Saturday, November 23, 2013

The Hidden Costs of Wind and Solar: Part I Power Density

Renewable energy is a strange term. What does it really mean to be renewable when even that giant nuclear reactor is the sky will burn itself out eventually? Looking at a dictionary we get this.

“Any naturally occurring, theoretically inexhaustible source of energy, as biomass, solar, wind, tidal, wave, and hydroelectric power, that is not derived from fossil or nuclear fuel.”

So renewable energy has to not be derived from fossil or nuclear fuel, yet the sun uses nuclear fuel, and most thing that go under the title renewable energy (i.e. biomass, solar, wind, tidal, wave, and hydroelectric power) are derived derived from sunlight so this definition contradicts itself.

Personally I think that “renewable” is just a rhetorical device used to try to make certain types of energy seem better than the alternatives, but my purpose here isn’t just to criticize people’s terminology. Really when people are talking about switching to renewable energy they are talking about wind turbines and solar pv. These are the two ‘white knights’ that people think are going to save us all from the evil nuclear and fossil fuel industry, so these are the two energy sources I’m going to talk about.

The cost of wind, and solar can be placed in two categories. One category includes things like the cost of things like wind turbines, solar panels and grid tied inverters. There are plenty of sources that talk about those costs so I’m not going to. Instead I’m going to talk about the second category which is the hidden costs of wind and solar pv.

The two biggest sources for the hidden costs of wind and solar are their low power density, and the intermittency of those sources.

Power density is the power per unit of area. It’s an important consideration because it describes how much space you need to fill your energy requirements. The power density of wind and solar are low, which means that to supply most of our power from such sources would require a lot of land. To give you an idea of how much consider this. One estimate of the power density of solar pv is 10 w/m^2(see page 41), and in 2011 the U.S. Generated 4,100,656 Gwh. These two facts taken together tell us that if we generating all of our electricity with solar pv in 2011 then it would have taken 46,811 square kilometers ( or 18,074 square miles). That’s roughly the area of all the roads in the US (roads occupy 17,879 square miles of the land in the contiguous 48 states). Those roads weren’t easy or cheep to build or maintain; can you imagine how much harder such a massive solar project would be? Of course this is all academic. You can’t really power the whole country with solar pv because solar pv doesn’t work at night.

Estimates for the power density of wind have been around 2 to 7 w/m^2, but new research suggests that the power density of large wind farms may actually be as low as .5 to 1 w/m^2 (Imagine wind turbines covering an area that is 10 to 20 times the area of all the roads in the US).

 “Energy from coal can also be made almost anywhere. But to make electricity from wind, the generator has to be where the resource is, and for wind, that means places with few major power lines. “ Matthew L. Wald in New York Times article

Power density is important (especially for wind) which is why people try to situate wind and solar in places where the power density is highest. This means a lot of transmission lines are needed to get the power to where it’s used and those transmission lines have a cost that needs to be accounted for. One source estimates that in the US between 2010 and 2030 the cost of without "renewables" would be $18.5 and the cost with "renewables" would be $115.2 billion.   Also, some people argue that there are aesthetic costs (i.e. it lowers property values) to having wind turbines, solar panels and transmission lines all over the place.

If you wish to know more about Power density please read this PDF Sustainable Energy — Without the Hot Air.   It is written by David J.C. MacKay FRS Regius Professor of Engineering Cambridge University Engineering Department. In it he explain many of the important issues in detail in a very reader-friendly way. I would recommend it to anyone interested in the subject. It’s a bit long, but it really is worth it.

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