News and analysis about renewable energy in California.

Saving Energy by Keeping The Sky Dark

S.F. Bay Area from the International Space Station | Photo: NASA Marshall Space Flight Center/Flickr/Creative Commons License

The beautiful photo here, taken with a standard still camera aboard the International Space Station (ISS) in 2010, may bring a pang of something like homesickness to those who love the San Francisco Bay Area. But it also shows one of the ways in which we waste energy most flagrantly. And we don't need to wait for new technology to curb that energy waste: we can do it cheaply and effectively with tech that's already here.

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The ISS is 220 miles above the surface of the Earth, and so every bit of light you see in the photo is bright enough -- either from a single source of a whole lot of smaller lights combined -- to be seen from 220 miles away. That's bright. And every bit of that light cost energy to generate, whether it's plugged into the grid or attached to the front of a vehicle.

Artificial lighting is a modern wonder we don't appreciate enough, until the power goes out. Without it our streets and sidewalks would be less safe, we'd get a lot less done during the short days of winter, and it would be a lot harder to run out to the grocery store to get cat food at 10:00 pm.

But almost every bit of that light you see in the photo is energy wasted. It's heading from the light source straight into space, where it benefits none of us except for passing ISS photographers and insomniac airline passengers trying to occupy the time by watching the pretty lights. And to add insult to injury, that extra light diffuses into the atmosphere, causing a glare that makes it difficult to see anything in the night sky aside from the moon and a couple of bright planets.

If we aimed our lights at the ground more effectively, we could keep our cities lit without beaming expensive energy into space for no good reason, and without obscuring our night skies.

Wasted outdoor lighting comes from a lot of sources, ranging from streetlights and household exterior lights to billboards and marquees to vehicle headlights. It even includes interior lighting bright enough to illuminate the outdoors through a window, as in some commercial districts.

It's hard to quantify outdoor light sources exactly because they're so diverse, but to get a general idea of the scale of the problem let's look at street lighting, which some estimates hold responsible for 30-50% of light pollution.

A 2012 report by UC Davis researchers calculates that California has 1.1 million streetlights. Of these, more than three quarters -- 852,000 -- are high pressure sodium vapor lamps. Sodium vapor lamps are reasonably energy-efficient compared to incandescents and old-style mercury vapor lamps, but it still takes a lot of energy to light streets and highways: sodium vapor streetlamps that consume 250 watts or more each are very common.

If every one of those 852,000 sodium vapor streetlamps were 250 watts -- some of them are a lot brighter, but let's make the math easier -- then all together they'd consume 213 megawatts of power, the equivalent of a mid-sized power plant's capacity.

As people become more concerned about keeping our night skies dark, a lot of municipalities have started to install street luminaires that direct more of the lamps' output downward. A so-called "full-cutoff" luminaire doesn't allow light to shine above the lamp, and reflects it back toward the ground. But there are still a lot of relatively wasteful luminaires out there. As much as 30 percent of roadside lighting is wasted and shines into the sky, according to the International Dark Sky Association. That's 71 megawatts of power beamed uselessly into space. And if we reflect that light downward instead of letting it head for the stars, then we don't need as bright a lamp, saving that energy.

There are other ways of saving street lighting energy, too: some cities have installed motion-sensitive dimmers that keep lights lower if no one's using the street. As a fair amount of street lighting reflects off pavement and them shines into space anyway, this can reduce light pollution even from lamps with full-cutoff luminaires, while saving a whole lot of power.

And that's just efficient sodium vapor lamps. If 30-50% of light pollution comes from street lighting, then 50-70% comes from other sources, which are almost all far less efficient than sodium vapor. In your travels each day you very likely see hundreds, perhaps thousands, of incandescent outdoor lamps that cast half or more of their light skyward, especially on homes and small businesses. It doesn't have to be that way. As "dark sky" towns proliferate -- Joshua Tree, where I live, is one such -- and full-cutoff light fixtures become widely available in hardware stores, it will become easier to keep our skies dark and our power bills lower. We can save a considerable amount of energy while making it possible for urban kids to go outside and see more than one star.

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About the Author

Chris Clarke is a natural history writer and environmental journalist currently at work on a book about the Joshua tree. He lives in Joshua Tree.
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