Water-Energy Nexus: Analysing the energy-for-water relationship in integrated energy systems

The volatility of renewable energies poses challenges to power system reliability and calls for more flexible electricity resources, both on the supply and the demand side. Energy-intensive water services such as wastewater treatment offer great demand flexibility potential in that regard. However, current demand response modelling approaches are insufficient for assessing this potential accurately. This study aims to fill the knowledge gap in industrial demand response modelling by introducing an integrated energy-water system model, which takes into account the constraints of the wastewater treatment process on power system scheduling in a joint system dispatch problem. The model is applied to a case study of the Irish wastewater treatment sector and power system. The objective of this study is to identify the benefits of energy demand and supply flexibility of wastewater treatment plants for power system operation, wastewater treatment operators and electricity consumers. The findings indicate that the wastewater treatment sector can be a valuable demand response resource for the power system. Wastewater treatment operators, electricity consumers and power system operators benefit from more flexible electricity demand from wastewater treatment plants, even in the presence of other flexibility measures in the system. Furthermore, it decreases the carbon intensity of domestic power generation. There is also a benefit for the power system operator in harnessing the flexibility of demand response and biogas production simultaneously. However, this can result in temporarily high electricity prices in the model, leading to increased electricity costs for consumer and wastewater treatment plants. Two main conclusions can be drawn from the findings of this study. First, wastewater treatment plants have untapped potential for demand response and utilising it for power system flexibility benefits wastewater treatment operators, electricity consumers and power system operators. The results inform policy makers on how to evaluate and support the electricity demand and supply flexibility of wastewater treatment plants. Given the benefits and minimal capital costs, policy makers should incentivise WWTP operators to tap into this readily available flexibility potential. Further, policy makers should carefully select the appropriate support schemes. In particular, smart demand response schemes should take into account possible interactions with electricity supply flexibility from biogas generation. Second, including wastewater treatment constraints in the system dispatch problem is crucial in order to estimate the flexibility potential accurately and uncover bottlenecks, which would probably be concealed by a black-box approach. Thus, this study provides a valuable case study for investigating the demand response potential of highly complex industrial processes, such as wastewater treatment.