International News
NOVEL COOLING MATERIAL
COULD PROVIDE REFRIGERANT ALTERNATIVE
By HEFEI Institutes of Physical Science , Chinese Academy of Sciences
The traditional refrigerant represented by Freon is a strong greenhouse gas that leads to serious climate implications .
Stimulated by the excellent cooling performance of plastic crystal materials , barocaloric ( BC ) cooling received much attention recently as a promising green refrigeration technology . However , problems like high driving pressure and low thermal response remain unsolved , which hindered their practical applications .
In recent decades , caloric cooling technologies based on the caloric effect have attracted great attention . The caloric effect refers to isothermal entropy change and adiabatic temperature change in response to a variety of external fields and is often associated with the first-order phase transition in materials . Depending on the types of external fields , such as the magnetic field , electrical field , uniaxial stress , and hydrostatic pressure , the caloric effects are sorted into magnetocaloric , electrocaloric , elastocaloric , and barocaloric ( BC ) effects , respectively .
Unlike the commercial refrigerants currently used in vaporcompression cycles , caloric materials have little environmental impact . To date , many achievements have therefore been made in a large variety of solid-state caloric materials .
In a new study , published on Nature Communications , scientists led by Prof . TONG Peng at the Institute of Solid State Physics , Hefei Institutes of Physical Science ( HFIPS ), Chinese Academy of Sciences ( CAS ), described how they discovered excellent BC cooling performance in n-alkanes with a self-developed BC test platform , which provided a new perspective for developing the application-oriented green refrigeration technology .
“ It ’ s safe and green ,” explained LIN Jianchao , first author of the paper , “ and the cost is low . Current refrigerating exploration tends to use solid-state phase transition materials . Now we have a future with solid-liquid phase transition ( L-S-T ) materials in mind .”
The material they found is called n-alkanes . By applying a low pressure of around 50 Megapascals , a colossal entropy change of ~ 700 J kg -1 K -1 was triggered . This is comparable to those of the commercial Freon-based refrigerants and at least three times larger than the reported values of existing BC materials driven by the same pressure . Aside from that , the adiabatic temperature change driven under this pressure also reaches the highest value of the existing BC materials . In this paper , they further revealed the possible BC mechanism . The Raman study and Molecular Dynamics Simulation reveal that applying pressure to the liquid state suppresses the twisting and random thermal motions of molecular chains , resulting in a lower configurational entropy . When the pressure is strong enough to drive the L-S-T , the configurational entropy will be fully suppressed and induce the colossal BC effect .
The excellent BC properties , tuneable operating temperatures , low-cost raw materials and well-known thermal properties all suggest n-alkanes are promising refrigerants for caloric cooling .
“ We expect more excellent BC performance by evoking L-S-T materials in the near future ,” said Jianchao . RACA
Comparision of BC performance between CnH2n + 2 and the existing BC materials . The maximum adiabatic temperature change ( a ) and isothermal entropy change ( b ) as a function of pressure for C18H38 and C16H34 , are shown along with the reported BC materials .
LIN Jianchao www . hvacronline . co . za RACA Journal I April 2022 7