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be heavy because of the robust construction necessary to withstand the high pressures and impacts. These factors make compressed hydrogen storage suitable for only short ranged applications or as a reserve fuel for liquid hydrogen powered vehicles. Liquefied hydrogen. Liquid hydrogen is formed by cooling hydrogen gas to -423°F (-253 °C) at atmospheric pressure. Storage of such low temperature fluids is achieved using a dual-walled cylinder with an evacuated space between the cylinder walls (Dewer’s flask). Due to the relatively high surface to volume ratio typical of the small tanks used in transportation applications, additional multi-layered radiation insulation sheets are also employed (Flynn, 1997). There are several technological challenges that must be overcome in order for liquefied hydrogen storage to come into widespread use. First is safe tank design to reduce weight and hydrogen boil off due to heat gains. The imperfect insulation of the inner tank supports, among other factors, causes a typical boil off rate of 3% per day (Clean Energy Research Center, 2003). Furthermore, improved methods of hydrogen liquefaction must be developed to reduce LH2 cost. Today, about 30% of the energy contained in LH2 is consumed by the liquefaction process (Fuel Cell Store, 2003). Lastly, re-filling stations must be developed such that the public can operate them safely. Liquefied hydrogen (LH2) is currently the optimum hydrogen storage method for vehicles in terms of tank size/weight and energy density. LH2 has the highest volumetric energy capacity of any commercially available storage system being only four times less than gasoline; and because hydrogen burns more efficiently than gasoline, LH2 tanks are not necessarily four times the size of typical gasoline tanks for a given vehicle range. This allows automobile manufactures to continue using current vehicle designs, easing the transition into a hydrogen economy. Carbon nanotubes and glass microspheres. Carbon nanotubes store hydrogen in microscopic surface pores and within the tube structures via adsorption. The mechanism by which they store and release hydrogen is similar to metal hydrides, however carbon nanotubes are lighter, cheaper, and are capable of storing 4.2 to 65% hydrogen by weight (Fuel Cell Store, 2003). Carbon nanotubes are still under research and development and currently store between one and ten percent hydrogen by mass (Clean Energy Research Center, 2003). Glass microspheres are currently being researched as a potential hydrogen storage method. Hydrogen is stored by first warming the tiny glass spheres to increase their surface permeability and then immersing them in high- pressure hydrogen gas. The spheres are then cooled, locking the hydrogen inside of the glass balls. Increasing the temperature of the spheres reverses this process. Experiments to increase hydrogen release rates by crushing the spheres are also being performed. The key advantage of glass microspheres is storage at ambient temperature. The technology exists today for the introduction of hydrogen-powered vehicles; however, the size, weight, and/or cost limitations imposed on storage systems by the low energy density of hydrogen must first be overcome. Liquid hydrogen holds the greatest promise for hydrogen- powered vehicles. These storage systems have the lowest weight and volume of those commercially available, and with improved tank design and hydrogen liquefaction methods, the relatively high costs will lessen over time. .Ammonia-Water Combined Power/Cooling Cycle The ammonia-water combined power/cooling cycle proposed by Goswami (1995) utilizes a binary ammonia/water working fluid to produce both power and refrigeration. The cycle is a combination of an ammonia- water refrigeration system and an ammonia-based Rankine cycle. An ammonia-water mixture is used because of its desirable thermodynamic properties. Binary mixtures have varying boiling points depending on the concentration of the more volatile species. This characteristic gives a good thermal match with a sensible heat source, thereby reducing the irreversibility associated with heat transfer (Hasan, Goswami, 2003). Additionally, the low boiling point of ammonia allows the utilization of low temperature heat sources such as low-grade waste heat from industrial processes, solar water heaters, and geothermal sources. In a theoretical investigation performed by Tamm et al., the cycle is shown to operate with heat source temperatures as low as 116.6 °F (47 °C) albeit with low first law efficiency (~ 5%). When operating with a heat source temperature of 224.6 °F (107 °C) and idealized parameters, however, second law efficiencies greater than 65% are possible (2003). The unique ability of this cycle to produce both power and refrigeration gives rise to two advantages for use in a hydrogen economy. First, the cycle can utilize low- grade renewable heat sources such as that available from inexpensive flat plate solar collectors to produce the power needed to drive an electrolyzer and liquefier. Second, the cooling produced by the cycle can be used to pre-cool hydrogen prior to liquefaction, thereby reducing the power requirement of the compressor. In this manner renewable energy source utilization is improved compared to technologies such as wind or PV electrolysis. Process Description Figure 4.3 gives a schematic of the cycle showing state points and flow paths. The fluid leaves the absorber at state 1 as a saturated solution at the cycle low pressure with a relatively high ammonia concentration. It is pumped to the system high pressure(state 2) before traveling through the recovery heat exchanger where it absorbs heat from the weak solution returning to the absorber. The solution is then partially boiled in the vapor generator by the heat sourcePDF Image | Optimization of a Scroll Expander Applied to an Ammonia/Water Combined Cycle System for Hydrogen Production - Paper No. 1645
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