Can you run an electric car on solar power?

23 August 2011

solar panels on a roof

Ever wondered if you can run an electric car solely on solar power? Well you can, of course, even in the not-so-sunny UK. The caveat is that you can’t run a car using solar panels that will fit on the car roof, sadly – or at least not the kind of car that can carry a couple of ordinary people and their luggage. Much like running a plug-in car from mains electricity, solar motoring is a case of feeding power into a conventional battery during long recharging sessions.

Let’s take the Nissan Leaf as our example electric car. Its battery holds 24kWh and takes roughly eight hours sucking from a socket to recharge. Evidently it will be drawing close to the full capacity of an ordinary UK plug, which can supply 240 volts at 13 amps which is equivalent to about 3.1 kilowatts. At full power, a UK plug takes about seven hours and 45 minutes simply to churn out 24kWh. It will take slightly longer to stuff equivalent stored charged into a battery, given that charging is only about 90 per cent efficient – about eight and a half hours at the nominal 13 amps.

To supply that single ordinary domestic socket with a constant 240V and 13A of pure solar power, you will need to install a solar array on your roof capable of delivering the necessary 3.1kW during the UK’s fleeting hours of daylight.

A photovoltaic (PV) solar panel capable of peaking at around 3.1kW will occupy about 28 square metres of south-facing roof space – or seven metres by four metres. Quite a large proportion of roof for most properties.

Of course it’s not always sunny, so you will only rarely see the panel hitting its peak output. An array of the above size will generate about 2,500kWh per year in the UK. Which is, unfortunately, only about 6.8kWh a day – enough to take the Leaf perhaps 28 miles each day. Which is, coincidentally, about 10,000 miles per year. So it looks like that 28 square metre solar array might be about right for powering a Leaf for most people, even if you might have a surplus in the summer and have to rely on the national grid in the winter.

The drawback? Well, it will obviously cost money to install the panels on your roof – probably around £15,000. The sweetener? Any PV installation will eventually pay for itself in saved energy costs as well as feed-in tariff payments, from supplying the grid during times when the panels generate more power than your household is using.

John Prescott tries out a Honda solar car

As an aside, it’s interesting to contemplate how big an array would be needed to supply energy to the Leaf in real time, without the buffer of a battery. The motor is rated at 80kW, and a panel that can peak at 80kW would cover 700 square metres – getting on for the size of three tennis courts. To work in overcast Britain, you’d better double that. So with the best will in the world, it’s not going to fit on the car itself.

And remember you can’t drive at night. And you’ll probably need a mortgage.

What about simply covering a Leaf in panels – would it actually move? At 4.4m by 1.8m, the Leaf has a footprint of roughly 7.9 square metres, enough for a panel that would peak at about 0.9kW (assuming the imaginary panel is on some sort of gimbals that can keep it angled towards the sun as you drive). About a kilowatt is enough to overcome the friction between tyres and tarmac and the losses in the powertrain, but would leave precious little beyond that. Pump up the tyres to reduce your contact patch and on a really sunny day you might hit 10mph on the flat. Forget hills, unless you’re going down them.

On balance, perhaps mounting the panels on the house is a better idea.

2 comments:

Brucedixon said...

I like your post.It is very informative.Using solar power for electric vehicles not only reduces loads on the grid, but also provides for a cleaner drive. This is one ways to get out from global warming. I love this.

power-adapter said...

Hello, I appreciate your writing on this green technology. I'm working the other way around, but the basic plan is the same: PV + EV. We're in the process of wiring our new 6.81 KW array. Will be leasing an EV in the not too distant future {I do not qualify for the full $7500 tax credit, but the lease company will, so I plan to buy it after I finish the lease.} It makes my family far more secure in the long run than putting the money in a 401k or IRA where scams abound. thanks~ Bob

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