Seasonal PCM heat storage for a solar space heating and domestic hot water combisystem

Background of the project:

Due to the mismatch of solar energy resources and the demand for heating, long term efficient storage of solar heat is essential for an innovative solar heating system that covers the heating demand with a high solar fraction in the range of 80%-100%. The goal of the project is the technological development and demonstration of a novel compact seasonal heat storage based on stable supercooling of phase change material. With the concept a partly heat loss free heat storage is achieved, i.e. if the PCM has been fully melted in summer when there is excess solar energy, it can cool down in its liquid phase to the surrounding temperature and still preserve the latent heat related to the heat of fusion. The heat storage can be left in this state with no heat loss until a heat demand occurs for example in winter in which case the solidification is activated, the heat of fusion is released and the heat storage temperature increases almost immediately to the melting temperature of the PCM.

Content of the project:

The following research activities will be carried out in the PhD project beyond the state of the art:
• development of efficient heat transfer methods that enable heat exchange capacity rates suitable for the application during charging and discharging of the PCM modules
• Long term tests will elucidate how to design the heat storage modules to ensure that the total heat of fusion is released from the module during discharge
• investigations on possible phase separation that could occur in the liquid phase at a low temperature level and development of a module design that avoid phase separation
• development of a simple and reliable method for the initialization of the crystallization of PCM
• development of an efficient control strategy for the charging and discharging of the storage system
• optimized integration into the conventional heating system by improved heat exchanging methods between the PCM store and the rest of the system
• proof of the developed concept and storage system in a demo building
• Monitoring of the demonstrated solar heating system with the seasonal heat storage modules
• Analysis of thermal performance of the demonstrated solar heating system with focus on efficiency, stability and reliability of the long term heat storage
• System optimization based on calculations with detailed simulation models
• Recommendations for development of improved PCM heat storage design and solar heating system design.

The project is co-financed by ongoing projects at DTU Civil: the FP7 supported COMTES project (Combined development of compact thermal energy storage technologies). It is expected that good synergy is created between the proposed PhD project and the ongoing projects.
The project is carried out in close collaboration with the Institute of Electrical Engineering, Chinese Academy of Sciences, China (CAS IEE). CAS IEE is among the world leading institutes in the field of high-medium temperature solar thermal application, concentrated solar power and heat generation. As a leader of the research group, Prof. Zhifeng Wang has extensive experiences on solar thermal power, heat storage technologies and solar space heating. Prof. Wang will be co-supervisor of the PhD project. It is expected that the PhD student will carry out part of the research activities in CAS IEE with a duration of in total 9 months during the whole PhD study. The planning activities to be carried out at CAS IEE are:
• Investigation of additives for PCM that can minimize the risk of phase separation in PCM heat storage modules
• Concept design of a novel seasonal storage system with enhanced heat transfer capacity rate during charge and discharge
• Investigations of the concept for different climates and building types
• Carry out limited education activities related to SDC

The project will result in a demonstration solar heating system with the seasonal heat storage that is significantly better than water based systems.