Hansen, Annette Kirstine

Grundvand er en vigtig, men ikke utømmelig kilde til forsyning af drikkevand. I forhold til kildepladsstyring findes der flere, og ofte modstridende, interesseområder. Dette Ph.d. projekt undersøger, hvordan kildepladsstyring kan forbedres med hensyn til forskellige beslutningsmål samtidigt, også kaldet multi-objektiv optimering. Der er udviklet en softwareplatform der bruger multi-objektiv opti- mering til at forbedre kildepladsstyringen. Optimeringen bruger "Strength Pareto Evolutionary Algorithm 2" (SPEA2) til at finde Pareto fronten mellem de mod- stridende beslutningsmål. En Pareto front er et sæt af ikke-dominerede optimale punkter, og er et vigtigt værktøj for beslutningstagere. Optimeringsplatformen er eksemplificeret og testet på to case studies, der begge indvinder cirka 20.000 kubik meter vand om dagen, men ellers er ret forskellige. Det første case study handler om kildepladsstyring af Hardhof vandværk i Schweiz, hvor kunstig infiltrering af flodvand i infiltrations bassiner og injektions brønde er essentiel for at sikre drikkevandsproduktionen. Tilstrømning af forurenet vand fra omgivelserne forhindres af infiltrationen, der opretholder en hydraulisk gradient væk fra kildepladsen. Beslutningsmålene for optimeringen er at minimere mæng- den af infiltreret vand og at minimere risikoen for at få forurenet vand ind i pro- duktionsbrøndene, imens der samtidigt skal leveres en daglig mængde drikkevand Konstant og sekventiel optimering er blevet foretaget på Hardhof vandværket. Den konstante optimering holder alle beslutningsvariabler konstante i modelevaluerings- perioden og fungerer godt, når de hydrologiske parametre og leveringskravet er relative konstante i evalueringsperioden. Resultaterne fra den konstante optimer- ingsmetode viser yderligere at kildepladsstyringen, sammenlignet med den his- toriske styring, kan blive forbedret med hensyn til begge beslutningsmål. Den sekventielle optimeringsmetode optimerer problemet skridtvis for daglige tids skridt, hvilket tillader den endelige løsning at variere i tid. Forskningen viser, at denne metode virker bedre end den konstante optimering, når der forekommer store vari- ationer i de hydrologiske parametre. Desuden kan den sekventielle optimeringsme- tode bruges ved real-tids operationer af vandværker, fordi de hydrologiske parame- tre til modellen kun skal leveres til et tids skridt frem i tiden. Resultaterne fra begge optimeringsmetoder viser, at hvis forureningstruslen holdes på det historiske niveau, er det muligt at spare på mængde af infiltreret vand. Det er også muligt at reducere forureningstruslen, hvis fordelingen af det infiltrerede vand ændres således, at mere vand bliver infiltreret i bassinerne og mindre i brøndene. Hvis vandværket vil være sikre på at undgå tilstrømning af forurenet vand er det nødvendigt at øge den samlede mængde af infiltreret vand. Det andet cast study handler om kildepladsstyringen af Søndersø vandværk i Danmark. Beslutningsmålene på Søndersø vandværk er at minimere vandværkets energiforbrug og at minimere risikoen for at få tilstrømning af forurenet vand fra den nærliggende forurenede grund, Værløse flyveplads. Beslutningsvariablerne er den relative hastighed af pumperne, mens vandværket skal levere en daglig mængde drikkevand. Der er udviklet en integreret hydraulisk kildepladsmodel, som forudsiger vandets strømning i grundvandsmagasinet, i brønden og i ledningsnettet. Kildepladsmodellen, WELLNES (WELL Field Numerical Engine Shell) kan forudsige effektforbruget i de enkelte brønde, og er i stand til at beregne de hurtige vandstands- og effektforbrugsændringer der sker i brøndene når pumpernes hastighes ændres. WELLNES er sat op og kalibreret for Søndersø vandværk. Der er god overensstemmelse mellem model simulering og observationer. Optimeringsresultaterne for Søndersø vandværk viser at den besparelse, der kan opnås ved at bruge de eksisterende tænd/sluk-pumper er lille. Hvis alle de eksisterende tænd/sluk-pumper erstattes med nye hastighedsregulerbare pumper, er det muligt at spare mellem 25 og 40%af det specifikke energiforbrug (energiforbruget per oppumpet mængde vand). Det svare til en reduktion i energiforbruget på cirka 200 MWh per år. Alle resultater viser, at der kan opnås en betydelig reduktion i forureningsrisikoen. Forskningen viser desuden, at det store energibesparelsespotentiale primært skyldes optimeringen af de hastigheds-regulerbare pumper frem for optimering af nye tænd/sluk-pumper. Tilbagebetalingstiden for at investere i de nye hastighedsregulerbare pumper til Søndersø vandværk er kun 3-4 år, hvilket er en interessant tidshorisont for vandværkerne. Den udviklede softwareplatform til multi-objektiv optimering af kildepladsstyring har vist sig at fungere godt, og det er med succes blevet brugt på de to case studies, Hardhof og Søndersø vandværk. Hvis optimerings metoden benyttes på alle de danske vandværker estimeres det, at der kan opnås besparelser på 20-32 GWh/år, eller det der svare til 17 til 27 millioner kroner.Groundwater is a limited but important resource for fresh water supply. Differ- ent conflicting objectives are important when operating a well field. This study investigates how the management of a well field can be improved with respect to different objectives simultaneously. A framework for optimizing well field man- agement using multi-objective optimization is developed. The optimization uses the Strength Pareto Evolutionary Algorithm 2 (SPEA2) to find the Pareto front be- tween the conflicting objectives. The Pareto front is a set of non-inferior optimal points and provides an important tool for the decision-makers. The optimization framework is tested on two case studies. Both abstract around 20,000 cubic meter of water per day, but are otherwise rather different. The first case study concerns the management of Hardhof waterworks, Switzer- land, where artificial infiltration of river water into infiltration basins and injec- tion wells are essential for securing the production of drinking water. Inflow of contaminated water from surrounding urban areas is prevented, because the in- filtration maintains a hydraulic gradient directed away from the well field. The objectives of the optimization problem are to minimize the amount of infiltration, and to minimize the risk of getting contaminated water into the production wells. The optimization problem is subjected to a daily demand fulfilment constraint. Constant and sequential scheduling optimization is performed on the Hardhof case. The constant scheduling keeps all decision variables constant during the evaluation period. This method shows good performance when the hydrological conditions and water demand are relatively constant during the evaluation period. Compared with historical operations the optimization problem can be improved with respect to both objectives. The sequential scheduling optimizes the management stepwise for daily time steps, and allows the final management to vary in time. The research shows that this method performs better than the constant scheduling when large variations in the hydrological conditions occur. This novel approach can be used in real-time oper- ation of the waterworks, because the hydrological parameters for the model only have to be provided for one time step ahead. If the contamination risk is kept at the historical level both optimization methods show that it is possible to reduce the amount of infiltration water. It is also possible to reduce the contamination risk if the distribution of the infiltrated water is changed, so that more water is infiltrated in the basins and less in the wells. However, if the waterworks want to be sure to avoid inflow of contaminated water it is necessary to increase the total amount of infiltration. The second case study considers the operation of Søndersø waterworks, Denmark. At Søndersø the optimization objectives are to minimize the energy consumptions of the waterworks, and to minimize the risk of getting contamination from the nearby contaminated Værløse Airfield into the well field. The decision variables are the relative speed of the pumps. The waterworks has to provide a certain amount of drinking water. A fully integrated hydraulic well field model, which predicts the flow of water in the aquifer, in the well, and in the pipe network has been developed. The well field model, WELLNES (WELL Field Numerical Engine Shell), is capable of predicting the power consumption at different wells. It captures the water level- and power dynamics in each well when pump speeds are changed. WELLNES is set up and calibrated for the Søndersø area. The WELLNES model shows good correspondence between observations and simulations in both calibration and validation periods. The optimization results for Søndersø shows that only minor energy savings can be achieved with the existing pumps. If all the existing on/off pumps are changed to new variable-speed pumps it is, however, possible to save between 25 and 40% of the specific energy (the energy consumption per cubic meter of abstracted water). This corresponds approximately to an energy reduction of 200 MWh per year. All optimization results shows that it is possible to obtain significant reductions in the contamination risk. The research shows that the large potential for savings is mainly due to optimizing the variable-speed pumps rather than optimizing the new on/off pumps. The payback period of investing in new variable-speed pumps for Søndersø waterworks is only 3-4 years, which is an interesting time horizon for the waterworks. The developed multi-objective optimization framework has shown to be useful in optimizing the management of well fields, and it has successfully been applied to the two case studies, Hardhof and Søndersø waterworks. If the method is applied to all Danish waterworks it is estimated that 20-32 GWh/year could be saved, corresponding to 17-27 million DKK

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Optimization of well field management

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Optimization of well field management

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