It can be argued that energy is a perspective on the context of existence.  Energy is not limited to radiation, combustion, or drain, which encompass oils, gases, or mineral used for energy production  The artifacts that we see as energy such as coal, petroleum , or plant based fuels are concentrations of easily convertible energy.  However energy is proportional with change, and the method by which energy is condensed into these artifacts is, or was, by change it self.

Any change in the state of nature is energetic.  This means that a change in mass, velocity, or context represents harvestable energy sources. 

This is particularly true in the context of existing gravitational fields, as demonstrated by my inventions the PEC (pressure energy converter, PRS (pressure rotation system), and GFG (gravity flotation generator) devices.  In each case the devices employ gravity harvesting to generate either electrical or mechanical energy.  These systems are described as E : abs(mass₁ - mass₂)

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