Geothermal handbook: PlanninG and Financing power Generation t e c h n I c a L r e p o r t 2 / 2

Binary Plants
Generating electricity from low or medium temperature geothermal fluids and from the waste hot fluids 
coming from separators in liquid-dominated geothermal fields has made considerable progress since 
improvements were made in binary fluid technology. Binary plants utilize a secondary working fluid, 
usually an organic fluid (typically n-pentane) with a low boiling point and high vapor pressure at low 
temperatures as compared to steam. The secondary fluid is operated through a conventional Rankine 
cycle: the geothermal fluid yields heat to the secondary fluid through heat exchangers, where the 
secondary fluid is heated and vaporizes. The vapor produced drives a turbine, then is cooled and 
condensed, and the cycle begins again.

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G e o t h e r m a l H a n d b o o k : P l a n n i n g a n d F i n a n c i n g P o w e r G e n e r a t i o n 
Binary plant technology is a cost-effective and reliable means of converting the energy available from 
liquid-dominated geothermal fields with temperatures up to 200°C into electricity. By selecting suitable 
secondary fluids, binary systems can be designed to utilize geothermal fluids with temperatures well 
below 100°C. However, such low temperatures would severely impact the financial viability of projects, 
depending on their location, their direct use options, and the power tariff offered.
Competing with the above mentioned Organic Rankine Cycle (ORC) plants, another binary system, 
the Kalina cycle, utilizes a water-ammonia mixture as the secondary working fluid. This technology was 
developed in the 1990s and is used commercially, particularly in Iceland and Japan. 
Binary power plants are commonly used as bottoming units. In these applications, the binary plant 
uses the waste fluids coming from the separators as well as the residual heat from a main power plant. 
For example, steam with a temperature of 250°C which is utilized by the main power plant (usually a 
conventional steam (flash) plant) can, depending on fluid chemistry, have a temperature when leaving 
the turbine of 120° to 170°C after expansion. Instead of condensing this steam by means of air-cooling 
or cooling towers, it can be efficiently used to generate more power by the bottoming unit and thereby 
increase the overall efficiency and economy of the entire power plant unit. However, bottoming units 
add significantly to total project costs. These costs affect the power generation costs per kilowatt hour 
and might reduce the margin between generation cost and the power tariff paid by the off-taker or 
utility company. The resulting reduction in operational profit is the reason why project developers do