Environmental Engineering Reference
In-Depth Information
40
Diesel
35
Biodiesel
Gasoline
30
Liquified petroleum
gas
25
Ethanol
Liquified natural gas
20
Coal
Methanol
15
10
Liquid hydrogen
Wood chips from
forest residues
5
Corn stover (bale)
Hydrogen -1 atm
Natural gas -1 atm
0
0
20
40
60
80
100
120
140
Gravimetric density (MJ/kg)
Figure 11.3
Gravimetric and volumetric energy densities for different forms of energy.
Data for corn stover and wood chips from Searcy et al., 2007; for the others, from Boundy et al., 2010.
produced (89.6 kg/MJ), whereas oil and gas produce 164 pounds of carbon dioxide per
1,000 Btu (70.6 kg/MJ) and 117 pounds of carbon dioxide per 1,000 (50.4 kg/MJ), respectively
(“Natural Gas 1998 Issues and Trends,” 1999). Other indicators for emissions include the
release of sulfur, nitrogen compounds, particle matter, and methane.
Power density is a metric that expresses power per unit of mass, volume, or area. Power
per unit of area becomes handy as an energy quality indicator for renewable resources. The
power density of sun radiation on Earth is approximately 1.4 kW/m
2
. Plants, through
photosynthesis, are able to capture between 3 and 6 percent of the incoming energy.
However, under physiological conditions photosynthesis efficiency drops to 0.7 percent
(Gebhardt, 1986). High quality PV solar collectors, on the other hand, are capable of
capturing and converting 14 percent of the incident radiation directly into electricity. From
the point of view of area utilization, solar collectors are a better choice to capture energy
from the sun than plants, and the use of biomass as the only substitute for fossil fuels seems
flaky. A shortcoming of PV collectors, which convert sunlight directly into electricity is
storage. Electricity is not easy to store in large quantities. In that regard, biomass has higher
quality than electricity but inferior quality in portability.
Portability refers to the ease of moving the fuel source around to power vehicles, other
mobile sources, and transportation vehicles. To be portable, a fuel needs to have a high
volumetric energy density and have a relatively low storage need. This is one disadvantages of
hydrogen because even when hydrogen has a high gravimetric energy density, its volumetric
energy density is low, requires special tanks for storage, and is explosive, which posses a risk
to humans and infrastructure.
