26-07-2012, 10:13 AM
Factors influencing the economics of the Kalina power cycle and situations of superior performance
Factors influencing the economics.pdf (Size: 180.93 KB / Downloads: 88)
Abstract
The Kalina technology has its advantages and disadvantages. Those in favor phrase it like new godsend while the opponents see in it only difficulties and additional complexity. The Kalina cycle is a power cycle and does as such compete with Rankine, Brayton, Diesel and Otto cycles. All theses cycles have their advantages and disadvantages and they are all a theoretical description of real-world machinery. Since the Kalina cycle is fully described in its physical values and material properties then we are able to compare it theoretically with other processes for different boundary conditions (in real life, energy situations). Accumulated cost-knowledge and operational experience, enables us to compare the real life performance of these cycles for a typical geothermal condition, that is source inlet and outlet temperatures and cooling fluid temperatures. The areas of superiority for the Kalina cycle are then presented.
Introduction
The second law of thermodynamics binds the conversion efficiency of low temperature heat into work or electricity. In addition to that, the conversion of heat from a heat source with a finite heat capacity has a lower upper bound for the efficiency due to the reduction of the source temperature as heat is removed from the source. This results in high cost for such low temperature power plants, as they have to handle large heat flows in order to produce acceptable power.
The Kalina cycle is a novel approach to increase this efficiency. The main benefit of the Kalina cycle is that the heat addition to the process happens at a variable temperature, and can thus be fitted to the falling temperature of a heat source with a finite heat capacity, reducing the generation of entropy in the heat exchange with the primary fluid. The temperature range for the boiling process of the ammonia-water mixture in the Kalina process may be as high as 100°C.
An Organic Rankine Cycle (ORC) is an alternative to the Kalina cycle and has found widespread use. There the boiling of the secondary fluid happens at a constant temperature, meaning that the vapour for the turbine in the process has relatively low temperature compared to the Kalina process.
The Models
Thermodynamic models have been established for the power cycles treated here. It is assumed that a fluid is available at temperatures ranging from 100 to 150°C. A heat customer is assumed for water at the temperature of 80°C, so that the temperature of the primary fluid after heat has been supplied to the power plant is fixed at that value. Flow of 50 kg/s is assumed. A large cooling water source is assumed, at the temperature of 15°C, and maximum cooling water temperature is fixed at 30°C. It is assumed that a cooling water pump has to overcome a pressure loss of 1 bar on the cooling waterside in the condenser.
The software Engineering Equation Solver (EES) is used to run the models for each operating condition, using the thermodynamic properties data supplied with that software package. Prof. Pall Valdimarsson described the modelling approach is in a paper (2002).
Further on the Kalina cycle
Flexibility
Both high-pressure level and ammonia content are design variables in the Kalina cycle. This gives the designer additional flexibility in the design of the cycle, and requires as well a certain design strategy. Market situations may demand that the plant is designed for absolute maximum power, and less regard paid to the cost per installed kilowatt, or if the load duration curve is not flat, strong demands on the investment cost, but less emphasis put on the total power. This is shown on Figures 5 and 6, where the contour lines for installed cost and power are drawn as a function of ammonia content and pressure. The source temperature is 100°C. The cost values are put at 100 for the lowest cost, and the power values at 100 for the highest power. An x denotes an infeasible solution, the cycle will not be able to run at these ammonia content - pressure combinations. In Geir Þórólfsson’s MSc thesis (2002) a thorough study of the influence of these combinations on the final cost figures is made.
Conclusion
The model results presented in Section 4 show that the Kalina cycle has similar installed cost to a high power ORC cycle. The maximum power generated for a given source is greater for the Kalina cycle.
This leads to the obvious conclusion that the Kalina cycle is well positioned against an ORC cycle for applications with high utilization time, a base load application.