Process design of micro hydro, expander, binary ORC and double flash cycle using separator pressure of unit V in Lahendong geothermal field, Indonesia

Abstract

The saturated liquid discharged from the production separator has the potential to drive a secondary turbine in Lahendong and other fields of the PT Pertamina Geothermal Energy company. The water from the separator flows directly to the reinjection wells due to a difference in elevation of about 60 metres. From well testing during the project’s initial phase in 2012, and tracer flow tests in 2016 and 2017, the flow rates of steam and brine are known. Four different ways to recover energy from this saturated liquid flow stream before re-injection are studied in this project. First is to let the saturated liquid run through a micro hydro turbine (Pelton type) with a working pressure of 8.63 bar-a (head substitute), temperature 174°C, flow rate 40.6 kg/s and proportion of flash steam inside the pipe 0.13%. Under the assumption that no flashing or boiling occurs, the resulting power would be 32.06 kW. Second option is to insert the boiling liquid into a two-phase expander (turbo expander type) and a biphase turbine. The turbo expander generates about 282.2 kW electricity with an outlet pressure of 5.92 bar-a. The biphase turbine produces 9 kW at atmospheric pressure (0.924 bar-a) as the outlet pressure. This type of turbine can remove the vapour energy from the system. The third and most economic option would be the use of a binary ORC either with a wet or dry cooling tower. The calculation using Scilab and CoolProp for numerical computations resulted in a net power of 1795.3 kW with Silica Saturation Index, SSI, value of 2.02 and 626.1 kW with SSI value of 0.99 (with an existing wet cooling tower as the cooling system). Fourth option is a double-flash cycle with a differential pressure of 3.8 bar-a which would produce 2447.6 kW.