Energy efficiency and flexibility in space heating: Thermal energy storage integration strategies in heat pump systems

The electrification of space heating through heat pump (HP) technologies plays a pivotal role in achieving low-emission and energy-efficient buildings. However, the resulting increase in electricity demand introduces challenges for grid stability, particularly during peak demand periods. Integrating thermal energy storage (TES) systems into HP configurations offers a promising solution to address these challenges by enhancing both energy efficiency and demand-side flexibility. This review study systematically investigates the integration of TES units into HP systems specifically for space heating applications, with a particular focus on energy efficiency improvement and strategies for demand management such as peak clipping and load shifting. The study discusses the concept of energy flexibility and its relevance to heating systems. The core of the review categorizes TES-HP integration based on the placement of the TES unit: either on the evaporator side or condenser side of the heat pump. Literature findings reveal that TES integration at the evaporator side, particularly using phase change materials (PCMs), significantly improves system efficiency and reduces compressor runtime. Conversely, TES at the condenser side is more effective in achieving load shifting and demand response targets. Key insights include the importance of PCM selection, TES configuration, and advanced control algorithms. While current applications show great promise, future developments are expected to focus on smart control integration, improved PCM thermal properties, and cost-effective implementation to optimize both performance and grid interaction.