Here is a perfect example of, "I can not believe this has not been done already."
Aalto University in Finland is testing the use of hybrid technology in non-road, mobile industrial machinery. Think Prius versions of cranes, bulldozers, tractors, and diggers. Their concept is to capture of the energy from the work being performed by each machine that is lost when in operation. Like in hybrid cars, deceleration, braking energies are being absorbed as well as energy from such tasks as lowering a load. For the record, no one has attempted making hybrids in this area.
Head researchers Professor Jussi Suomela spoke of the impetus of the project, "These heavy duty machines are operated for long periods of time, so by the end of the day emissions and fuel consumption have added up. Being able to target them is a significant improvement." From recent experimental data, Suomela has show that involving such combustion/electric hybrids in work machinery has the potential to save 50% of the fuel costs in stand alone operation, and even more if the system is plugged into the power grid.
Now it is not a question of can it be done, but when can it be implemented. Great work, Aalto University.