Modern Engineering Thermodynamics

700 CHAPTER 17: Thermodynamics of Biological Systems
Work done (W)
Heat loss (Q)
Waste energy out
Food energy in
FIGURE 17.5
Energy flows in living systems.
Life processes all have some degree of irreversibility. Therefore, _Q normally is negative since the internal irreversibilities
generally produce internal heat generation, which must be removed from the system if the system is
not to overheat.
Classically, the concept of work in thermodynamic analysis has been somewhat ambiguous. As discussed in
Chapter 4, during the development of thermodynamics, it was convenient to separate the changes in kinetic and
potential energies from the work term. These energy terms are written separately and usually grouped with the
system’s total internal energy change, as shown in Eq. (17.11). Thus, the work term in the thermodynamic
energy balance encompasses all the work transport of energy into or out of a system except the work associated
with changes in the system’s kinetic and potential energy. This can be quite confusing when analyzing biological
systems, since one of their major work modes in a social or cultural context is that of mobility, that is, running
and climbing, which are the kinetic and potential energy terms we are discussing. Also, whereas a classical thermodynamic
system can either do work or have work done on it, in general, a biological system only does work
(i.e., the work term is always negative).
As with nonliving work-producing systems, we can define an energy conversion efficiency as
Desired energy result
Energy conversion efficiency = η E = (17.12)
Required energy input
The term energy used in this equation must include relevant kinetic and potential energy changes. For example,
the energy conversion efficiency of a human climbing a hill could be calculated by choosing the change in
potential energy of the person as the desired energy output, while ignoring other types of energy output simultaneously
performed (such as aerodynamic drag against the atmosphere). This is acceptable, providing the meaning
of the efficiency is clearly defined in each case.
The required energy input part of Eq. (17.12) is more difficult to evaluate. Since, for warm-blooded animals,
the net _Q is always out of the system, it cannot be considered as a source of energy input. Also, one cannot
generally input useful energy into a biological system via changes in the system’s kinetic or potential energies.
Thus, what remains is
Required energy input = − dU
dt
Then, we may write Eq. (17.12) as
η E =
_W + d
mV 2
+ d
mgZ
dt 2g c dt g c
= 1 +
−dU/dt
_Q
−dU/dt
(17.13)
and since both _Q and dU/dt are always negative, it is clear that Eq. (17.13) gives an energy conversion efficiency
that is always less than 100%.