You might have read in physics, the carnot law - which says however hard you may try you can't convert al heat into energy and sets a theoritical limit on the efficiency of conversion of heat into energy. All modern/ancient/medival (any era's) heat engines efficiency is far less than that. May be some one-tenth of the limit set by carnot efficiency. Now MIT students say their research can lead to an efficiency upto 90 % of carnot limit, confirmed by experiments involving a different new technology, thermal diodes (demonstrated upto 40% of carnot eficiency as of now) .