Numerical Investigation of Pressure Retarded Osmosis with Brines for Power Generation
Author
Petersen, Kasper Juel
Term
4. term
Education
Publication year
2018
Submitted on
2018-06-01
Pages
83
Abstract
I 2016 var det globale elforbrug 15.567 TWh. Et nyere skøn anslår, at salte reservoirer rummer energi fra saltholdighedsgradienter svarende til 647 GW, dvs. 5.668 TWh om året eller 36,4% af det globale forbrug. Trykretarderet osmose (PRO) er en lovende metode til at udnytte denne ressource. I PRO adskilles en saltholdig væske fra en ren eller lav-saltholdig væske med en semipermeabel membran, så forskellen i saltholdighed kan omdannes til strømning og dermed mekanisk arbejde. Denne afhandling fokuserer på geotermiske saltlage og viser, at temperaturforskelle på tværs af membranen ligeledes kan bidrage og omsættes til mekanisk arbejde. For at beskrive dette udvikles en fænomenologisk membranmodel, der kan forudsige gennemtrængningshastigheder baseret på de drivende gradienter, og modellen kobles til et beregningsværktøj for strømningsmekanik (computational fluid dynamics). Modellen kan kvantificere effekten af intern og ekstern koncentrationspolarisering—ophobning eller udtynding af salt nær membranen—som et tab i energiflux. Simulationer viser, at selv en lav massefraktion af opløst stof på 1,39% i den lav-saltholdige strøm giver et energitab på 47%. Derudover konkluderes det, at temperaturgradienter over membranen ikke er ubetydelige, og at gennemtrængningshastigheden afhænger ikke-lineært af disse. Modellen er den første, der eksplicit repræsenterer effekterne af temperaturgradienter, og arbejdet danner grundlag for en samlet ramme, der kan forudsige membraners ydeevne under alle fundamentale drivkræfter. Dette er værdifuldt for at modne teknologien til kommerciel anvendelse.
In 2016, global electricity consumption was 15,567 TWh. A recent estimate suggests that saline reservoirs contain salinity gradient power equivalent to 647 GW, or 5,668 TWh per year—36.4% of global use. Pressure retarded osmosis (PRO) is a promising way to tap this resource. In PRO, a saline fluid is separated from a pure or low-salinity fluid by a semi-permeable membrane, converting the salinity difference into flow and thus mechanical work. This thesis focuses on geothermal brines and shows that temperature differences across the membrane can likewise be harnessed and converted into mechanical work. To capture these effects, a phenomenological membrane model is developed to predict permeation rates from the driving gradients and is coupled to a computational fluid dynamics framework. The model quantifies the impact of internal and external concentration polarization—salt build-up or depletion near the membrane surfaces—as a loss in energy flux. Simulations indicate that even a low solute mass fraction of 1.39% in the low-salinity stream causes a 47% energy loss. The study also finds that trans-membrane temperature gradients are non-negligible and that permeation depends nonlinearly on them. The model is the first to represent temperature gradient effects, providing a basis for a comprehensive framework that predicts membrane performance under all fundamental driving forces. This will help advance the commercial implementation of the technology.
[This abstract was generated with the help of AI]
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