Combined short and long-term heat storage with sodium acetate trihydrate for solar combi-systems

Abstract

With increasing utilization of energy from renewable sources, solar heating has become a promising technology for reducing fossil fuel consumption in the building sector. Due to the mismatch of solar energy resources and the demand patterns of single family houses in central and northern Europe, long-term heat storage is essential for solar combi-systems that cover hot water supply and heating with a solar fraction larger than 50%. Therefore, a concept for combined short and long-term heat storage utilizing stable supercooling of sodium acetate trihydrate (SAT) was examined for its applicability in combi-systems. The concept enables use of the sensible heat capacity of liquid SAT composites while preserving its heat of fusion at room temperature. Thus, loss-free heat storage is achieved, which can be utilized for on-demand supply in periods without solar irradiation available. The research objective was to develop a heat storage prototype and to test it in a full scale laboratory solar heating system. Findings were used in system simulation as well as to design inexpensive heat storage units. SAT composites containing thickening agents and liquid polymers were identified to be suitable heat storage materials. Their heat of fusion, available in supercooled state at 20 °C, was determined to be in the range of 205–216 kJ/kg. Material tests showed that SAT composites in glass bottles supercool down to temperatures of about −24 °C, and crystallized in the range of −9 °C to −15 °C in contact with steel. Thus, devices employing mechanical seed crystal injection and local cooling by Peltier elements or evaporating carbon dioxide were applicable to initiate crystallization of supercooled SAT composites when heat was on demand. Four heat storage units, each containing 200–220 kg of SAT composites, and a 735 L water tank formed a segmented heat storage prototype. A system control strategy was developed for charging the heat storage with an array of 22.4 m2 (aperture) evacuated tubular solar collectors and to enable hot water supply and space heating. System demonstration with heat demand patterns of a Passive House in Danish climate and verified control parameters proved applicability of the heat storage concept. System simulations for a solar heating system for the house with a yearly heat demand of 3977 kWh with optimized component specifications and 1 m3 of SAT composites and 22.4 m2 (aperture) collector area resulted in a yearly solar fraction of 71%. The system was found to perform best with SAT composite volumes below 1 m3, heat storage units of 200 L, a 0.6 m3 water tank and with collector areas of 12.8−22.4 m2 with a collector tilt of 70°. Inexpensive tank-in-tank units, built with standard componens of water stores, could be applied in the system. Laboratory tests showed that their heat transfer properties were sufficient for hot water supply and space heating with liquid SAT composite, whereas discontinuous discharge would be needed during solidification. For the first time, a solar heating system utilizing stable supercooling of SAT composites was demonstrated. This work will provide reference for solar heating in energy efficient buildings with solar fractions above 70%. Further studies are needed to elucidate optimal application of the heat storage concept for different climates considering different collector types, improved heat storage units and electric grid stabilization via power-to-heat conversion.