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Integrating Latent Heat Storage into Residential Heating Systems

A study from material and component characterization to system analysis

Time: Mon 2021-05-31 13.00

Location:, Stockholm (English)

Subject area: Energy Technology

Doctoral student: Tianhao Xu , Tillämpad termodynamik och kylteknik

Opponent: Professor Tianshu GE, Shanghai Jiaotong University

Supervisor: Docent Samer Sawalha, Tillämpad termodynamik och kylteknik, Energiteknik; Björn Palm, Energiteknik, Tillämpad termodynamik och kylteknik; Justin NingWei Chiu, Kraft- och värmeteknologi

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Latent heat thermal energy storage (LHTES) systems can be coupled with heat pump (HP) systems to realize heat load shifting on demand side. With phase change material (PCM), well designed LHTES components exhibit high storage energy density and thus have large potentials to be integrated in residence where a compact energy storage solution is needed. However, real installations of LHTES-HP integrated systems are still rare nowadays; feasibility of this technology in achieving technically sound and economically viable load shifting operations should be demonstrated and understood by stakeholders to promote its implementation. Therefore, this thesis presents an exemplary feasibility study for three selected off-the-shelf macro-encapsulated PCM products, encompassing in-depth experimental and numerical modelling investigations on three levels—material, component, and system. The feasibility is studied with a specific scenario where the macro-encapsulated LHTES systems are designed to integrate with HP-based heating systems in common Swedish residential buildings. 

On the material level, three commercial PCMs (C48, C58, and ATP60) are selected by the operating temperature levels in typical HP-based space heating systems. Differential Scanning Calorimetry and Temperature-History method are employed to measure PCM enthalpy-temperature profiles; Transient Plane Source method is used to measure the thermal conductivity of ATP60. C58 based on sodium acetate trihydrate is prioritized for in-depth feasibility analyses because of its highest volumetric heat storage capacity.

On the component level, three full-scale macro-encapsulated LHTES components (Component 1: cylindrical encapsulation of C48; Component 2: cylindrical encapsulation of C58; Component 3: ellipsoidal encapsulation of ATP60) are developed for integration in single-family houses to achieve full peak load shifting. A test rig is built for characterizing the three components under possible operational conditions in practical systems. The heat transfer enhancing effects from increasing the temperature difference between heat transfer fluid (HTF) inlet temperature and phase-change temperature as well as from increasing the HTF inlet flowrate are quantified. Performance indicators, such as completion time of charge/discharge, energy storage density, and capacity enhancement factor, are evaluated at different operating temperature ranges. Overall, Component 2 is feasible in delivering around 90% of storage capacity (the capacity loss is due to phase separation). However, storage design and control improvements are still needed for realizing full peak load shifting over a three-hour discharging process. For Component 2, an improved storage solution with a reduced capsule diameter and time-increasing HTF flowrate profiles is developed through numerical simulation using an experimentally-validated two-dimensional heat transfer model. Furthermore, a one-dimensional model is developed and validated for simulating storage thermal output of Components 2 and 3. 

On the system level, a numerical model is developed to calculate electricity input to the LHTES-HP integrated systems for technical, economic, and environmental load shifting evaluation. Three new integration layouts are developed to charge scaled-up Component 2 with a de-superheater and/or a booster heat pump cycle. The new layouts can improve the weekly heating performance factor by 22%–26%, compared with a conventional layout using the condenser for charging. Savings in operational expenses can justify a capital expense of 25,000 Swedish Krona (about 2,500 €) for the LHTES system with a 15-yr operation. Although this justifiable capital expense is lower than the storage component cost alone estimated with the cost of Component 2, it is anticipated that similar LHTES solutions may gain more economic feasibilities with larger peak-valley electricity price differences foreseeable in the future. 

Through presentation of the multi-level feasibility evaluation, this thesis identifies key design and operational issues which might be neglected in single-level investigations. Furthermore, the thesis develops two new LHTES-HP integrated solutions with improved storage design/control strategies and enhanced system coupling methods from the existing solutions. This provides application-oriented insight for design and operation of the load-shift based LHTES installations in residential buildings, potentially contributing to decarbonisation of the increasingly electrified heating sector.