CAMBRIDGE, England — A few centuries ago, the developing world got most of its energy from windmills, from water mills, from whales and from plants. Plants contributed both wood for making heat and biofuels to power the labor of humans and other animals.
Nowadays, the “developed” world gets most of its energy from fossil fuels — coal, oil and natural gas — along with nuclear power and hydroelectric dams.
But this era of easily accessible fossil fuels is likely to be but a brief blip in the history of humanity. The peaks of oil and gas production are expected to be reached within the next 50 years, and coal production is likely to peak about the end of this century, if business continues as usual.
Moreover, scientists advise us that we would be wise to leave a lot of the fossils in the ground, to reduce the risk of climate change.
So where will the world get its energy from next — when, inevitably, humans stop using fossil fuels?
Is it realistic to imagine a comfortable return to renewable energy sources? Whales are not a realistic source of oil anymore, but could windmills, water mills, energy crops and all the other renewable options provide enough energy for today’s vastly increased population to maintain today’s lifestyle?
Discussion of those questions is astonishingly polarized. On the one side, advocates point out that the earth receives more energy from the sun in just one hour than the world’s population uses in a whole year or argue that wind power could theoretically supply all the world’s electricity.
The other side of the cacophony responds with a litany of its own: Renewables are pathetic, intermittent and expensive. They are not good but bad for the environment. And ugly.
Neither is very illuminating. To have constructive conversations about the world’s energy options, one needs to take a calm look at the numbers.
How much energy does our chosen lifestyle use? How much land area do we have? And how much could we produce, from each source, and at what cost?
The average energy consumption on earth is 56 kilowatt-hours per day per person. You can visualize this personal energy consumption in terms of light bulbs: 56 kilowatt-hours per day is the energy consumption of 56 ordinary 40-watt bulbs left switched on all the time. The world’s population density is roughly 50 people per square kilometer, or 0.4 square mile. Countries, of course, vary significantly around the world average.
When considering the potential of renewables to power a country, it’s critically important to know that country’s power consumption per unit area. The graphic accompanying this article plots where a variety of countries fall on the scale.
The countries with the highest power consumption per area are those, like Bahrain, that have high population density and high per-capita consumption. Bahrain consumes more than 10 watts per square meter, or 11 square feet. The lowest are countries like Botswana and Sudan, at less than 0.01 watt per square meter.
Then there are those that, while widely varying in their population densities and energy consumption, fall in the middle. Saudi Arabia, South Africa and Mexico all have a power consumption per unit land-area roughly equal to the world average, about 0.1 watt per square meter. China, in 1990, was close to the average, but it has since moved considerably away from that line.
The United States consumes power per land-area at a rate three times the average. Even though they are more energy efficient, densely populated industrial countries like Germany, Britain and Japan have even bigger power consumption per area.
Why is it useful to know how much power is consumed by land-area?
Well, almost all renewables are harvested on land, and it is possible to quantify the potential power production from renewables in exactly the same units as consumption: watts per square meter.
Concentrating solar power stations in deserts, for example, can produce 15 or 20 watts per square meter, on average, year-round, day and night. Germany’s famous solar parks in Bavaria produce about 5 watts per square meter of land area, on average. A hydroelectric facility in Scotland has power per surface area of 11 watts per square meter of lake.
Wind farms, if they are in windy locations, produce roughly 2.5 watts per square meter of land or sea, on average. The best energy crops in Europe deliver about 0.5 watt per square meter.
So, can the world “easily” live on renewables, setting aside environmental, economic, and social constraints?
Just taking physical factors into account, if a country’s energy consumption per unit of land is the same as the world average, 0.1 watt per square meter, then the power densities of the renewables just listed — 0.5, 2.5, 5, or 20 watts per square meter — are all bigger.
That means such countries could match today’s power consumption if they covered, for example, 20 percent of their land with energy crops; or 4 percent of their land with wind farms; or 2 percent of their land with solar parks in the traditional Bavarian style; or 0.5 percent of their land with desert solar power stations (assuming they have desert).
For average countries, therefore, it is technically possible to live on renewables, and solar power in deserts, solar parks and wind farms are all feasible solutions.
But not all countries are average. And even average countries tend to have growing power consumption.
Countries whose power consumption per unit area is bigger than 0.1 watt per square meter, like those where most people in the developed world live, are countries that should expect renewable facilities to occupy a significant, intrusive fraction of their land, if they ever want to live on their own renewables.
Countries with power consumption per unit area of more than 1 watt per square meter, like Britain, Germany, Japan, the Netherlands, Belgium and South Korea, would have to industrialize much of their countryside to live on their own renewables.
Alternatively, their options are to radically reduce consumption, use nuclear power and buy additional renewable power from other, less densely populated, countries.
The power per area of nuclear power facilities, by the way, is about 1,000 watts per square meter — much higher than that of renewables. When it comes to the land area required, nuclear power stations and uranium mines are relatively small and unobtrusive.
If the world is to move to a sustainable energy future, one that also limits the risk of global warming, each country will need to work out its own post-fossil-fuel energy plan. And the numbers will have to add up.
Any such plan should anticipate the lifestyle aspired to and the energy consumption required. Today, the average European consumes 120 kilowatt-hours per day. That’s more than double the world average, but lifestyle changes and determined switches to more efficient technologies for transport and heating might enable a modern European quality of life to be enjoyed for an energy cost close to the current world average, perhaps 60 kilowatt-hours per day a person.
What sort of building project is required to deliver that much energy?
For illustration, imagine getting one-third of that energy from wind, one-third from desert solar power and one-third from nuclear power.
To obtain 20 kilowatt-hours per day from wind, one person would require roughly 330 square meters of wind farm — or, to put it another way, would need to share a big 2-megawatt turbine with 600 friends. To get the same power from deserts would require roughly 50 square meters of concentrating solar power station — the same area as a typical British house. And 20 kilowatt hours per day from nuclear power would require roughly a one-millionth share of a modern nuclear power station.
If a country with the size and population of Britain — 61 million people — adopted that mix, the land area occupied by wind farms would be nearly 10 percent of the country, or roughly the size of Wales. The area occupied by desert solar power stations — in the case of Britain, they would have to be connected by long-distance power lines — would be five times the size of London. The 50 nuclear power stations required would occupy a more modest 50 square kilometers.
The effort required for a plan like that is very large, but imaginable. Countries that claim to be serious about creating an alternative energy future need to choose a plan, stop arguing and get building.