# Can you run an electric car from the rain that falls on your roof?

As we all know, England is a rainy place, it is not the hills, or lack of cycle lanes, or even the thought of sweaty fatties in Lycra that stops us cycling, it is the rain. As some of you know, I live in Cornwall. On average it rains 151 days a year here.

There is also a mean rainfall of 88mmm a month, with a peak of 124mm in December and a low of 60mm in May.

Just like wind, rain is caused by solar energy. The sun heats up water somewhere, which then evaporates and rises into the sky. Air currents then transport this cooling water vapour until it can hold onto it no longer. Then it rains, on my house, a lot.

So this got me thinking, could I collect most of the water that falls on my tiny hovel and use it to generate enough electricity to power an electric car.

The same caveats apply as my last two posts about this cartoon car, 12,000 miles a year and 220Wh/mile after charging losses.

Thankfully the calculations for hydro power are pretty simple.

PE = MGH

All that is saying is that potential energy, which can be compared to the charge in a battery, is equal to mass in kilograms, multiplied by gravity, which is, as we all remember is 9.81 metres per second squared, multiplied by height in metres. The answer comes out in joules, J, the SI unit for energy.

There are 3.6 million joules in a kWh. At an average 33 miles a day, 7.2 kWh/day is needed to run the car.

Now I think with a bit of jiggery pokery, some serious reinforcement of my loft floor and a large water tank, I could store nearly all the rainfall, six metres above a pico hydro turbine.

Punching in the numbers for my roof area of 32 square metres, that gives me 32 tonnes of water after allowing for some losses to evaporation, leaks and times the wind blows the rain sideways. 32 tones sounds a lot, but when converted to potential energy it is just 1.88 MJ or 0.52 kWh. That is about enough to keep my mobile phone going for 45 days. So the answer is no, I can’t run a car on it.

If I had a more sensible sized house, say one with a 60 metre squared roof, and it was 3 stories high, I have seen one I like and it is close to the park and sea, that would change the numbers to 1.64 kWh, or enough to travel about 7.5 miles, a year.

So how does the country manage to have a successful hydro electric generation industry.

Simple really, very large reservoirs that get run off water from rivers and streams, over thousands of square miles of land. And that reservoir is very high compared to the turbines.

If we take England’s largest hydro electric power station at Kielder, which is somewhere up north, it stores 200 billion litre of water, a litre of water has a mass of 1 kg, 186 metres high. It has two turbines, a relatively small 500 kW Francis turbine that is run constantly and a much larger 5,500 kW Kaplan turbine that runs for a shorter time when demand requires it. Overall Kielder generates around 20,000 MWh/year.

That is enough to run about 7,500 cars for a year.

Keilder has a surface area of about 4.2 miles squared.

So to run 30 million cars in the UK, we would need to flood an area a third the size of England, below 650 feet of water. That is pretty deep, about halfway up Brown Willy.

Don’t seems to bad does it.

Bring it on.

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