
11Introduction to Sustainable Energy
© 2008 Taylor & Francis Group, LLC
hydrothermal resources that are seldom above 300°C. Depending on the pressure of the
resource, some of the geothermal brine is ashed and expanded through a steam turbine.
The remaining brine and condensate from the ash steam turbine provide the boiler heat
for an organic Rankine cycle (ORC), so-called because the working uid is a hydrocarbon
like pentane. Organic uids, ammonia, and refrigerants boil at much lower temperatures
than water and also condense at higher temperatures. ORCs are also used for solar thermal
power plants and waste heat power conversion.
The amou