Cogeneration Fuel Cell-Sorption Air Conditioning Systems (Green Energy and Technology)


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Table of Contents

Gamboa, P. Sebastian, W. In this chapter the basic thermodynamic and electrochemical principles behind fuel cell operation and technology are described. The basic electrochemistry principles determining the operation of the fuel cell, the kinetics of redox reactions during the fuel cell operation, the mass and energy transport in a fuel cell, etc. The ideal and practical operation of fuel cells and their efficiency are also described.

This will provide the framework to understand the electrochemical and thermodynamic basics of the operation of fuel cells and how fuel cell performance can be influenced by the operating conditions. The influence of thermodynamic variables like pressure, temperature, and gas concentration, etc. Understanding the impact of these variables allows system analysis studies of a specific fuel cell application. Before going into the details of cogeneration combined heat and power CHP processes, it is necessary to establish the operation principles and performance characteristics of the most common fuel cells for consideration in CHP processes.

According to the phenomena governing the performance of fuel cells, it is worth noting that fuel cells are electrochemical energy conversion devices, where redox reactions occur spontaneously and the fuel and oxidant are consumed, and the electrochemical energy is transformed into electricity to produce work. The thermodynamic reversible potentials and over-potential losses are the principal factors that control the net efficiency to convert chemical energy to electrical energy.

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The fuel cell operating conditions depend on the electrochemical nature of the electricity production. Normally, activation over-potentials predominate at low current densities and they are also controlled by mass transport over-potentials at high current densities.

Thus, the application of fuel cells in CHP processes implies a more detailed knowledge of the operation of every kind of fuel cell. Therefore, in this chapter, the fundamentals of the four typical fuel cells considered for CHP applications are explained, taking electrochemical operation principles and the consequent heat and electricity production into consideration. Generally, fuel cell classification is according to the type of electrolyte used and the operating temperatures. Sorption cooling systems have been used commercially for some decades for different applications including air conditioning and refrigeration, using a diverse range of thermodynamic cycles and technologies for many size and capacities.

However, their use has been limited mainly because of their low efficiency and high investment costs, at least compared with compression systems that are widely used all over the world. Because of this, sorption and desiccant systems have been used, in general, only when large amounts of waste thermal energy that can be used as the energy supplied to the system are available, and recently with, for example, solar and geothermal technologies.

As will be explained in the next chapter, desiccant cooling DEC and sorption systems are in fact heat pumps since they have the capacity to absorb heat from a source at low temperature and to pump it to a heat sink at a higher temperature level. Depending on the use, common sorption systems are classified as sorption refrigeration systems when they are used for refrigeration and air conditioning, heat pumps when they are used for heating and heat transformers when they are used also for heating but the temperature of the useful heat is higher than the temperature of the heat supplied to the system.

The knowledge of the basic principles of thermodynamics allows us to understand the conditions and necessary limitations in order to transform heat in work, transferring heat from a thermal source of high temperature to a smaller one. Thermal machines work under this principle, however, there are machines that consume work external and produce heat, that is to say in the inverse sense of a thermal machine operation according to the cycle of Carnot, this it is the case of a refrigerating machine.

In most cases, no thermal load exists. Only in the rarest of circumstances would it be economically feasible to generate power while not recovering any thermal energy. If it does appear that pure power generation is an economic possibility, a detailed study of the power company rate structure that serves the facility should be performed. It is likely that changing to another rate structure would lower electrical costs enough to make the pure power generation option economically undesirable.

Another criterion is the size of the electrical and thermal loads relative to each other. This should not be confused with the first criteria. We are assuming that the magnitude of each type of load is sufficient to consider a cogeneration system. For high electrical usage vs. If the opposite is true, the thermal load outpaces the electrical load, and then a steam turbine would better suit the application. Finally, if both are relatively equal, then a gas turbine system might be the initial system to analyze. The relative magnitudes of the thermal and electrical loads are not the only criteria, but also the time dependent nature of each load.

Loads that vary considerably with respect to time can cause undesirable effects on certain systems, much. Comments Very large industrial Usually multiple smaller units Custom engineered systems Industrial and large commercial Usually multiple smaller units Custom engineered systems Commercial and light industrial Single to multiple units Potential packaged units Small commercial and residential Appliance-like.

A reciprocating engine generator responds much better to changing loads than a gas turbine does, not only in terms of efficiency, but also reliability. Steam turbines can match loads well by simply throttling the steam flow through the turbine.

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An important consideration when choosing a cogeneration system is what type of fuel is most readily available. For almost every fuel, there is a system capable of using it. Gaseous fuel, such as natural gas, is most commonly used in gas turbines, but it is also used in natural gas fired reciprocating engines.

Fuels such as No. Solid fuels, such as coal and biomass, are exclusively used in the Rankine cycle. Except for the solid fuels, any fuel can be used in any system, so a certain amount of flexibility exists. However, using a fuel other than the ideal will cause increased operating costs and decreased equipment life. The type of industry choosing to cogenerate will often determine the fuel, and thereby cogeneration system to be used. The paper industry, which generates a great deal of biomass and chemical by product fuel, generally opts for a Rankine cycle system to utilize the readily available fuel source.

The huge boilers burn both bark removed from the incoming logs and chemical liquor generated in the pulp making process. Similarly, the petroleum industry most often relies on fuel oil as a heat source. Because of the available supply and low cost associated with using ones own fuel oil, it makes excellent economic sense to do so. However, for those industries that do not generate a fuel source in their production process, natural gas is often the best choice, due to the low cost, high efficiency, ease of transport, and low capital cost of the storage and distribution equipment Bretton Pollution concerns have become particularly important in recent years, especially in heavily populated areas.

Gaseous fuels tend to have the lowest emissions, followed by fuel oils, and finally solid fuels. However, in large industrial locations using solid fuels, exhaust stacks are equipped with scrubbers or precipitators to remove particulate matter or other pollutants from exhaust, thus minimizing pollution concerns.

It should be borne in mind that these scrubbers add considerable cost to the overall system, and any economic analysis should include the additional capital outlay. Heavier grade fuel oils can have a high sulfur content, and unless special steps are taken, sulfur emissions can be considerable. Low sulfur oils are available, but again at a higher cost. Finally, the efficient operation of each of the systems will minimize the pollutants generated in the combustion process.

If the combustion process for any of the systems is poorly managed through combustion air, etc. The physical space available for a cogeneration system will often affect which type of equipment is used. Gas turbines and reciprocating engine generators are compact, packaged units which are simply dropped into place, attached to the fuel, steam, and electrical systems, and started.

Steam turbine systems usually require more on-site preparation, but only because drop-in packaged units do not exist. For completely new systems, steam turbine cogeneration systems are the most. The operational cost is a key factor in choosing a cogeneration system. Systems that have high fuel, maintenance, or supervisory costs will undermine any savings gained from cogenerating. Generally speaking, reciprocating engine generators have the highest operating costs, in terms of downtime and preventive maintenance, due to the high number of moving parts in the system.

Steam turbine systems have lower maintenance costs than reciprocating engines, with gas turbines having the lowest costs of all. Some general guidelines have been developed through experience with regard to selecting a prime mover Dyer Specifically, reciprocating engines, micro gas turbines, and fuel cells tend to prosper in smaller systems micro and mid systems , up to 3, kW, or systems where a peak shaving operational strategy is used because of the relatively short operational time.

Gas turbines perform best in moderately larger applications med and large systems , from approximately 5, kW up to several hundred MW. Steam turbines are ideal for the largest applications large and mega systems or applications where solid fuel is used, because the large boilers that use this type of fuel produce enough steam to allow for huge extraction turbines to produce sizable amounts of electricity. Steam turbines will also perform well in any situation in which steam is required at different pressures.

Cogeneration is among different kinds of technologies that allow the waste heat utilization for power generation, where electricity and heat are produced simultaneously. If some cooling type is required and this is produced by the same energy source, this process is known as trigeneration electricity, heat, and cold.

The trigeneration process increases the energy efficiency due to better utilization of waste heat into cooling power. If sorption refrigeration systems are integrated the environmental impact is reduced due to the use of natural refrigerants ammonia, water, methylamine, ammonium nitrate, alcohol, etc.

The trigeneration plant can be evaluated as a cogeneration plant, considering all the heat used in producing cold. This cooling can be done through sorption absorption or adsorption refrigeration cycles. These systems are adapted in order to recover industrial and commercial waste heat, hot liquid or hot gas, and steam, to provide cold for air condi-. These sorption systems can be operated with thermal residual flows with a temperature range from C and low pressure steam, or up to C, if a double effect configuration is considered.

In the case of gaseous flow, we need minimum temperatures of the order of C, due to the need for intermediate heat exchange circuit in order to generate hot water at a temperature up to C. To generate cooling power for air conditioning application using sorption refrigeration cycles, a heat source with a temperature range between C single and double effect is required, depending on the technology selected. The chemical energy is transformed into electricity, heat, and water. These devices have high efficiency, low emission and noise, and a modular design. Their main practical applications are in the transport sector.

Fuel cells are classified by the electrolyte used and the operating temperature. In order to optimize the efficiency of these devices, various projects are being carried out for the use waste heat for air conditioning systems in residential, commercial, and industrial sectors. The waste heat released by a PEM fuel cell enables one to obtain hot water with temperatures up to 80 C, which is suitable for the operation of sorption refrigeration cycles. References Bretton DJ Cogeneration in the new deregulated energy environment.

Bibliographic Information

US Department of Energy, Office of Energy Efficiency and Renewable Energy Rosen MA Comparison based on energy and exergy analyses of the potential cogeneration efficiencies for fuel cells and other electricity generation devices. Accessed 30 Dec United Nations World economic situation and prospects. The basic electrochemistry principles determining the operation of the fuel cell, the kinetics of redox reactions during the fuel cell operation, the mass and energy transport in a fuel cell, etc. The ideal and practical operation of fuel cells and their efficiency are also described.

This will provide the framework to understand the electrochemical and thermodynamic basics of the operation of fuel cells and how fuel cell performance can be influenced by the operating conditions. The influence of thermodynamic variables like pressure, temperature, and gas concentration, etc. Understanding the impact of these variables allows system analysis studies of a specific fuel cell application.

The energy balance analysis in the fuel cell should be based on energy conversion processes like power generation, electrochemical reactions, heat loss, etc. The energy balance analysis varies for the different types of fuel cells because the various types of electrochemical reactions occur according to the fuel cell type. The enthalpy of the The energy balance analysis is done by determining the fuel cell temperature at the exit by having information of the reactant composition, the temperatures, H2 and O2 utilization, the power produced, and the heat loss Srinivasan The fuel cell reaction inverse of the electrolysis reaction is a chemical process that can be divided into two electrochemical half-cell reactions.

Analyzing from a thermodynamic point of view, the maximum work output obtained from the above reaction is related to the free-energy change of the reaction. Treating this analysis in terms of the Gibbs free energy is more useful than that in terms of the change in Helmholtz free energy, because it is more practical to carry out chemical reactions at a constant temperature and pressure rather than at constant temperature and volume. The above reaction is spontaneous and thermodynamically favored because the free energy of the products is less than that of the reactants.

Where G is the free energy change, n is the number of moles of electrons involved, E is the reversible potential, and F is Faradays constant. The value of G corresponding to 2. The enthalpy change H for a fuel cell reaction indicates the entire heat released by the reaction at constant pressure. The electrochemical reactions taking place in a fuel cell determine the ideal performance of a fuel cell; these are shown in Table 2.

It is very clear that from one kind of cell to another the reactions vary, and thus so do the types of fuel. The minimum temperature for optimum operating conditions varies from cell to cell. This detail will be discussed in subsequent chapters. Low to medium-temperature fuel cells such as polymer electrolyte fuel cells. PEMFC , alkaline fuel cells AFC , and phosphoric acid fuel cells PAFC are limited by the requirement of noble metal electrocatalysts for optimum reaction rates at the anode and cathode, and H2 is the most recommended fuel.

For hightemperature fuel cells such as molten carbonate fuel cells MCFC and solid oxide fuel cells SOFC the catalyst restrictions are less stringent, and the fuel types can vary. Carbon monoxide can poison a noble metal electrocatalyst such as platinum Pt in low-temperature fuel cells, but it serves as a potential fuel in hightemperature fuel cells where non-noble metal catalysts such as nickel Ni , or oxides can be employed as catalysts. The ideal performance of a fuel cell can be represented in different ways. The most commonly used practice is to define it by the Nernst potential represented as the cell voltage.

The fuel cell reactions corresponding to the anode and cathode reactions and the corresponding Nernst equations Simons et al. The Nernst equation is a representation of the relationship between the ideal standard potential E0 for the fuel cell reaction and the ideal equilibrium potential E at other temperatures and pressures of reactants and products.

Once the ideal potential at standard conditions is known, the ideal voltage can be determined at other temperatures and pressures through the use of these equations. According to the Nernst equation for hydrogen oxidation, the ideal cell potential at a given temperature can be increased by operating the cell at higher reactant pressures. Improvements in fuel cell performance have been observed at higher pressures and temperatures. The symbol E represents the equilibrium potential, E0 the standard potential, P the gas pressure, R the universal gas constant, F Faradays constant and T the absolute temperature.

Table 2. In general in a fuel cell the reaction of H2 and O2 produces H2O. When hydrocarbon fuels are involved in the anode reaction, CO2 is also produced. For molten carbonate fuel cells CO2 is consumed in the cathode reaction to maintain the invariant carbonate concentration in the electrolyte.

Since CO2 is generated at the anode and consumed at the cathode in MCFCs, and because the concentrations of the anode and cathode flows are not necessarily equal, the Nernst equation in Table 2. This value is normally referred to as the oxidation potential of H2. The potential can also be expressed as a change in Gibbs free energy for the reaction of hydrogen and oxygen. The change in Gibbs free energy increases as cell temperature decreases and the ideal potential of a cell is proportional to the change in the standard Gibbs free energy.

This will be discussed in more detail in the thermodynamics sections of the other chapters. The variation of the standard potential in a fuel cell with temperature is shown in Figure 2.

It is very clear that the influence of temperature on the standard potential is more pronounced for high-temperature fuel cells. This case corresponds to low, medium, and high-temperature fuel cells. Hence the ideal potential is less than 1. The ideal and actual performance of a fuel cell is quite different, especially when one analyzes the potential current response of a fuel cell. Figure 2. Electrical energy is obtained from a fuel cell when a current is drawn, but the actual cell potential is lowered from its equilibrium potential because of irreversible losses due to various reasons.

Several factors contribute to the irreversible losses in a practical fuel cell. The losses, which are generally called polarization or over potential, originate primarily from activation polarization, ohmic polarization, and gas concentration polarization Chase et al. These losses result in a cell potential for a fuel cell that is less than its ideal potential.

The first of these three major polarizations is the activation loss, which is pronounced in the low current region. In this region electronic barriers must be overcome before the advent of current and ionic flow.

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The activation loss is directly proportional to the increase in current flow. Where act is the activation polarization, R the universal gas constant, T the temperature, the charge transfer coefficient, n the number of electrons involved, F the Faraday constant, i the current density, and i0 the exchange current density. Activation polarization is due to the slow electrochemical reactions at the electrode surface, where the species are oxidized or reduced in a fuel cell reaction. Activation polarization is directly related to the rate at which the fuel or the oxi-.

In the case of fuel cell reactions the activation barrier must be overcome by the reacting species. The ohmic polarization varies proportionally to the increase in current and increases over the entire range of currents due to the constant nature of fuel cell resistance. Where ohm is the ohmic polarization and Rc is the cell resistance. The origin of ohmic polarization comes from the resistance to the flow of ions in the electrolyte and flow of electrons through the electrodes and the external. The dominant ohmic loss is in the electrolyte, which is reduced by decreasing the electrode separation, enhancing the ionic conductivity of the electrolyte and by modification of the electrolyte properties.

The concentration losses occur over the entire range of current density, but these losses become prominent at high limiting currents where it becomes difficult for gas reactant flow to reach the fuel cell reaction sites. The concentration polarization can be represented as. Where con is the concentration polarization, iL is the limiting current density. As the reactant gas is consumed at the electrode through the electrochemical reaction, there will be a potential drop due to the drop in the initial concentration of the bulk of the fluid in the surroundings.

This leads to the formation of a concentration gradient in the system. Several processes are responsible for the formation of the concentration polarization. These are 1 slow diffusion of the gas phase in the electrode pores, 2 solution of reactants into the electrolyte, 3 dissolution of products out of the system, and 4 diffusion of reactants and products, from the reaction sites, through the electrolyte. At practical current densities there is slow transport of reactants to the electrochemical reaction and slow removal of products from the reaction site, which is a major contributor to the concentration polarization.

The net result of concentration polarization in current flow in a fuel cell is to increase the anode potential and to decrease the cathode potential. This will result in the reduction of the cell voltage. These polarization curves are typical for each type of fuel cell. For the above reaction the volume change is negative, hence the reversible potential increases with an increase in pressure. The influence of temperature on the fuel cell voltage is shown schematically in Figure 2. The reversible potential decreases with increasing temperature, but the operating voltages of these fuel cells actually increase with an increase in operating temperature.

PEFC exhibits a maximum in operating voltage. The lower operating temperature of SOFC is limited to about C since the ohmic resistance of the solid elec-. However, advances in materials science in developing new solid oxide electrolytes and thin film solid electrolytes have succeeded in lowering the minimum operating temperature of SOFC below C. Normally fuel cells operate at voltages considerably lower than the reversible cell voltage. The better performance is related to changes in the types of polarizations affecting the cell as the temperature varies. An increase in the operating temperature is beneficial to fuel cell performance because of the increase in reaction rate, higher mass transfer rate, and usually lower cell resistance arising from the higher ionic conductivity of the electrolyte.

In addition, the CO tolerance of electrocatalysts in low-temperature fuel cells improves as the operating temperature increases. An increase in operating pressure has several positive effects on fuel cell performance. The partial pressures of reactant gases, solubility, and mass transfer rates are higher at higher pressures.

The electrolyte loss by evaporation is reduced at higher operating pressures. The system efficiency is increased by the increase in pressure. The benefits of increased pressure may be compared with the problems associated with fuel cell materials and other associated system instrumentation. Pressure differences must be minimized to prevent reactant gas leakage through the electrolyte and seals. High pressure favors carbon deposition and methane formation in the fuel gas. The overall reactions given in Table 2. The maximum work available from a fuel source is related to the free energy of reaction in the case of a fuel cell, whereas the enthalpy of reaction is the pertinent quantity for a heat engine, i.

This entropy change is manifested in changes in the degrees of freedom for the chemical system being considered. The maximum amount of electrical energy available is G as mentioned above, and the total thermal energy available is H. The amount of heat that is produced by a fuel cell operating reversibly is TS. Reactions in fuel cells that have negative entropy change generate heat, while those with positive entropy change may extract heat from their surroundings. Differentiating Equation 2. Hydrogen fuel and oxygen oxidant can exist in each others presence at room temperature, but if heated to above C and at high pressure they explode violently.

The combustion reaction for these gases can be forced to occur below C in the presence of a flame, such as in a heat engine. In the case of a fuel cell, a catalyst can increase the rate of reaction of H2 and O2 at temperatures lower than C in the ambient of an electrolyte. In high temperature fuel cells a noncombustible reaction can occur at temperatures over C because of controlled separation of the fuel and oxidant. The process taking place in a heat engine is thermal, where as the fuel cell process is electrochemical.

The difference in these two processes in energy conversion is the fact behind efficiency comparison for these two systems. In the ideal case of an electrochemical energy conversion reaction such as a fuel cell the change in Gibbs free energy of the reaction is available as useful electric energy at the output of the device.

Thus, the thermal efficiency of an ideal fuel cell operating reversibly on pure hydrogen and oxygen at standard conditions would be. The efficiency of an actual fuel cell can be expressed in terms of the ratio of the operating cell voltage to the ideal cell voltage. The actual cell voltage is less than the ideal cell voltage because of the losses associated with cell polarization and the iR loss, as discussed in the earlier section. As mentioned earlier, the ideal voltage of a fuel cell operating reversibly with pure hydrogen and oxygen in standard conditions is 1.

A fuel cell can be operated at different current densities; the corresponding cell voltage then determines the fuel cell efficiency. Decreasing the current density increases the cell voltage, thereby increasing the fuel cell efficiency. In fact, as the current density is decreased, the active cell area must be increased to obtain the desired amount of power. Changing the fuel cell operating parameters can have either a beneficial or a detrimental impact on fuel cell performance and on the performance of other system components. Changes in operating conditions may lower the cost of the cell, but increase the cost of the peripheral components.

Generally, a compromise in the operating parameters is made to meet the required application. It is possible to. Operating conditions are optimized by defining specific system requirements such as power requirement level, voltage, current requirement etc.

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From this and through life cycle studies, the power, voltage, and current requirements of the fuel cell stack and individual cells are determined. It is a question of choosing an optimum cell operating point as shown by Figure 2. This figure shows the relation between voltage and current density and between output power and current density.

For example, a design point at high current density will allow a smaller cell size at lower capital cost to be used for the stack, but a lower system efficiency results. This type of operating point would be required by a vehicle application where light weight, small volume, and efficiency are important parameters for cost effectiveness. Fuel cells capable of higher current density operation would be of special interest.

Operation at a lower current density, but higher voltage would be more suitable for stationary power plant operation. Operation at a higher pressure will increase cell performance and lower cost. It is normal and seems logical to design the cell to operate at the maximum power density that peaks at a higher current density. However, operation at the higher power densities will mean operation at lower cell voltages or lower cell efficiency. Setting the operating point at the peak power density may cause instability in power control because the system will have a tendency to oscillate between higher and lower current densities around the peak.

It is normal practice to operate the cell at a point towards the left side of the power density peak and at a point that yields a compromise between low operating cost and low capital cost. References Angrist SW Direct energy conversion, 3rd edn. Springer, New York. According to the phenomena governing the performance of fuel cells, it is worth noting that fuel cells are electrochemical energy conversion devices, where redox reactions occur spontaneously and the fuel and oxidant are consumed, and the electrochemical energy is transformed into electricity to produce work.

The thermodynamic reversible potentials and over-potential losses are the principal factors that control the net efficiency to convert chemical energy to electrical energy. The fuel cell operating conditions depend on the electrochemical nature of the electricity production. Normally, activation over-potentials predominate at low current densities and they are also controlled by mass transport over-potentials at high current densities.

Thus, the application of fuel cells in CHP processes implies a more detailed knowledge of the operation of every kind of fuel cell. Therefore, in this chapter, the fundamentals of the four typical fuel cells considered for CHP applications are explained, taking electrochemical operation principles and the consequent heat and electricity production into consideration. Generally, fuel cell classification is according to the type of electrolyte used and the operating temperatures.

They can be classified by operating temperatures as low C , intermediate C , and high-temperature C fuel cells. However, the name definition of every fuel cell is due to the electrolyte used where half cell reactions occur to produce electricity. In this case, the classification is proton exchange membrane fuel cell, direct methanol fuel cell, alkaline fuel cell, phosphoric acid fuel cell, molten carbonate fuel cell, and solid oxide fuel cell. A schematic representation of a single PEM fuel cell is shown in Figure 3. One important performance parameter of the currently most used polymer electrolyte in PEM fuel cells Nafion, a registered trade mark of Dupont Co.

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The integration of fuel cells in highly integrated microelectronics is possible due to the thickness of MEAs, so an appropriate and compact fuel cell can be designed for supplying energy to these kinds of devices and the heat and water formation of the fuel cell can be safely vented. However, this electrolyte has some inherent problems to the nature of its synthesis, for example, the water management of the polymer and the high dependence of ionic conductivity as a function of membrane hydration content. It is possible to think that the other types of fuel cells operating at low temperature are going to be used in specific applications, until the solid electrolyte can solve the above mentioned problems in addition to the specific cost of the development of catalyst technology.

The technology to develop new electrocatalysts is not just based on using less platinum as the principal catalyst used in PEMFC; it also implies the technology to develop more powerful fuel cells, improving all components of the devices. The most important advances in PEM fuel cell technology were carried out in the last 10 or 20 years. One part of this advance has been the discovery of a new technology, for example, the understanding and use of applied nanotechnology. Another advance is due to the effort of Ballard Power Systems and its partnerships with research centers Koppel These improvements were conducted to obtain high power fuel cells higher power density and a significant reduction in cost per kilowatt of electric power.

PEMFCs have a potential and real use in automobiles, aerospace applications, portable devices phones and laptops , and a more significant use in CHP systems Zhang et al. The versatility of PEMFCs is the possibility to obtain fuel cells from few watts to hundreds of kilowatts for powering electronic and domestic devices up to vehicular and industrial systems. Nafion has been a reference or a standard based on sulfonated fluoropolymers, in particular flouroethylene widely used in fuel cells. Nafion is a polymer similar to polytetrafluoroethylene, also called Teflon.

This polymer has played an important role in the development of fuel cells. Teflon is formed by very resistant and stable chemical bonds between.

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This principle is also used in other types of low and intermediate temperature fuel cells Larminie and Dicks Sulfonation of molecules is a powerful technique used in chemical processes for obtaining extraordinary materials with excellent characteristics, for example, the detergent industry.

In the case of the electrolyte for fuel cells, the HSO3 group is normally ionically bonded in the form of SO3 like an ionomer. An interesting property of sulfonic acid is its hydrophyllic characteristic for absorbing water molecules. Nafion membranes show both characteristics in one, and the effect is a solid electrolyte where hydrophilic islands exist in a hydrophobic bulk; it is expected to be responsible for the interesting properties of the Nafion membrane as electrolyte in fuel cells.

However, the electrical conductivity of Nafion depends strongly on the hydrating value. For the highest conductivity conditions it is necessary to have a complete hydrated membrane showing a conductivity value of about 0. When the membrane hydration decreases, the conductivity decreases much faster, and the transport of ions through the membrane is affected, causing a power loss in the performance of the fuel cell.

The high cost of the catalyst was balanced with the advances in platinum reduction in the electrodes to around 0. The use of very small catalyst particles, nanoparticles allows the reduction of the amount of platinum catalyst layer in fuel cell electrodes. Probably the success of the active catalyst layer on the electrode is due to the effectiveness of the composition and preparation of the electrodes. In general, it is possible to say that the anode and cathode electrodes in a PEMFC are identical, and if the fuel and oxidant are pure, then the catalyst used can be considered as elemental platinum.

The platinum catalyst in an electrode is normally supported and spread out on carbon powder; the most widely used is XC Cabot. The purpose of this procedure is to maintain the catalyst as an active area where contact with the reactant gases is possible and easy, and also the possibility to take out the electrogenerated electrons through the electronic conductor carbon powder.

Small Wind Turbines David Wood. Alternative Energy Sources Efstathios E. Ocean Wave Energy Joao Cruz. Decarbonising Cities Vanessa Rauland. Back cover copy Although conventional cogeneration systems have been used successfully in the last two decades, most of them have been large units using mainly hydrocarbon fuels that are becoming increasingly expensive.

Topics covered include: selected fuel cells for cogeneration CHP processes; state-of-the-art sorption refrigeration systems; potential applications in demonstration projects; and profitability assessment of the cogeneration system. Table of contents 1. Energy and Cogeneration. Thermodynamics of Fuel Cells. State-of-the-art Sorption Refrigeration Systems.

Sorption Refrigeration Systems. Potential Applications in Demonstration Projects. Profitability Assessment of the Cogeneration System. About I. Pilatowsky I. Dr Pilatowsky works for the Centro de Investigacion en Energia from the Universidad Nacional Autonoma de Mexico and has 25 years of teaching experience in undergraduate and graduate courses, and 37 years of research in the fields of applied thermodynamic, heat and mass transfer and solar thermal applications at low temperatures, particularly solar refrigeration and air conditioning, solar drying, and solar water heating systems design.

He works for the Universidad Autonoma del Estado de Morelos. Dr Romero has 10 years of research experience in applied thermodynamics, in heat pumps, particularly in heat transformers and in the applications field. He has teaching experience in chemical and mechanical engineering in undergraduate and graduate level courses.

Dr Isaza has 10 years of teaching experience in undergraduate and graduate level courses, and 15 years of research experience in the fields of refrigeration, gas combustion, solar energy, rational and efficient energy use, energy auditing and new energy technologies. He has worked for the Centro de Investigacion en Energia from the Universidad Nacional Autonoma de Mexico in projects related to renewable energy resources for more than 15 years. Dr Gamboa has research experience in electric engineering integrated and hybrid systems , semiconductor devices, Schottky barrier solar cells and synthesis of nanomaterials for electrochemical energy conversion systems, rechargeable batteries, supercapacitors and low temperature fuel cells.

He has 10 years of teaching experience in undergraduate and graduate level courses.

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