Figure 1 shows the vapour-liquid equilibrium diagram of ammonia/water mixture. Ammonia/water vapour mixture (ammonia mass is about 70% of the mixture) at outlet of turbine has the lower condensing temperature to complete condensation in a Rankine cycle, and the temperature level is not easily able to get from surrounding. Kalina cycle using ammonia/water non-azeotropic mixture can bring to condense the working fluid under ambient temperature level, from utilizing the absorption-distillation-condensation system instead of the condenser of the Rankine cycle.
Figure 2 shows the schematic diagram of Kalina cycle, and the absorption-distillation-condensation system, which is the novelty of this cycle, is drawn in broken lines as can be seen in Fig.2.
This cycle consists of a boiler, a turbine, a pump and one absorption-distillation-condensation system. The absorption-distillation-condensation system consists of an absorber (lower-pressure condenser), a condenser (higher-pressure condenser), a separator (vapour liquid separator), one pump, one diffuser (expansion device) and 3 heat exchangers.
In this cycle, all processes of absorption, condensation and separation are operated at environment temperature level.
Weak ammonia solution from vapour-liquid separator absorbs vapour mixture from turbine at absorber and it changes to ammonia rich solution. This strong solution flows to separator; on changing to the vapour-liquid two-phase flow by being heated at the heat exchangers, that is pressed up by the pump.
In separator, this two-phase flow is separated to weak ammonia solution and strong ammonia vapour. Weak ammonia solution is led to absorber as absorbent solution, and the other strong vapour joins the other line of strong solution from absorber, and flow to condenser. This strong two-phase flow is completely condensed in condenser, pressed up by pump and led to boiler. Vapour heated in the boiler that flows to turbine, and expand in turbine to produce electric power.
In this way, Kalina cycle can condense turbine exhaust vapour under environment temperature level by two lower and one intermediate pressure levels in the cycle without decreasing turbine output.
Figure 2 Schematic of Kalina cycle.
3. Cycle simulation
Cycle simulation was carried out for the combined cycle as shown in figure 3 with the following assumptions:
(1) The cycle is in the stationary state, and temperature, pressure, liquid concentration and vapour concentration are in equilibrium at each condition point.
(2) Both the boiler efficiency, ηBLR and the turbine efficiency, ηTBNare 100%.
(3) For each heat exchanger, heat exchange efficiency is 100% and the heat exchange area is considered as the ideal condition.
