A full decarbonisation in Britain or Germany, using 100% renewable energy, would use up more than the entire land mass of those two  countries, says Vaclav Smil, a well regarded energy professor based in Canada, in his new book, “Power Density: A Key to Understanding Energy Sources and Uses”.

A low-carbon energy future does not have to be 100% renewable energy; it could have nuclear too, and who knows what else. But it is important to think through the consequences of each scenario, which Professor Smil does in his new book (which I recommend for summer reading).

He concludes his analysis of power density by saying that a large-scale conversion to renewable energy would require “a profound spatial restructuring of the existing energy system”.

Power density is the amount of power made available by a particular energy technology or fuel, per unit of the Earth’s surface area. Power density can be very high for fossil fuel power plants, coal mines and oil fields, which can yield lots of energy from just a couple of hectares of land at the surface, but lower for wood fuel from forests, or for that matter for electricity generated by wind and solar farms.

For sure, a full decarbonisation of the global economy would take many decades, given the longevity of energy infrastructure, but Professor Smil’s finding that it would also face a fundamental land shortage seems worth putting to the test.

Britain seems to be a good test case, given it has a greater population density and energy use than many nations.

Professor Smil estimates the UK power density of solar farms at 5 watts per square metre (accounting for their very low capacity factor), and uses an estimate for U.S. wind power density of 50 watts per square metre. For the area of wind farms, he only includes the area of the turbine pad and access roads, not the area between the turbines.

In a 100% renewable energy future, Britain would also need battery and pumped hydropower storage, and more interconnectors to the rest of Europe. A source of flexible baseload would be important, such as waste-to-energy, tidal power or biomass, but I will ignore all that, to focus on the two main sources of electricity.

Total primary UK energy consumption in 2014 was 193 Mtoe (million tonnes of oil equivalent), including electricity, heat and transport. That is equivalent to 2,244,590 gigawatt hours, or about 260 gigawatts of electricity generating capacity, running 24-7 through the year.

Applying Professor Smil’s power density estimates, and dividing this generating capacity equally between wind and solar, I calculate that they would use up nearly 2.9 million hectares. That compares with a UK land area of 24.1 million hectares, or 12 percent of the total. I am not suggesting this is a definitive answer, more a wild guesstimate.

The point, though, is that there is enough land in Britain. Professor Smil made his conclusion, that renewable energy would use up all available land, by assuming that a low-carbon transition would substitute wind and solar for fossil fuel electricity – as I have done – but that biofuels would replace gasoline, and wood would substitute for gas heating.

I assumed an electrification of the entire energy system.

Importantly for the calculation of land use, biofuels and biomass have very small power densities, an order of magnitude lower than wind and solar power. In other words, they need much more land.

In this way, Professor Smil’s book demonstrates why the world would eventually have to shift almost entirely to electricity, rather than biofuels or biomass, under a high renewable energy scenario.

Renewable energy supporters may forget the implications of that electrification, focusing instead on a low-carbon transition in the electricity sector. In fact, electricity accounts for less than 20 percent of Britain’s final energy consumption (see chart below). By far the biggest energy consumption is of oil, for transport, followed by gas (for heating and industry) and solid fuel (coal and biomass, for industry and heating). Replacing 80 percent of Britain’s energy consumption with electricity, of any kind, whether renewable or fossil fuels, would be an enormous challenge.

Total final UK energy consumption by fuel type

Professor Smil points out the problem of achieving electrification of air transport and heavy trucks, or electric smelting of pig iron (which at present is based on coal burning, to remove impurities).

His book is useful in highlighting the issue of power density, which is very relevant even in a fully electrical future without biofuels and biomass. Even if renewable electricity only consumed 5% of the land area of Britain, and not more than 10% as I calculated, that is still vastly more than the present oil, gas and coal infrastructure. Such large impacts show why it’s important to think through the consequences of a possible transition to 100% renewables, even if it would take decades to achieve.

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