Science Interviews

Interview

Thu, 11th Jul 2013

World Solar Challenge: Across Australia

Stephen Pendrigh, Peter Mildon, Cambridge University Eco Racing

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Later this year, the World Solar Challenge will see 47 teams from 26 countries Eco racer carracing across the Australian outback from Darwin to Adelaide in purpose-built solar-powered cars. To complete this 3,000-km long journey, the teams have to design a lightweight vehicle that can use solar panels to travel at 80 kph in one of the world’s most harsh conditions and terrains. Chris Smith spoke to two members from the Cambridge team, Cambridge University Eco Racing, Stephen Pendrigh and Peter Mildon about their entry.

Chris - First of all, tell us a bit about the race, Peter. How did it get started and how did Cambridge University got involved?

Peter - So, the World Solar Challenge has been going on since the ‘80s when it was first setup and teams have been entering cars every 2 years since then. Cambridge first entered back in 2009 for the first time with our car Endeavour. We did okay, we were about 2/3 down the packing order at that point. We then did some work to redevelop Endeavour and sent her back in 2011. There was a small improvement but due to some things like bushfires, the race was slightly disrupted and the finishing position was again, about 2/3rds of the way down. So, for this year, we are building a completely new car. We’ve got a completely new concept and we’re hoping that that’s going to mean that we can fight for the top step of the podium this year round.

Chris - Fighting talk. Stephen, what's the new technology you're introducing to win you poll position?

Stephen - So, when you're designing a new car, the two main important aspects of it are to look at the solar and also the aero. So, you're looking into the power into the car and the losses out of it. What most cars before do is they try to focus on the solar and increase it as much as they can and then try to fit the aerodynamics around that. What we’ve done with ours, is we’ve completely decoupled the two effects and mainly focused on the aerodynamics, and then afterwards fitted the solar cells inside the body of the car inside a clear chassis. Obviously, the actual solar area for this is much smaller, so we’ve then decided to actually get the solar cells and to track the sun from east to west throughout the day.

Chris - How ingenious! Because when you normally see people, they’ve basically turned a car into a mobile solar panel to maximise the collecting area, but there will of course be some bits of panel that are not really in the sun at all, so they're just adding weight and contributing nothing. So, you're saying, “Let’s go for just a tight light small compact solar collecting area, but make it directional.”

Stephen - Yes, definitely. The key point for this is that the efficiencies have gained by about 20% and we’re using a much smaller and therefore, cheaper array than what we would’ve done if we wanted the same efficiency with the same cells outside.

Chris - So, how are you going to track the sun? Have you got the panel at the front of the car, back of the car, where is it mounted and how will it make sure it’s maximum efficiency all the time?

Stephen - We’re mainly driving from Darwin in the north, to Adelaide in the south and so, the sun will be directly to the north and behind us. So, we have the driver sitting in the front of the car and the majority of the solar cells are behind the driver, basically down a long central pole, and these rotate around that axis, basically going round from east to west.

Chris - So, if they reverse the finishing line are you going to have to do the race backwards?

Stephen - Yes, we actually will have a much bigger problem if we were going back up further north. We will be at slight disadvantage there.

Chris - What's the energy consumption of the vehicle when it’s doing 80 kilometres an hour. That’s some speed.

Peter - So, around about our speed which is somewhere between 80 and 90 kilometres an hour, we’ll be draining about 1 kilowatt. It depends on the time of the day as to how much power is coming in from your array and how much is going to be charging up or draining the battery that we have. So, we do have the battery there to sort of stabilise the total power consumption. So that means, in terms of driving at our race target speed, we’re looking at about a kilowatt whereas in the middle of the day, we’ll be getting more power in and obviously in the morning and the evening, it will be slightly less.

Chris - So, you mentioned that there's a battery there. How sensitive is it to – if there is a cloud day because I mean in Australia, you should be pretty okay, it should be pretty sunny all the time. But if it does sort of get cloudy or dark, how much will that incumber you?

Peter - We actually have a few people looking into the race strategy and we get weather data in that we can work out what the optimum solution is going to be in that situation. So, our battery has got about 5 kilowatt hours’ worth of energy in it. That will allow us to drive at our race speed for 5 hours or slower for a prolonged period of time. If we know that we’re going to be in a cloud for the next entire day, we’ll probably drive slower whereas if we know that there's sun coming at the end of the day or there's sun further ahead on the road, we can actually afford to speed up a bit, use up some of our 5 kilowatt hours’ worth of energy and then get to the sun quicker.

Chris - Stephen, what is the car made of?

Stephen - The majority of the chassis is a carbon fibre monocoque chassis which basically means that the membrane just on the outside can take all the structural loads, and it also ensures that the car is as lightweight as possible.

Chris - How much does it weigh?

Stephen - It’s about 120 kg without the driver and then we say, about 200 kg with the driver.

Chris - Do you have to be really small to drive this car like the Cambridge Cox in the boat race? You select for small people.

Stephen - I mean, I'm 5 foot 10 and I can actually fit in the car, not for the race itself. I wouldn’t be legal for the race. However, I have driven the car myself.

Chris - It’s going to get pretty hot in there, isn’t it?

Stephen - It does get very, very hot, yes.

Chris - No aircon on there, so what do you do about that?

Stephen - The heat is very bad for both the driver and also for the solar cells. We have some active cooling so we have fans going through to try to cool both of them down.

Chris - I wouldn’t like to be in that car. What about though Peter, the sort of wider implications of this?

Peter - So, I think the key thing also that our powered cars are looking at is again inefficiency. So, if we look at a typical road car, it’ll run at about 40 kilowatts. We’re driving at comparable speeds on 1 kilowatt, so we’re more than an order of magnitude more efficient than a normal car. To put that into some perspective, our battery is quite small. It’s limited by the rules. We could easily have a battery 4 or 5 times the size of that, but if we were to drive at around 30 miles an hour, we could go about 800 km with no solar panels on the car whatsoever. And basically therefore, beat most petrol driven cars in terms of range. So, the key thing here is where we’re getting the efficiency savings from. If you could transfer them into a normal car, you would see, even if it was still a petrol car, a significant reduction in emissions.

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Interesting.  I had wondered about solar tracking in a vehicle. 

The multi-junction cells actually are supposed to work better with concentrated sunlight.  I wonder if there would be benefits of adding Fresnel lenses to the array, which might also allow making a smaller, and thus cheaper array.

I like the idea of long-distance solar vehicles, but I do think the largest benefit will be local hops, driving the car to work in the morning, letting it sit in the sun all day, then driving back home in the evening. CliffordK, Mon, 15th Jul 2013


Will there come a time when drivers will pay a premium for parking bays that remain unshaded all day long, eh!*

NB: not in the UK, I suspect. peppercorn, Mon, 15th Jul 2013

I find that the top level of car parks are often emptier than lower levels.
Solar vehicles may reverse that trend. evan_au, Tue, 16th Jul 2013

Of course, the other option is to have fixed solar panels, and plug the vehicle in, and thus much less emphasis on carrying the panels around everywhere.  Then, in the car-parks, one can shade the vehicles with the solar arrays.  Not that we have an excess of renewable electricity, so one can always just feed the grid.

Anyway, I like the Cambridge concept of a smaller solar vehicle with solar tracking high efficiency cells.  Just don't toss a bag of groceries onto the $200K solar array. CliffordK, Tue, 16th Jul 2013

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