Master thesis Jonathan Orbie

Modeling of part load behavior of an Organic Rankine Cycle (ORC) with a direct evaporator
By
 | Jonathan Orbie | Universiteit Gent, Faculteit Ingenieurswetenschappen en Architectuur, Vakgroep Mechanica van Stroming, Warmte en Verbranding, Applied Thermodynamics and Heat Transfer
Promotor(s)
 | Prof. dr. ir. Michel De Paepe

Abstract
The objective of this thesis is to develop a generic model for the part load behavior of an Organic Rankine Cycle (ORC) with a direct evaporator. The Organic Rankine Cycle (ORC) converts low temperature heat into electricity. In this way, renewable energy sources can be valorized and the e ciency of existing systems can be increased by waste heat recovery (WHR). Usually, an intermediate heat transfer (HTF) loop is used to transfer the heat from the heat source to the ORC because of reasons of stability and controllability. Feeding the ORC directly from the heat source is more challenging but yields a lower investment cost and a higher power output of the ORC. In this thesis, a thermo-economic design model for an ORC with a direct evaporator is developed  rst to determine the design with the lowest specific c investment cost (SIC). The boundary conditions in WHR applications often  uctuate strongly over time. This means the ORC will not work under the operating conditions for which it was designed. To predict the ORC’s part load behavior, a generic off -design model is developed in this thesis. From the analysis performed with this static model, it is concluded that it’s possible to operate the ORC over a very wide range of off -design operating conditions. The particular modeling technique used for the turbine is essential to obtain this result. Finally, two possible WHR applications in a steel factory are considered. In each case, the optimal design for the ORC is determined. The off -design model is then used to perform a quasi-steady state simulation of the ORC’s part load behavior over an entire year. This makes it possible to judge the investment by calculating the discounted payback period.