HPM
To create energy from heat powered machines is an old idea, our fossil fuel fired power plants are using this principle today, a steam engine.
In 1824 Sadi Carnot stated, that everywhere where a temperature diffierence occurs an engine can be applied to generate energy. He proved that an ideal engine (efficiency of 100%) can not exist. Every engine will generate some amount of waste heat and can not utilize 100% of the fuel.
The Carnot-cycle is the cycle process of an ideal engine. The closer an engine comes to that cycle, the more efficient the engine is.
There are machines, invented more than 100 years ago, which had a market share at their time. But lost over time when internal combustion engines (gasoline and diesel) entered. Those engines could be built comparable cheap and the fuel was cheap.
Those engines had not been an esoteric idea, but working types.
Stirling Engine
The Stirling engine was invented before 1816 (patented in 1816). This engine therefore is the second oldest engine for energy generation, only the steam engine is older.
This engine has some advantages than a steam engine: higher efficiency, lower working pressure and no danger of steam burns.
Due to inherent inertia this engine is slower to change the speed and power output. This is one of the major reasons, why such an excellent engine lost its market share.
Our energy hungry world is rediscovering this engine as it offers benefits, when fuel price is continously increasing.
Tesla Turbine
The Tesla Turbine was one of many inventions of the genius Nikolai Tesla. Unfortunately esoteric circles occupied Tesla and a lot of his inventions as esoteric knowledge, leading to a bad reputation. Scientist and engineers refused to work on Tesla's principles in fear of being laughed at and loosing their own reputation.
Still, the Tesla Turbine is nothing esoteric, but hard science and engineering. Tesla built several of his turbines and marketed the principle successful in Tesla pumps.
One disadvantage of this turbine is the requirement of high speed and temperature. With the available material the Tesla turbine can not achieved the highest possible efficiency.