A simple experiment that to prove the potential energy is unlimited.
 

Drop a bottle of liquid gas (V=1M3 ) into a deep pool of 10 meters, burned the gas out, The buoyancy of the empty bottle will do work: W=G(water)H=10000N*10M=100000J.
The shortcoming: The operation is not working continually.
(It's the rudiment of perpetual motion machine)

Why the potential energy is unlimited?

The definition of potential energy is not correct and the conservation law of mechanical energy is faultiness for the following reasons.
Potential energy can be converted into kinetic energy, the resistance removed. Otherwise, the kinetic energy can't be changed into potential energy at all. Mechanical energy is just produced by the movement of objects according to the definition of it.
Force is the cause to change the movement of the objects by the first law of Newton. Therefore force is the real source of mechanical energy.
The force (outside force) is limited. However, gravity is a special force which is unlimited.
As a result, mechanical energy can be divided into two types; one is called kinetic energy (limited) the other potential energy (unlimited).
Potential energy of objects is unlimited because it got from gravity, and it is general knowledge that gravity is eternal.
Gravitational potential energy of objects is created by gravity. If there is no gravity, the potential energy will disappear.
At present, the potential energy is generally agreed that it comes from the movement of objects being taken from low to high.
But people have ignored that the kinetic energy has already been consumed during the period of movement.
The movement of objects takes place depending on the force acted on.
Without force acting on the objects maintain static.
Even with equal forces from opposite directions act on each other, they can't move as well. Gravity is just in this condition.
The potential energy is always equal to the weight of objects no matter the objects is low or high.
Potential energy will be zero only when the gravity disappears. In the absence of resistance, the potential energy will be released to do work.
The formula of potential energy: U = mgh, U represents work more suitable than energy.
In a word, force is energy itself or a form of expression of energy just like the electric energy, the heat energy and so on.
According to the following formulas: W=F·S, F=ma=1/S(E2-E1), Work or energy is made up of force while the quantity of force is measured by work or energy.
The force consumed has turned into work; unconsumed, energy.