Assessing Alternatives to Fossil Fuels

Three Important Features of Renewable Energy

But before you begin with MacKay, make sure you understand these three important features of renewable energy before you go on:

1.) Form of Energy Supply (Store vs Flow)
Fossil fuels are concentrated stores of chemical energy (stored in carbon bonds). One can store and transport coal, oil, and natural gas over long distances without losing the quality of this energy. Moreover, they contain significant amounts of energy per unit mass or volume (something that our transportation system has benefited from).

Renewable energy systems generally derive directly from the energy absorbed by the earth system from the sun. Solar and wind energy are flow systems — they must be used as they are produced. For these energy sources, an external energy storage system needs to be added to save energy for when they are not flowing. Both wind and solar are most abundant at certain times of day, and so must be stored for use when they are absent. Biomass (e.g biofuels) are chemical stores of energy but with a much shorter shelf life compared to fossil fuels. They decompose relatively quickly. Hydroelectric is generated from converted solar energy that drives the water cycle and does have storage abilities, since water can be held behind a dam rather than being released.

2.) Form of Energy as Delivered
Many of our renewable options (including tidal and geothermal not listed in table) supply energy to end uses in the form of electricity. Considering that electricity now supplies less than 40% of our energy end uses (the rest being largely fossil fuels), a renewable future would require not only significant changes in energy generation and storage but energy supply systems.

Our transportation sector, which consumes more than a quarter of US energy supply, currently depends on petroleum-based liquid fuels (94%). Think about our dependence on the internal combustion engine….Movement away from fossil fuels will require some combination of drastically increased electricity use (fuel cells, hybrids, electric vehicles) and biofuels in the transportation sector. The infrastructure changes associated with such a transformation would be significant.

3.) Power Density
Power density refers to the rate of energy flow per unit land area. Why should you care about what seems like a dry engineering concept? You should care because this is a characteristic of energy supply that shapes our infrastructure and settlement patterns, In many ways we can say that our cities are in part the outcome of the HIGH power densities provided by fossil fuels.

We can think of the power density of energy supply as the energy flow (watts) that can be captured or produced per unit land area through a particular energy supply. We can also think about energy consumption in this way — the amount of energy consumed per unit land area. For instance, one would expect the a skyscraper to have a higher energy power consumptive density than a single-story house.

Take a look at the table to the right. Watts are units of the rate of flow of energy (power) and therefore power density is the energy flow per unit land area (m2). The table presents power density of energy-producing (wind farms, hydroelectric plants, corn fields) and consuming sources and entities houses, cities..etc) facilities. Please note that renewable energy power densities are much lower than fossil fuel facilities — renewable energy supply is diffuse.

Reflecting about this table should help you answer questions such as:

  • Can our cities be powered with roof-top photovoltaics?
  • How can our cities be powered through renewables?
  • What kinds of energy supply and transport infrastructure would be required?


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