RENEWABLES - Energy: What Everyone Needs to Know - page 48

usable form, so the technical potential is much smaller than the theoretical poten-
tial but still considerable. In contrast, the amount of renewables being used today is
small, as indicated in Table 5.1 . It is approximately 62 EJ/year, mostly in the form
of biomass, so there are enormous opportunities for increasing the use of renew-
able forms of energy.
How much land is needed to produce energy from renewables?
Concerns have been raised over the amount of area needed to produce large
amounts of renewable energy as compared to the amount needed to produce fossil
fuels, which have a very large energy density. One can produce approximately 3
kWh per kilogram of fossil fuels, an amount which would fit in a small cubic box
of less than 10 cm on each side. To generate the same amount of electricity from
photovoltaics, one would need typically 50 m 2 of surface over the course of a day.
The area needs for renewables is therefore very large and could lead to space lim-
itations.
Table 5.1 Renewable energy production, technical and theoretical potentials
The production of biomass energy also requires a large amount of space since
photosynthesis is an inherently inefficient energy-conversion process. Even with
an intensively cultivated plantation of fast-growing trees, a wood-burning electri-
city generation plant would not have power densities higher than 0.6 W/m 2 . Space
demand for such facilities, then, would be two to three orders of magnitude (100 to
1,000 times) greater than for coal- or gas-fired electricity generation.