GEOTHERMAL POWER PLANT CYCLES

Single flash cycle schematic

 

The geothermal fluid enters the well at the source inlet temperature, station 1. Due to the well pressure loss the fluid has started to boil at station 2, when it enters the separator. The brine from the separator is at station 3, and is re-injected at station 4, the geothermal fluid return condition. The steam from the separator is at station 5, where the steam enters the turbine. The steam is then expanded through the turbine down to station 6, where the condenser pressure prevails. The condenser shown here is air cooled, with the cooling air entering the condenser at station c1 and leaving at station c2. The condenser hot well is at station 7. The fluid is re-injected at station 4

              T-s diagram of a single flash cycle

Typically, such a process is displayed on a thermodynamic T-s diagram, where the the temperature in the cycle is plotted against the entropy. The condition at station 1 is usually compressed liquid. In vapour dominated fields, such as Lardarello in Italy, the inflow is in the wet region close to the vapour saturation line.

              T-h diagram of a single flash cycle

Double flash cycle schematic

A flow sheet for the DF cycle is shown in Figure above. The geothermal fluid enters the well at the source inlet temperature, station 1. Due to the well pressure loss the fluid has started to boil at station 2, when it enters the separator. The brine from the separator is at station 3, and is throttled down to a lower pressure level at station 8. The partly boiled brine is then led to a low pressure separator, where the steam is led to the turbine at station 9. The turbine is designed in such a way, that the pressure difference over the first stages is the same as the pressure difference between the high and low pressure separators. The mass flow in the lower pressure stages of the turbine is then higher than in the high pressure stages, just the opposite of what happens in a traditional fuel fired power plant with a bleed for the feedwater heaters from the turbine.

The brine from the low pressure separator is at station 10, and is then re-injected at station 4, the geothermal fluid return condition. The steam from the high pressure separator is at station 5, where the steam enters the turbine. The low pressure steam enters the turbine a few stages later, at station 9. The steam is then expanded through the turbine down to station 6, where the condenser pressure prevails. The condenser shown here is air cooled, with the cooling air entering the condenser at station c1 and leaving at station c2. The condenser hot well is at station 7. The fluid is re-injected at station 4.

              T-s diagram of a double flash cycle

 
              T-h diagram of a double flash cycle

ORGANIC RANKINE CYCLE (ORC)

               Flow diagram for an ORC cycle with regeneration

A flow sheet for the ORC cycle is shown in Figure. The geothermal fluid enters the well at the source inlet temperature, station s1. The fluid is frequently liquid water. If the pressure is kept sufficiently high, no non-condensable gases will be separated from the liquid, and a gas extraction system is not necessary. The fluid is then cooled down in the vaporizer, and sent to re-injection at station s2.

Pre-heated (in the regenerator) ORC fluid enters the vaporizer at station 2. The fluid is heated to saturation in the vaporizer, or even with superheat in some cases. The vapour leaves the vaporizer at station 3, and enters the turbine. The exit vapour from the turbine enters the regenerator at station 4, where the superheat in the steam can be used to pre-heat the condensed fluid prior to vaporizer entry. The now cooled vapour enters the condenser at station 5, where it is condensed down to saturated liquid at station 6.

A circulation pump raises the pressure from the condenser pressure up to the high pressure level in station 1. There the fluid enters the regenerator for pre-heat before vaporizer entry. The condenser shown here is air cooled, with the cooling air entering the condenser at station c1 and leaving at station c2.

             T-s diagram of an ORC cycle with regeneration 

 

              T-h diagram of an ORC cycle with regeneration

KALINA CYCLE

The Kalina cycle is patented by the inventor, Mr Alexander Kalina. There are quite a few variations of the cycle. The Kalina power generation cycle is a modified Clausius-Rankine cycle. The cycle is using a mixture of ammonia and water as a working fluid. The benefit of this mixture is mainly that both vaporization and condensation of the mixture happens at a variable temperature. There is no simple boiling or condensation temperature, rather a boiling temperature range as well as condensation range. This is due to the fact, that the phase change process is a combined process, both the phase change of the substance and absorption/separation of ammonia from water.

                 Flow diagram of a saturated Kalina cycle

A flow sheet for the Kalina saturated cycle is shown in Figure. The cycle is “saturated” because there is no superheat in the cycle. The fluid is not boiled entirely in the vaporizer, and the vapourliquid mixture is then separated afterwards. This is done in order to maximise the vapour temperature at the vaporizer outlet. The geothermal fluid enters the well at the source inlet temperature, station s1. The fluid is frequently liquid water. If the pressure is kept sufficiently high, no non-condensible gases will be separated from the liquid, and a gas extraction system is not necessary. The fluid is then cooled down in the

vaporizer, and sent to re-injection at station s2. Pre-heated (in the regenerators) liquid ammonia-water mixture enters the vaporizer at station 3. The fluid is boiled partly in the vaporizer. The liquid-vapour mixture leaves the vaporizer at station 4, and enters the separator. The separated liquid leaves the separator and enters the high temperature regenerator at station 7. After the high temperature regenerator the liquid is throttled down to the condenser pressure in station 8, and mixed with the turbine exit vapour from station 6. The ammonia-rich vapour enters the turbine at station 5, and is expanded to the condenser pressure at station 6. The exit vapour mixed with the throttled liquid (now at the average ammonia concentration) from the high temperature regenerator enters the low temperature regenerator at station 9. The cooled fluid from the low temperature regenerator enters the condenser at station 10. The fluid gas now started to condense, and the ammonia concentration is not the same in the liquid or vapour phase. An absorption process is going on, where the ammonia rich vapour is absorbed into the leaner liquid, in addition to condensation due to lowering of the mixture temperature. The kinetics of the absorption process determines the rate of absorption, whereas heat transfer and heat capacity controls the condensation process.

Finally all the mixture is in saturated liquid phase in the hot well of the condenser at station 11. The circulation pump raises the fluid pressure up to the higher system pressure level, and the liquid is then preheated in the regenerators in stations 1 through 3. The condenser shown here is water cooled, with the cooling water entering the condenser at station c1 and leaving at station c2.