To support the large­scale integration of renewables in electricity grids, power­to­X­to­power (P2X2P) systems have been proposed. These systems serve to increase the flexibility of thermal power plants while potentially providing both economic and environmental benefits by allowing power from the plant to be redirected into an electrolyzer and converted to a gaseous energy carrier. In this study, the feasibility of a P2X2P system consisting of a combined cycle gas turbine (CCGT) power plant coupled with a PEM electrolyzer in the Italian power sector has been investigated. A dynamic technoeconomic model has been developed for both hydrogen and ammonia­based systems together with a profit maximizing dispatch strategy for operation in both day­-ahead and balancing electricity markets. As a part of this, a PEM electrolyzer model was also developed and validated against experimental data. Notable technical improvements were observed as a consequence of the implementation of a P2X2P system in the form of avoided shutdowns and a more even power output. However, any economic and environmental benefits of such improvements were not observed as the addition of the P2X2P system led to a reduction in net present value as well as higher specific emissions of carbon dioxide. When the gaseous energy carrier was utilized as fuel in the CCGT, similar technical performances were achieved by the hydrogen­based and ammonia­based systems. Due to the increased investment cost demanded by the ammonia production process the hydrogen­based system thus seems most suitable for this setup. / För att möjliggöra en storskalig utbyggnad av förnyelsebar energi har power­to­X­to­power­system (P2X2P) föreslagits som en potentiell lösning. Genom att omdirigera electricitet från kraftverket till en elektrolysator och därmed omvandla denna till vätgas kan dessa system förbättra den tekniska flexibiliten hos värmekraftverk samtidigt som de har potential att medföra både ekonomiska och miljömässiga fördelar. Detta examensarbete har undersökt den tekno­ekonomiska potentialen hos ett P2X2P­system bestående av ett gaskombikraftverk i anslutning till en elektrolysator i det italienska kraftnätet. En dynamisk, tekno­ekonomisk modell av både vätgas­ och ammoniakbaserade P2X2P­system samt en vinstmaximerande kontrollstrategi har utvecklats. En modell över en PEMelektrolysator har även utvecklats och validerats gentemot experimentella data. Införandet av ett P2X2P­system till kraftverket påvisade en teknisk förbättringspotential genom ett minskat antal uppstarter samt en mer jämn uteffekt. Huruvida denna tekniska förbättring också medför ekonomisk and miljömässig förbättring eller ej kvarstår att påvisa. Detta då nuvärdet minskade samtidigt som koldioxidutsläppen per producerad kilowatttimme ökade vid införandet av P2X2P­systemet. Då den producerade energibäraren, i form av vätgas eller ammoniak, enbart användes för att ersätta fossilgas som bränsle i kraftverket påvisades marginell skillnad i presetanda mellan de två systemen. De större kostnaderna som medförs av ett ammoniak­baserat system pekar därför på att ett vätgas­baserat system vore att föredra under sådana förutsättningar.

This report travels through multiple disciplines to seek innovative and sustainable energy solutions for wastewater treatment plants. The first subject is a report about increased global temperatures and an over-exploitation of natural resources that threatens ecosystems worldwide. The situation is urgent where the current trend is a 2°C increase of global temperatures already in 2040. Furthermore, the energy-land nexus becomes increasingly apparent where the world is going from a dependence on easily accessible fossil resources to renewables limited by land allocation. A direction of the required transition is suggested where all actors of the society must contribute to quickly construct a new carbon-neutral resource and energy system. Wastewater treatment is as required today as it is in the future, but it may move towards a more emphasized role where resource management and energy recovery will be increasingly important. This report is a master’s thesis in energy engineering with an ambition to provide some clues, with a focus on energy, to how wastewater treatment plants can be successfully integrated within the future society. A background check is conducted in the cross section between science, society, politics and wastewater treatment. Above this, a layer of technological insights is applied, from where accessible energy pathways can be identified and evaluated. A not so distant step for wastewater treatment plants would be to absorb surplus renewable electricity and store it in chemical storage mediums, since biogas is already commonly produced and many times also refined to vehicle fuel. Such extra steps could be excellent ways of improving the integration of wastewater treatment plants into the society. New and innovative electric grid-connected energy storage technologies are required when large synchronous electric generators are being replaced by ‘smaller’ wind turbines and solar cells which are intermittent (variable) by nature. A transition of the society requires energy storages, balancing of electric grids, waste-resource utilization, energy efficiency measures etcetera… This interdisciplinary approach aims to identify relevant energy technologies for wastewater treatment plants that could represent decisive steps towards sustainability.