Hot Fractured Rock (HFR) and Engineered Geothermal Systems (EGS) refer to man-made geothermal systems.

HFR sources are usual deep granite rocks  from 4500 m to 5000m depth.  These sometimes show a higher than usual geothermal gradient as the granite rocks contain minor amounts of radioactive materials.  Those materials, while quite safe, generate heat from naturally occurring radioactive decay.

The best known examples of attempts to commercialise HFR are:

• The Upper Rhine Graben on the France-Germany border.  At a project site at Soultz-sous-Fort's in France, researchers have been progressively developing a demonstration system since 1987 with good results.
• The Cooper Basin in central Australia.  This too is a sedimentary basin, but the HFR geothermal project is focussed on the underlying basement rocks which are granites.  The Australian company Geodynamics is currently testing its man-made reservoir system .

HFR is different from sedimentary and volcanic geothermal systems in that there is no natural reservoir.  Water cannot flow through the granite rocks at reasonable rates.  So, HFR developers artificially create a reservoir with suitable permeability using a process known as "hydraulic stimulation".

To date, no HFR operation has been able to demonstrate flow rates at profitable rates.  However, this type of geothermal resource has huge potential as a longer term source of clean energy as it has great scope for expansion.