We have all heard of geothermal power. This typically refers to steam produced by geothermal. This steam is in turn used to power turbines. However, much of the geothermal accessible to us is warm but not boiling. In this case refrigerants or refrigerant gases can play a pivotal role.
Refrigerants are designed and sometimes mixed to produce a liquid and vapor phase conducive to a heat exchange within a particular range. However if you were to reverse the process and allow a heat source to boil the refrigerant say at room temperature then capture that produced energy via a piston engine (essentially a compressor) or turbine. Then you would have power. However , it is not quite that simple. For that process you would need an endless supply of refrigerant gas.
To solve the problem you must make your system a loop. To return the gas to the boiling tank without the same amount of power used to spin the turbine you have to cool the gas so it returned to its liquid state after it exits the turbine. Theoretically one could make power from any temperature gradient using this method. However the greater the temperature the greater the ROI for the machine. In Chena, Alaska just this machine has been built and it powers the whole town.
In Chena there is a geothermal pocket that produces temperature of 165 degrees Fahrenheit. Not enough to boil water and make steam but plenty to flash the refrigerant R-134a into vapor. The refrigerant is flashed in the geothermal pocket and then fed through a turbine to produce 200 Kw of power / assembly of 2 currently. This company imagines using many 100 degree temperature gradients on the planet to produce power.
This reverse refrigeration system may herald a new era of power production and a new breed of refrigeration technician. We look forward to seeing more clean energy on the planet no matter what form it comes in.