This is one more attempt to kill a zombie myth: the notion that local generation of renewable electricity can substitute for long-distance transmission. I can see where this comes from — the sun shines almost everywhere, and the wind blows strong within a few hundred miles of most places where it doesn’t, right? If we are going to use renewable electricity at all, it’s hard to understand why we wouldn’t get it from rooftops, parking lots, or at worst surrounding rural areas.

But if we generate renewable electricity locally (locally being anything from your own rooftop to a wind farm a few hundred miles away), we end up with a huge monthly variation. Even in one of the nation’s best solar sites [PDF] you end up with slightly over half the power available in the worst months compared to the best. Wind sites can be worse. For example, the Northfield, Minn. school district recorded a more than a 3 to 1 ratio between best and worst months.

In contrast, if you connected all existing U.S. wind farms [Excel] in a national transmission grid, in 2007 the ratio between best and worst months would vary by no more that 1.7 to 1. Add solar electricity (which tends to be high in months wind is low) and that ratio would be reduced to a 30 or 40 percent difference between best and worst production months. Long-distance transmission can reduce daily variation as well.

In short, a mostly renewable grid without long-distance transmission will cost at least double one that includes such transmission. Note that this applies to rooftop generation and giant solar or wind farms alike.

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Aren’t long-distance transmission lines even more expensive?

Not than doubling or tripling generation. Also very high prices quoted for transmission installation and maintenance conflate transmission and distribution. In the U.S., long-distance transmission costs are about half of distribution. Maintenance is even a smaller percentage. For example, a few years ago Western Washington suffered a major outage where wind storms took down both transmission and distribution. Long-distance transmission was mostly up within 24 hours, whereas distribution was fixed over the course of almost two weeks. Even though transmission is much more expensive to build and repair per mile, distribution requires many times the line miles. Transmission is point to point, or perhaps a few points to a few points. Distribution has to branch out to ever county, municipality, unincorporated area, and ultimately to every home.

Don’t long-distance lines reduce reliability?

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Not if they are modern High Voltage Direct Current lines. The most common HVDC type installed today actually can help guard against spikes and improve power quality.

Aren’t HVDC lines an environmental disaster?

Not compared to coal and natural gas generation. But they can be installed in a much more environmentally benign way than at present. It has been suggested that we use existing railroad rights of ways for long-distance HVDC transmission — both as part of electrifying freight rail, but also as a way to create a true national grid without harming wilderness. Ultimately we need to remember there is no such thing as zero impact electricity. There is no Kilowatt fairy, no BTU bunny. Efficiency and renewables, including transmission are the lowest impact energy sources we know of.

Won’t demand shifting in a smart grid make long-distance transmission unnecessary?

Demand shifting for a few hours or even a day won’t compensate for monthly variations in production. Even on a day-to-day basis, long-distance transmission is probably needed to produce a smooth enough production curve for smart-grid demand shifting and electricity storage (including vehicle to grid) to match.

Don’t most such variations match local seasonal peaks? That is, isn’t wind strongest in winter in cold climates, and sun strongest in summer in hot ones?

If we were just changing electricity supply this would be valid. But climate control has huge potential for both efficiency improvements and non-electrical renewable supplies. Improved insulation, weather sealing, air circulation, along with ground source heat pumps can reduce demand. Direct solar heating (in commercial buildings and multi-unit residences cooling) also has significant potential. Dollar for dollar, these save or provide more energy than renewable electricity. This does not mean we don’t need renewable electricity. By the laws of physics, doing more with less won’t ever extend to the point where we can get something for nothing. But it does mean that renewable electricity only makes sense in a context where we are also massively investing in efficiency improvements and direct solar heating. These efficiency techniques will reduce seasonal variation in electricity demand.