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As the world warms, it's also getting more urban - more than half the world's population now lives in and around cities. So when it comes to sustainable living, cities pose a growing challenge.
In one sense, cities have a lot going for them - good public transit, efficient power distribution, and a density that means you often don't need cars to get around. Plus, living and working in tall buildings rather than spread-out exurbs saves a huge amount of energy per person. But cities also have one big problem: they're already built. We can invent all the green technologies we like, but we can't tear down blocks full of drafty old structures and start from scratch - to say nothing of the networks of streets lined with wiring, pipes, and tunnels that might be decades, even centuries old. The problem is especially acute in older cities such as
So how to improve the cities we've got? The answer: retrofitting. In the past several years, engineers, urban planners, and entrepreneurs have come up with imaginative new ways to take what we now know about living more energy-efficiently, and grafting that technology onto cities without clearing away what's already there. Here are some ideas already being tried, including some that might work in
BIKES ON DEMAND
Bicycles use sweat power, not fossil-fuel power, which means they're a favorite of green-transit planners everywhere. But unless you're a dedicated cyclist, you don't always have your bike with you when you need to make a quick jaunt. Enter bicycle-sharing programs, which offer racks of public bikes that can be used for one-way rides around town. Though such programs have mostly failed over the last 20 years,
SMART GRID
The power grid that delivers our electricity might be complex, but it's not too bright. Think of it as that phone in grandma's house - you know, the one that doesn't give you caller ID, let alone receive text messages or video of your friend's graduation party. If it were smart, it could communicate with your house, and vice versa. At the household level, this means you'd know exactly which appliances are hogging power, and how to manage them more efficiently. At the city level, a smarter grid could change how power gets consumed, in part by charging more money at high-demand times. You could even sell excess renewable power back to the grid. A few pioneering cities and utilities already have smart grids under way - in Boulder, Colo., the utility Xcel Energy is piloting smart meters that let consumers see up-to-the-second statistics on their power usage, and change temperature and other usage settings automatically.
MOBILITY HUBS
As cities sprout new alternative transit ideas, like car-share companies and bike-sharing services, one problem looms: they don't necessarily connect very well, especially as you get farther away from downtown. Well-planned small hubs could link these services, much the way urban train stations connect buses, subways, and taxi stands.
RE-SKINNING BUILDINGS
Efficient new buildings are great, but not if you need to knock down existing buildings to construct them. Instead, developers are starting to "re-skin" older concrete buildings, adding new thermal covers to reverse concrete's normal tendency to capture heat in summer and release it in winter. Careful re-skinning might cut energy usage by as much as half, and the new skin can also house better pipes, ducts, and cables.
SOLAR RENTALS
For homeowners who want to switch to solar power, one of the biggest obstacles is the cost: an average of $25,000 for a set of roof panels. Cities like
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