DEFINITIONS  Exergy ( Availability energy):- The maximum portion of energy which can be converted into useful work by reversible that can be obtained from a system at a given state in a given environment; in other words, the most work you can get out of a system  Dead State:- when a system is in thermodynamic equilibrium with the environment, denoted by a subscript zero; at this point no more work can be done

 Unavailable energy:- It is that portion of energy which can not be converted into work even by reversible process which reduces the system in a state of equilibrium(dead state)  Available energy:- It is theoretical maximum amount of work which can be obtained from a system at any state.  High grade energy :- It is energy that can be completely converted into shaft work without any loss.  Example:- mechanical work, electrical energy,water power, wind power.  Low grade energy :- It is energy of which only a certain portion can be converted into mechanical work.  Example :- heat thermal energy, heat from nuclear fision or fusion, heat from combustion of fuels like coal, wood, oil, etc..

= T env 1 - THTH Infinite heat source at T H K Available energy refered to an infinite heat source Environment at T env K η thermal = W out Q in W out Q in Q out η Carnot = W max Q in Exergy = W max = η Carnot Q in = Q in T env 1 - THTH

EXERGY OF A FLOW STREAM  Flow Exergy  Energy needed to maintain flow in pipe  w flow = Pvwhere v is specific volume  Exergy of flow work = exergy of boundary work in excess of work done against atom pressure (P 0 ) to displace it by a volume v, so  x = Pv-P 0 v = (P-P 0 )v

EXERGY OF A FLOW STREAM  Giving the flow exergy the symbol ψ  Flow exergy :- Ψ=(h-h 0 )-T 0 (s-s 0 )+½Vel 2 +gz  Change in flow exergy from state 1 to state 2 is :- Δψ = (h 2 -h 1 )-T 0 (s 2 -s 1 )+ ½(Vel 2 2 – Vel 1 2 ) +g(z 2 -z 1 )

 The exergy of an isolated system during a process always decreases or, in the limiting case of a reversible process, remains constant.  This is known as the decrease of exergy principle and is expressed as The Decrease of Exergy Principle

 Irreversibilities such as friction, mixing, chemical reactions, heat transfer through finite temperature difference, unrestrained expansion, non-quasi- equilibrium compression, or expansion always generate entropy, and anything that generates entropy always destroys exergy.  The exergy destroyed is proportional to the entropy generated as expressed as Exergy Destruction

 The decrease of exergy principle does not imply that the exergy of a system cannot increase.  The exergy change of a system can be positive or negative during a process, but exergy destroyed cannot be negative.  The decrease of exergy principle can be summarized as follows: