Power-to-water Battery: Integrated Energy Storage and Water Harvesting

Abstract

This thesis explores the integration of energy storage and water production technologies, examining how Power-to-Water (P2W) technologies can be optimized to enhance the efficiency and cost-effectiveness of both energy storage and water production, offering sustainable solutions to the challenges posed by renewable energy intermittency and water scarcity, emphasizing scalability, efficiency, and economic feasibility.

The first section of the thesis is finding the best configuration (e.g., storage medium and conversion routes) of Power-to-X (P2X) system. We first focuses on the optimization of energy storage systems for residential and commercial buildings. By employing the bi-level expanded P-graph optimization method (BEPGOM), the study demonstrates thermal energy storage (TES) systems integrated with heat pumps can be configured to minimize costs and reduce carbon emissions. The results indicate that, through optimization, the net present value of the systems can increase by 18%, while CO2 emissions are reduced by 34%. Compared to traditional methods, BEPGOM realizes cost savings improving by 234% and emissions reduced by 60%.

Building on the optimization of energy systems, the research then examines power-to-heat (P2H) batteries to enhance energy flexibility and provide a cost-effective solution for renewable energy integration. Through the use of the BEPGOM, the study evaluates various P2H systems configurations in efficiency and economics. Key findings show that using high-temperature sensible TES reduces the levelized cost of storage (LCOS) by up to 70% compared to other counterparts. Moreover, configurations employing high-temperature sensible TES achieve storage arbitrage values (SAV) of up to $150,000 annually, showcasing the potential for significant economic benefits through efficient energy management.

The investigation then expands into the broader realm of P2X technologies, which include systems that convert surplus renewable electricity into various forms of energy, including power-to-power (P2P), power-to-heat (P2P), power-to-heat-and-power (P2HP), power-to-combined-heat-and-power (P2CHP). Notably, TES-based P2H systems, when paired with fire bricks and absorption heat pumps, present better performance over other energy storage configurations, exhibiting an efficiency that is 20% higher than that of electrochemical storage systems and achieving a remarkable reduction in LCOS of less than $0.01/kWh, providing a cost-effective solution for large-scale energy storage and renewable energy integration.

Building on the foundation of energy storage system configuration optimization, the findings suggest that the use of high-temperature TES for energy storage offers high cost-effectiveness. Moreover, regions with abundant new energy sources are usually arid. The P2X field has seen less focus on water production, motivating the introduction of the P2W concept, which address both energy and water challenges simultaneously. During off-peak hours, when renewable energy is abundant, the P2W battery stores surplus energy as high-temperature thermal energy. This stored energy is later used to provide a flexible solution for AWH and dehumidification processes. The preliminary research demonstrates that the P2W system is capable of producing 10.2 g/L_device•h, significantly outperforming current AWH technologies. The efficiency of the P2W battery reaches 90%. These results highlight the P2W battery potential as a highly efficient and cost-effective solution for water production in regions with abundant renewable energy but limited freshwater resources.

The thesis keeps refining the P2W systems, introducing the multi-stage power-to-water (MSP2W) battery, a system combining high-temperature TES and multi-stage AWH. The thesis presents a theoretical analysis of the multi-stage AWH performance limits, showing that the water production of multi-stage AWH systems is strongly influenced by factors such as the number of stages and the sorbent properties, especially the sorption capacity and kinetics. This theoretical model guided sorbent material development, sorbent bed invention, and experimental design, ensuring that the system operates efficiently without unnecessary complexity. The prototype demonstrates impressive performance, producing 3060 g_water/day, which fully meets an adult’s daily water needs, with a record-low specific energy consumption of 1.13 kWh/kg_water. The performance improves by 40% in water production and reduces energy consumption by 26% compared to single-stage P2W battery. The flexibility of MSP2W battery allows it to store energy during low-price periods and produce water during high-demand periods (0.44 USD/ton during periods of energy surplus), offering an economically viable solution for regions with abundant renewable energy but limited freshwater, such as northern China, the Middle East, and North Africa. Overall, the MSP2W battery provides a scalable, low-cost, and sustainable way to address water scarcity while optimizing the use of renewable energy resources.

In conclusion, this research provides a comprehensive framework for optimizing energy storage systems and integrating them with innovative water production technologies. This thesis introduces novel technologies such as P2W and MSP2W to provide sustainable, cost-effective solutions that integrate renewable energy systems with water production, addressing the growing demand for both energy and clean water, the two of the most pressing global challenges of the 21st century.

Date of Award	24 Sept 2025
Original language	English
Awarding Institution	City University of Hong Kong
Supervisor	Wei WU (Supervisor)