The Geothermal Rankine Steam Cycle Compared to the Geothermal Flash Cycle

Abstract

This paper compares studies of the common geothermal flash steam cycle and the Rankine steam-cycle powered by high-temperature geothermal two-phase fluid, with the aim of generating electricity. The concept of the geothermal Rankine steam cycle is based on the utilization of high-temperature geothermal two-phase fluid at the Nesjavellir high-temperature geothermal area. Small shell-and-tube heat exchangers have been in periodical operation during the past eight years, generating hot water by the geothermal two-phase fluid for local district heating utility.

Despite considerable content of amorphous silica in the geothermal fluid, no extensive scaling has been detected nor have the heat exchangers needed to be cleaned. It is therefore assumed feasible to generate steam, as well as the heating of water, through the use of geothermal two-phase fluid in heat exchangers at Nesjavellir. It is also considered feasible to generate steam in various geothermal fields with similar chemical composition as the fluid at Nesjavellir.

Technical and economical comparison of the two cycles indicates that if the heat extraction of the geothermal two-phase fluid is feasible, the geothermal Rankine steam cycle could be considered as an alternative to the geothermal flash cycle. If the geothermal fluid has a high content of non-condensable gasses (NCGs), the geothermal Rankine steam cycle becomes even more advantageous in comparison.

The comparison takes notice of the geothermal Rankine steam cycle's benefits of a clean steam operation and the disadvantage of increased costs due to increased requirements of heat exchangers.