(26b) Integrated Approach for the Design of Refrigeration and Power Systems

The use of vapour-compression refrigeration in low temperature processes creates significant demands for mechanical power. Therefore, the power system supplying this energy will have an impact on the performance and economics of these processes. This is particularly true for LNG.

This paper follows previous work on the optimal design of refrigeration systems (Del Nogal et al., 2005a) and power systems (Del Nogal et al., 2005b) individually. In the first case, the optimal compositions and operating conditions in mixed refrigerant cycles were obtained by using a genetic algorithm approach and a process model that considers multiple compression stages and full enforcement of temperature feasibility. In the latter, a mathematical programming formulation was presented for the optimal driver and power plant selection in power-demanding industrial processes with proper consideration of discrete gas turbine and power plant options as well as availability and energy efficiency implications.

As an effort to improve the overall design of low temperature processes, a systematic methodology for the integrated design of refrigeration and power systems was recently produced as an extension to the previous work above. Additional key degrees of freedom such as stage pressure ratios and plant capacity are optimised, alongside other design variables, in order to provide greater flexibility in the matching of power supply and demands. The optimisation framework is genetic algorithm-driven and features an inner optimisation for the design of the power system. This strategy is applied to an LNG case study with cascaded mixed refrigerant cycles and shows the convenience of this approach as the interactions between the refrigeration and power systems are systematically exploited.