Thermal management of the adsorption-based vessel for hydrogeneous gas storage

Abstract

Thermal management is a design bottleneck in the creation of rational gas storage sorption systems. Ineffi- cient heat transfer in a sorption bed is connected with a relatively low thermal conductivity (0.1-0.5 W/(m⋅K)) and an appreciable sorption heat of activated gas storage materials. This work is devoted to the development of a thermally regulated onboard system of hydrogenous gas (methane and hydrogen) storage with the use of novel carbon sorbents. A hydrogenous gas storage system based on combined gas adsorption and compression at moderate pressures (3-6 MPa) and low temperatures (from the temperature of liquid nitrogen of about 77 K to a temperature of 273 K) is suggested. Introduction. Progress in the development of new hydrogen technologies, such as fuel cells, transport systems on hydrogen and methane, and sorption heat pumps, has shown that the use of hydrogenous gas results in qualitatively new solutions of ecological and power problems. The advantage of the use of hydrogen and natural gas is associated with their ecological cleanliness. The adsorbed hydrogenous gas is a promising alternative to the existing technologies for gas storage, since in this case the same gas amount can be stored at a much lower pressure (2-6 MPa) in a thin- ner-walled tank, and the method does not require expensive and cumbersome compression or liquefaction equipment. The key points of the adsorption gas technology are the high sorbent property and the thermal management of vessels. The problem is that, according to the thermodynamic laws, the sorbent and gas temperatures are increased during the process of adsorption, and hydrogen (methane) uptake by the sorbent diminishes with the filling time. Correspondingly, the lowering of the temperature during adsorption provides for a reduction in the time of vessel filling and an increase in the hydrogenous gas uptake. At high gas output, the fraction of the recoverable gas can decrease to 50-60%. This is in contrast to hydrogen and methane storage by cryogenic liquefaction and compression at very high pressure, where the stored gas is easily accessible for use. Hydrogen can potentially be stored at a high density and low pressure by absorption in metal hydrides. By now, metal hydrides have been commercialized, and hydride tanks are commercially available. Daimler Benz, Honda, and Mazda are among the several car companies to have tested metal hydrides for vehicular propulsion. Daimler Benz has demonstrated a combination of low-temperature (FeTi) and high-temperature (Mg 2 Ni) hydride tanks to store hydro- gen (1). The uptake capacity of hydrogen in the case of hydrides is dependent on temperature, pressure, and alloy composition. It is expected that hydride tanks will be charged in ambient conditions. While most metal hydrides are too heavy and too expensive or bond too strongly to hydrogen, recent research has identified sodium alanate (NaAlH 4 ) as a potentially practical hydride for vehicular applications. However, many of the problems remain. Hydrides release a considerable amount of thermal energy as they adsorb hydrogen and require significant thermal energy input to re- lease hydrogen. Therefore, hydride beds typically need to be built with heating and cooling passages to allow fast re- fuelling and desorption, as well as reducing the volumetric and gravimetric energy storage density of the system. Many metals and alloys can also reversibly adsorb large amounts of hydrogen, but their adsorption energy is reasonably high (50-100 kJ/mol) (2). The problems of technical design of metal hydride-hydrogen accumulators are the following: 1) necessity of introducing gas filters or elaborating metal hydride composites without dusting wear; 2) possible appear- ance of substantial stresses in the container walls at a too high density of the powdered bed (the maximum allowable powder density of a metal hydride material in the container should not be more than 60% of its true density).