More on Renewable Energy: Where Does It Come From?

My previous blog talked about the history of renewable energy at the U.S. Department of Energy. This and succeeding blogs will get more into the current status of renewables and address some of the policy issues surrounding their development and deployment. It will be the first of a series of blogs that will focus in greater detail on each of the broad array of renewable energy technologies.

Wikipedia defines renewable energy as “…energy that comes from resources which are continually replenished such as sunlight, wind, rain, tides, waves and geothermal heat.” A list of renewable energy technologies would include the following:

Solar Energy
o photovoltaics
o solar thermal
– dish Stirling
– parabolic trough
– power tower
– solar hot water
– passive solar
Wind energy
o onshore wind
o offshore wind
Geothermal energy
o power generation
o direct use
o ground source heat pumps
Biomass energy
Ocean and tidal energy
o wave energy
o tidal energy
Hydrogen and fuel cells

What is interesting about this list is that most of the entries are direct or indirect forms of solar energy. PV and solar thermal are direct forms. Wind is an indirect form arising from uneven heating of the earth’s surface. Biomass is organic matter grown with the aid of sunlight. Hydropower depends on water delivered by the hydrological cycle which is solar-driven. OTEC (ocean thermal energy conversion) depends on solar heating of the ocean’s surface. Wave energy is partly wind-driven but is also affected by the gravitational attraction between the earth and the moon. Tidal energy is water energy that is driven by ocean heating as well as gravitational effects. And finally, hydrogen, which is currently derived largely from steam reforming of natural gas, will eventually be derived from electrolysis of water using renewable electricity sources that are largely solar-derived. The one exception is geothermal energy that derives from radioactive decay in the earth’s core.

As an introduction to a more detailed look at renewables in subsequent blogs I am attaching the introductory section to a chapter entitled ‘Solar Energy” that I was invited to write for a new energy encyclopedia (Macmillan Encyclopedia of Energy, December 2000). It reflects my strong interest in understanding where all this solar-derived energy comes from. While the specific details on solar technologies in this chapter may be largely out of date by now (we’ve made significant progress since 2000), the introductory section (Macmillan – introduction) remains valid and informative. More to come, including a discussion of the promise of renewable energy.