Environmental Engineering Reference
In-Depth Information
What Carnot demonstrated is that when transferring heat from a high-temper-
ature source to a lower-temperature reservoir to produce work, 100% efficiency is
never achieved, that is, some heat always dissipates. For example, Carnot showed
that if the initial temperature is 177 °C and the final is 27 °C, the efficiency is 0.25.
In other words, only 25% of work is performed and the remaining 75% is lost in
low-temperature heat plus a minor fraction in friction. The efficiency of conversion
is higher when the initial temperature (T
0
) is much higher than the final temperat-
ure (T).
It is impossible to convert energy completely into work.
This is one of the ways to state the Second Law of Thermodynamics: there is a
time direction in the events that occur in nature. Heat from high-temperature bod-
ies can flow to low-temperature bodies, but the inverse does not happen. Thermal
events are not reversible, while mechanical events are reversible.
More recently, other, higher-efficiency systems have been developed, such as
turbines, internal combustion engines (such as Otto and Diesel engines), jet tur-
bines, reactors, and jet rockets. Original steam engines had a maximum efficiency
of 5%, but, over time and with the introduction of technical improvements, their
efficiency has reached approximately 50%. Modern thermal machines such as gas
turbines operate at temperatures on the order of 1,000 °C and are therefore highly
efficient.
A Sankey diagram is a type of flow diagram used to visualize energy, materials,
cost transfers, and even movement of troops in battle. The width of the arrows is
proportional to the amount of flow.
