High performance of carbon nanotube elastocaloric refrigerators over a large temperature span

Abstract

Compression of greenhouse gases still dominates the market of refrigeration devices. Although well stablished and efficient, this technology is neither safe for the environment nor able to be scaled down to nanoscale. Solid-state cooling technologies are being developed to overcome these limitations, including studies at nanoscale. Among them, the so-called elastocaloric effect (eC) consists of the thermal response, $\Delta T$, of a material under strain deformation. In this work, fully atomistic molecular dynamics simulations of the eC in carbon nanotubes (CNTs) are presented over a large temperature span. The efficiency of the CNTs as solid refrigerators is investigated by simulating their eC in a model of refrigerator machine running under Otto-like thermodynamic cycles (two adiabatic expansion/contraction plus two isochoric heat exchange processes) operating at temperatures, $T_\mbox{O}$, ranging from 300 to 2000 K. The coefficient-of-performance (COP), defined as the ratio of heat removed from the cold region to the total work performed by the system per thermodynamic cycle, is calculated for each value of $T_\mbox{O}$. Our results show a non-linear dependence of $\Delta T$ on $T_\mbox{O}$, reaching a minimum value of about 30 K for $T_\mbox{O}$ between 500 and 600 K, then growing and converging to a linear dependence on $T_\mbox{O}$ for large temperatures. The COP of CNTs is shown to remain about the same and approximately equal to 8. These results are shown to be weakly depend on CNT diameter and chirality but not on length. The isothermal entropy change of the CNTs due to the eC is also estimated and shown to depend non-linearly on $T_\mbox{O}$ values. These results predict that CNTs can be considered versatile nanoscale solid refrigerators able to efficiently work over a large temperature span.