Sustainability assessment of technology systems that address the energy-water nexus: the case of desalination in the Western Cape

Swart, Liesel (2017-12)

Thesis (MEng)--Stellenbosch University, 2017.

Thesis

ENGLISH ABSTRACT: In recent decades, the critical interdependencies that exist between the energy and water sectors, referred to as the energy-water nexus, have been investigated. It has become apparent that a policy change or intervention in one of the sectors can have a significant impact on the other. The impact on the entire energy-water nexus, therefore, needs to be considered when implementing changes in either sector. The Western Cape Province needs to increase its available water supply to ensure that the future water demand can be met. The aim of this research was to investigate the appropriateness of seawater desalination used in conjunction with renewable energy as a possible water supply intervention. The objectives of this research were to: select an appropriate modelling approach; compile, verify and validate the model; simulate different scenarios with and without the technology; and provide policymakers with recommendations regarding the sustainability of implementing a desalination technology system. It was determined that the Western Cape Province’s energy-water nexus is a complex system, because of the number of subsystems that exist within the system. A literature survey of the previous efforts that had been made to model similar systems was conducted. System dynamics modelling was found to be the most appropriate modelling tool, given the objectives of the research and the complexity of the problem. A systems thinking and modelling process was followed to develop a model of the Western Cape Province’s energy-water nexus. The first phase was problem formulation, and the second phase was the development of the conceptual model using causal loop diagrams. The construction, verification and validation of the dynamic computer simulation model was done in the third phase. Once confidence in the dynamic model was established, the fourth phase of the modelling process was completed. For this phase, a number of scenarios were developed and simulated to determine the effect of different desalination technology systems on the Western Cape Province. The impact of multi-effect distillation (MED) and reverse osmosis (RO) were compared. The effects of combining these technologies with different renewable energy sources, including thermal waste heat and solar energy, were also investigated. It was seen that the Western Cape Province’s water supply would be unable to meet the future water demand if no intervention was made. The results indicated that RO with photovoltaics would be the most sustainable and that the system’s life cycle cost up until 2040 is the lowest of all the simulated systems. In phase five, it was recommended to policymakers that, of all the systems that were considered, this system would be the best to implement. Additional water supply interventions, however, need to be investigated, as the recommended desalination system would not be enough to ensure sufficient future water supply. This research provides a better understanding of the complexities involved in the installation of a new technology system, such as desalination, in the Western Cape Province’s energy-water nexus. This research can be used as a platform to further explore the impacts of a desalination system or to investigate the sustainability of other technology systems that will affect the nexus.

AFRIKAANSE OPSOMMING: Die kritiese skakels tussen die energie en water sektore, waarna verwys word as die energie-water nexus, word in die afgelope dekades ondersoek. Dit is duidelik dat die beleidveranderings en ingrypings wat in een sektor uitgevoer word ‘n beduidende impak op die ander sektor kan hê. Die impak op die hele energie-water nexus moet dus oorweeg word, wanneer veranderinge in enige van die sektore uitgevoer word. Die Wes-Kaapse Provinsie moet sy beskikbare watervoorsiening verhoog om aan die toekomstige wateraanvraag te kan voorsien. Die doel van hierdie navorsing is om die toepaslikheid van seewater ontsouting, gepaard met hernubare energie, te ondersoek as ‘n moontlike watervoorsieningsingryping. Die doelwitte van die navorsing was om: ‘n toepaslike modelleringsbenadering te kies; die model saam te stel, te verfieer en te valideer; verskeie scenarios te simuleer; en beleidsmakers met aanbevelings rakende die volhoubaarheid van ‘n ontsoutingstegnologiestelsel te voorsien. Daar is bevind dat die Wes-Kaapse energie-water nexus ‘n komplekse stelsel is as gevolg van die aantal substelsels waaruit die stelsel bestaan. ‘n Literatuuroorsig van vorige pogings om soortgelyke stelsels te modelleer is uitgevoer. Daar is bevind dat stelseldinamika die mees toepaslike modelleringstegniek is, gegewe die doelwitte en kompleksiteit van die probleem. ‘n Stelseldenke en modelleringsproses is gevolg om die model van die Wes-Kaapse energie-water nexus te ontwikkel. Die eerste fase was om die probleem te formuleer en die tweede fase was om die konsepsionele model te ontwikkel. Die dinamiese rekenaarsimulasie model is in die derde fase ontwikkel, geverifieer en gevalideer. Nadat vertroue in die dinamiese model bevestig was, is die vierde fase van die modelleringsproses voltooi. Vir die fase is ‘n aantal scenarios ontwikkel en gesimuleer om die gevolge van verskillende ontsoutingstegnologiestelsels vir die Wes-Kaapse Provinsie te bepaal. Die impak van multi-effek distillasie (MED) en omgekeerde osmose (RO) is vergelyk. Die gevolge van die kombinasie van die tegnologieë met hernubare krag, soos termiese afvalhitte en sonkrag, is ook ondersoek. Die simulasieuitslae het aangedui dat die Wes-Kaapse watervoorsiening nie voldoende is vir die voorspelde wateraanvraag nie. Die uitslae het ook aangedui dat RO met fotovoltaïese panele die mees volhoubare stelsel is en dat die stelsel se lewenssikluskoste tot en met 2040 die laagste van al die gesimuleerde stelsels is. In die vyfde fase is beleidsmakers aanbeveel dat hierdie stelsel, van al die stelsels wat oorweeg is, die beste is om te implementeer. Addisionele watervoorsieningsingrypings moet egter ondersoek word, aangesien die aanbevole ontsoutingsstelsel nie voldoende sal wees om aan toekomstige wateraanvraag te voorsien nie. Die studie het gelei tot ‘n beter begrip van die kompleksiteit rakende die installering van ‘n nuwe tegnologiestelsel, soos ontsouting, in die Wes-Kaapse energie-water nexus. Die studie kan gebruik word as ‘n platform om verder die gevolge van ‘n ontsoutingsstelsel te ondersoek, of om die volhoubaarheid van ander tegnologiesisteme wat die nexus sal raak, te ondersoek.

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