Exergy Transfer by Heat, Work and Mass
By Alva Murphy
During a process exergy can be lost or gained, through heat transfer, work or mass.
From the second law we know only a fraction of energy during heat transfer can be converted into work. The maximum work that can be taken from a system at T0 is equivalent to the carnot efficiency
. This fraction of energy is the exergy.
o where T0 = environment temp T = heat source temp Q = heat energy transferred
o When the temp T isn’t constant
o Exergy is destroyed when entropy is created. This happens when there is heat transfer across a finite temp difference.
o Is ZERO for adiabatic systems
Exergy is the useful work potential
o For boundary work, the work used to move a boundary which encloses a fluid, changing the volume of the fluid……
o For other forms of work (with no boundary work)……..
o The work W is equal to the shaft and electrical work (e.g. from a piston-cylinder device) and is equivalent to the exergy from work. Whereas, the surrounding work Wsurr is equal to
where P0 is atmospheric pressure and V1/V2 are the systems initial and final volumes. This is the work used to push when air out of the way, causing exergy losses in systems where there is boundary work.
Mass contains energy, entropy and exergy. Therefore, when mass flows into or out of a system there is a transfer of all these things
o where m is the mass transferred and is the flow exergy
o Is ZERO for systems with no mass flow (i.e. closed systems)