Environmental Engineering Reference
In-Depth Information
Energy-generation capacity and capacity factor
Electricity-generation capacity is measured in watts, which is a unit of power equivalent to
one joule per second (J/s). Because a watt is a small unit, generation capacity is expressed in
megawatts (MWs), which is the equivalent of 1 million watts, or for a small power generation
unit in kilowatts (kWs), which is the equivalent of 1,000 watts.
When consumers buy electricity from utility companies, they are billed for the power
consumed in a period of time, which is equivalent to energy. For instance, 1 kW-h, a unit used
in residential metering, is equivalent to 3.6 megajoules (MJ):
⎛⎞
J
J
3600
s
1
kWh
10
3
×=×××
h
10
3
h
=⋅
3.6 10
6
J
=
3.6
MJ
⎝⎠
s
s
h
Because industrial consumers use massive amounts of energy, they are billed in megawatts
per hour (MWh) instead of kWh.
The concept of generation capacity is applicable not only to public utilities but also to on-site
projects. For instance, a 40 kW peak solar PV array, installed on the roof of a warehouse,
produces 40 kW of electricity per hour at its peak (equivalent to 40 kWh). However, solar panels
produce energy intermittently, and the output depends on the solar radiation. Thus,  the total
energy production needs to be corrected by a capacity factor, which is defined as:
Actual energy output
[11.3]
Capacity factor
=
Output of the system operating at full capacity
Then,
[11.4]
Energy production (kWh)
=
Capacity (kW)
×
Capacity factor
×
Time
Assume a 200-kW wind turbine is installed. If the turbine runs at full power 24 hours a day
for 365 days, then the energy produced in a year would be:
200 kW
×
(24 h/day
×
365 day)
=
1, 752, 000 kWh
Now, suppose that the real measured energy produced by the turbine was 473,040 kWh; then
the capacity factor is:
Capacity factor
=
473, 040/1, 752, 000
=
0.27
=
27 percent.
Energy outputs of all power plants are affected by a capacity factor. Typical capacity factors
are 20 to 40 percent for wind sources, between 30 and 80 percent for hydro around 60 for
nuclear and 70 to 90 percent for base-load coal plants (Renewable Energy Research Laboratory
([RERL], n.d.). For solar PV, the capacity factor is a function of the insolation at the location
(amount of solar energy at that latitude), the efficiency of the solar panels, the orientation of
the panels, and efficiency of electronic equipment, such as inverters (Fraas and Partain, 2010).
In Arizona, the calculated capacity factor for solar PV over a 2-year period for a 4.6-MW PV
plant was 19 percent (Apt and Curtright, 2004), whereas in Massachusetts, the capacity factor
varied from 12 to 15 percent (RERL, n.d.) and in California, averages were 24.6 percent (DOE,
2009). The DOE estimates a capacity factor of 0.83 for biomass, 0.90 for landfill gas applica-
tions, and 0.9 for geothermal projects (DOE, 2009).
 
Search WWH ::




Custom Search