Basic Research Challenges for Solar Fuels
Because of the day/night variation of the solar resource, the practical use of solar energy faces
two overarching technological challenges: economically converting sunlight into useful energy,
and storing and dispatching that converted energy to end users in an economical, convenient
form. To provide base load power, solar electricity and any other solar energy conversion system
will require a tightly integrated storage and distribution technology to provide energy to end
users in accord with demand. Additionally, there must be a means to cost-effectively convert this
energy into forms useful for transportation, residential, and industrial applications. Because these
sectors currently use chemical fuels as their primary energy feedstock, one of the following three
options must occur: (1) solar electricity must be converted into chemical fuels that could be used
in the existing distribution and end-use infrastructure; (2) the distribution and end-use
infrastructure must be converted to one that allows widespread, direct use of electricity, which
also must be stored until use; or (3) solar energy must be directly converted into useful chemical
fuels. Put simply, without cost-effective global transmission, storage, and/or fuel formation, solar
electricity can only be a (large) niche market serving as a supplement for other primary sources
of energy.
Conversion of electricity into chemical fuels, through electrolysis of water to produce H 2 and O 2 ,
is an existing technology. However, it is a very expensive method of making H 2 (as discussed
below), and the catalysts that are used in current electrolyzers cannot readily scale to the levels
that would be needed to support a TW-level implementation of solar electricity use in
H 2 production. Direct production of fuels from sunlight is advantageous because it inherently
provides a method for extracting energy during the night and for dispatching and distributing
energy cost effectively in the existing infrastructure for use in the residential, industrial, and
transportation sectors. The ability to use sunlight to produce CH 4 or H 2 from abundant, non-toxic
resources such as CO 2 and water, respectively, would revolutionize the economical,
environmentally sound production of fuels.
Photosynthetic solar energy conversion has produced the vast majority of the energy that fuels
human society and sustains life on Earth. This global-scale, time-tested energy conversion and
storage process produces all current biomass and, over geologic time, has produced all the fossil
fuels available today. The drawback is that, with current plant types, a large-scale
implementation of biomass as a primary energy source would require very large areas of land to
make a material contribution to meeting current energy demands. Using the best-known plant for
energy production, switchgrass, as an example, production of 10 TW of average power would
require covering 10% of the land on Earth (i.e., essentially all of the cultivable land on Earth that
is not currently used for agriculture) would have to be covered with biomass farms. Such a large
deployment would also clearly stress our ability to provide fresh water to grow such crops;
would constrain land use on a global scale; and would impose serious infrastructural constraints
to effectively and constantly manage, harvest, and optimally exploit all of the crops over such a
large land area. Hence, practical constraints dictate that (1) the efficiency of photosynthesis be
increased so that less land area (likely by a factor of 5–10) is required and/or (2) that artificial
photosynthetic systems be developed that either borrow components from natural systems or are
inspired by the natural system’s components to produce useful chemical fuels directly from
sunlight with higher efficiencies than the natural system and with an acceptably low cost.