Environmental Engineering Reference
In-Depth Information
entail some dissipation of energy, usually in the form of heat. Strictly speaking, the energy
converted into heat has not been lost. However, it is not easily recovered. Staring into an
open wood fire on a cold winter's evening, it is easy to become mesmerised by the sparks
rising with the smoke. What we are witnessing is the chemical energy stored in the wood
being converted into heat and light. However, the second principle prevents the opposite
occurring: heat and smoke cannot be converted back into a woodpile. Part of the energy
has been so widely dispersed that it cannot be retrieved.
The Low Efficiency of Energetic Conversions
Strictly speaking, the terms 'energy production' and 'energy loss' are incorrect, as -
according tothe first law ofthermodynamics -energycan beneither created nordestroyed.
What we observe in physics or chemistry is merely a conversion from one form of energy
into another. Fully efficient energy conversion is possible only in theory, and indeed
most conversions are highly inefficient. The engine of a car provides a good example of
how energy gets 'lost' in conversion. Cars run thanks to a controlled explosion in the
combustion chamber. Thus, chemical energy (fuel) is first converted into thermal energy
(heat), and then into kinetic energy (motion). However, within this threefold conversion
process only 10 per cent of the chemical energy contained in the petrol or diesel is
converted into motion. So, what happens to the other 90 per cent? About three-quarters of
it is lost either as heat or consumed by the car's cooling system, while the remainder is
lost as a result of friction (of tyres gears and air drag), idling, and auxiliary functions such
as air-conditioning and power steering. Some conversions are even more inefficient (for
example, a candle transforms no more than 0.01 per cent of the chemical energy in the wax
into light), while others are considerably more efficient: an electric motor transforms about
80 per cent of the electricity consumed into mechanical energy.
Table
1.1.
Comparison
of
different
forms
of
energy
conversion
and
their
efficiencies
Process/technology
Conversion
Efficiency
Energy
loss
light
→
chemical bonds 0.2-0.3% heat
Photosynthesis (wild
plants)
light
→
chemical bonds 2-5%
Photosynthesis (crops)
heat
