Jorrit Wronski from DTU Mechanical Engineering defends his PhD "Design and Modelling of Small Scale Low Temperature Power Cycles" Monday, 30 November 2015, at 13:00. The defence takes place in Building 306, Auditorium 37, DTU. Professor Fredrik Haglind is supervisor, Associate Professor Brian Elmegaard and M.Sc. Morten Juel Skovrup, IPU, are co-supervisors.
The work presented in this report contributes to the state of the art within design and modelling of small scale low temperature power cycles. The study is divided into three main parts: (i) fluid property evaluation, (ii) expansion device investigations and (iii) heat exchanger performance.
The traditional approach to fluid property modelling requires users to choose between computational efficiency and high accuracy since the complexity of many of the modern equations of state makes the evaluation of fluid properties time consuming. This work contributed to the new fluid property library CoolProp that provides automated routines for fluid property evaluations based on Taylor series expansion and bicubic interpolation. The internal structure was redesigned completely to include mixtures and the library currently contains binary interaction parameters for many refrigerants and natural working fluids. It also includes more than 100 pure and pseudo-pure fluids as well as over 100 pure and binary secondary heat transfer fluids.
The reformulation of the equations for the incompressible fluids allows the calculation of a full thermodynamic state, including entropy and selected partial derivatives. The accelerated property evaluation by means of table-based interpolation was shown to be up to 120 times faster than solving the full equation of state (EOS) for enthalpy and pressure as inputs, which enhances the simulation experience for many system models.
Regarding expansion devices for small scale organic Rankine cycle (ORC) systems, this work focuses on reciprocating machines. A prototype of a reciprocating expander with a swept volume of 736 cm3 was tested and modelled. The model was written in object-oriented Modelica code and was included in the ThermoCycle framework for small scale ORC systems. Special attention was paid to the valve system and a control method for variable expansion ratios was introduced based on a cogeneration scenario.
The machine ran with n-pentane as working fluid and delivered up to 2.5 kW of shaft power. Operating with variable admission valve timing, the expander exhibited a stable isentropic efficiency around 70% for expansion ratios from 8 to 15. The simulation code could predict the expander efficiency within 6% points, larger deviations of up 30% occurred for the produced work per revolution. he final part of this report deals with the performance of plate heat exchangers. A framework for dynamic heat exchanger modelling was developed and different single phase and two phase correlations for pipe flow and for plate heat exchangers were implemented. The heat exchanger model was used with a dynamic heat source and the behaviour of the plate heat exchanger correlations could be reproduced with a correctly parametrised model with a mass flow exponent and fixed heat transfer coefficients for single phase and two phase flow could reproduce the results. Conducted heat transfer measurements could not be used to validate different heat transfer correlations. The existing test rig could not be controlled sufficiently well and the design of a new test is proposed.