First Principles - Sustainable Design: The Science of Sustainability and Green Engineering - page 39

Figure 2.4 The building as a
control volume. Note the various
ways that contaminants can enter
the volume and the numerous
physical and chemical
mechanisms that can transform
the material that enters.
From U.S. Department of Energy,
Lawrence Berkley Laboratory,
http://eetd.lbl.gov/ied/ERA/
CalEx/partmatter.html, 2003.
The thermodynamics term for the description of the change of a system
from one state (e.g., equilibrium) to another is known as a process . Processes
may be reversible or irreversible, and they may be adiabatic (no gain or loss of
heat, so all energy transfers occur through work interactions). Other processes
include isometric (constant volume), isothermal (constant temperature), iso-
baric (constant pressure), isentropic (constant entropy), and isenthalpic (constant
enthalpy).
Thermodynamic terms are crucial to engineers, architects, and other design
professionals working collaboratively with them. For example, you may attend
a seminar or meeting where the engineer refers to conditions. Often, these are
of two types: initial conditions and boundary conditions. You may also hear
the terms assumptions , constraints ,and drivers . These are all rooted in thermody-
namics. An initial condition is where we start. For example, differential equations
require an initial condition before calculating changes. A boundary condition is
imposed on the solutions of differential equations to fit the solutions to the
actual problem. In models, the boundary conditions describe what is expected
to occur along the edges of the simulation region. Thus, initial and boundary
conditions are similar to Figure 2.2. However, instead of the system within the
boundary, the region inside the boundary is what is explained by the differential
equation, and the boundary is where this is no longer valid (i.e., the bound-
ary value given along the boundary curve). Everything outside the boundary
is not explained by the differential equation, analogous to the thermodynamic
surroundings. Constraints are those factors that must be considered as part of what
could affect the energy transfer or changes within the boundaries. Drivers are