Comprehensive Evaluation Of The Heat Storage Performance And Its Improvements For Sugar Alcohols As Phase Change Materials For Low-to-medium Temperature Latent Heat Storage

Thermal energy storage has long been studied and practiced in many industrial applications,like industrial waste heat recovery and solar thermal utilization and,etc.The primary requirement for latent heat storage applications is the exploration and identification of appropriate PCMs.The sugar alcohols were proposed as potential phase change materilas towards latent heat storage in the low-to-medium temperature range?70–250??due to its high latent heat of fusion.However,it was also found that the sugar alchols suffer from difficulty in crystallization and other crystallization related issues,which limits its application.Therefore,it is of significance to identify these phase change related issues through the comprehensive evaluation on their performance.In addition,the underlying mechanisms governing these issues need to be clarified and the effective methods will be proposed to address them.The six affordable candidates,i.e.,xylitol,D-sorbitol,erythritol,D-mannitol,D-dulcitol and inositol were preliminarily screened out based on the costs and melting points,followed by the preparation of 15 binary eutectic mixtures.The non-isothermal phase change behaviors were then tested by a differential scanning calorimeter for the 21 candidates.It was found that,in spite of the wide melting point range?70–230??and high latent heat of fusion?up to 350 kJ/kg?,the two pure sugar alcohols and nine mixtures with the melting points less than 100?C are unable to crystallize from the melt during cool-down.Four the three mixtures containing erythritol as one of the components and D-dulcitol?69mol%?/inositol,were also shown to be unable to crystallize upon cooling,with the crystallization occurring during the reheating process instead,referred to as cold crystallization.The rest four pure sugar alcohols and two mixtures were found to be able to crystallize upon cooling,although they all suffer from severe supercooling?up to 100?C for erythritol?.Based on the non-isothermal phase change behaviors,the isothermal tests were further performeded for five selected pure sugar alcohols?except D-sorbitol?and their five binary eutectic mixtures to make an advanced screening of these candidates.It was found that the incrystallizable xylitol and its eutectic mixture are also unable to crystallize during isothermal cool-down at any degrees of subcooling?30–90??.Moreover,the mixture of D-dulcitol?69mol%?/inositol was also identified to be crystallizable at low degrees of subcooling?60–120??,and still to experience cold crystallization at relatively high degrees of subcooling?150–180??.The rest seven crystallizable candidates all suffer from severe supercooling.The duration of crystallization was shown to decrease with increasing the degree of subcooling due to the enhanced driving force for crystallization.The rheological behaviors of 5 pure and 5mixture sugar alcohols were characterized over a wide range of shear rates(10^-3–10⁴ s^-1)and temperature range?60–280??covering the molten and supercooled liquid states.The flow curves show that the four linear-structured sugar alcohols and their mixtures exhibit typical shear thinning behaviors at low shear rates(<10 s^-1).The dynamic viscosity curves of inositol,only cyclic-structured sugar alcohol and its mixtures were found to agree with the Cross model,where the first Newtonian plateau,non-Newtonian regime and the second Newtonian plateau can be classified.An Arrhenius-type dynamic viscosity-temperature curve was observed for these candidates and their activation energies of viscous flow were determined.The incrystallizable xylitol and mixture of xylitol?75mol%?/erythritol possess the highest activation energies of viscous flow of 83.5 kJ/mol and 92.4 kJ/mol,respectively among these candidates.The high activation energies of viscous flow can cause difficulty in nucleation and crystallization.The dynamic viscosities of the mixtures appear to be higher than those of their respective pure compounds,which will deteriorate melting and crystallization performance.For instance,the durations of crystallization of the mixture sugar alcohols are found to be extended compared to those of their pure compounds.The evaluation on the thermal endurance of crystallizable 4 sugar alcohols and 3 mixtures shown that both the melting point and latent heat of fusion of these candidates degrade with the heating duration,and a higher degree of superheat leads to faster degradation.The decrease of melting point mainly attributes to the shrinking range of hydrogen bonding,whereas the degradation of latent heat is likely resulted from the disruption of hydrogen bond network and incomplete crystallization.The infrared spectra demonstrated that sugar alcohols suffer from oxidation and tend to generate aldehydes upon heating when exposed to air.It was also found that the duration for 10%degradation of the latent heat of inositol gets extended by approximate 9times when being protected by nitrogen gas,indicating the improvement of thermal endurance.An experimental system was built to test the long-term cyclic stability of 2 representative candidates?D-dulcitol and inositol?under isothermal heating(?T_sh=20?)and cooling?T?20??process.The results show that the phase change temperatures and latent heats tend to degrade during the cycling process.The melting point and crystallization temperature of D-dulcitol decline23?and 67?,respectively after 70 cylces,and its latent heat of fusion and crystallization degrade 50%and 58%,respectively.The introducing N₂ can also improve the cyclic stability of sugar alcohols.Above all,it can be confirmed that the difficulty in crystallization,cold crystallization and supersooling are identified as the main issues that the sugar alcohols suffer from.Fortunately,the difficulty in crystallization,and supercooling can be effectively addressed by introducing air bubble and addition of nucleation agents.In addition,the erythritol can be recommended as a promising medium-temperature candidate due to its high latent heat of fusion and desirable thermal stability after its supercooling behavior gets suppressed.