Cities of the future may not be able to make new forms of energy, but they will find ways to use a lot less of it.
Take Bottrop, Germany, whose 70,000 residents have seen the collapse of their coal industry over the past few decades, but who still sit in a sweet spot of transportation links, nearby universities and some remaining manufacturing.
Five years ago, city leaders won a competition to become an "Innovation City" with the goal of cutting carbon emissions in half by 2020.
The EU kicked in 500,000 euros ($553,000) to get things started, and a group of German companies like Bayer and GE agreed to use Bottrop as a testbed for more than 300 new energy-saving technologies in seven neighborhoods. Total cost so far: 240 million euros ($265 million), mostly private funding.
"We are convinced we will reach our CO2 reduction," said Rudger Schuman, a spokesman for the project said during a recent tour of Bottrop.
It's an ambitious goal, but there are positive signs that it may work. A commercial office downtown dating from the 1950s has been turned into a "zero-plus" building that produces more energy than it uses. Tenants arrived in January after a two-year retrofit.
Of course, it's been helped by triple-glazed windows, fancy new kinds of insulation, solar panels on the roof and battery storage in the basement.
A similar zero-plus four-unit apartment building has also recently opened. With solar PV panels on the top and sides, it looks like a corporate showroom plunked in the middle of a mid-century neighborhood of sturdy brick homes topped with tile roofs.
The four apartments produce 23,000 kilowatt-hours/year, but only use 18,000 kw-hr/year.
Overall, 12,500 buildings lie within the pilot area of downtown Bottrop and the surrounding neighborhoods. It's not realistic to tear them all down, or build expensive zero-plus buildings, so instead, they are being renovated to use less energy, explained Schumann.
The project has also installed 100 co-generation units that produce both heat and electricity in residential and commercial buildings.
Industrial firms are also making a go at energy savings. Technoboxx, a steel fabricating plant in Bottrop, put up an array of 300 solar panels on the roof to generate half its power. The other half comes from burning wood pellets, and the plant is saving 1,000 euros ($1,200) per month.
A nearby steel coke plant has figured out how to capture waste heat and transport it to local schools.
To cut down on traffic, families in the Bottrop target zone have also been given special leasing deals on electric scooters and cars. Delivery trucks have been re-directed to follow new routes that cut down on bottlenecks and idling.
It's still not clear whether these improvements will result in the massive cut in overall CO2 emissions that everyone is hoping for, or just make it easier to burn more energy.
Experts from a local university will be conducting an audit later this year to check the progress of Bottrop's energy challenge. But officials here say they have made a big improvement for residents, and new green energy firms that are taking advantage of the changes.
Building a future, "smart city" like Bottrop isn't rocket science, says Tim Lehmann, an architect and urban planner at Berlin's "InnoZ" Innovation Center for Mobility and Societal Change.
People have to find new ways to keep themselves warm, live comfortably and get around without relying so much on fossil fuels.
"There is no energy transformation without mobility transformation," Lehmann said. "Future cities have to be nice cities and enjoyable cities and easy to share things. The smart city must network its systems."