From Thermodynamics to Economics and Ecology - Thanatia: The Destiny of the Earth's Mineral Resources - page 69

Fig.3.1Exergyofaheatflowasafunctionoftemperature
It can be observed that when T is greater than T 0 , exergy is always smaller
than Q. In fact, only at infinite temperatures will it ever be equal to the value
of energy. For instance, the exergy that the Earth receives from the sun at an
equivalent temperature of about 5,700 K constitutes 95% of its energy with respect
to the average ambient temperature at 288 K. As the heat flow approaches T 0 , its
exergy tends to zero which is why the thermal exergy of the human body at 37 o C
(310 K) constitutes only 7% of its energy. Or in other words, the best thermal
engine that one could design for extracting work from the heat of the human body
would only ever have a maximum e ciency of 7%. So, even if it is theoretically
possible to move the hands of a watch using solely body heat, any such item would
certainly be very expensive and as uncomfortably bulky as a conventional thermal
engine.
Nevertheless, as can be observed in Fig. 3.1 exergy never surpasses zero to
become negative. This is due to the fact that heat at this point flows in the opposite
direction, i.e. T 0 to T, as heat always flows from hot to cold bodies. The exergy of
a system reaches infinity when the temperature tends towards absolute zero Kelvin.
This implies that cooling a body below ambient temperature is very di cult and
that this di culty increases as T decreases. Indeed even if scientists had the whole
energy of the universe at their disposal, it would not be enough to force absolute
zero. This is in effect the formulation of the true meaning of “it is impossible by any
procedure, no matter how idealised, to reduce the temperature of any system to zero
temperature in a finite number of finite operations”, a statement which constitutes
the Third Law of Thermodynamics.