Investigation Of The Durability Of Oxalic Acid Dihydrate Phase Change Material

With the development of society,the contradiction between the large consumption of fossil energy and the worsening of environment is becoming more and more prominent.Phase change materials(PCMs)can realize the space-time conversion of thermal energy,which are effective solutions to address this issue.Dicarboxylic acids are very important medium temperature PCMs.Among them,oxalic acid dihydrate(OAD)shows great application potential due to its characteristics of high latent heat and low price,which can be widely used in the fields of solar energy storage systems,heat pump and waste heat recovery.However,severe degradations of thermal properties limit applications of OAD.Thus improving the cyclic stability is the key to enlarge applications of OAD.In this paper,OAD was used as the matrix material,and accelerated thermal cyclic test was carried out by different experimental methods.Reasons of the degeneration of thermophysical properties were analysed by using differential scanning calorimetry(DSC),X-ray diffraction(XRD),Fourier transform infrared spectroscopy(FT-IR)and so on.Measures of improving the cyclic stability of OAD were made according to aforementioned reasons.In addition,the corrosion behaviour of the common metals and alloys were tested to screen out the appropriate corrosion-resistant packaging material.Results appear as shown below:(1)Compared with the measurement results before and after accelerated thermal cycle of OAD,it can be found that there is a large amount of water vapour when OAD is heated to melt,which will increase the internal pressure of the system and cause the leakage.The seal of the system has a great influence on its cyclic stability.Decreasing melting point of OAD to reduce the generation of steam and preparation of shapestabilized composite PCM of OAD to inhibit the leakage during the melting process are good ways to improve its cyclic stability.(2)After screening a large amount of organic and inorganic materials,sodium chloride(NaCl)is selected as the suitable additive.9 wt% NaCl can reduce the melting point of OAD from 101.2°C to 88.7°C.The phase change enthalpy still remains above 320 J/g and the change of heat of fusion was 4.7% after 100 thermal cycles.The cyclic stability was greatly improved.In addition,it was found that polyethylene glycol(PEG)could improve the stabilities of OAD as a stabilizer,although it did not reduce the melting point of OAD.The addition of 2.5 wt% PEG 6000 showed the best effect.(3)The composite porous PCM was prepared by using the porous expanded graphite and OAD+9 wt% NaCl.Scanning electron microscope(SEM)and DSC were used to analyse the structure and properties of the composite.6 wt% expanded graphite can completely absorb the OAD+9 wt% NaCl composite PCM into its pores.Results of DSC showed that the heat of fusion of the composite PCM was reduced by only 1.7% after 100 repeated thermal cycles.The cyclic stability was further improved.(4)As OAD has a relatively strong acidity,eight kinds of metals and alloys were used to study the corrosion resistance of OAD composite PCM.Results showed that composite PCM has relatively strong corrosivity.In view of the low corrosion rate and cost,copper is the best encapsulation material.