Renewable Revolution

France has approved legislation that will require all car parks with more than 80 spaces to be covered over by solar panels. This is part of a wider program that will see solar panels occupy derelict lots, vacant land alongside roads and railways, as well as some farmland. This is expected to add 11 gigawatts to the French electricity grid equal to ten nuclear reactors. Do the numbers add up? And should other countries do the same?

Several countries, most notably Germany, have already mandated developers of new buildings to incorporate renewables into their designs, like roof-mounted solar panels, biomass boilers, heat pumps, and wind turbines. The French policy would apply to new and existing car parks.

The average car parking space is about 4.8m by 2.4m, or 11.52m sq. Assuming an output of 120 watts per m sq that works out at roughly 1.4 kilowatts of power per bay. There would be further space over walkways and traffic lanes within the car park, but the solar panels would need to be kept far enough apart to stop them shading each other.

For an output of 11 gigawatts, you’d need to cover about 7.7 million car parking spaces. Are there that many in France that would qualify? The UK has between 3 and 4 million spaces and 40 million vehicles. France has a similar sized fleet of 38 million. So, 7.7 million spaces seems unlikely.

But the legislation covers a lot of urban land, not just car parks. In theory, 92km sq of French urban land (defined as any built-up area with more then 5,000 people) could provide 11 gigawatts of solar power.

That might sound like a lot, but it’s only 0.106 per cent of France’s total urban land area of 86,500 km sq. Accounting for the difference in capacity factors (how much energy each source generates a year compared with its maximum theoretical output) between French nuclear (70 per cent) and French solar (15 per cent), 430 km sq of solar would supply the same amount of power each year in gigawatt-hours as those ten nuclear plants.

These panels need only cover 0.5 per cent of French urban land, or about 0.07 per cent of France’s total area. So it’s possible, though car parks will make up a tiny portion of the overall program.

The UK and countries further north receive less sunlight per m sq and the sun sits lower on their horizon, which makes the issue of shading on panels bigger, although the longer days in summer do compensate for this to some extent.

Also, while a lot of car parks in southern Europe already have sun shades over them (which allow solar panels to be mounted onto existing structures), this is rare in cooler countries. As a result, it would probably be a lot easier to mount panels on the roofs of buildings than over the surrounding car park in some countries. Where solar panels aren’t practical, other options, like wind turbines, might well be viable alternatives.

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