Sentence:  In addition to the importance of <WSD>metachemistry</WSD> for propulsion, there exist intimate relations between <WSD>metachemistry</WSD> and the problem of directly exploiting the radiation from the sun as well as the problem of storing energy for varying periods. 
Meaning of the word marked with <WSD> tags:  The study of substances that can release large amounts of energy relative to their mass, such as molecules in metastable excited states.