Energy Density

An important issue with regard to any primary source of energy is how "dense" it is. A second consideration is how transportable it is, and a third issue is how easily it can be changed into other forms. By these measures, oil is remarkably versatile, since it can be burned directly or in other forms, it can be transported in pipelines, and it can be transformed to electrical energy, which is perhaps the most versatile form of energy since it can be transmitted over stationary power lines and used for so many purposes.

By these measures, wind energy has a low density (we need a large turbine to capture sufficient kinetic energy of moving air to convert it to electricity). Solar energy also has low density. We can use photovoltaic cells to convert it directly to electrical energy or plant crops or trees to capture it and use it to convert atmospheric CO₂ to plant carbon (e.g., wood) which then can be burned to give electricity. For comparison, 10 TW would require area of 220,000 km² (about 1.5 times the area of the state of Iowa) for solar collectors. If we looked to biomass for this 10 TW per year, we would need more than 10% of the land area of the Earth, which is comparable to all that currently is used for agriculture (Hoffert et al, 2002).

Hydroelectric energy is a more dense form of primary energy. Since water is about 1,000 times as dense as air, it takes a wind turbine of area 1,000 times larger than a water turbine to get energy out for the same fluid speed.

Renewable sources of energy contribute less than 1% to global total energy consumption if we exclude wood and hydroelectric. Geothermal energy is a possible source in isolated locations where high temperature water is accessible from the Earth's surface, such as in Iceland. Other primary forms of energy (e.g., ocean thermal and ocean tidal) are very diffuse and difficult or expensive to concentrate.

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