Environmental Engineering Reference
In-Depth Information
Table 11.1
Energy return on investment (EROI) for selected energy sources.
Resource
EROI
Reference
East Texas oilfields, 1930
100:1
Cleveland, 2005
US oil today
11:1 to 18:1
Alberta oil sands, surface
7.2:1
“Fact Sheet,” n.d.
Alberta oil sands, in-situ
5:1
US oil shale, nonelectric heat
6.9:1
US oil shale, electric heat
2.5:1
Nuclear
5:1
Kubiszewski et al., 2006
Coal
8:1
Hydro
12:1
Wind
18:1
Photovoltaic solar
8:1
Sugarcane-based ethanol, United States
1.12:1
Pimentel and Patzek, 2007
Sugarcane-based ethanol, Brazil
1.38:1
Sugarcane-based ethanol, Brazil
8:1
Smeets et al., 2006
Corn-based ethanol
0.78-1.29
Randolph and Masters, 2008
Energy quality
Does a Btu of electricity have the same value than a Btu of coal? Definitely it does not
because electricity has a higher energy quality than coal, and that is the reason why people
are willing to pay a premium for electricity in relation to other energy sources. Electricity
(a secondary source of energy) is a more refined energy that has undergone a conversion
process with a yield factor in between. For example, to produce 1 kW-h of electricity, it
takes much more than one 1 kW-h of heat contained in coal, biomass, or a liquid fuel.
Assuming an efficiency of 40 percent for a coal-fired plant, then it takes about 2.5 units
(1/0.4) of energy to produce one unit of electricity. This conversion loss is reflected in the
price as a premium, and the public is willing to pay that premium because of the versatil-
ity of electricity. Consider an electric motor compared with a gasoline engine of the same
power. An electric motor is smaller, quieter, cleaner, generally lighter, simpler, and more
durable than a homologous gasoline engine. Energy quality is not universal, though, and
it depends on the application. To run an automobile, gasoline has a higher quality than
electricity because of its higher energy density in reference to its volume and weight.
Energy quality is a measure of the capacity of a specific unit of energy to perform tasks for
the society. It is not represented with just one parameter, but instead with metrics that capture
physical, technical, economic, and social attributes. For the gasoline example, besides energy
density, other important metrics are emissions, power density, cost and efficiency of conver-
sion, ease to storage, portability, and risk to human health (Cleveland, 2008).
Energy density represents the amount of energy stored in a system per unit of volume or
per unit of mass. Figure 11.3, which shows the energy density for selected fuels, demonstrates
why the high content of energy per unit of volume in liquid petroleum-derived fuels (diesel
and gasoline) makes them preferred transportation fuels.
Energy density is an important parameter in terms of portability; however, it does not
indicate anything about the conversion efficiency or environmental impacts to produce the
energy. An indicator that measures one aspect of emissions is the carbon per heat unit. This
indicator provides the amount of carbon dioxide emitted to the atmosphere for each unit of
heat or energy. For instance, coal produces 208 pounds of carbon dioxide per 1,000 Btu of heat
