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Question

Variables Affecting Specific Steam Consumption

Gama Gilang  Adiarte's picture
Student University of Indonesia

I'm a final year physics student with the specialization of system and instrumentation at the University of Indonesia. I currently working on my research topic about geothermal power plant...

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  • Dec 24, 2020
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Hello, I am a university student and currently working on my undergraduate thesis. I need some opinion from the professionals who have been working in geothermal power plant or studying geothermal power plant efficiency. There is a parameter called 'specific steam consumption (SSC)' which determines the geothermal power plant(GPP) efficiency. My question is, what are the variables in the sub-system GPP that affect SSC? Thanks for the help. 

 

 

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Here is a real-life example of the increased efficiency of a geothermal power plant in Mexico using SSG and some additional references with the math.

 Definition of Specific Steam Consumption: Specific steam consumptions depend on the absolute pressure ratio over the turbine and the size of set considered.

 For example, see the math in the PDF link Science Direct topics

 Specific Steam Consumption - an overview (pdf) | ScienceDirect Topics

 24.4 Power plants and output

 The first power plants at Los Azufres began operation in 1982 (Table 24.2), Unit 1 in July and the others in August. Units 1 to 5 were 5-MW back-pressure, wellhead type, built by Mitsubishi. Every turbine was composed of one horizontal cylinder with five reaction or impulse stages running at 3600 revolutions per minute (rpm). The steam entered to the turbine at 8 bar,a (8.16 kg/cm2) and 170.4°C and was discharged to the atmosphere through a silencer at 0.7 bar,a (0.714 kg/cm2) and 91°C. The units did not need a condenser, cooling system and system for removal of NCGs (Fig. 24.5). The electric generator, also manufactured by Mitsubishi, rotated at 3600 rpm and worked at 4160 V at a power factor of 80% (Table 24.3). All these units had a low efficiency; when new, each one required 59 t/h (16.4 kg/s) of steam at full load, or around 11.8 t/h (3.28 kg/s) per MW, but in the last years of operation, the specific steam consumption (SSC) rose to more than 12 t/h per MW. Unit 1 was dismantled and moved to Los Humeros in 1996 and the other four units were dismantled in early 2015 when the new Unit 17 started to operate (Table 24.2). Table 24.2. Some historic and current data on the Los Azufres power unit

 

Specific steam consumptions depend on the absolute pressure ratio over the turbine and the size of set considered.

 From: The Efficient Use of Energy (Second Edition), 1982

 

 

Steam Gathering Process SGS is the most important part of the Specific Steam Consumption SSC process see link to the Menengai project in Kenya Africa

 SGS is used in geothermal projects to collect and deliver steam from the steam fields to the power plants. GDC has completed the set-up of an elaborate SGS in the Menengai Geothermal Project.

 GDC| Geothermal Development Company

The term "specific steam consumption" comes from the railroad industry in the late 1800s. It is defined as the amount of steam consumed by the locomotive's cylinders per unit output of power. The units are typically kg of steam per kilowatt hour (kg/kWhr). In using this definition and applying it to a steam turbine, it would be the amount of steam consumed by the turbine to generate one kWhr.  That is what is meant by the term "specific", as opposed to the total amount of steam consumed by any given engine.  The more steam that is needed, the less efficient the steam turbine system is. There are two parts to this issue.  The first is the steam conditions.  The work output from a turbine can be calculated by integrating the Ideal Gas Law.  Since the bulk of the output work in a steam turbine comes from the low pressure portion of the turbine, the Ideal Gas Law can be used, as steam is reasonably close to an ideal gas at low pressures.  The result is that the Work output is equal to 

             - nRTln(P2/P1)

Thus, from the thermodynamic side, the higher the temperature and pressure of the inlet steam, the greater the work output of the turbine.  If the steam conditions degrade over time (ie temperature and pressure is reduced) from the geothermal well, it will take more steam to get the same output.  The other part of this issue is the isentropic efficiency of the turbine itself.  This means, basically, the physical design of the turbine.  The steam turbine consists of a shaft, with a number of blades attached to the shaft, along with shaft seals.  The mechanical design of the blades, the blade attachments, and the seals will determine the actual performance compared to the theoretical performance.  In this case, the theoretical performance would be the performance with no change in entropy to the system.  That means no leaks, no friction, no slippage, perfect airfoils (ie the blades), etc.  Today's steam turbines have isentropic efficiencies in the range of 91 - 94%.  The higher efficiencies are typically experienced in the high pressure sections of the steam turbine.  The lower efficiencies are more prevalent in the low pressure sections.  Once the turbine is mechanically designed, this efficiency is essentially fixed, barring wear and tear on the turbine.  With decent quality steam, a major steam turbine overhaul takes place every 10 years or so.  That is when the casing is opened up and the turbine is inspected for worn or damaged parts.  There may be deposits on the blades that upset both the steam flow over the blades (lift) as well as the balance on the rotating shaft (vibration).  Condensation in the last rows of blades may have eroded the blades in that section.  The 3600 rpm rotation of the shaft may have caused the seals to be worn, allowing more steam leakage in the system.  The goal of the major overhaul is to find these problems and fix them, thus recovering the performance that was lost over the years.  For your question, the steam conditions have to be known as well as the isentropic efficiency of the actual turbine that is in place at the geothermal well.  Once those are known, the isentropic performance can be calculated using the Ideal Gas Law and then corrected by the isentropic efficiency of the actual turbine.  The final number that is needed is the temperature at the back end of the turbine.  A power turbine usually has a condenser in the back end to reduce the final pressure (P2) as low as possible (ie a vacuum).  That pressure is nominally the vapor pressure of water at the temperature in the condenser.  The temperature in the condenser will be set by the cooling water temperature used in the condenser.  If no condenser is used, then the final pressure is atmospheric pressure and the exhaust steam is vented to the atmosphere. 

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