First Law of Thermodynamics
The First Law of Thermodynamics states:
Energy can neither be created nor destroyed, only altered in form.
For any system, energy transfer is associated with mass and energy crossing the control
boundary, external work and/or heat crossing the boundary, and the change of stored energy
within the control volume. The mass flow of fluid is associated with the kinetic, potential,
internal, and "flow" energies that affect the overall energy balance of the system. The exchange
of external work and/or heat complete the energy balance.
The First Law of Thermodynamics is referred to as the Conservation of Energy principle,
meaning that energy can neither be created nor destroyed, but rather transformed into various
forms as the fluid within the control volume is being studied. The energy balance spoken of here
is maintained within the system being studied. The system is a region in space (control volume)
through which the fluid passes. The various energies associated with the fluid are then observed
as they cross the boundaries of the system and the balance is made.
As discussed in previous chapters of this module, a system may be one of three types: isolated,
closed, or open. The open system, the most general of the three, indicates that mass, heat, and
external work are allowed to cross the control boundary. The balance is expressed in words as:
all energies into the system are equal to all energies leaving the system plus the change in storage
of energies within the system. Recall that energy in thermodynamic systems is composed of
kinetic energy (KE), potential energy (PE), internal energy (U), and flow energy (PL); as well as
heat and work processes.
S (all energies in) = S (all energies out) + D(energy stored in system)
S Ein S Eout DE storage
For most industrial plant applications that most frequently use cycles, there is no change in
storage (i.e. heat exchangers do not swell while in operation).
First Law of Thermodynamics Summary
• The First Law of Thermodynamics states that energy can neither be
created nor destroyed, only altered in form.
• In analyzing an open system using the First Law of Thermodynamics, the
energy into the system is equal to the energy leaving the system.
• If the fluid passes through various processes and then eventually returns
to the same state it began with, the system is said to have undergone a
cyclic process. The first law is used to analyze a cyclic process.
• The energy entering any component is equal to the energy leaving that
component at steady state.
• The amount of energy transferred across a heat exchanger is dependent
upon the temperature of the fluid entering the heat exchanger from both
sides and the flow rates of thse fluids.
• A T-s diagram can be used to represent thermodynamic processes.
Second Law of Thermodynamics
With the Second Law of Thermodynamics, the limitations imposed on any process can be studied
to determine the maximum possible efficiencies of such a process and then a comparison can be
made between the maximum possible efficiency and the actual efficiency achieved. One of the
areas of application of the second law is the study of energy-conversion systems. For example,
it is not possible to convert all the energy obtained from a nuclear reactor into electrical energy.
There must be losses in the conversion process. The second law can be used to derive an
expression for the maximum possible energy conversion efficiency taking those losses into
account. Therefore, the second law denies the possibility of completely converting into work all
of the heat supplied to a system operating in a cycle, no matter how perfectly designed the
system may be. The concept of the second law is best stated using Max Planck’s description:
It is impossible to construct an engine that will work in a complete cycle and
produce no other effect except the raising of a weight and the cooling of a heat
reservoir.
The Second Law of Thermodynamics is needed because the First Law of Thermodynamics does
not define the energy conversion process completely. The first law is used to relate and to
evaluate the various energies involved in a process. However, no information about the direction
of the process can be obtained by the application of the first law. Early in the development of
the science of thermodynamics, investigators noted that while work could be converted
completely into heat, the converse was never true for a cyclic process. Certain natural processes
were also observed always to proceed in a certain direction (e.g., heat transfer occurs from a hot
to a cold body). The second law was developed as an explanation of these natural phenomena.
Losses due to inefficiencies:
Turbines, pumps, and compressors all behave non-ideally due to heat losses, friction and windage losses.