Interstage Heating Strategies for Double-Flash Geothermal Power Plant

English Abstract:

More than 60% of the electricity generated by geothermal power plants in the world is produced from the heat of high temperature hydrothermal reservoirs. These high temperature power plants are typically of the Flash category (Single, Double or Triple-Flash), and it is well known that their efficiency is relatively low. Hence, to ensure economic sustainability of these power plants, it is desirable to improve their design so that they can provide more energy with the same amount of geothermal fluid.

Recently, we proposed a novel strategy to improve Double-Flash power plants, which consists in extracting heat present in warmer parts of the thermodynamic cycle in order to use it in colder parts of the system, by means of an additional heat exchanger. These new designs are named "Interstage Heating", and are inspired from the work of DiPippo and Vrane (1991).

A numerical model was developed to simulate Interstage Heating processes in Double-Flash thermodynamic cycles, and the model was validated with results reported in literature. That numerical model was then used to optimize the operating parameter values (i.e., separator pressures) and to maximize the specific work output of the geothermal power plants.

Numerical results showed that by using Interstage Heating in Double-Flash designs, the specific work output of power plants may be increased by about 5%, the liquid content in the low pressure turbines may be decreased by about 50%, and the waste heat to power output ratio of power plants may be reduced by about 10 %. In other words, Interstage Heating provides a higher energy output from the same geothermal wells, it reduces the erosion rate of turbine blade, and it may reduce the initial cost of cooling systems in Double-Flash power plants. To summarize, Interstage Heating has the potential to provide significant benefits to owners of Double-Flash power plants.