Environmental Engineering Reference
In-Depth Information
Photovoltaic plants can generate electricity with much higher power densities
than wood-burning stations. Converting solar radiation to new biomass, overall,
has an efficiency no better than 1%, while even relatively inefficient PV cells have
efficiencies around 5%, and today's best commercial facilities go above 10%. Tak-
ing only the PV cell area into consideration, this translates to power densities of
mostly between 10 and 20 W/m
2
. But when all ancillary space requirements are
included, the typical density range declines to 4-9 W/m
2
, an order of magnitude
higher than for wood-powered generation but one to three orders of magnitude
lower (that is demanding 10 to 1,000 times more space) than the common modes
of fossil fuel electricity production.
Power densities for central solar power are slightly higher, with rates as high as
45-55 W/m
2
, when only the area of solar collectors is considered, but with overall
power densities (including spacing, access roads, and tower facilities) on the order
of 10 W/m
2
. Finally, wind-driven electricity generation has power densities similar
to, or slightly higher than, wood-burning stations, with most new installations us-
ing powerful (1-6 MW) turbines fitting into a range between 0.5 and 1.5 W/m
2
.
For power plants alone such densities are commonly in excess of 2 kW/m
2
and
can be as high as 5 kW/m
2
. When all other requirements (coal mining, storage, en-
vironmental controls, settling ponds) are included, the densities inevitably decline
and range over an order of magnitude from as low as 100 W/m
2
to as much as
1,000 W/m
2
(1 kW/m
2
).
In contrast, compact gas turbine plants, which can be connected to existing gas
supplies, can generate electricity with power density as high as 15 kW/m
2
. Larger
stations (>100 MW) using the most efficient combined-cycle arrangements (with a
gas turbine's exhaust used to generate steam for an attached steam turbine) will op-
erate with lower power densities. If new natural gas extraction capacities have to be
developed for their operation, then the overall power density of gas and electricity
production would decline to a range similar to that of coal-fired thermal generation
Most of the area occupied by large wind farms could be used for crops or graz-
ing but other land uses would be excluded, and large areas dotted with wind tur-
bines would require construction and maintenance of access roads as well as the
creation of buffer zones not suitable for permanent human habitation. And in all
cases of renewable energy conversion, much more land would be needed for more
extensive transmission rights-of-way in order to export electricity from sunny and
