 'Twente One'. The data is used for monitoring and for determining the strategy (together with the weather

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forecast and route). Based on the strategy the set point for the cruise control can be set from the car behind

the solar car. Likewise the tilting of the 'solar wing' and the translation of the cell arrays underneath the

concentrators can be controlled remotely.

Picture 11: imc CS7008, Data Acquisition System with CANSAS modules

Picture12: First test with Acquisition Computer

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Analog signals

Besides all the digital information gathered by the imc system, also analog signals are measured. This also

played an important role in the design phase of the vehicle, when strain gages were used to measure the me-

chanical load on the wheel suspension. During the race the following sensor were connected:

• 3 x Potentiometric Displacement Sensor – wheel suspension movement

• 3 x infra red thermocouple sensor – tire temperature measurement

• Strain gage bridge on rear wing mounting joint

With the infra red thermocouples directed towards the surface of the tires, it is easy to detect any problems.

Even a little puncture will show an increasing temperature of the tire.

Software

In the early stages of development the team used the standard imc Devices software. Quickly they discovered

the ease of use and flexibility of the on-line signal processing (On-line Famos). All measurement data was eval-

uated using imc’s analysis software FAMOS.

For use during the race a dedicated program was developed based on the imc COM interface. This program

facilitated an easy link to a database. It was important to have this data available for the strategic people.

Race results

Coping with high tech design challenges is one thing, finishing in the World Solar Challenge is something

completely different. During their journey the Solar Team Twente faced many problems, most of them

mechanical. The rough road conditions causing a broken front wheel suspension and other problems, didn’t

stop the team from finishing. After many, many hours of hard work, they proudly finished on the sixth place.

Picture 13: Finish in Adelaide

For information on the Solar Team Twente see:

www.solarteam.nl

For information on the imc equipment please see:

www.imc-berlin.de

Your contact in the Benelux:

Daqpoint Benelux B.V.

www.daqpoint.nl