Environmental Engineering Reference
In-Depth Information
increases until a phase change occurs. During the phase change, the solid
bonds are broken at a constant temperature as heat is added to the material.
This is the fundamental difference between sensible and latent heat. After
the material has completed the phase change, in this case from a solid to
a liquid, the material again undergoes an internal energy and temperature
increase with the application of heat.
In general, it is desirable for TES to use materials that possess a large
change in internal energy per unit volume and/or mass to minimize the
space needed to store the desired amount of energy. To be economically com-
petitive in commercial solar systems, it is important to employ substances
with a high internal energy change per unit cost. Other properties such as
vapor pressure, toxicity, and corrosiveness also must be considered since
they influence the price of material containment and operation in commer-
cial systems.
Sensible Heat Storage
Sensible heat can be hot or cold potential energy with respect to the ambi-
ent temperature, stored in a solid, liquid, or dual medium that consists of
some combination of liquid and solid media. The internal energy change
in sensible heat is dependent upon mass, specific heat, and temperature
change:
∆
U mc T T
p
=
(
−
)
(7.1)
1
2
∆U
represents the change in internal energy of the material in kilojoules,
m
represents the mass of the material in kilograms,
c
p
is the specific heat capac-
ity (kJ/kg·K) and
T
1
and
T
2
are the initial and final temperatures of the mate-
rial, respectively, in Kelvin.
Sensible Heat Storage Materials
The material chosen for TES with sensible heat must be thermally stable
and undergo no phase change during temperature extremes. The substance
should also have a high heat capacity, high density, and an acceptably low
vapor pressure. To be economically viable, it must be inexpensive. Several
common sensible heat storage materials and some of their thermal proper-
ties are listed in Table 7.1.
Latent Heat
Latent heat is the amount of energy in the form of heat released or absorbed
by a material during a change of state or phase transition such as solid to
liquid (melting) or liquid to gas (vaporization). Latent heat energy storage is
