Actually energy harvesting is a technique used to promote the energy usage of a node of a sensor..Nodes become unusable after its battery(energy) has been used up,,,so there are techniques such as energy harvesting whereby the cell in the sensor node can use energy from the nature to use the sensor for a longer time..hope u understood wat u actually meant!!!
The simplest way is to operate the sensors in sleep mode, receive mode and transmit + receive mode, with different power settings at each. Alternately depending on your application, you can have the sensor sense only at a certain frequency (say for two seconds) a minute.