Kalina Cycle Power Plant

Working fluid - Mixture of 2 fluids

What makes Kalina cycle different from conventional Rankine cycle of power production is its choice of working fluid. Kalina cycle uses mixture of 2 fluids as working fluid, most commonly used is ammonia and water mixture. To get answer why Kalina uses a mixture as its working fluid, have a look at T-s diagrams of ordinary Rankine cycle and Kalina cycle.

Fig.1 Comparison of Rankine and Kalina cycles

The major difference of Kalina cycle from Rankine cycle is that in Kalina heat addition and heat rejection happen at varying temperature even during phase change, since the fluid is a mixture. But in Rankine heat addition and heat rejection happen at uniform temperature during phase change. This is the one thing which makes all the difference in performance of Kalina cycle.

Comparison with Carnot Engine - Reason for high efficiency

In a Carnot engine heat addition and rejection happen at uniform temperature.

Fig.2 In a Carnot engine heat addition and rejection happen at uniform temperature

Efficiency of such an engine can easily be proved as

So it is clear that if heat rejection temperature (Tc) decreases or heat absorption temperature (Tb) increases thermal efficiency of Carnot engine will increase.Same analysis can be done for Rankine and Kalina cycles, using average temperature of heat addition and rejection as reference temperatures. This is marked in dotted lines in following figures.

Fig.3 Average heat addition and rejection temperatures of Kalina cycle is much wider than a Rankine cycle

So it is clear from the diagrams that Kalina cycle has got lower average heat rejection temperature (Tc) and higher average heat addition temperature (Tb) compared to Rankine cycle. It will obviously lead to high thermal efficiency. This forms theoretical background of Kalina cycle, but in order to implement it we have to overcome some practical hurdles.

Difficulty at Condenser - Use of Separator

Kalina cycle uses high concentration ammonia mixture (around 70% ammonia) at steam turbine part, but such a mixture has got very low condensing temperature.

Fig.4 Phase diagram of Ammonia-Water mixture

Means you have to supply a very low temperature cooling water at condenser for this purpose. Production of such low temperature cooling water is not economical. You can observe from Fig.4 that condensing temperature of ammonia-water mixture increases drastically with decrease in ammonia concentration. So in a Kalina cycle power plant, we will decrease ammonia concentration at condenser side. An equipment called separator will produce 2 streams of fluid from condenser outlet, one with high concentration and other with low concentration (30% ammonia). Low concentration ammonia mixture will get mixed with exist fluid at turbine and will produce a medium concentration (40% ammonia) ammonia mixture. This mixture will have fairly high condensing temperature and can be condensed with supply of ordinary cooling water. This is shown in following figure.

Fig.5 Use of recuperator in producing low concentration ammonia mixture at condenser

Concentration of fluid is brought back to original state by mixing high concentration ammonia stream from separator with fluid exit at condenser.

Use of Recuperator

It is clear from T-s diagram of Kalina cycle that temperature at exit of steam turbine (point 4) is greater than temperature at inlet of boiler (point 2). So there exists a chance of heating up boiler liquid by virtue of this high temperature steam turbine output. This is accomplished with help of a heat exchanger called recuperator. This is shown in following figure.

Fig.6 Increase in further thermal efficiency with help of a recuperator

Thus with use of recuperator one need not supply same amount of heat at boiler side as supplied in previous case, this will further increase efficiency of Kalina cycle power plant. But this opportunity of heat transfer is not there in Rankine cycle based power plant. You can notice that in Rankine cycle temperature at point 4 is always less than temperature at point 2, thus there is no chance of heat transfer from steam turbine outlet to boiler inlet.

Advancements in Kalina Cycle Power Plants

Instead of ammonia-water mixture industries have started implementing organic mixture based Kalina cycles in order to harness maximum from given condition. Some examples are mixture of R22 & R114 and mixture of Hexamethyldisiloxane & Decamethyltetrasiloxane.
Thanks to its unique feature of varying thermo-physical properties by varying mixture concentration at different parts of cycle, Kalina cycle power plants are widely used in Geothermal stations and waste heat recovery units. They can easily match to any source (heat addition) and sink (heat rejection) condition by varying mixture concentration in the cycle.