In addition, the SEM results of 10h stability test shows no microstructure change or degradation as well as the same element atomic percentage on Ni-YSZ on anode surface from Energy Dispersive X-Ray spectra. Fuerte et al. research showed similar behavior for ammonia fed SOFC compared to hydrogen fed SOFC at 700°C. However, Yang et al. studied ammonia fed Ni/YSZ anode at 600°C and 700°C and their results demonstrated Ni was partially nitrated under the ammonia atmosphere. Literature findings on effect of ammonia on degradation of SOFC is inconsistent, as a result further investigation specially for long term degradation seems necessary. This work evaluates the long-term degradation effect of direct ammonia conversion on SOFC. EIS characterization,SEM Imaging and EDX quantifications are tools used in investigating the degradation of SOFC.These benefits allow LDACs to overcome problems with the conventional air conditioning. The most commonly used air conditioning is Vapor Compression Systems due to stability in performance, lower cost, long life and resendable COP of between 2-4. However, VCS use harmful refrigerants such as R-22, R-41-A, and R- 134A which have high global warming potential and consume significant amount of electrical energy to drive the compressor. Conventional cooling system such as chilled water or VCS cannot meet the latent cooling load on humid days.
The other popular air conditioning systems are absorption chillers,hydroponic channel which replace the electric driven compressor with a heat driven absorber and generator. The COP of these systems is in the range of 0.5 in single effect cycles and up to 1.2 in double effect cycles. All conventional chillers and air conditioners cool the air sensibly and, dehumidify the air by lowering the air temperature below its dew point so that moisture condenses and can be removed. The air that leaves this cooling coil in these traditional cooling systems is thus close to saturated conditions. For supplying air at 18℃ and 50% relative humidity, a conventional cooling system cools the air to 8℃ as and then reheats the air to 18℃ as it is shown with blue lines in Figure 10. In this case the air conditioner is overcoming roughly 30% more than is required to meet the load, and then also using even more power to reheat the air to desired supply temperature. A LDAC uses liquid desiccant to dehumidify the air the supply air to 25% relative humidity and a temperature that is 10C above the desired temperature. It performs almost 100% latent cooling , then relies on a separate cooling for remaining sensible cooling and eliminate the over cooling/heating process. Table 1 shows the comparison of liquid desiccant dehumidifier and VCS. Liquid desiccants are material that have high attraction to water. Lithium chloride , Calcium Chloride and Lithium Bromide are common types of liquid desiccants. CaCl2 is the cheapest solution but has the poorest absorption ability. The strength of a desiccant is measured by its vapor pressure. When liquid desiccant is in equilibrium with the humid air, the temperature and vapor pressure of the humid air and the liquid desiccant should be the same. At any given temperature, desiccant that has a water vapor pressure in equilibrium with the humidity ratio of the air , produces a line that is closely in line with a constant relative humidity line on psychometric chart.
Lower concentrations of desiccant come into equilibrium at higher ambient air Relative Humidity levels. Liquid desiccant dehumidification is the inverse of evaporative cooling. Liquid desiccant can either absorb or desorb water from the air based on its relative humidity. If the equilibrium relative humidity of desiccant is below the relative humidity of the air, the desiccant absorbs water from the air and its temperature increases. The temperature that the air-desiccant interface approaches is called the brine-bulb temperature. This temperature is dependent upon air temperature and humidity, and liquid desiccant concentration in solution with water. The brine temperature is slightly higher than the temperature of intersection of constant enthalpy line from air state to equilibrium relative humidity of desiccant. This is due to the chemical heat of mixing between water and desiccant that is released in addition to latent heat of condensation. During the dehumidification process, liquid desiccant absorbs the water from the air and releases heat, which decreases its water vapor pressure and ability to absorb water. Desiccant’s ability to absorbs water also decreases by increasing its temperature. For example, if a 43% LiCl at 26.6oC can dry the air to 23.2 grains. But, absorbing the amount of water that dilutes it from 43% to 42% would increase the desiccant temperature to 50.5oC which decreases the ability of desiccant to dry the air to 107.6 grains, whereas if we keep the desiccant temperature at 26.6oC, the 42% solution can dry air to 25.7 grains. The red arrow in Figure 12 shows the ambient air driving to equilibrium with LiCl in case of keeping the free surface of the desiccant at 26.6 C temperature.
Increasing the temperature has the highest effects on desiccant losing its drying potential. Liu XH et al., showed that in order to obtain better humidification performance, liquid desiccant should be heated rather than the air, and counter flow configuration is recommended. Design of a liquid desiccant system depends on the choice of desiccant. Both glycols and solutions halide salts are used in LDAC. LiCl, CaCl2 and LiBr are three of the most widely used halide salt desiccant solutions. Different Studies measured thermodynamic properties of these three desiccants, and they found that LiCl solution is the most stable liquid desiccant, which offers the lowest water vapor pressure and dehydration concentration 30-40%. However, the cost of LiCl is relatively high. LiBr solution offers similar characteristics during dehumidification and regeneration process, but the cost of LiBr is about 20% higher than LiCl. Lithium based desiccants can be expensive especially when the storage of concentrated desiccant is important. LiBr and LiCl are strong desiccants that can respectively dry the air to 6% relative humidity and 11% if they are saturates solution. LiBr is mainly used in closed system that doesn’t have direct contact with air due to their higher oxidation potential and LiCl is mainly used in open systems. CaCl2 solution is the cheapest and most widely available desiccant, but it can be unstable, depending on the air inlet conditions and the solution concentration rate. Halide salts are very corrosive to metals, so all the equipment is typically made of fiberglass, plastic, or other nonmetallic materials. For the cases that metal must be used, titanium is commonly specified. As a less corrosive and more environmentally friendly solution, Potassium Formate solution has recently been applied in desiccant cooling unit. KCOOH solution has low toxicity and viscosity, and they are neither corrosive nor volatile. However, Longo and Gasperella experimentally tested the performances of a liquid desiccant cooling system using LiBr, LiCl and KCOOH solutions. Results show that LiCl and LiBr solutions demonstrated better dehumidification performance compared with KCOOH solution, which performed better in regeneration process. Glycols are mainly used in industrial equipment. Glycols such as triethylene and propylene have low toxicity and compatible with most metals, but they are volatile. A mixture of 96% triethylene glycol and 4% water the same effect as a 42% lithium chloride solution. However, at equilibrium, the molar concentration of the glycol in air will be in the order of 1% that of the water vapor. The environmental impact and economic penalty of the triethylene glycol volatility in unacceptable in LDACs. Liquid desiccant systems can be classified as direct-contact or indirect contact, as adiabatic or intercooler,hydroponic dutch buckets and by flow pattern: parallel flow, cross flow, counter flow and counter-cross flow. Contactors are typically used to bring the desiccant solution and air stream into contact for both the dehumidification and regeneration processes. In direct-contact dehumidifiers/regenerators, heat and water vapor transfer take place between air and desiccant solution streams through direct contact. Desiccant solution usually flows downward due to gravity. The processed air can either flow in a parallel, counter or a cross flow manner across the contactor surface. However, counter flow systems are generally showing better dehumidification ability compared with other two flow patterns due to the fact that the outlet air stream is located at the entrance of the fresh desiccant solution’s inlet.This can be either in a direct or in direct manner.
In an adiabatic dehumidifier, the solution temperature increases which gradually decreases the solution dehumidification capability. By introducing internal cooling, the heat can be removed from liquid desiccant, which keeps the solution vapor pressure at lower level and improving the dehumidification process. Experimental and theoretical research results indicate that internally cooled dehumidifier offers higher dehumidification effectiveness. The most commonly used and studied liquid desiccant dehumidifier/regenerator systems amongst the various direct contact types incorporate packed beds. In this design, solution is usually distributed from the top of the packed bed with the help of spray nozzles and flows over the packed bed where it comes in direct contact with the air stream. This design is inexpensive and widely used for commercial and residential because it allows large contacting area between air and desiccant with simple configuration. High heat and mass transfer efficiency but the air flowing through the highly flooded porous beds causes a large pressure drop on process air side and, thus, high parasitic power consumption while flowing through the packing material. It also requires a separate heat exchanger to cool the desiccant before it is delivered to the porous bed, and the air may entrain droplets of desiccant as it flows through the highly flooded porous bed. This last disadvantage is particularly important because of the corrosiveness of the desiccant. Carryover of desiccant droplets can be eliminated by droplet filters, but at the expense of additional pressure drop. In an internally cooled/heated packed bed, the solution is continuously cooled/heated by a third fluid as it passes through the packed bed. As a result, the solution experiences no change in its temperature as it passes through the packed bed, and consequently the potential for mass transfer is improved compared to an adiabatic packed bed . Bansal et al. compared the experimental performances of adiabatic and internally-cooled packed-bed dehumidifiers. It was found that the effectiveness of the internally-cooled packed bed is 28–45% higher than the effectiveness of the adiabatic packed bed, depending on the operating conditions. It was found that the counter flow configuration has the highest effectiveness under air dehumidification and hot desiccant- driven regeneration conditions, compared to parallel-flow and cross-flow configurations, while, the parallel-flow configuration resulted in the highest effectiveness under hot air driven regeneration conditions. Although the packed-bed design is characterized by several advantages, the transfer of liquid droplets to the air stream is a drawback of the packed-bed design especially when it is operated under high flow rates. Thus, a mist eliminator is usually installed at the air exit to capture any entrained desiccant droplets, which increases the air-side pressure drop, as well as the capital and operating costs of the packed bed LDAC system. AIL Research patented a low flow LDAC with more than an order of magnitude less flow of desiccant compared to flooded-bed systems in 1994. This design replaces the bed of porous contact media with a plastic heat exchanger that has internal cooling/heating. In high flow LDAC, depending on the whether it’s a conditioner or regenerator, the liquid desiccant is either cooled or heated before it is sprayed or dripped onto the packed bed. Low flow LDAC has internal cooling or heating in which heat transfer fluid flows within the plates of the heat exchanger, desiccant flows on the outer surface of the plates, and air flows between the plates which are spaced 0.25in. This system does not rely on thermal mass of liquid desiccant to control its temperature [83]. Figure 13 shows the main three components of internal cooling systems: the conditioner, the regenerator, and the Interchange Heat Exchanger . A very thin film of desiccant flow in wicks on the outer surfaces of the plates, and air flows in the gaps between the plates. For conditioner a coolant typically water from cooling tower flows within the plates, however, for regenerator, hot water which can be supplied by boiler or solar thermal collectors, recovered heat from an engine or fuel cell, or other energy source flows within the plates. An interchange heat exchanger is used to increases the efficiency of the regenerator and decreases the cooling load on the conditioner.