Thermochemical heat storage materials – Performance of mixed salt hydrates. (15th October 2016)

• Record Type: Journal Article
• Authors: Rammelberg, Holger U.; Osterland, Thomas; Priehs, Boris; Opel, Oliver; Ruck, Wolfgang K.L.
• Is Part Of: Solar energy. Volume 136(2016)
• Page Start: 571
• Page End: 589
• Publication Date: 2016-10-15
• Subjects: DSC differential scanning calorimetry -- gh mass of hydrated form -- i cycle number -- mW, i mass of water uptake of cycle number i, mg -- mw, y mass of water uptake of standard cycle y, mg -- msalt1, salt2 mass of salt 1 or 2, mg -- mmixture, simulated mass of mixture simulation, mg -- Nm normalized water uptake -- NΔH, R normalized reaction enthalpy -- n.d. not determined -- p water vapor pressure, mbar -- T temperature, C -- t time, min -- TGA thermogravimetric analysis -- Xsalt1, salt2 share of salt 1 or 2 in the mixture -- ΔHe expected reaction enthalpy of a mixed salt hydrate, kJ/mol -- ΔHR reaction enthalpy, kJ/mol -- ΔHR, i reaction enthalpy of cycle number i, kJ/mol -- ΔHR, y reaction enthalpy of standard cycle y, kJ mol -- ΔHs1, s2 reaction enthalpy of pure salt hydrate 1 or 2, kJ/mol -- (s) solid -- (l) liquid -- (g) gas -- ↑ increasing value -- ↓ decreasing value; Thermochemical energy storage -- Cycling stability -- Salt hydrate -- Thermal analysis; Solar energy -- Periodicals; Solar engines -- Periodicals; 621.47
• DOI: 10.1016/j.solener.2016.07.016
• Languages: English
• ISSNs: 0038-092X

Abstract

Highlights: Salt mixtures based on CaCl2, MgCl2 and MgSO4 are synthesized. Thermal and X-ray characterizations are performed. New thermochemical material shows high cycle stability and reaction enthalpy. Different hydration cycle methods are compared. Abstract: Thermochemical heat storage is highly promising, in particularly with a view to long-term heat storage. For the implementation of heat storage in households, thermochemical reactions in the low temperature range below 120 °C are important. Especially salt hydrates such as MgCl2, CaCl2 or MgSO4 were tested with micro gravimetric methods for their suitability. However, the cycle stability of consecutive charging (dehydration) and discharging (hydration) reactions of these materials was low and could be improved only by control of the water uptake (i.e. discharging time) to prevent overhydration. In contrast, mixtures of these salt hydrates showed significant improvements in cycle stability, mass and enthalpy balances. The experiments also showed that the cycleability of all investigated materials increased if hydration and dehydration reactions were performed under constant vapor pressure of 21 mbar. Contrary to other materials, the mixture of CaCl2 and MgCl2 showed good cycleability under all tested conditions. In addition, the mixture showed superior kinetic properties. Additionally, there is evidence of tachyhydrite (CaMg2 Cl6 ⋅12H2 O) formation during cycling of the mixture by the use of XRD after the thermal analysis. Further investigations will be performed to identify further synergies, ideal mixing ratios and formed phases.