A study of thermodynamic modelling and gas extraction system design for Aluto Langano geothermal power plant II in Ethiopia

Abstract

This report describes thermodynamic modelling and gas extraction system design for the Aluto Langano geothermal power plant phase II expansion project. The Aluto Langano geothermal field is presented as well as information on the condition of the Ethiopian electric market. Basic design data, like the well production curve of previously drilled wells as well as geographical and meteorological data of the area, are used to design the expansion of the power plant. The well pads and pipe layouts are hypothetically sited and pressure drop in the pipelines is estimated. Two flow models are presented for two-phase flow pipe calculations. The processes of other power plant equipment such as separators, turbines, the condenser and cooling towers are discussed. The basics of possible standard-flash power plant thermodynamic models, with the aid of EES software, were built. The model for a single-flash plant, a double-flash with two condensing turbines and a double-flash with one back pressure unit and one condensing turbine unit are discussed and compared. Non-condensable gases (NCG) have great impact in the power production efficiency of geothermal power plants. In Aluto Langano geothermal field, the total gas flow in one typical well is about 6.3% by weight of the total steam phase. Studies of gas extraction system design in regard to the plant model with the highest gross turbine output are presented. Four gas extraction system methods are presented. Economic evaluations, using the net present value method of these gas extraction systems, are performed. The results from the power calculations show that the double flash with a back-pressure turbine unit has the highest power output with a generation capacity of 25 MWe. A centrifugal compressor is the most economic method of gas extraction with a total updated net present cost of 2.5 million USD.