Carbon dioxide semi-clathrate hydrate for cold storage based air conditioning systems: materials and applications

Abstract

The increasing use of household air conditioning has been claimed to be one of the main drivers for peak electricity grid load over the past decade. Cold thermal energy storage (CTES) allows excess cooling capacity to be collected and conserved for later use; thus it is proposed to alleviate peak diurnal electricity demand problems. This thesis investigates the potential for CTES equipped air conditioning systems to time shift demand on electricity grids. In particular, CTES allows air conditioners to be operated overnight using off-peak electricity. It also provides backup for solar cooling systems. A major component of this thesis is the development, modification and application of a phase change material (PCM) based on CO2 hydrate. The first part of this work demonstrates the feasibly of CTES for residential space cooling. A TRNSYS model is composed, in which a CO2 hydrate cold store is integrated into a photovoltaic (PV) cooling system as a backup. This configuration is compared to a PV cooling system with an electrical energy store. The inclusion of CTES in the system strongly contributes to electricity savings. Electricity savings of the system with cold store is slightly lower than that of the system with battery. Moreover, the CO2 hydrate cold storage provides superior electricity savings compared to conventional ice storage or fatty acid cold storage. The second part of this work involves the development of a CO2 hydrate based cold storage medium. In order to operate within the practical working pressure range of conventional cooling systems, the pressure adopted in this study is moderate (<8 bar). Tetra-n-butylammonium bromide (TBAB), tetra-n-butylammonium fluoride (TBAF) and tetra-n-butylphosphonium bromide (TBPB) are employed to reduce the equilibrium pressure of the hydrate. Instead of using complex calorimeters, the formation enthalpy of CO2 hydrate is measured using a modified T-history method in a self-fabricated pressure tube. The T-history is applied for the first time to determine the enthalpy of gas hydrate under pressure. Through this test, the enthalpy of CO2 hydrate is found to be high compared to other PCMs. However, the hydrate suffers a large supercooling degree and a long induction delay before the hydrate formation really takes place. Besides, there is a non-negligible difference between the freezing and thawing temperature, which would result in a temperature loss in the cold store. These are the main challenges to overcome in this study. By using secondary promoters, such as sodium dodecyl sulfate (SDS) and titanium dioxide nanoparticles, formation behaviour of CO2 hydrate is optimised. As a result, a composition is composed which has suitable formation temperature, acceptable supercooling degree and induction time. This composition consists of CO2 + TBAB (20 wt%) + TBAF (0.25 wt%) + SDS (0.15 wt%). The last part is the application of the developed composition in an emulated cold storage system. The aim is to evaluate the cyclic charging-discharging performance of the CO2 hydrate based cold store. The 17.5 L cold store is equipped with a heat exchange coil and external ultrasonic vibrator. The working pressure of the hydrate is around 5 bar, which enables moderate cost storage vessels to be used in real cold storage systems. In this configuration, the cold storage medium showed further reduced supercooling degree, rapid formation and small difference between the freezing and thawing temperature. It would be reasonable to claim that solutions to the serious impediments to trialling CO2 hydrate cold storage technology are presented in this thesis. Further investigations are required to determine the heat transfer properties and longer term cyclic longevity of the material. Based on small scale costs and some assumptions, the cost of such a hydrate cold storage is in the order of $250 per kWh of electricity. This compares favourably to the contemporary battery electric storage cost of $1500 per kWh of electricity. Additionally, through instantaneous pressure control, the charging and discharging temperature can be adjusted for requirements of different chillers and air conditioning terminals. Show more