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Large solar collector fields are very popular in district heating system in Denmark, even though the solar radiation source is not favourable at high latitudes compared to many other regions. Business models for large solar heating plants in Denmark has attracted much attention worldwide. Denmark is not only the biggest country in both total installed capacities and numbers of large solar district heating plants, but also is the first and only country with commercial market-driven solar district heating plants. By the end of 2017, more than 1.3 million m 2 solar district heating plants are in operation in Denmark. Furthermore, more than 70 % of the large solar district heating plants worldwide are constructed in Denmark. Based on the case of Denmark, this study reviews the development of large solar district heating plants in Denmark since 2006. Success factors for Danish experiences was summarized and discussed. Novel design concepts of large solar district heating plants are also addressed to clarify the future development trend. Potential integration of large solar district heating plants with other renewable energy technologies are discussed. This paper can provide references to potential countries that want to exploit the market for solar district heating plants. Policymakers can evaluate the advantages and disadvantages of solar district heating systems in the national energy planning level based on the know-how and experiences from Denmark.

Document Version Peer reviewed version Link back to DTU Orbit Citation (APA):Dannemand, M., Schultz, J. M., Johansen, J. B., & Furbo, S. (2015). Long term thermal energy storage with stable supercooled sodium acetate trihydrate. Applied Thermal Engineering, 91, 671-678. DOI: 10.1016/j.applthermaleng.2015 requires that the sodium acetate trihydrate is heated to a temperature somewhat higher than the melting 20 temperature of 58°C before it is cooled down. Expansion and contraction of the phase change material in a 21 closed tank compromises the stability of the supercooling. An expansion device that allows for the phase 22 change material to expand and contract without pressure changes in a closed chamber makes it possible to 23 achieve stable supercooling. Initializing the crystallization of the supercooled sodium acetate trihydrate can 24 be done by cooling the phase change material locally to the maximum level of supercooling or by 25 mechanically providing a seed crystal. Supercooled sodium acetate trihydrate at 20˚C stores up to 230 26 kJ/kg of thermal energy. The energy discharged is affected by the storage temperature of the supercooled 27 phase change material before crystallization and the final discharge temperature. Internal cavities are 28 formed during the contraction of the phase change material when crystallizing decreasing the heat transfer 29 rate. A thermally conductive liquid that does not mix with the phase change material can be added to fill 30 the cavities and enhance the heat transfer. Graphite powder can improve the thermal conductivity of the 31 sodium acetate trihydrate and needs to be kept dispersed evenly in the phase change material by a 32 thickening agent that is stable at the maximum temperature of the storage during the charging. A TRNSYS 33 simulation of a solar thermal combi system including a storage utilizing stable supercooling of sodium 34 acetate trihydrate elucidates the system size to achieve 80 % solar fraction of the low energy house in 35Danish climatic conditions. 36

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