In 2007 Constanze Bongs completed her degree in Industrial Engineering and Management with distinction at the Technical University of Berlin. The main focus of her studies was on energy technology and energy economics. She spent one year studying economics at the Italian university Libera Università Internazionale degli Studi Sociali (LUISS) Guido Carli in Rome (2003/04), working in Vancouver, Canada (semester break 2004) and completing her diploma thesis at the Commonwealth Scientific and Industrial Research Organization (CSIRO) Energy Technology in Newcastle, Australia (2006/07). In her diploma thesis she also gained first experience with the simulation of a thermally driven air conditioning cycle "Modelling of a Desiccant Evaporative Cooling System". After completion of her university degree, Constanze Bongs started working in the group "Thermally Active Materials and Solar Cooling" at the Fraunhofer Institute of Solar Energy Systems (ISE) in Freiburg, Germany. In the course of her PhD project carried out at Fraunhofer ISE she works on the development of a novel thermally driven ventilation unit for building air-conditioning. The PhD thesis is supervised by Prof. Dr.-Ing. Felix Ziegler, head of the Institute of Energy Technology at the Technical University of Berlin.
Short description of the doctoral thesis:
„Optimisation of evaporatively cooled sorptive-coated heat exchangers (ECOS) by experiment, simulation and exergo-economic analysis"
The rise in the demand for active air-conditioning leads to an increase in the demand for electricity needed to drive the conventional air-conditioning units. Thermally activated air-conditioning cycles offer an opportunity to use solar or waste heat as driving energy. Substituting the high grade electric energy with low grade available heat therefore offers a climate friendly alternative to common air-conditioning units. A novel system concept for a ventilation unit to provide air dehumidification and cooling was developed at Fraunhofer ISE. A research project with partners from industry focuses on the development of such a ventilation unit for small scale installations in offices and residential buildings, characterised by a capacity range of air volume flows up to 400 m³/h.
The main component of the ventilation unit is an evaporatively cooled sorptive-coated heat exchanger (ECOS – Evaporatively COoled Sorptive Heat Exchanger). This heat exchanger consists of sorptive-coated air channels and cooling air channels in which water evaporation takes place. Sorptive and cooling air channels are in thermal contract. Fresh air flowing through the sorptive channels is dehumidified by contact with the desiccant material – typically a silicagel. The dehumidification capacity of the desiccant is limited by its temperature which increases during adsorption due to the release of the phase change energy. Simultaneous evaporative cooling in the adjacent cooling channels allows maintaining the desiccant temperature low, enhancing its dehumidification potential and cooling the air. Once the desiccant is saturated (wet) the water held on its surface must be driven out by heating the desiccant. To provide continuous air dehumidification and cooling, two sorptive-coated heat exchangers alternate between dehumidification and desiccant regeneration (drying of the desiccant).
The main focus of the PhD project is optimising the thermodynamic behaviour of the sorptive-coated heat exchanger. Important design parameters such as the choice of sorptive material, relation of thermal masses and the impact of the evaporative cooling process on the achievable outlet air conditions shall be identified. To this purpose, three methodologies are applied: experimental characterisation of different sorptive-coated heat exchanger prototypes, dynamic heat exchanger simulation and a combined thermodynamic-economic analysis. The thermodynamic analysis is conducted from a first-law, energetic perspective, but more importantly also from a second-law, exergetic perspective. The concept of exergy allows assessing the value of energy as exergy is the amount of energy which may be transformed into available work. An exergetic analysis helps identifying and consequently minimising system irreversibilities. The paradigm that exergy expresses the usefulness of energy allows considering it as a measure of its economic value. Allocating an economic value to the exergy flows in the desiccant system shall enable a combined economic and thermodynamic optimisation needed for reaching the main goal of the PhD thesis: identifying a thermodynamically sensible and economically feasible alternative to conventional air-conditioning cycles.
Experimentelle und mathematisch-numerische Untersuchung on verdunstungsgekühlten, sorptiv beschichteten Wärmeübertragern für die Luftentfeuchtung und -kühlung