Exergy analysis of solar radiation processes - Solar Energy Sciences and Engineering Applications - page 35

equal to the energy surplus relative to the reference state. This surplus has to account
for the latent heat of eventual change of the substance phase.
In addition, the components of negligible or constant value must not be taken
into account. For example, the energy of surface tension can be included only in
consideration of a fluid mechanics process of liquid atomization or of a mechanical
process of solid material comminution. Both processes have been analyzed by Petela
(1984a,b).
Energy exchange ( E in and E out ) with system can occur on different ways.
Electrical energy can be delivered for heating the system, for driving an electric
motor, or generating an electromagnetic effect within the system (e.g. a strong electric
field affects combustion). In reverse processes the electric energy can be obtained, e.g.
with use of an electric generator the energy is obtained from the system. The energy
flux of electric energy (power) is measured by a wattmeter.
Mechanical work can be exchanged with a system by means of a piston rod of
reciprocal motion or with a rotating shaft.
The energy balance of a system should comprise the mechanical work performed
by all forces acting on system boundary. Therefore, if a substance flux passes through
the boundary then the work performed by the force acting in the place of passing
should be taken into account. Such work of transportation of substance through the
boundary is expressed by enthalpy. For some kind of substance the enthalpy can be
calculated with specific formulae, e.g., formulae for plasma are discussed by Petela
and Piotrowicz (1977).
If the considered system is moving relative to the coordinate system determining
the location and velocity, then the work done by the forces causing the system displace-
ment, has to be accounted for. The energy balance should also include the work done
by deformation of the system boundary if its shape changes during consideration.
Kinetic energy should be accounted if substance passes the system boundary with
significant velocity relative to the boundary.
Potential energy of a substance exchanged with a system is included in the energy
balance if the substance has significant elevation above the reference level. This energy
component results from the presence of the gravity field.
Energy transferred by heat occurs by direct contact of system with body at a
temperature different from the system temperature, or can occur without contact by
radiation. The effect of contact during heat exchange appears in heat conduction as well
as in heat convection. The model of pure conduction occurs when the particles of the
contacted body do not change their location (solids). The energy is then transferred
by free electrons and oscillations of atoms in the crystal lattice. Still pseudo pure
conduction can be recognized between fluids of very laminar flow; conduction occurs
in the direction perpendicular to the ordered motion of particles at component velocity
only in the flow direction. In such cases, excluding the possibility of diffusion, there is
no perpendicular substance flow and in spite of the medium flow this heat is transferred
by conduction.
The essence of heat convection is the motion of substance (fluids), during which
mixing of hot and cold fluids occur. However, the micro-mechanism of this mode of
heat transfer, also depends on direct effective contacts (conduction) between the hot
and cold fluids portions being replaced. If mixing is caused by non-uniform distri-
bution of density (temperature profile) then convection is called natural convection .