REVERSE ELECTRODIALYSIS HEAT ENGINE: Low-grade Waste Heat into Electricity

Our society is undergoing a progressive change about the life style and habits. The world population is continuously increasing with 7.6 billion of human beings in 2018, resulting in an increasingly demand of resources in terms of food, water and energy. The exploitation of the planet resources since the first Industrial Revolution, results today in an unsustainable condition, which requires fundamental changes. In particular, in the energy sector the adoption of fossil fuels as the main energy source for human beings’ activities resulted in a strong impact on our planet, leading to climate changes and environmental pollution. Nowadays these aspects have induced society to a substantial challenge to find new sustainable energy sources for the future of human civilization. Low-grade thermal energy, derived from industrial or geothermal sources, represents an interesting resource for energy production. Indeed, huge amounts of low-grade thermal energy are available. Considering the industrial sector an amount ranging between 20-50% of the energetic input of industrial plants, are lost every day, in the form of hot gasses and liquid streams. However, the recovery and re-use of low-grade thermal energy or waste heat is limited due to the lack of efficient technologies for converting low-temperature heat sources into electrical power. Recently, Salinity Gradient Power Heat Engines (SGP-HEs) have been proposed as a viable process for the recovery of low-grade heat. In particular, this PhD thesis focuses on the analysis of Reverse Electrodialysis Heat Engines (RED-HEs), contributing to the activities of the European project “RED-Heat-to-Power” funded by the European Union’s Horizon 2020 Research and Innovation Programme (www.red-heat-to-power.eu). The aim of the project is to study and develop the first prototypes for the conversion of low-grade heat into electricity through a reverse electrodialysis (RED) unit. A reverse electrodialysis heat engine consists of two main units: (i) a power generation unit based on the reverse electrodialysis process, where the salinity gradient between two salt solutions is exploited to produce electricity, and (ii) a regeneration unit where low-grade heat is used to restore the salinity gradient of the reverse electrodialysis solutions exiting from the power generation unit. The restoring of the two solutions can be achieved by means of different strategies, e.g. solvent extraction and salt extraction, as summarized in the following. (a) In solvent extraction, the salt exchanged within the reverse electrodialysis unit is integrated by adding a part of the exhausted dilute stream to the exhausted concentrate stream, then, the resulting solution is fed to the regenerative unit where solvent is recovered by a thermal separation process (e.g. multi-effect distillation, membrane distillation) and transferred to the dilute solution. (b) In salt extraction, the exhausted dilute solution is fed to the regenerative unit where the salt is recovered by using low-grade heat as an energy source and transferred again to the concentrate solution. Rebalancing of the solvent is eventually carried-out to restore the solvent amount in the two streams. The main objective of this thesis is to provide a proof of the ground-breaking concept of a reverse electrodialysis heat engine, demonstrating the technological readiness level of such technology at a lab-scale prototype level. To this aim, both configuration, solvent and salt extraction reverse electrodialysis heat engines were investigated. The first part of this PhD thesis was dedicated to the development of a mathematical model for the RED process, based on a multi-scale modelling approach, considering two different scales of description: (i) a lower-scale model, describing the main phenomena involved in a single repeating unit (cell pair) and (ii) a higher-scale model related to the whole system, including all cell pairs and the relevant interconnections. The mathematical model was validated against experimental results and integrated with exergy analysis tools useful to evaluate the main causes and location of irreversibility sources. Ad hoc experimental campaigns were carried out in order to characterize the behaviour of different salt-water solutions, membrane properties and the operability of lab-scale units. In particular, a purposely-developed test rig was built in order to evaluate the osmotic and activity coefficients of salt-solutions from vapour pressure measurements, then used to determine novel data for caesium and potassium acetate salt solutions. This activity was performed at the University of Edinburgh, during a period abroad of six months. Furthermore, several experimental results on lab-scale reverse electrodialysis unit were carried out in order to provide membrane properties and experimental results for the mathematical model validation. In the case of solvent extraction strategies, validated mathematical models were developed for a reverse electrodialysis heat engine implementing either membrane distillation or multi-effect distillation as regeneration unit. These activities were performed in collaboration with the “Centro de Investigaciones Energéticas, Medioambientales y Tecnológicas (CIEMAT)” of Almeria. Finally, an exergy analysis at component level was performed for the integrated RED-MED HE, analysing the impact of the main operating conditions and membranes properties on the exergy efficiency of the system and identifing the main irreversibility sources. In the case of salt extraction strategies, thermolytic salts were selected and studied as working solutions of the RED-HE. Solutions of thermolytic salts have the peculiarity that their ions dissolved in water undergo a degradation process into gaseous species when the temperature is increased over a certain limit. Then, the dissolved thermolytic salt can be removed nearly completely from the solution by means of a thermal desorption process. The stripped gaseous species can be dissolved again in the aqueous solutions through an absorption step, resulting in an aqueous solution consisting of the very same ions of the thermolytic salt. Experimental test-rigs were designed, built and tested to investigate the stripping process of thermolytic salts. Mathematical models were developed in ASPEN Plus® and validated by comparison with experimental results. The model was also used to perform a sensitivity analysis and investigate the performance of the whole regeneration unit, including also the absorption process. Then, a mathematical model of a RED-HE fed by thermolytic salts was developed by coupling the ASPEN Plus® process model for the regeneration unit to the RED process model and used to perform sensitivity analyses. Finally, all the knowledge gained by theoretical and experimental works have made possible the design, construction and operation of the first world prototype of the thermolytic RED HE, demonstrating for the first time the feasibility of the process. The present thesis has been organised in order to cover the main aspects of the RED process and describe the aforementioned objectives. Chapter 1 introduces the concept of salinity gradient power (SGP) and related SGP-HEs, describing the SGP-HE concept and analysing the state of the art of the technology. Section I - Reverse Electrodialysis process Chapter 2 presents an overview of the reverse electrodialysis process, describing the technological fundamentals, experimental tests and modelling. Chapter 3 reports an original exergy analysis of the RED unit and the potential applications of the process in real environments. Chapter 4 presents the analysis on the influence of salt-solution properties on the RED-HE performance, reporting novel osmotic and activity coefficients for potassium acetate and caesium acetate water solutions. Section II - Solvent Extraction: RED with evaporative regeneration unit Chapter 5 and Chapter 6 are dedicated to the solvent extraction reverse electrodialysis heat engine. In particular, Chapter 5 reports the simplified mathematical models used to evaluate preliminary performances of (i) theoretical SGP unit fed by different salt solutions, (ii) RED-MED HE and (iii) RED-MD HE fed by NaCl solutions. Chapter 6 describes the advanced RED-MED-HE model developed to perform exergy analysis on the integrated system. Section III - Salt Extraction: Thermolytic salts Chapter 7 and Chapter 8 reports the analysis on thermolytic salts. In particular, Chapter 7 is focused on the regeneration unit of thermolytic salts presenting mathematical models and experimental assessments. Chapter 8 presents the first operating thermolytic RED heat engine (t-RED HE) and a perspective analysis based a validated process model for the t-RED HE