An integrated cryogenic energy system with cold energy storage

Li, Yunren (2022). An integrated cryogenic energy system with cold energy storage. University of Birmingham. Ph.D.

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Abstract

Environmental concerns due to greenhouse gases emissions from burning fossil fuels call for the development of low or zero carbon technologies, particularly renewable energy sources. Deep penetrations of renewable energy bring in significant challenges in managing the mismatch between fluctuating and intermittent supply and demand. Various energy storage technologies have been proposed and developed to address this challenge. Liquid Air Energy Storage (LAES) has attracted significant attention in recent years due to its high energy density and no geographical constraints.

The round-trip efficiency of a standalone LAES system is limited to 50-60%, one of the major reasons for this is related to the high-grade cold energy storage. Several studies have attempted to address this. However, most of these studies are focused on sensible heat storage (SHS), and little attention has been paid to other storage methods, such as the latent heat storage (LHS) at the cryogenic temperature range, which is also known as the phase change materials (PCMs) based storage. This work aims to develop an integrated sensible and latent heat storage system to store and utilise high-grade cold energy (below -150℃) based on suitable cryogenic thermal energy storage (CTES) materials.

Firstly, this work focuses on the screening and characterisation of suitable CTES materials. In the study of SHS materials, temperature variations were found to have a significant effect on the specific heat values of the glass bead, the Dorset gold gravel and Brittany bronze gravel, decreasing by ~21.7%, ~13.08% and ~20.9% separately at -40℃ compared to that at the room temperature. Borosilicate glass bead is selected as the SHS material due to its better specific heat capacity and thermal conductivity than quartz-based gravels. For the LHS material study, hydrocarbons with C\(_5\) and C\(_6\) were chosen as the LHS material with a phase transition temperature of approximately -160℃. The results from the DSC and T-history measurements showed that the sample volume has a significant influence on the supercooling degree. To improve the thermal conductivity of the PCMs, expanded graphite (EG) and two different thickeners were used in the formulation to form composite PCMs. Both SiO\(_2\)-based (3 wt.%) and SEBS-based (2 wt.%) composites with 0.5 wt.% EG enhance the thermal conductivity by 34.5% and 38.1% in the solid state and 64.2% and 52.3% in the liquid state compared with the pure substance, respectively. Furthermore, the addition of fire retardants was studied and found to be able to inhibit the burning intensity of the PCM with the flammability limits observed to be proportional to the amount of fire retardants added.

An experimental system was then designed based on the CTES materials to store and utilise high-grade cold energy for studying the charging (storage of the cold energy) and discharging (use of the cold energy) processes. The design scheme of the latent heat cold storage (LHCS) device was developed according to the simulation results of the one-dimensional transient tube model with cylindrical fins in terms of fin length, fin thickness, fin gap, and tube material. The simulation results indicated that although the heat transfer coefficients for boiling and condensation varied in different patterns with the proportion of vapour in the two-phase flow, both the inner tube diameter and the mass flow rate were positively related to these coefficients. In addition, the heat transfer rate was significantly influenced by fin thickness, fin gap, fin length and number of fins, but the effect of different tube materials was negligible.

Various experiments were performed using the experimental system, including the high-grade cold energy storage and utilisation processes. The system was found to be able to liquify nitrogen (the working fluid). The charging experiments were conducted at an operating pressure of 13 barg and 8.5 barg, respectively. The CTES devices were found to be completely charged within 10 hours using 110 L of LN\(_2\). In the system discharging experiments, the real-time pressure and temperature of the system were monitored and recorded to evaluate the N\(_2\) liquefaction performance. The results indicate that the total amount of LN\(_2\) produced during the non-phase change period is 1.08 kg at a system pressure of 16.5 barg. During the phase transition period of the PCM, the effective time of N\(_2\) liquefaction is approximately 24 minutes with the mass of the condensed N\(_2\) of 0.72 kg at a system pressure maintained at 17 barg.

Type of Work:

Thesis (Doctorates > Ph.D.)

Award Type:

Doctorates > Ph.D.

Supervisor(s):