Richard Heinberg writes: A process for designing the energy system to meet society’s future
needs must start by recognizing the practical limits and potentials of
the available energy sources. Since primary energy sources
will be the most crucial ones for meeting those needs, it is important
to identify those first, with the understanding that secondary sources
will also play their roles, along with energy carriers (forms of energy
that make energy from primary sources more readily useful—as
electricity makes the energy from coal useful in millions of homes).

We can define a future primary energy source as one that meets, at a minimum, these make-or-break standards discussed above:

• It must be capable of providing a substantial amount of energy—perhaps a quarter of all the energy currently used nationally;
• It must have a net energy yield of 10:1 or more;
• It cannot have unacceptable environmental impacts; and
• It must be renewable.

The most cursory examination of our current energy mix yields the
alarming realization that about 85 percent of our current energy is
derived from three primary sources—oil, natural gas, and coal—that are
non-renewable, whose price is likely to trend higher (and perhaps very
steeply higher) in the years ahead, whose EROEI is declining, and whose
environmental impacts are unacceptable. While these sources
historically have had very high economic value, we cannot rely on them
in the future; indeed, the longer the transition to alternative energy
sources is delayed, the more difficult that transition will be unless
alternatives can be identified that have superior economic and
environmental characteristics.

Assuming therefore that oil, natural gas, and coal will have rapidly
diminishing roles in our future energy mix, this leaves fourteen
alternative energy sources with varying economic profiles and varying
environmental impacts. Since even the more robust of these are
currently only relatively minor contributors to our current energy mix,
this means our energy future will look very different from our energy
present. The only way to find out what it might look like is to
continue our process of elimination.

If we regard large contributions of climate-changing greenhouse gas
emissions as a non-negotiable veto on future energy sources, that
effectively removes tar sands and oil shale from the discussion.
Efforts to capture and sequester carbon from these substances during
processing would further reduce their already-low EROEI and raise their
already-high production costs, so there is no path that is both
economically realistic and environmentally responsible whereby these
energy sources could be scaled up to become primary ones. That leaves
twelve other candidates.

Biofuels (ethanol and biodiesel) must be excluded because of their low
EROEI, and also by limits to land and water required for their
production. (Remember: we are not suggesting that any energy source
cannot play some
future role; we are merely looking first for primary sources—ones that
have the potential to take over the role of conventional fossil fuels.)

That leaves ten possibilities: nuclear, hydro, wind, solar PV,
concentrating solar thermal, passive solar, biomass, geothermal, wave,
and tidal. (03/29/09)
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