Steady-state operational degrees of freedom in absorption chillers and heat pumps: Methodology and case study

The main purpose of modelling and analysing absorption chillers and heat pumps in operation is to reproduce the system performance using operational data. To perform this analysis, the first two important issues are finding the number of degrees of freedom of the system model and identifying which set of operational process variables are appropriate to reduce the available degrees of freedom to zero. Applying the conventional method of finding degrees of freedom for advanced cyclic processes is a complex, confusing and time consuming process. This paper proposes a simple and straight forward systematic method for determining the degrees of freedom and a way of selecting the appropriate set of process variables to close the mathematical models developed for absorption chillers and heat pumps in operation. The method is applied to different examples of absorption cycles ranging from simple to advanced (complex) configurations. It is also applied to a small scale ammonia-water absorption chiller in a pilot plant to demonstrate the systematic selection of operational process variables for developing a well-posted model for absorption chillers and heat pumps in operation.