(courtesy Australian National University)
PRI's system has 2 closed circuits
   Producton Field Circuit
This loop circulates water down an injection well where it passes through the fractured intrusive body and is heated. The superheated water is then returned under pressure to the surface by four surrounding production wells  This keeps dissolved solids from the formation (silica or carbonates) in solution.
At the surface, the superheated water is passed through a heat exchanger where most of the heat is removed.
The now cooled water is returned, via the injection well, to the formaion where it recovers more heat.

Working Fluid Circuit
WIthin the plant, a second closed circuit containing a proprietary mixture of organics carries the energy from the heat exchanger to the turbine.  This fluid generates pressures in excess of 50 bar at temperatures of  120oC..converting the heat into pressure.  This pressurized fluid is expanded in the turbine, causing it to rotate and generate electricity.
Renewable Energy
The heat used in the PRI power plant is continually replaced by heat escaping from the Earth's mantle. and PRI's plants qualify for registration under the Clean Development Mechanism of the Kyoto Protocol.  This registration permits client companies to sell Carbon Credits(CERs) on the world market.  PRI's 100 Mwe  plant can generate
12,000,000 / yr in CER sales, in addition to power sales of 49,144,369 / yr.
Median global heat flow through Earth's crust ranges from 2 - 5 W/m2.  Though this figure is small in comparison to mean global insolation (solar energy), which is ~ 1400 W/m2, the temperature in Earth's crust nevertheless increases at a measured rate between 20 - 30oC/km.  In much of the western USA it is greater than this, often approaching 40oC/km.. This means that temperatures high enough to produce commercial power are accessible worldwide using modern drilling techniques, which permit drilling to 5000m, and advanced power cycles which permit power production from sources > 70oC..
Because the cost of drilling grows exponentially with depth, and is a major cost of geothermal power production, the Institute recommends prospects where energy extraction is possible at moderate depths.
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