Wilhelm Tegethoff

Modelling of heat pumps with an object-oriented model library for thermodynamic systems

Novel CO2 heat pump systems for domestic hot water supply are modelled using an object-oriented thermodynamic model library written in Modelica. Due to the variety of mathematical structures of thermodynamic models and the possibilities of object-oriented modelling languages, powerful libraries are often very complex and hard to understand. The scope of the presented library lies on good readability and usability for both users and model designers. The basic structure and design concepts of the library are outlined and also the most important model concepts are explained. The heat pump system is modelled and simulation results are compared with test stand measurements. Simulation results are used to analyse the design of the system and to get information about possible further improvements.

Nonlinear Model Predictive Control of a Vapor Compression Cycle based on First Principle Models

Abstract Vapor compression cycles are broadly used for air-conditioning, refrigeration and heat-pump applications and are responsible for a large part of primary energy consumptions. Increasing availability of electrical actuators leads to a high number of possible control inputs of vapor compression cycles. Due to their highly nonlinear behavior and strong cross coupling of inputs and outputs modeling and control of this systems is a nontrivial task. Classical control design methods based on linear plant models seem not to be able to achieve the best possible control in terms of energy efficiency and disturbances rejection. In this contribution we present a Nonlinear Model Predictive Control scheme for vapor compression cycles, which takes nonlinearities and cross coupling explicitly into account. First principle models are used to formulate a system of Ordinary Differential Equations (ODE) describing the dynamic behavior of the underlying process. Inside these models a new highly efficient method for refrigerant fluid properties computation based on bicubic spline interpolation is used. Based on the derived ODE an Optimal Control Problem is formulated and solved by a fast Direct Multiple Shooting algorithm. Finally a Nonlinear Model Predictive Control (NMPC) scheme with a sampling rate of only 0.5 s is derived. Simulation experiments show the ability of the method to immediately react to disturbances and at the same time keeping the system at the most energy efficient working point.

Visual Investigation of an Ejector Motive Nozzle

Previous investigations by other authors, e.g. Lorentzen [1], have shown that in a conventional refrigeration cycle significant throttling losses occurs. With the help of an ejector, these losses can be reduced. As a result, the energetic efficiency (COP) of the refrigerant system will be improved. Investigations show that CO2 ejector cycles are feasible and that some systems have already been commercialized successfully. The key issues in the optimization of the ejector used in a refrigeration cycle are the geometries of the different ejector parts. To optimize the geometry, a deeper understanding of the physical effects and the flow conditions within the ejector are essential. So far there are only a few investigations published on this issue, e.g. Elbel [2], investigated the flow in the mixing section of the ejector. This paper presents experimental results for different ejector nozzle geometries and operational condiditons. The motive nozzle was investigated separately from the other ejector parts. Investigated were multi-hole nozzles and the effect of the jet shape. Parameters were chosen according to the typical conditions in ejector refrigeration systems. Based on these conditions, the free jet exiting the motive nozzle was observed. To investigate the jet shape, an new experimental setup was designed. The motive jet was visually observed in a glass cylinder. The combination of both the contraction and compressibility effect on mass flow rate was also investigated.Copyright

A Limiter for Preventing Singularity in Simplified Finite Volume Methods

Abstract System simulation with equation based and object-oriented modelling languages has become an important tool for system and control design of vapour compression cycles. A common approach of modelling one-phase and two-phase fluid flow is 1-D spatial discretisation according to the Finite Volume Method. In order to avoid stiff systems and to speed up simulation, pressure loss and momentum balance are usually handled in a simplified way. There exist different approaches. But all approaches suffer from one drawback. There are points where the resulting set of equations is not solvable and simulation fails. In this contribution the solvability problem is analysed in detail. It is shown that these problems are closely connected to fluid properties. Based on this result a modelling approach is suggested to avoid singularities.

Verbrauchsbestimmung von Kälteund Wärmepumpenkreisläufen durch hochdynamische Verdichtermessungen gekoppelt mit Echtzeitsimulationen

Der Verdichter in Kalte- oder Warmepumpenkreislaufen ist haufig der groste energetische Nebenverbraucher in Fahrzeugen. Die Bestimmung des genauen Anteils am Kraftstoffverbrauch ist allerdings schwierig, da dieser von zahlreichen Randbedingungen abhangt. Bedingt durch die langsame Dynamik der thermischen Speicher in einem Fahrzeug genugt es nicht, ausschlieslich stationare Betriebspunkte zu betrachten. Um realistische Aussagen zum Kraftstoffmehrverbrauch durch den Verdichter zu treffen, ist die Berucksichtigung des transienten Verhaltens des Gesamtsystems wichtig. Hierzu kann mit der im Folgenden vorgestellten Methode ein wichtiger Beitrag geleistet werden.

Developing Flow Correlations for Different Valve Geometries Using Reference Media for R-744

This paper presents experimental data of three different prototype valves with cone-, ball-, and slide-valve geometries that have been tested using R-744 and two reference media, ARAL 4005 and R-729, under one-phase flow conditions. Three different test rigs were used to carry out the tests using boundary conditions with the objective to achieve more or less similarity regarding the Reynolds number and the Mach number. One aim was to find physical-based flow correlations to describe the flow behavior of the three investigated valve geometries with carbon dioxide (R-744), hydrocarbon-based low-viscosity fluid (ARAL 4005), and air (R-729) as the fluid. The main goal of this research was to find a correlation between the flow coefficients of both (a) R-744 and ARAL 4005 and (b) R-744 and R-729 to enable the transfer of a flow coefficient measured for ARAL 4005 or R-729 into a flow coefficient for R-744.