Second Law of Thermodynamics

The second law of thermodynamics states that energy always seeks a lower level, or, in a manner of speaking, only runs downhill. Energy is only useful when it moves through a device from a higher level to a lower level. The level of obtainable benefit is proportional to the decrease in level that is available. The decrease in level is denoted by an increase in molecular disorder, or entropy (s)

Entropy is the thermodynamic property of the system held constant in a reversible adiabatic process. The entropy is a useful parameter in accounting of energy conversions that result in production of work as well as energy unavailable to do work (i.e., heat loss from the system and loss due to internal irreversibilities). It measures the relative molecular disorder of a given system. Entropy can be expressed as:

Where:

T = Temperature at the boundary

Commonly used units for expressing entropy are Btu/lbm per °F and kJ/kg per °K.

This increase in entropy is unavoidable because energy never develops enough work to restore itself to the original level. The conversion of energy is, therefore, never completely reversible, hence, there are no perpetual motion machines.

The second law of thermodynamics can be stated in a number of equivalent ways. One statement is: no heat engine, either actual or ideal, when operating in a cycle, can convert all the heat energy supplied to it into work. Some of the heat energy must be transferred to a heat sink at a temperature lower than the temperature at which the heat energy is supplied. A consequence of the second law is that for any actual process, ds , > 0

system

Where:

s = Entropy

The principle of the increase of entropy, based on the second law of thermodynamics, is that the only processes that can occur are those in which the net change in the entropy of system plus the surrounding region increases. Therefore,

system unding

The important concepts operating in a thermo-dynamic system are then energy and entropy. A unit of energy represents a certain potential to do work. The entropy change of the system is a measure of the irreversibility of that process and the degree to which energy has not been made available to perform work.

Guide to Alternative Fuels

Guide to Alternative Fuels

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