The best bet for cities of the future is to use what they already have plenty of to power their growth

Amid the hum of machinery and warm odour of putrefying autumn leaves, official Pierre Hirtzberger is explaining how three giant fermenters can convert household food waste, trimmings from parks and gardens and the slops from school and hospital canteens into enough methane gas to power about a third of the buses in the French city of Lille.

“The process is exactly the same as in the stomach of a cow,” he said, gesturing towards three ‘biodigesters’ which each hold 20,000 cubic metres of rotting liquefied waste.

“The objective is to fuel 100 of Lille’s buses on this biogas, out of a total fleet of 350,” Hirtzberger, head of the city’s urban waste research and development, said.

From San Francisco to Malmo in Sweden, cities around the globe are preparing for a new imperative: to accommodate the mass of world population growth and thrive, without further accelerating the release of carbon dioxide that threatens their existence.

With half the world’s population already living in cities and the urban population projected to reach almost five billion by 2030, it is not just growth that puts them in the front line of climate change.

Even if populations escaping drought migrate to urban centres, the fact that 60 per cent of the world’s 39 largest metropolises are located in coastal areas puts the cities themselves at risk in future centuries, from rising seas.

Sunshine, tech creativity and a clued-in population help widen the range of options for places like San Francisco - the first city to make it a crime not to compost food and waste in city bins, in a bid to cut landfill use to zero.

Plenty of money on top of abundant sun are allowing Abu Dhabi to showcase a futuristic eco-city: Masdar City is a vision of solar panels powering pilotless taxis and trams and feeding desalinated water to citizens and its verdant palms.

Such visions make dazzling prospectuses for those eyeing a market which analysts expect to be worth a record $200 billion next year, and sunshine will be a major source of clean power as the cost comes down to make it competitive with fossil fuels.

But for many cities, particularly older centres in gloomier climates, the reality will be more like Lille - distilling energy from the excrement of citizens, the waste from restaurants and the mountains of unsold sandwiches left in supermarket fridges at the end of each week.

Much of it will just be plain boring - pumping insulation foam into loft spaces and wall cavities, fitting double or triple glazing - the stuff that can keep small builders busy even if economic slowdown stalls grand construction projects.

In all, it will require myriad diff-erent approaches to whittle down society’s impact on the planet.

Cities in France, Sweden, Australia and the US are looking at an exotic mix of energy sources, and their choices prove that what looks good in architects’ promotional literature is not necessarily what works on the ground.

In Australia, the government plans seven ‘Solar Cities’ and is putting $1.37 billion into four large power stations driven by the sun.

But a temperate city like Melbourne will have a very different approach from that of sun-bathed Brisbane, 1,700km north and just 600km from the Tropic of Capricorn.

“If you’re in Brisbane, you’ll probably have solar hot water and solar air-conditioning and a bit of electric power as your mix,” said Jim Smitham, a renewable energy expert at Australian state research body CSIRO.

“But if you’re in Melbourne, you’ll be much more interested in heating and power and a little bit of air-conditioning for the summer.”

Even within cities, the density of solar generation will vary accord-ing the value of land, he added.

In pricey central business districts, solar panels will be stacked on rooftops, but in the suburbs small-scale solar plants will help supplement households’ own generation. Outside the cities, where land is cheapest, solar power stations will find a niche, feeding power into the metropolis.

As solar power costs have fallen due to economies of scale, an initially subsidised power source is becoming viable in some places.

“In countries like Spain, southern Italy and Greece, the cost of energy from solar is already, or will soon be, at parity with the cost of electricity from the grid,” said Winfried Hoffmann, president of the European Photovoltaic Industry Association.

“Germany is less sunny so it will take longer, but it will reach parity by 2016 at the latest,” he added.

But where Brisbane gets about 2,790 hours of sunlight a year, Lille gets about half that, as moist air sweeps in from the North Atlantic. So Lille is focusing hard on waste.

Biogas - the fuel that will power some Lille buses - is actually an ancient energy source. It captured the attention of 13th-century adventurer Marco Polo in China, where he noted covered pots of sewage stored to generate energy, and it earned a mention by 17th-century writer Daniel Defoe.

Lille is also looking at that option.

“We’re studying the possibility of getting biogas from sewage sludge at one of the city’s two sewage treatment plants, and that has the potential to do at least 150 more buses,” said Hirtzberger.

“Potentially, one could run the entire bus system with biogas from sewage and rubbish. This would be typical of most cities in Europe.”

Other cities, such as Malmo in Sweden, use waste to heat and power buildings.

In Malmo, 50 per cent of heat is produced from its 550,000 tonnes of waste a year - a level that could be replicated in most north European cities, said Richard Bengtsson, project manager of EON Nordic, which developed Malmo’s heat and power system.

“Waste is an interesting fuel due to the fact that you don’t have to pay for it,” said Bengtsson. “You get paid to take care of it.”

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