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Eric

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3 hours ago, gwalchmai said:

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The concept of net energy must be applied to renewable sources of energy, such as windmills and photovoltaics. A two-megawatt windmill contains 260 tonnes of steel requiring 170 tonnes of coking coal and 300 tonnes of iron ore, all mined, transported and produced by hydrocarbons. The question is: how long must a windmill generate energy before it creates more energy than it took to build it? At a good wind site, the energy payback day could be in three years or less; in a poor location, energy payback may be never. That is, a windmill could spin until it falls apart and never generate as much energy as was invested in building it.
 
As Hughes observed, his point was that “the concept of net energy is crucial if we want to find a policy that will see us through the Energy Sustainability Dilemma,” not that windmills should be eschewed for using up more energy than they ever create.
 
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13 minutes ago, pipedreams said:
The concept of net energy must be applied to renewable sources of energy, such as windmills and photovoltaics. A two-megawatt windmill contains 260 tonnes of steel requiring 170 tonnes of coking coal and 300 tonnes of iron ore, all mined, transported and produced by hydrocarbons. The question is: how long must a windmill generate energy before it creates more energy than it took to build it? At a good wind site, the energy payback day could be in three years or less; in a poor location, energy payback may be never. That is, a windmill could spin until it falls apart and never generate as much energy as was invested in building it.
 
As Hughes observed, his point was that “the concept of net energy is crucial if we want to find a policy that will see us through the Energy Sustainability Dilemma,” not that windmills should be eschewed for using up more energy than they ever create.
 

One must also consider the question "What problem is this solution intended to solve?" In the case of windmills, many of us assume the answer is "Provide an effective alternative to fossil fuels", when the actual answer is "Get votes".

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