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Vaclav Smil and power densities

Source: Vision Earth

As you can see from my earlier posts on this blog, I recently wrote a couple of articles on Professor Vaclav Smil's calculation of power densities and the implications for renewable technology. These articles, mostly through my sharing of this blog through links with other energy sites, have come to the attention of the blogsite Vision of Earth, a link to which is included on this site (see below). As a result, Ben Harack from Vision of Earth has written a response which is most interesting as it appears to contradict Smil's findings. I will attempt to summarise this reply but bear with me as, despite my interest in energy issues, I am not a scientist but a journalist.
Harack explains that Smil uses the method of trying to calculate how much energy can be generated from a square meter of land (W/m2) and states that this method has distinct disadvantages in that it leaves out large portions of an energy story, for example concerning the reliability of the energy being produced. He states that the term 'Power density' may confuse as it has other technological definitions and so uses the term 'A real power density' which may be expressed in terms of watts per metre squared (W/m2). This means essentially that higher power densities are more desirable because more power can be generated from the same amount of land. Renewable technologies such as solar power suffer from being intermittent and therefore Smil tries to calculate the yearly averaged power output. The full potential power output (nameplate capacity) differs from the averaged power output by a factor called the capacity factor.
Since land use is crucial in determining the relative merits of various power sources, the power density method is very useful and may be analogous to similar calculations involving, for example, land requirements for food production with which conventional power generation such as coal competes for space. However, Harack argues that renewable energy technology can be utilised in synergy with other land uses and therefore may not need to compete although since wind turbines need sufficient spacing between towers, because of the rotors, its power density may be reduced. Wind turbines for example uses sites such as hilltops, ridges and shallow ocean, all of which may simultaneously be used for livestock grazing or fishing. Wind turbines also need to be spaced properly in order to be cost effective since turbines currently cost a lot more than land does and therefore they need space in order to be able to operate efficiently. On this basis Harack argues that Smil isn't considering the potential of wind as an energy source at all but in actuality the cost effectiveness of turbines. He states that this may change in the future with larger, slower turbines or greater market penetration of wind.
Interesting stuff!


  1. Hey Robin, just a quick note, it is "Areal power density" instead of "A real power density". Areal is a word meaning essentially 'pertaining to an area'. We wanted to use that word to say that this is a power density pertaining to this case land usage.


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