The ejector recompression absorption cycle, which has recently been developed by Eames and Wu (2000), is similar to the conventional single-effect lithium bromide absorption cycle. The difference between them is that there is a steam ejector in this novel cycle for enhancing the concentration process. Because of the use of the steam ejector, the performance and the operating characteristics of the novel cycle are different from the conventional cycle.
The steam ejector recompression absorption cycle is shown schematically in Figure 3.51a. In this figure, the expansion of the high-pressure steam causes a low pressure at the exit of the primary nozzle of the steam ejector, therefore, the vapor at point 8 in the concentrator is entrained by the primary flow. The two streams are mixed in the steam ejector and condensed in the heat exchanger of the concentrator. The condensation heat is used to heat the solution in the concentrator. Obviously, the heat of the entrained vapor is recovered by the steam ejector in this process. Water at point 3 splits into two streams; one flows back to the steam generator and the other flows into the condenser. In stable operation, the mass flow rate of the first stream equals that of primary flow while the mass flow rate of the second stream equals that of the entrained vapor. The rest of the cycle is similar to that of the conventional single-effect lithium bromide absorption cycle. Figure 3.51b shows the novel cycle on a P-T-C diagram. As shown in Figure 3.51b, the cycle 6-7-9-10-6 takes up water at the absorber (10-6) and releases it as vapor at the concentrator (7-9). In the conventional absorption cycle, the vapor is condensed at 8′ and the condensation heat is rejected to the surroundings. In the novel cycle, this vapor undergoes a compression process through the ejector to point 2. Since the vapor temperature is greater than the solution temperature in the concentrator, this vapor is used to heat the solution by condensation to point 3. Therefore the heat otherwise wasted is recovered and the energy efficiency is improved.
Eames and Wu (2000) investigated the energy efficiency and the performance characteristics of the novel cycle and the theoretical results showed that the COP of the novel cycle is better than that of the conventional single-effect absorption cycle. The characteristics of the cycle performance show its promise in using high temperature heat source at low cost.
Recently, Kang et al. (2000) have undertaken a study to propose and evaluate advanced absorption cycles for the COP improvement and temperature lift enhancement applications. The characteristics of each cycle are assessed from the viewpoints of the ideal cycle COP and its applications. The advanced cycles for the COP improvement are categorized according to their heat recovery method: condensation heat recovery, absorption heat recovery, and condensation/absorption heat recovery. In H2O-LiBr systems, the number of effects and the number of stages can be improved by adding a third or a fourth component to the solution pairs. The performance of NH3-H2O systems can be improved by internal heat recovery due to their thermal characteristics such as temperature gliding. NH3-H2O cycles can be combined with adsorption cycles and power generation cycles for waste heat utilization, performance improvement, panel heating and low temperature applications. The H2O-LiBr cycle is better from the high COP viewpoint for evaporation temperature over 0°C while the NH3-H2O cycle is better from the viewpoint of low temperature applications. This study suggests that the cycle performance would be significantly improved by combining the advanced H2O-LiBr and NH3-H2O cycles.
