The first loop uses water to extract heat from the buried hot rocks. The simplest hot dry rock power plant comprises one injection well and two production wells. Cold water is pumped under pressure down an injection well where it flows through an underground heat exchanger in the hot granite. It is then returned to the surface through production wells.
Because this loop operates under pressure, the super heated water remains a liquid, with no steam generated.
The heated water (> 200oC) returns to the surface under pressure. The high temperatures of the hot water are transferred to the geothermal power station loop via a heat exchanger. This second closed loop uses liquids with a low boiling point (similar to those used in fridges and air conditioners) and this drives a turbine system.
The cooled geothermal water returns to the underground heat exchanger where it is reheated.
The twin loop system is known as a binary geothermal power plant.
3 Well HFR Geothermal Power Station. The hot granite needs an insulating blanket of sedimentary rocks at least 3km in thickness.
Schematic diagram of binary geothermal powerplant
From: http://www.geodynamics.com.au/IRM/content/hfr_hfrexplain_hfrgeothermal.html