Ionocaloric refrigeration cycle:A new breakthrough in efficient refrigeration technology
Developing safe,efficient and environmentally friendly refrigerants or refrigeration technology is of great importance for mitigating global warming.Among the various refrigeration methods,caloric effect refrigeration technology,which is based on caloric effect,is considered one of the most promising.However,high external field strength,small adiabatic temperature changes,and low efficiency have limited these techniques.Recently,a new refrigeration technology based on the ionocaloric effect,showing low applied field strength and high energy efficiency,has broken the bottleneck of current caloric effect refrigeration.The associated research is proposed by Lilley and Prasher,and published in the Science.Hence,this article summarizes the principle behind ionocaloric effect and ionocaloric refrigeration cycle based on their earlier research,delves into the potential scientific challenges related to the ionocaloric refrigeration cycle,and provides potential solutions for these issues.The principle of the ionocaloric effect is that a substance might undergo a phase transition in response to variations in the species or concentration of surrounding ions.Temperature change is achieved through heat exchange,which is the result of the enthalpy of fusion being absorbed or released during the phase transition.This process is similar to how salt is used to melt snow and ice in daily life.Based on this principle,Lilley and Prasher established a matching thermodynamic cycle using EC and NaI as the working ingredients.In this cycle,the ion separation step is the decisive factor for the cycle efficiency.Thus,electrodialysis,a low energy consuming and high efficiency separation technology,was utilized for ion separation,and continuous cooling was successfully achieved.The calculated isothermal specific entropy change reaches up to 802.08 J/(kg K),and the adiabatic temperature change reaches up to 28 K.Remarkably,the ionocaloric refrigeration cycle is achievable with an only 0.22 V applied field.These results outperform the majority of current caloric effect refrigeration systems.Despite its excellent energy efficiency,currently the ionocaloric refrigeration cycle still faces limitation of low cooling power.This is because the ionocaloric refrigeration cycle is a cyclic process involving mixing and separating of ionocaloric material(also called solvent in this article)and ions(also called salt),which requires minutes to hours for the ionocaloric material to melt,crystallize,and mix with salt.In this respect,we propose improvement measures from the standpoints of material characteristics,electrochemical system architecture,and membrane separation.Firstly,new solvent-salt systems can be developed to enhance the cycle performance.Herein,we have provided a series of selection criteria for ionocaloric material and salt.For instance,significant enthalpy of fusion,cryoscopic constant,and dielectric constant are typical features of ionocaloric materials,whereas strong conductivity,high solubility in ionocaloric materials,and monovalent dissociation are typically required of the corresponding salts.Secondly,designing an electrodialysis cell in conjunction with electrode/electrolyte interface modulation and electrochemical reactions is also a viable strategy to extend the electrodialysis cell's life and elevate efficiency of separation.Lastly,lowering membrane resistance promotes the ion transport and separation and thus is another potential way to boost the cooling capacity of the ionocaloric refrigeration cycle.Specifically,modulating the ion exchange membrane's structure and developing new types of ion exchange membrane would have hopeful prospects in diminishing membrane resistance.We expect that these three measures would point out the further direction to achieve high cooling power in ionocaloric refrigeration cycle.In summary,given the existing performance,it is critical that scientists keep working to enhance the ionocaloric refrigeration cycle in order to create a smart,safe,secure,and environmentally conscious refrigeration system.
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