We do not need to exchange at all, with the tech We have now, some of which is hidden avidly as the psychopaths in control know full well the VERY intimate relationship between money and energy. Money is Their ONLY tool to widespread power over Others, and free energy makes that tool pointless and powerless. So it is NOT "the mere CONCEPT of a convenient medium of exchange," but the implementation itself that creates the power-grabs that benefit psychopaths and not Humanity.