Experimental study on cool release process of gas-hydrate with additives
Introduction
Cool storage technology can be used to shift electrical demand from on-peak to off-peak hours, and cool storage air conditioning systems can store a sizeable quantity of “cool” thermal energy that helps meet the cooling load of a building [1], [2], [3], [4], [5], [6], [7], [8]. Gas-hydrate, also called clathrate, is an ice-like crystalline compounds which occur when water molecules form a cage-like structure around smaller “guest molecules” of gas or easily volatile liquid at certain temperature and pressure [8], [9], [10]. Chatti et al. [11] reviewed the benefits and drawbacks of clathrate hydrates in their areas of interest in detail.
Hydrate crystals can form from the interaction between the most refrigerants (hydrate-former) and water under a hydrate-formable temperature (5–12 °C) condition. In 1982, American scientists suggested taking Freon (refrigerant) and forming a hydrate as cool storage medium in air conditioning systems, and thereafter, the so-called “gas-hydrate warm ice” technology has been rapidly developed in USA and Japan [12]. The phase change latent heat of the gas-hydrate is similar to that of ice [13]. Therefore, most refrigerants are considered to be ideal cool storage medium in air conditioning systems.
Alternative refrigerant R141b is the first gas-hydrate cool storage medium that has been chosen in industry because of its lower saturated vapor pressure, low cost, and the negative pressure character avoiding the difficulty of pressure vessel. The ideal critical decomposition temperature and pressure of R141b are 8.44 °C and 43.0 kPa, respectively, and the latent heat of phase change is 344 kJ/kg [14]. In order to reduce further the degree of subcooling of gas-hydrate R141b formation and increase the rate of crystallization, a new gas-hydrate cool storage system had been built by Guo et al. [15] and Lv et al. [16]. In the system, the inner-heat exchange/outer-crystallization technology and the integrated condenser/evaporator structure design were adopted. The cool storage tank is filled with a heat exchanger pipe coil. The heat transfer medium flowed through inside the pipe coil to discharge and charge the thermal energy to the cool storage medium which is mingled of water, refrigerant and some additives. When there is no disturbing, the water, refrigerant and hydrate will be stratified into three regions due to the gravity. Under cool storage condition, the water from the above region and the refrigerant from the bottom region are suctioned into the crystallizer through the down-flow pipes, and mixed to form hydrate nuclei. Then, the mixture of hydrate, water and un-reacted refrigerant is fed back to the cool storage tank through the return pipes and sprayers. Along with the hydrate formation, the gas-hydrate in middle region becomes larger and larger and the fluidity in this region becomes lower and lower. However, as long as the water and refrigerant being stratified and exiting in the above and bottom regions, respectively, they can be suctioned into the crystallizer to form hydrates, until all the cool storage medium inside the cool storage tank becoming hydrates and the process completed. The outer-crystallizer of this system can bring larger cool storage density and higher efficiency of charge and discharge processes only in a certain range of volumetric-flow rate of the crystallizer [17], [18]. The influence of additives and surfactant is one of the most important researching work in the field of gas-hydrate cool storage [19], [20], [21], [22]. Bi et al. [22] studied the influences of different proportions of calcium hypochlorite or benzenesulfonic acid sodium salt on the crystallization process of gas-hydrate HCFC141b, i.e. the cool storage process. However, the purpose of cool storage is cool release, and cool release process includes the dissolution process of gas-hydrate R141b. Gas-hydrate may be decomposed by heating or decompressing. For the cool storage systems, dissolution process by heating is concerned. In this paper, the influences of different proportions of calcium hypochlorite or benzenesulfonic acid sodium salt on the dissolution process of gas-hydrate R141b are experimentally studied.
Section snippets
Experimental system and experimental process [18,22]
Gas-hydrate R141b cool release experiments are performed using an efficient gas-hydrate cool storage/release experimental system, which is illustrated in Fig. 1. This experimental system includes cold/hot reservoirs, a gas-hydrate cool storage tank and a data acquisition system.
Cold/hot reservoirs are composed of a chiller and cold/hot water tanks. A 30% glycol solution is used as the secondary refrigerant. Its volumetric-flow rate is measured through a LZB-15 rotameter (range ability: 40–400
Description of dissolution process of R141b gas-hydrate
The exchanger surface temperature is higher than that of the other positions in the cool storage tank, therefore, gas-hydrate R141b on the exchanger surface decomposes firstly. The dissolution of gas-hydrate causes a great deal of gas R141b bubbles rising, and bubbles contact with the above un-decomposed gas-hydrate. The condensation of bubbles to liquid occurs on the gas-hydrate surface because the temperature of bubbles is higher than that of gas-hydrate, and the condensation heat absorbed by
Temperature variation of the cool release process
The cool release process of gas-hydrate R141b can be divided into two stages, i.e. the phase change with latent heat transfer period and sensible heat transfer period. In the phase change period gas-hydrate R141b decomposes, and the dissolution process of gas-hydrate is similar to nucleate boiling. Phase change heat transfer is driven by the temperature difference between exchanger coil wall and the ideal critical decomposition temperature of gas-hydrate. Gas-hydrate R141b keeps basically at
Conclusion
The experimental study has confirmed that the cool release processes of gas-hydrate R141b are obviously quicker than the corresponding cool storage processes, and the dissolution is easier than the crystallization of gas-hydrate. This is mainly due to the heat transfer temperature difference of the discharging and charging process. As to the dissolution process of the gas-hydrate by heating, the effect of the heat transfer process is the main influence factor. The temperature difference between
Acknowledgements
This paper is supported by Program for New Century Excellent Talents in University of PR China (Project No. NCET-04-1006), The National Natural Science Foundation of PR China (Project No. 59836230), Shanghai Leading Academic Discipline Project (Project No. T0503) and Beijing Municipality Key Lab of Heating, Gas Supply, Ventilating and Air Conditioning Engineering. The authors wish to thank the reviewers for their careful, unbiased and constructive suggestions, which led to this revised
References (22)
- et al.
Energetic, environmental and economic aspects of thermal energy storage systems for cooling capacity
Applied Thermal Engineering
(2001) On thermal energy storage systems and applications in buildings
Energy and Buildings
(2002)Recent advances in research on cold thermal energy storage
International Journal of Refrigeration
(2002)- et al.
A theoretical study of new-style cool storage air-conditioning
Energy and Buildings
(2006) - et al.
Benefits and drawbacks of clathrate hydrates: a review of their areas of interest
Energy Conversion and Management
(2005) - et al.
Ice-water two-phase flow behavior in ice heat storage systems
International Journal of Refrigeration
(2001) - et al.
Formation, growth and dissociation of clathrate hydrate crystals in liquid water in contact with a hydrophobic hydrate-forming liquid
Journal of Crystal Growth
(1999) - et al.
Influence of volumetric-flow rate in the crystallizer on the gas-hydrate cool-storage process in a new gas-hydrate cool-storage system
Applied Energy
(2004) - et al.
Influences of additives on the gas hydrate cool storage process in a new gas hydrate cool storage system
Energy Conversion and Management
(2006) Review on sustainable thermal energy storage technologies. Part I. Heat storage materials and techniques. Part II. Cool thermal storage
Energy Conversion and Management
(1998)