A promising energy source is hydrogen. In principle hydrogen is a completely clean energy source and its only emission at the end user is clean water. However, the problem is that the most efficient way to produce hydrogen is electrolysis, which demands electricity usually obtained from the fossil fuels. A possible solution is to obtain the electricity from a renewable source. In this case hydrogen acts basically as substance to transmit energy. Starr suggests using the nuclear energy as a source for the Continental Super Grid, where the energy is transmitted to the end user via hydrogen, which feeds fuel cells . A better solution would be the use of biologically derived hydrogen. While the research in this field is still in the initial phase, there are some promising results with bacteria  and green algae . The research in nanotube with a possible application for hydrogen storage is beeing developed by the laboratories at J. Stefan Institute.
Fuel cell cars have recently been introduced as a possibility to use hydrogen as a clean energy source. The advantage of these cars is that they create no polution in the city, although there is still polution created in producing hydrogen.