A review of heat pump water heater system demand flexibility

With the rapid growth of renewable energy generation, demand flexibility from heat pump systems hot water systems has emerged as a key strategy to support energy system decarbonization, grid stability, and efficient energy use. This review systematically examines the open literature related to characterising and quantifying energy flexibility of heat pump hot water delivery systems. The review considers several aspects including system design and building, district, and sector details as well as four main flexibility types: load shifting, shaving, modulation, and generation. The review highlights a significant diversity in modelling approaches with approximately half of studies using physics-based models, followed by data-driven, hybrid, and statistical models. Control strategies are also diverse, with model predictive control being the most prevalent, while others like rule-based control and reinforcement learning are less common. Load shifting is the most widely studied approach, while other flexibility types remain underexplored. A major gap identified is the lack of standardisation in flexibility metrics—73 unique metrics were found with inconsistent definitions and application. A quarter of studies did not include a mathematical formulation for flexibility. The findings underscore the need for standardised metrics, greater application of experimental validations, and broader exploration of the flexibility types beyond load shifting. Social acceptance, user comfort, and equity considerations are also key areas for future research and deployment.