First Principles - Sustainable Design: The Science of Sustainability and Green Engineering - page 37

Also seemingly obvious is what separates the system from the surroundings: the
boundary .
Systems are classified into two major types: closed and open. Both exist and
are important in the environment. A closed system does not allow material to
enter or leave the system (engineers refer to a closed system as a control mass ).
An open system allows material to enter and leave the system. Such a system is
known as a control volume . Two control volumes that are commonly considered in
green design are the organism and a defined volume around the organism. Thus,
scientists commonly calculate mass balances for the classic control cube and adapt
it to the environment (see Fig. 2.3). The human body is a control volume. For
example, physiologically-based pharmacokinetic models (called PBPK models )
consider the amount of a contaminant or nutrient entering a body, its changes,
and the amount leaving the body. By definition, this is a control volume. Each
of these volumes meets the same criteria as those for the cube in Figure 2.3, fully
accounting for the mass in and out, as well as the processes that occur within
them.
A few special thermodynamic considerations must be taken into account when
dealing with an organism as a control volume. Body burden is the total amount
of the contaminant in the human body at a given time of measurement. This is
an indication of the behavior of the contaminant in the control volume (i.e., the
person). Some contaminants accumulate in the body and are stored in fat or bone,
or they simply are metabolized more slowly and tend to be retained for longer
periods. This concept is at the core of PBPK models. These models attempt to
describe what happens to a chemical after it enters the body, showing its points
of entry, its distribution (i.e., where it goes after entry), how it is altered by the
body, and how it is ultimately eliminated by the body. This is almost identical to
the processes that take place in a stream, or a wetland, or other system.
In our freshman green engineering course at Duke, for example, one of the
studios addresses the indoor environment. In variably the students are concerned
about volatile organic compounds (VOCs) as a group. However, upon further
investigation, they find that VOCs vary considerably in how they behave in
buildings and in organisms (including humans).
The building is an important focus of green engineering and sustainable de-
sign. Recently, engineers and scientists have applied mass balance approaches to
the individual home. Unlike the well-defined boundary conditions of the small
control volume, a home has numerous inflows and outflows as well as sources,
sinks, and transformation reactions. Some are shown in Figure 2.4. Modeling
these dynamics is useful in estimating the exposure of people to toxic sub-
stances. Thus, to design a building properly, a thorough understanding of systems
is essential.
Another thermodynamic concept is that of property . A property is some trait
or attribute that can be used to describe a system and to differentiate that system