Environmental Engineering Reference
In-Depth Information
1.1 Introduction
Renewable energy refers to energy sources that natural processes replenish in a
short term. It has the potential to decrease our dependence on fossil fuels and
reduce global warming. Renewable energy currently supplies approximately
14% (or 57 EJ/year) of the energy consumed by humans around the world [1],
and the contribution of renewable energy in the global market is growing.
Incident solar energy carries 173,000 TW of energy to Earth; that corresponds
to 5.5 million EJ/year, which is more than 10,000 times greater than the current
annual consumption of primary energy by human activities (418 EJ/year) [1].
Photosynthesis to produce biomass captures an average of about 140 TW, or
less than 0.1% of the incident energy.
Although the average conversion percentage may seem small, photosynth-
esis actually produces a significant amount of biomass that embodies renewable
energy. Annually, photosynthesis produces 220 billion dry tonnes of biomass in
primary production, and the corresponding energy value for that biomass is
4500 EJ/year [2]. The energy value of terrestrial biomass available for human
use has been estimated as 67-450 EJ [2] and, thus, corresponds to 25-110% of
the current primary energy consumption.
Chynoweth et al. [3] estimated the energy potential from domestically avail-
able biomass in the United States. Energy crops can provide 22 EJ/year, and
marine crops have a more significant energy potential of
>
100 EJ each year. On
a yearly basis, 7.5 EJ of energy value is available fromwaste sources that include
municipal solid waste, biodegradable industrial wastes, crop residues, animal
wastes, sewage sludge, and sludge-grown biomass. This energy equals 7%of the
U.S. energy use of 105 EJ in 2005 [4]. Among the wastes, food-processing,
brewery, and agricultural wastewaters are ideal candidates for energy genera-
tion, because they contain high levels of easily degradable organic material that
converts to biofuels easily using microorganism-based systems.
Wastes are produced at all human habitats, and their local consumption
reduces energy losses due to transportation. In 2006, the United States gener-
ated 57.8 million tonnes of municipal organic waste, which includes food and
yard trimmings [5]. Since transporting 1 tonne of municipal organic waste for
1 km by truck requires approximately 48 MJ of energy [6], transporting 57.8
tonnes for 1 km requires 2.8 PJ of energy or 459 million barrels of crude oil.
Local consumption of organic wastes for energy generation can potentially save
PJs of energy and provide double benefit by treating the pollutants so that they
are no longer harmful to the environment.
The technological challenge is capturing this sustainable biomass energy with-
out creating serious environmental or social damage. In the next sections, we
introduce the general cycle for biomass-based renewable energy and discuss
where two contemporary alternatives for capturing biomass energy - bioethanol
and anaerobic digestion to methane - fit in the context. Then, we introduce the
microbial fuel cell (MFC) and showwhere it fits in the context of the energy cycle.
