Political cost
.. I imagine the barrier to this and similar technologies from making it to fruition are the political incentives to keep fossil fuels alive and well for some time to come. I can't imagine a day where for a one-off $500 or even $5000 you will be able to buy a clean power source which is practically infinite and also free. The taxation system will have to change and the global patents will most likely be bought up by oil barons.
The economic cost of realising sustainable, eco-friendly, cheap/free fuels is greater than most governments, oil-sheik's and billionaire dynasties would be willing to bear.
