Design and Modelling of Small Scale Low Temperature Power Cycles

he 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 effciency 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 allowed 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 enhanced the simulation experience significantly while keeping the associated relative error below 10−4 at all times and below 10−7 away from the phase boundaries.Regarding expansion devices for small scale organic Rankine cycle (ORC) systems,this work focussed on reciprocating machines.

A prototype of a reciprocating expander with a swept volume of 736 cm3 was tested and modelled. he model was written in object-oriented Modelica code and was included in the thermo Cycle 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.

Admission control based on evaporator and condenser conditions was found to be suitable for an optimisation of the expander operation. During the experiments,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 effciency around 70% for expansion ratios from 8 to 15.

The simulation code could predict the expander effciency within 6% points, larger deviations of up to 30% occurred for the produced work per revolution. An analysis of the heat transfer occurring in the expansion chamber showed that also large heat losses only had a limited impact on the work output of the expander.The final part of this report deals with the performance of plate heat exchangers.

Several plate heat exchanger correlations were reviewed focussing on their applicability to ORC systems. A framework for dynamic heat exchanger modelling was developed that includes both single-phase and two-phase flow in pipes and plate heat exchangers. Four different pairs of heat transfer correlations were compared based on a test case with a dynamic heat source suggesting that a simplified modelling approach could besufficient to model the dynamic response of a small scale plate heat exchanger.

Working towards a validation of heat transfer correlations for ORC conditions, a new test rig was designed and built. The test facility can be used to study heat transfer in both ORC and high temperature heat pump systems.