The Modelica Association Modelica 2006, September 4 th – 5 th Using Modelica as a Design Tool for an Ejector Test Bench Christoph Richter, Christian Tischendorf, Ricardo Fiorenzano, Peterson Cavalcante, Wilhelm Tegethoff, Jürgen Köhler Technical University Braunschweig, Institute for Thermodynamics Hans-Sommer-Straße 5, 38106 Braunschweig, Germany ch.richter@tu-bs.de Abstract As a result of the Kyoto Protocol, the use of R134a in air conditioning system of new cars is going to be forbidden in the EU due to the high Global Warming Potential (GWP) of this substance. Carbon dioxide (CO 2 ) is one of the possible alternatives as a cooling agent in mobile air conditioning applications and is desirable since it is a natural refrigerant. Cooling cycles using CO 2 are currently achieving COP simi- lar to those of R134a cycles but there are promising options to further improve COP. One possibility is the application of an ejector instead of the valve. A Modelica library was developed that allows compu- tation of ejector cooling cycles in steady state with simplified component models that can be used as a design tool for the construction of an ejector proto- type. The library uses a new object-oriented library that serves as an interface to external medium librar- ies to compute thermodynamic and transport proper- ties for the refrigerant. Keywords: Fluid properties, cooling cycle, ejector 1 Introduction As a result of the discussion about reducing the worldwide emission of greenhouse gases, carbon dioxide (CO 2 ) was re-discovered as a natural refrig- erant with promising thermodynamic properties. Among a few others, Lorentzen [1] pointed out early that trans-critical CO 2 refrigeration cycles encounter significant throttling losses reducing their coefficient of performance (COP). One way to overcome this problem is to use an ejector instead of the throttling valve. A lot of research is currently carried out in this field [2], [3] and the obtained results are very promising [4]. This paper describes the development of a Modelica library that was used in the design process of a test bench for an ejector cooling cycle and that will be extended to allow the deeper analysis of the ejector refrigeration process. The developed library is kept as simple as possible. The fluid properties for CO 2 are computed using a new fluid property library that provides a convenient interface to an external library written in C. 2 Problem description To be able to simulate an ejector cooling cycle one needs models for the components of the cooling cy- cle as well as for the fluid properties of the chosen refrigerant. Although it would be possible to have one library for both requirements it usually is a good idea to have separate libraries for the components and for fluid properties. Well-known Modelica li- braries such as Modelica.Media/Modelica_Fluid and AirConditioning Library/ThermoFluidPro follow the same basic concept. Before looking at libraries that are capable of model- ing the given problem the most important require- ments are compiled in the following list: • The component models should be as simple as possible for the first rough analysis during the design process. The component models should only require the definition of a mini- mal set of parameters and efficiencies be- cause exact geometries are usually not known in the first rough analysis. • There are a lot of engineers that can only spend a certain amount of their work time on understanding Modelica libraries. The de- veloped library should enable those users as well as students with no or little background in modeling to get started with developing new models as easily as possible. • The simple models used in the first steps of the design process should be designed in such way, that they can be replaced by more detailed models in future steps when an in- depth analysis of experimental results is re- quired. 501 Using Modelica as a Design Tool for an Ejector Test Bench