Page 119 - Biosystems Engineering

100 Cha pte r T h ree

carbon uptake from the atmosphere. When stomata are open, plants
transpire water. Almost 95 percent of the water taken by the roots is
lost back to the atmosphere by transpiration. Water loss from the
leaves causes the osmotic potential of cells to become negative. This
increases the driving force from stem to leaves and stimulates closure
of stomata.
Water yield could be increased if water loss through evaporation
could be minimized without jeopardizing other natural resources.
One such approach is the application of some solvents to lake sur-
faces to minimize water losses through evaporation. Forest cutting
has shown to increase water yield significantly. Motivated by this
fact, some scientists propose removal of massive forest areas to
increase stream flow in rivers. However, we should also look at the
other side of the coin. In spite of an increase in water yield, forest
removal could have significant adverse impacts on the environment,
the most obvious one being increased soil erosion. Damage to wild-
life habitat and ecosystem is another aspect that also needs to be con-
sidered. The irrationality of this idea is obvious.

3.4.1 Some Relevant Concepts and Terminology

Sensible Heat Portion of internal energy proportional to tempera-
ture. This is the heat that can be sensed by contact.

Specific Heat Capacity (C ) Measure of how internal energy changes
p
with temperature. It is often called specific heat. It represents the
increase in internal energy per unit mass and per unit increase in tem-
perature. Mathematically, it can be defined as
Δ E
u
C = mT (3.3)
Δ
p
2
2
where E is the internal energy [ML /T ], m is the mass [M], and T is
u
the temperature. The specific heat of water at 20°C is approximately
3
4.2 × 10 J/(kg· K) or, equivalently, 1 cal/(g ·K).
Latent Heat (k) Portion of internal energy that cannot be sensed.
This energy is responsible for phase changes. In other words, latent
heat is the amount of internal energy either absorbed or released dur-
ing phase change at a constant temperature. Because evaporation is
the phase change from liquid form to gaseous form, it requires energy
added to the water. Typical values of latent heat for water are

6
Latent heat of vaporization (λ ) = 2.45 × 10 J/kg (at 20°C)
v
5
Latent heat of melting (λ ) = 3.34 × 10 J/kg (at 0°C)
m
Latent heat of sublimation (λ ) = λ + λ
s v m