Environmental Engineering Reference
In-Depth Information
In Fig.
5.2
, CO
2
is in its supercritical state when conditions of temperature and
pressure are greater than
T
c
and
P
c
—shown as the supercritical region. Through
variation of temperature and pressure, it is possible to change the state of a fluid
from liquid to vapor and vice versa without abrupt phase change by letting the fluid
pass through the supercritical region as also shown in Fig.
5.2
. During the change
of conditions of a fluid through the critical region, the fluid phase remains homoge-
neous and the path is referred to as
transcritical
. The lack of an abrupt phase change
for fluids in their supercritical state is highly-advantageous in heat and mass trans-
port and it is a key concept in developing highly-efficient energy systems.
5.3
Supercritical Fluids and Their Use in Energy Systems
Supercritical fluids are being studied for use in heating, cryogenic exergy recovery,
geothermal and waste heat energy, refrigeration, ultra-supercritical steam genera-
tors, biofuel synthesis and biomass conversion types of energy systems (Smith et.al.
2013
; Machida et.al.
2011
). In some of these applications, the supercritical fluid is
used as a
working fluid
, which means that the substance acts to transport energy in
a cycle through various devices and only undergoes heat and work exchange with
the environment. The homogeneous phase conditions of the supercritical region
give highly-efficient energy transport. Energy systems that use a working fluid in
its supercritical state are referred to as transcritical cycles since conditions traverse
the critical region. In other applications, the supercritical fluid is used as a reaction
solvent or as a solvent to enhance mass and heat transport to efficiently transform
raw materials into chemicals or products. The next sections provide some basic
definitions used to evaluate the efficiency of a device and provide an overview of
some of the applications of supercritical fluids in energy systems.
5.3.1
Thermal Efficiency
Consider an energy system in which the objective is to produce heat, for example,
a natural gas burner to produce warm air. The thermal efficiency of the system can
be defined as a ratio of energy obtained to that supplied:
Energy obtained
Energy supplied
η
=
thermal
(5.1)
Not all of the energy value of the fuel (energy supplied) can be obtained so that
η < 1 . The thermal efficiency depends on the device and also depends on the
type of energy being supplied. Thermal efficiencies for coal and fossil fuel combus-
tion are typically 40-90 %, with the higher values being for natural gas burners- since
natural gas can achieve more complete combustion than coal among other practi-
cal considerations. Heating by a device that uses electrical resistance, on the other
hand, is almost 100 % efficient. However, the electricity used in the device must be
generated by power generation processes that are generally only about 40 % efficient.
thermal
