Energy can be recycled, but doing so can be inefficient. Because many devices produce excess heat, which is energy, when they run, recycling the excess heat can lead to significant savings.
Many power plants operate by recycling heat. After burning fuel, power plants capture the heat they generate and use it to convert water into steam. This steam then spins turbines, which produce electricity. Hybrid vehicles achieve greater efficiency by recycling energy. The forward momentum of cars is energy, so capturing it when braking allows the vehicle to recycle energy that is otherwise lost heating up the brakes. When the car brakes, the vehicle increases the resistance in the tires to generate electricity, which the vehicle then stores in a battery. Hybrid vehicles have a second engine that runs on electricity stored in the battery.
In the industrial field, "waste heat" is a term used to describe the excess heat machinery generates. Companies can sometimes use waste heat to warm up water or another substance to send to other parts of the building. Combined heat and power systems are designed to use as much energy from fuels as possible by converting some of it to electricity and using some of it to produce heating for machines or for warming rooms.