About the second law of thermodynamics

Second law of thermodynamics provides the criterion as to the probability of various processes. Sadi Carnot established this law in 1824. But it got importance from last two decades (1980 onwards) in view of conservation of energy. First law dictates that energy in a system is always conserved. There is no way to conserve energy by use of first law of thermodynamics. However, it is the exergy, which is a consequence of the second law that is never conserved. Unlike energy, exergy always decreases. So minimization of exergy loss is nothing but the principle of energy conservation. The net exergy output to the actual exergy input to the system is the second law efficiency.

1.2

where A = exergy

To improve a system, we always try to improve the second law efficiency. Exergy is a tool to identify the loss of energy. In complicated systems, exergy loss at different locations can give an estimation of such losses and measures can be taken up to reduce the same. Exergy analysis will be discussed in lectures 7-12 of Module 1.

Apart from the energy conservation, two important implications of second law of thermodynamics are:

Directional flow of energy

A spontaneous process occurs only in one direction. Heat always flows from a body at high temperature to a body at lower temperature, water always flows downhill, time always flows in forward direction. Reverse of these processes never happen spontaneously. The spontaneous process is due to a finite driving potential called FORCE or CAUSE. The outcome or result is called the FLUX, CURRENT or EFFECT.

Table: 1.5 Forces and fluxes

Sl No	Force (Cause)	Conjugate fluxes (Effect)
1	Temperature Gradient	Heat Transfer
2	Concentration Gradient	Mass Transfer
3	Electric Potential Gradient	Flow of Electric Current

TRANSFER PROCESSES CAN NEVER SPONTANEOUSLY OCCUR FROM A LOWER TO A HIGHER POTENTIAL. SECOND LAW OF THERMODYNAMICS PUTS LIMITATION ON DIRECTION OF PROCESS OCCURANCE.

Qualitative measurement of energy

The second law distinguishes energy in two different forms (1) High grade energy and (2) Low grade energy. High grade energy is an orderly form of energy whereas low grade energy is in random form. Some high and low grade energy forms are given in Table 1.6.

Table: 1.6 Different form of energy

High Grade Energy	Low Grade Energy
Electrical energy	Thermal Energy from Fossil Fuel
Wind energy	Nuclear Fission
Hydropower	Nuclear Fusion
Kinetic energy of a water jet	Waste Heat from Thermal Power Plant
Mechanical work	Solar Thermal Energy
Tidal power	Geothermal Energy

Quality of energy can be ascertained by applying the second law of thermodynamics to a process or a system.