Building Services Engineering 5th Edition Handbook

Energy economics 35
This detailed analysis can be made for each building or department of a large site, each large
room or factory area, each type of heating, air-conditioning or power-using system, each industrial
process and each item of plant. The most serious deficiency in the acquisition of data is likely
to be the lack of sufficient metering stations. Electricity, gas and other fuels are metered by the
supply authority at the point of entry to the building or site; further metering is the responsibility
of the site user. Frequently, no further meters are installed and capital expenditure is needed to
obtain data. A careful cost benefit approach is required to assess the viability of this equipment
(Moss, 1997).
Unity brackets
In order to deal with the numbers involved, a degree of familiarity with the units and conversion
between the various types is needed. A handling technique known as the unity bracket helps to
avoid errors being made when dealing with unfamiliar combinations of units. Suppose that we
wish to convert 1260 mm into metres.
length = 1260 mm
Now, 1000 mm ≡ 1 m. Divide each side by 1 m: thus
1000 mm
1m ≡ 1
The left-hand side is now a unity bracket exactly equal to 1 (or unity). Similar unity brackets can
be formed for any suitable conversion problem. Now, multiply length by the unity bracket:
length = 1260 mm ×
= 1.26 m
1m
1000 mm
Notice that the unity bracket is arranged so that its denominator units cancel the original units.
A long chain of conversions can easily be handled and the method avoids errors of logic that can
occur if an attempt is made to cope with the problem using mental arithmetic.
EXAMPLE 2.1
British Gas sold gas by the therm up to 1992. Harmonization of the units of measurement
in Europe caused the change to kWh units. If 1 therm is equal to 105.5 MJ, how many
kilowatt-hours are there in 1 therm?
1 therm = 105.5 MJ
1MJ= 10 6 J
1h= 3600 s
1 Watt = 1J/s