Environmental Engineering Reference
In-Depth Information
consumption rate was 4.1 9 10 20 J, or 13 terawatts (TW) in 2000. By year 2050,
the number will have doubled to 28 TW and tripled to 46 TW by the end of the
century based on current estimates of population growth and energy consumption
[ 2 ]. Currently, this huge energy demand is supplied by oil (35 %), coal (23 %) and
natural gas (21 %), which in total yields a ratio of around 79 % from fossil fuels.
While biomass provides only 8 % of the energy supply, nuclear energy only
6.5 %, and hydropower a mere 2 % share [ 1 ]. Excessive extraction is leading to
gradually decreasing reserves of conventional, nonrenewable, energy resources,
such as oil, coal, and natural gas. Moreover, the increasing energy production
concurrently impacts the environment through the production of greenhouse gases
(i.e., carbon dioxide, methane, nitrous oxide, and other gases) from fossil fuels,
which contribute to climate change and even global warming concerns [ 3 ].
Therefore, in order to deal with the increasing energy demand and provide a long-
term solution to the energy crisis in the future it is essential to develop environ-
mentally and economically clean alternative energy resources in order to achieve a
globally sustainable society.
Renewable energy resources, including hydroelectricity from tides and ocean
currents (2.5 TW), geothermal energy (12 TW), wind power (24 TW), and solar
energy striking the earth (170,000 TW) are considered highly promising options
[ 3 ]. Among these, solar energy is clean, endlessly abundant, and has the largest
potential to satisfy the future global demand for renewable energy sources (under
ideal conditions, radiation power on a horizontal surface is 1000 Wm -2 )[ 2 ].
However, to date, the energy converted from sunlight remains far less than that of
the total energy demand. For instance, the total average annual installed energy
capacity in 2009 was about 7 gigawatts (GW), which only contributed to 0.2 % of
global electricity usage [ 4 ]. Thus, the efficient, direct conversion of solar energy
into electricity and fuels should, and must, be one of the most important scientific
and technological pursuits of this century [ 5 ].
1.1 Applications for Solar Cells
First of all, to become a major contributor to future renewable energy, solar energy
must be cost-effective and priced competitively relative to conventional energy
resources, nuclear power, and other renewable energy resources [ 6 ]. Currently, the
direct conversion of incident solar photons to electricity is achieved through pho-
tovoltaic (PV) devices (or solar cells) [ 7 , 8 ]. Single crystal silicon-based PV devices
are the first-generation solar cells, which are commercially available for installa-
tion, deliver power with a 15 % efficiency and make up about 90 % of the current
PV market. However, first-generation solar cells still suffer from several inherent
deficiencies as a result of the complicated and energy intensive fabrication process,
inevitable use of toxic chemicals, heavy cell weight, and the high cost of manu-
facturing and installation [ 9 ]. A retail price of about $2 per peak watt (Wp) with a
corresponding production price of about $0.5/Wp could make PV cost-competitive
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