Liquid-dominated systems are abundantly available and do not require development of special technologies.
The following types of power systems are discussed:
1. The flashed-steam system,
2. Binary cycle, and
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3. Total-flow system.
1. The Flashed-Steam System:
Hot water is available above 150°C to 315°C underground. When tapped, the water can flow naturally under its own pressure or be pumped to the surface of the earth. The drop in pressure causes it to partially flash into steam and a liquid-dominated, low-quality, two-phase mixture of water and stream is available at the well head. The water contains dissolved solids. The flow diagram and T-s diagram of a flash-steam system are shown in Fig. 7.3 and 7.4 respectively.
Hot water from reservoir (1) reaches the well head (2) Pressure p2 is lower than P1 and process 1-2 is a constant enthalpy throttling process. The two-phase mixture of low quality (2) is passed through a flash separator (3) the quality of steam is higher at point 3. The dry saturated steam (4) at pressure of about 8 bars is expanded in the steam turbine. The separated brine (5) is re-injected into the ground.
The exhaust steam from turbine is mixed with cooling water in a direct- contact condenser. The mixture is cooled in a cooling tower.
Improved Flashed-Steam System:
In order to recover large quantity of heat energy from brine at point (5), some improvements in the cycle are carried out.
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i. Double Flash System:
Depending upon the original water conditions, the brine at point 5 is admitted to a second, lower-pressure separator, where it flashes to a lower pressure steam. This steam is admitted to a low-pressure stage in the turbine as shown in Fig. 7.5. The new low-pressure brine contains less energy and therefore cycle energy loss is reduced.
The corresponding T-s diagram is shown in Fig. 7.6. It uses an innovative steam condenser and gas extraction system and a dual – admission steam turbine. An example of the double-flash system is the 50 MW Hatchobane plant, Kyushu in Japan.
ii. Water Turbine:
The spent brine at point 5 is at high pressure and can be used to drive a water turbine in parallel with steam turbine.
iii. Rotary Separator Turbine (RST):
The fluid leaving the well head (point 2) is a two-phase mixture of steam and water. It is partially expanded in a nozzle. The quality and kinetic energy of fluid increase. This also helps in the separation of two phases. The separation takes place in a rotating drum by centrifugal action. The steam is admitted to the steam turbine and high velocity water is used in a special liquid turbine. The discharge from the turbine is re-injected into the ground.
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2. Binary Cycle:
Water in lower temperature ranges is unsuitable for power production. It can be used directly for domestic and industrial process heating. Hot water can also be used in heating an organic fluid with low boiling point and can be used to run a Rankine Cycle. The working fluids can be isobutene, Freon-12, ammonia or propane.
The flow diagram of a binary cycle is shown in Fig. 7.7.
Hot brine from underground reservoir circulates through a heat exchanger (HX) and is pumped back to ground. The organic fluid is heated to superheated vapour and is used in a standard closed Rankine cycle. The vapour drives the turbine and is condensed in a surface condenser. The condenser is cooled by water from a natural source or a cooling tower circulation system. There is no problem of corrosion and scaling in the working cycle components.
Kamchatka binary cycle plant in Russia is 680 kW plant using hot water at 80°C. The working fluid is Freon-12.
Hot brine from the well is throttled and full flow is expanded in a two-phase expander as shown in Fig. 7.8. A barometric condenser is used. As seen from T-s diagram, thermodynamically maximum available energy can be converted into mechanical work.
Comparison:
The analytical comparison of various liquid-dominated geothermal systems is given in Fig. 7.10. The total-flow system produces the highest specific power per unit mass-flow rate at the well head.
These calculations are based on the following assumptions:
Condenser temperature = 45°C
Turbine efficiency = 85%
In-plant power requirements = 30% of gross.
