General Formula for Comparison of Clock Rates -- Applications to Cosmos and Solar System

In this paper we deduce a quite general formula which allows the relation of clock rates at two different space time points to be discussed. In the case of a perturbed Robertson-Walker metric, our analysis leads to an equation for the comparison of clock rates at different cosmic space time points, which includes the Hubble redshift, the Doppler effect, the gravitational redshift and the Rees-Sciama effects. In the case of the solar system, when the 2PN metric is substituted into the general formula, the comparison of the clock rates on both the earth and a space station could be made. It might be useful for the discussion on the precise measurements on future ACES and ASTROD.Comment: 17 pages. CQG submitte

A Novel Hybrid Battery Thermal Management System for Prevention of Thermal Runaway Propagation

Lithium-ion batteries (LiBs) are extensively used in electric vehicles (EVs) because of their high energy density and long service life. Designing a battery thermal management system (BTMS) that prevents thermal runaway (TR) propagation in the event of abusive accidents is crucial. The goal of this study is to design a novel hybrid BTMS with both active liquid cooling (LC) and passive cooling for preventing TR propagation in the battery module. A numerical model for a battery module (16 cylindrical 18 650 cells) was developed in COMSOL multiphysics software to examine the TR propagation caused by a single cell. Copper foam and expanded graphite-paraffin (EG-PCM) composite material were used for passive cooling. In addition to the TR scenario, the thermal behaviors of the battery module with the hybrid BTMS were evaluated under the 3C discharging and driving cycle circumstances. A conventional BTMS with natural air cooling is chosen as the baseline. The findings reveal that the proposed hybrid BTMS, which uses EG-PCM with a melting temperature of 52 °C and thermal diffusivity of 9.68 mm2/s and copper foam with a porosity of 0.7–0.9, is capable of limiting the maximum cell temperature below the thermal safety threshold (80 °C) to prevent TR propagation. Under the New European Driving Cycle (NEDC) load cycle, the battery module can be maintained within an optimal working temperature range by passive cooling only. By applying active LC with a flow rate of 0.3 m/s for BTMS, the average temperature reduction of the battery module at a 3C discharging rate can be up to 72.5% and 52.7% compared to passive cooling with copper foam and EG-PCM, respectively. The study highlights that the combination of active LC and appropriate passive cooling is an efficient thermal management solution for Li-ion battery applications in EVs, notably in the consideration of thermal safety

Experimental study of battery passive thermal management system using copper foam-based phase change materials

Lithium-ion batteries (LiBs) have been widely applied in electric vehicles (EVs) and energy storage devices. The battery thermal management system (BTMS) critically impacts the safety and degradation of LiBs. Phase change material (PCM) is a promising passive BTMS solution owing to its high latent heat and non-parasitic power consumption requirements. In this paper, paraffin (PA) as the PCM was embedded in the copper foam to enhance the heat dissipation of the cooling material. The thermal responses of the battery module were comparatively investigated under different thermal management solutions, including natural air, pure PCM, and copper foam-PCM. A battery module consisting of 16 thermal dummy cells (TDC) was designed, built, and calibrated to replace real commercial 21700 NMC battery cells. The findings indicates that the proposed copper foam-PCM solution effectively enhances heat dissipation and improve the temperature uniformity of the battery module. For instance, in the condition of intensive operation (60% depth of discharge and 3C discharge), copper foam-PCM composite material reduces the maximum temperature rise from 57.4°C to 51.4°C (-10.4%) compared to pure PCM. At ambient temperatures of 25°C and 35°C, the temperature inhomogeneity of the battery module with copper foam-PCM is maintained within 5°C and 2°C, respectively. Besides, the effect of copper foam-PCM cooling on the cell-to-pack conversion efficiency was evaluated. The gravimetric cell-to-pack ratio (GCTP) and volumetric cell-to-pack ratio (VCTP) of the battery pack employing the proposed BTMS reached 53.1 % and 45.6 %, respectively