Two types of energy conversion I favor and see no reference to are,

Geothermal and Hydrogen power.

A third i find little information on is microwave accumulators.

These three are 0 to little emmission generators.
Hydrogen is portable and microwave would be as well. Geothermal is converted thermal energy and electicity produced is very portable and once converted can be transmitted by further conversion to microwave energy.

I have a theory and have read about a possible use of Mercury in it's liquid state we find on Earth as a catalyst for power generation as well as a fuel. This type of energy production is hazzardous yet very powerful.