While conspiracy theorists may want to believe that Big Oil, Big Coal and Big Nuclear are stifling the growth of renewables, the simple truth is that coal, oil, natural gas and nuclear can satisfy the Four Imperatives: power density, energy density, cost and scale.

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Opponents point out that unlike areas in other countries, mass transit is only economic viable in areas with sufficient density and that on a large scale, massive public transit programs have no chance of becoming self-sufficient.

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If you take the numbers counted, give consideration to their catholic tastes in terms of habitat, the density at which they can breed, and scale it up, the estimate is 53, 900 pairs!

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Our ability to store electricity in batteries is improving, but current low energy density makes batteries an inefficient choice for large-scale storage.

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To compare alien planets, Dr. Mendez at the Planetary Habitability Laboratory and his colleagues are creating an Earth Similarity Index that ranks worlds by their resemblance to our own planet, on a scale from zero to one, based on their mass, radius, density and surface temperature.

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While the residents of Samso can take advantage of their island's blustery weather, open spaces and low-population density to be carbon neutral, the success of wind energy on a global scale may come down to simple economics.

CNN: Striking a blow for wind power

United Technologies is a fuel cell leader, and David Parekh, director of the United Technologies Research Center, says that expertise has allowed the company to improve the energy density of a standard flow battery by 10 times, albeit on a small scale.

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