Environmental Engineering Reference
In-Depth Information
Table 1.1
Conversion Factors for Energy
kJ
kcal
kWh
kg ce
kg oe
m
3
gas
BTU
1 kilojoule
(kJ)
1
0.2388
0.000278
0.000034
0.000024
0.000032
0.94781
1 kilocalorie
(kcal)
4.1868
1
0.001163
0.000143
0.0001
0.00013
3.96831
1 kilowatt-
hour (kWh) 3600
860
1
0.123
0.086
0.113
3412
1 kg coal
equivalent
(kg ce)
29,308
7000
8.14
1
0.7
0.923
27,779
1 kg oil
equivalent
(kg oe)
41,868
10,000
11.63
1.428
1
1.319
39,683
1 m
3
natural
gas
31,736
7580
8.816
1.083
0.758
1
30,080
1 British
Thermal
Unit (BTU) 1.0551
0.252
0.000293
0.000036
0.000025
0.000033
1
is the first derivative of the work,
W
, with respect to the time,
t
. Thus, power
describes the period of time in which the correlated work is performed. For
instance, if a person lifts a sack of cement 1 metre, this is work. The work
performed increases the kinetic energy of the sack. Should the person lift the
sack twice as fast as before, the period of time is half. Hence the power needed
is twice that of before, even if the work is the same.
The units of both energy and work according to the SI unit system are
joules (J), watt seconds (Ws) or newton metres (Nm), and the unit of power is
the watt (W). Besides SI units a few other units are common in the energy
industry. Table 1.1 shows conversion factors for most units of energy in use
today. Older literature uses antiquated units such as kilogram force metre kpm
(1 kpm = 2.72
•
10
-6
kWh) or erg (1 erg = 2.78
•
10
-14
kWh). Physics also
calculates in electronvolts (1 eV = 4.45
•
10
-26
kWh). The imperial unit BTU
(British Thermal Unit, 1 BTU = 1055.06 J = 0.000293071 kWh) is almost
unknown outside the US and the UK. Common convention is to use SI units
exclusively; this topic follows this convention apart from using electronvolts
when describing semiconductor properties.
Many physical quantities often vary over many orders of magnitudes;
prefixes help to represent these and avoid using the unwieldy exponential
notation. Table 1.2 summarizes common prefixes.
Errors often occur when working with energy or power. Units and
quantities are mixed up frequently. However, wrong usage of quantities can
change statements or cause misunderstandings.
For example, a journal article was published in the mid-1990s in Germany
describing a private photovoltaic system with a total installed power of 2.2
