Investigation On The Modification Of Barium Hydroxide Octahydrate As Phase Change Energy Storage Material

Energy shortage and environmental pollution are global issues,and energy storage technology plays a crucial role in improving energy efficiency and protecting the environment.Phase change energy storage materials as it can effectively solve the mismatch between energy supply and demand,overcome the intermittent and fluctuation of renewable energy,and recover and reuse waste heat.Especially,hydrated salts as phase change energy storage materials have received much attention because of its suitable phase transition temperature,large heat capacity and low cost.However,most of hydrated salts have two drawbacks as supercooling and phase separation,which influence their functionality and durability.Meanwhile,the phase change temperature of this type of phase change material is constant,the corresponding application is relatively fixed.Its strong corrosiveness also causes damage to the packaging material.Consequently,these issues that have limited widespread use of hydrated salts as phase change materials.In view of the merits and drawbacks of intensive research on these kinds of phase change materials,Ba（OH）₂·8H₂O was selected as research object.Melting-solidification experiments were carried out to record temperature-time data,the effect of different types and contents of nucleating agents and thickeners on supercooling degree of Ba（OH）₂·8H₂O was studied.The eutectic mixtures and eutectic/expanded graphite composite phase change materials were fabricated by melt-blending and melting impregnation method,respectively.The thermal properties and performance were assessed using differential scanning calorimeter and thermal conductivity test.The crystalline structure and morphology were revealed by X-ray diffraction and scanning electron microscopy.Given that the strong alkaline characteristics of the materials prepared above,common metal encapsulation materials were selected for corrosion full-immersion test to determine whether the phase change material is compatible with the encapsulation metal materials.The main conclusions are as follows:（1）In the comprehensive compounding experiments which were taken out to reduce the supercooling degree of Ba（OH）₂·8H₂O as phase change material,Ca F₂,Na₂HPO₄ and sodium tripolyphosphate were chosen as nucleating agents and the optimized combinations were 1C1G（1%Ca F₂+1%gelatin）,1A1G（1%Na₂HPO₄+1%gelatin）and 2S1H（2%sodium tripolyphosphate+1%hydroxyethyl cellulose）with the crystallization temperature of 76.2°C,76.5°C and 75.9°C,the supercooling degree of the three systems were 0.3°C,0.7°C and 0°C,respectively.（2）Two novel eutectic mixtures of BHO-KCl and BHO-KNO₃ were prepared and their mass ratio at eutectic point were determined.The eutectic compositions were 10wt%KCl and12wt%KNO₃ with phase transition temperatures of 66.25°C and 67.71°C,and latent heat of melting process were found to be 206.4J/g and 231.5 J/g for the BHO-KCl and BHO-KNO₃binary systems,respectively.Expanded graphite added in the phase change material provided effective nucleation sites and changed the crystallization method,effectively reduced the supercooling degreeΔT of the phase change material from 12.4°C to 4.8°C,which remarkably improved the thermal performance of the phase change material.（3）In view of the strong alkalinity of Ba（OH）₂·8H₂O,three common metal encapsulation materials,namely 304 stainless steel,brass and copper,were selected for a uniform corrosion full immersion test.The results showed that the corrosion rates of the three metals were arranged and the order is:304 stainless steel<purple copper<brass.Consequently,brass should not be considered as packaging material.