Advocates claim, ‘Renewable energy has a bright future,’ and the skeptics quickly respond, ‘and always will (have a bright future, that is).’ The truth about renewable energy, of course, lies somewhere in between.

Advocates claim, "Renewable energy has a bright future," and the
skeptics quickly respond, "and always will (have a bright future, that
is)." The truth about renewable energy, of course, lies somewhere in between.

First, let’s clarify what we really mean by "renewable energy" and
define three arbitrary categories:

  • Group I : Truly renewable energy sources – As long as the sun is shining,
    the rains keep coming, the winds are blowing and the tides are churning, we
    will be able to exploit the potential of solar (both solar-thermal and photovoltaic),
    hydro, wind and tidal energy.

  • Group II : Virtually renewable energy sources – In this category are those
    sources that (i) can be replenished over less-than-geologic time scales (so-called
    biomass sources like trees, forestry and crop residues, and crop-derived ethanol)
    or (ii) have very long useful lifetimes (e.g., geothermal) or (iii) rely on
    continuous supplies of combustible streams (e.g., landfill gas, sewage sludge,
    discarded auto tires, and certain other municipal, industrial or agricultural
    wastes).

  • Group III : Indirectly renewable energy sources – Energy conversion devices
    (like fuel cells and other hydrogen-fueled systems) and storage devices (like
    batteries) are often considered to be renewable technologies but only in the
    sense that the hydrogen fuel is derived from renewable sources (e.g., separated
    from water by wind- or solar-powered electrolysis) or the batteries charged
    by renewables-generated electricity.

The potential environmental benefits of renewable energy sources (especially
Groups I and III) are readily apparent: "free fuel" and zero emissions.
Moreover, in selected applications, renewable energy sources can displace conventional
power generation that is polluting and contributory to global warming and does
consume fossil fuel supplies that are ultimately not inexhaustible.

However, not even renewables are completely environmentally benign. For example,
hydropower installations can seriously disrupt marine ecosystems, navigation
and commerce. Wind machines can be noisy, unsightly and a threat to migratory
birdlife. Biomass and waste fuels typically are nonhomogeneous and contain constituents
that may create their own pollution problems when burned.

In our post-9/11 world, domestically available renewables might contribute
to lessening our dependence on foreign oil supplies; in addition, they are readily
"scalable," lending themselves to distributed generation applications
that may further enhance our energy security.

Despite their attractiveness, renewable energy sources (Groups I and III, at
least) have a number of severe limitations that have so far restricted widespread
adoption in practical applications :

  • Renewables are diffuse energy sources – The potential energy available
    from Group I renewables is – by any convenient measure of energy density –
    orders-of-magnitude less than that available from the combustion of conventional
    fossil fuels. For example, the solar capacity equivalent of a 1000 MWe power
    plant would require a photovoltaic array of roughly 4 square miles; replacing
    the total installed U.S. generating capacity with photovoltaic cells would
    require roughly 3 times the land area of Rhode Island, but only about 3 percent
    of Arizona (which may or may not seem impractical, depending on your point
    of view).

  • ‘ Renewables are not, in general, dispatchable – Energy sources are
    most useful when they can be relied upon to provide a continuous and predictable
    supply of power when and where it is needed. With the notable exception of
    large-scale hydropower facilities, Group I renewables cannot pass this test.
    Because of their intermittent nature, these Group I renewables must be coupled
    with an effective energy storage system to have any value as stand-alone reliable
    power sources.

  • ‘ Renewables have unfavorable economics – Renewable energy technologies
    are generally characterized by very low operating costs (including zero fuel
    costs) but very high capital costs. It is difficult to compare with the economics
    of conventional generation technologies in a meaningful way because renewables
    are typically location-specific, have poor capacity factors and cannot be
    relied upon for ‘firm’ capacity. However, using levelized costs
    of electricity with suitable assumptions for capacity factors, one can conclude:
    solar power has traditionally been much more expensive than conventional power
    generation, hydropower is competitive with baseload generation, with other
    renewable options falling somewhere in between. Probably the most useful way
    to evaluate the economic benefits of renewables is to consider the value of
    displaced generation from conventional sources.

Because of these characteristics, renewables can compete practically in only
selected applications and in selected locations. Currently, renewables account
for roughly 13 percent of total U.S. electric generation, with hydropower representing
the largest portion (roughly 10 percent of total U.S. generation). The remaining
renewables generation comprises – in descending order of generation – biomass,
geothermal, wind and solar.

We will likely continue to see incremental improvements in renewables technologies
that will expand practical applications and make them more economically competitive.
However, more widespread adoption will likely depend on significant breakthroughs
in energy storage technologies and/or the development of a hydrogen production/distribution/storage
infrastructure – which will be crucial for transportation applications.

As the world’s appetite for energy continues to grow, as we continue to deplete
our most useful and least-polluting fuel sources, and as the environmental implications
of business-as-usual become more acute, renewables are likely to be an increasingly
important part of the solution – especially in the developing world. Indeed,
responsible energy planners must consider renewables as part of any long-term
energy portfolio to promote continued economic prosperity and protect the quality
of our environment.


Author

Paul Weinberger has led Technology Management, a Farmington CT consulting
firm, since 1987.
He can be reached at tech_mgt@attbi.com.