By "Energy Recovery" we mean the process by which part of the heat is discharged by a process (eg from a heat generating unit or into the air coming out of the air conditioning system). Recovery is done using heat exchangers. A heat exchanger is a device used to transfer heat energy between two fluid media (air, water, exhaust gas, etc.) at different temperatures.
The purpose of a heat recovery system is to make the most of the waste heat with the simplest - most economically feasible - solution, which has the least possible interference with the existing system.
Recovered heat can be used to meet hot water or hot air needs, to create a pre-air conditioned air conditioning or ventilation system for the production of hot water, etc.