Characteristics of Bottoming Cycle using Ammonia/Water Mixture
Mitsuo KOJIMA*, Masao GOTO* and Takao KASHIWAGI**
In order to improve power generation efficiency for environmental protection, development of innovative technology for energy utilization is required. Especially, to improve generation efficiency, combined cycle is necessary. In general, this cycle utilizes steam turbine (Rankine cycle) as the bottoming power recuperation cycle. Furthermore, when heat source of low temperature is used, the cycle using ammonia/water mixture, as working fluid, (Kalina cycle) is effective. This cycle can be operated with not only high temperature heat sources but also various low temperature heat sources such as ocean thermal energy, geothermal heat, solar energy or waste heat, by exploiting effectively the characteristics of non-azeotropic mixture. This study describes the bottoming cycle using ammonia/water mixture with DCSS (Distillation Condensation Subsystem), which is called Kalina cycle. A cycle simulation program was developed. For the cycle simulation, cooling water temperature, turbine inlet vapours concentration, vapour liquid separator temperature, etc. was considered as key parameters. The efficiency of this cycle was obtained about 28% with a driving temperature of 400℃. Therefore, for the case of combined cycle power plant, the total efficiency will be increased to a value above 50%.
Key Words: Bottoming Cycle, Ammonia/Water Mixture, Advanced Energy System, Power Recuperation
Recently, the energy systems having high conversion efficiency is called for as a countermeasure of environmental problems that include climate change, ozone depletion and primary energy crisis.
For the combined cycle that is one of the highest efficient energy conversion cycles, the performance of the combined cycle as a whole depends on the performance of the bottoming cycle. Kalina cycle works as the bottoming cycle of gas turbine cycle, which uses the ammonia/water mixture as working fluid. As ammonia/water mixture is a non-azeotropic mixture, the boiling point is lower than that of pure water. Ammonia is added with the ammonia-water mixture that is considered as the operating parameter to temperature and pressure on pure water. So it is possible to use heat source of low-temperature and to be utilize the various low-grade heat source temperatures, as the using waste-heat of trash plant and ironworks, geothermal energy and ocean thermal energy.
Some proof test plants, have been made for evaluation, and some plants have been in practical operation.
In this study, for the bottoming cycle, which is called the Kalina cycle, using ammonia-water non-azeotropic mixture as working fluid, a cycle simulation is carried out. The effect of operating conditions such as temperatures and concentrations of working fluid on cycle efficiency is described.
P : pressure
T : temperature
x : ammonia mass fraction
BLR : at boiler
TBN : at turbine
1〜22 : point
2. Kalina cycle
As ammonia/water mixture can change its boiling point by changing the ammonia concentration of the mixture, this can use the sensible heat along with the waste heat like exhaust gas for vapour generation more effectively. Both pure ammonia and pure water have the constant saturation temperature under constant pressure, while ammonia/water mixture, which is a non-azeotropic mixture, changes it saturation temperature in according to the change of concentration under the constant pressure. So this mixture makes the characteristic of heat transfer in the countercurrent flow type heat exchanger and thus improves heat transfer efficiency.
* Tokyo University of Mercantile Marine
2-1-6, Etchujima, Koto-ku, Tokyo, 135-8533, Japan
Phone & FAX: +81-3-5245-7470
** Tokyo University of Agriculture and Technology