Ozone is a respiratory tract irritant and can cause shortness of breath, pain when inhaling, exacerbated asthma symptoms, wheezing and cough in children. Ozone also causes airway inflammation and decreased pulmonary function in adults . Exposure to NOx can also enhance the allergic response to allergens. Particulate pollution contributes to excess mortality and hospitalization for cardiac and respiratory tract disease in adults [5—8]. Other studies have found that exposure to diesel exhaust, a major source of particulate emissions, is Fig.
In children, this type of pollution affects lung function and growth . It also increases the symptoms of bronchitis and other studies found associations between particulate pollution and post-neonatal infant mortality, low birth weight and preterm birth [10—17].
In industrialized nations, an uninterruptible source of energy is critical to economic stability and energy security. As an example, Fig. As many industrialized countries are highly dependent on other countries for their energy, their economies are dependent on the willingness of other countries to supply it. Lastly, fossil fuel supplies are limited. Although fossil fuels can be consumed more efficiently through improved energy conversion techniques and other means, these supplies are finite.
They are not the ultimate solution to the problem of energy demand and an alternative to meet this demand will need to be developed. The EIA anticipates significant capacity addition to electric generation capacity to meet the growing demand for electricity and to replace old plants.
As shown in Fig. According to EIA, the choice of technology for projected capacity additions is based on the least expensive option available at rates that depend on the current stage of development for each technology. Therefore, EIA estimates assume that the current cost of renewable resources is hindering its potential to play a significant role in the electricity generation market.
Even with conservation and energy efficiency measures, energy demand is projected to double to 30 TW by and more than triple the demand to 46 TW by the end of the century. A large driver of this increase in energy demand will be the growing economies of non-industrialized countries and the large populations of the world that are beginning, for the first time, to experience the benefits of living in electrified communities.
As the lives of people and the economies of the non-OECD countries become more energy intensive, there will be an accelerated growth in energy demand. This is not due to a reduction in energy use by OECD countries, however. The recent expansion of natural gas power plants is consistent with this pathway and should continue depending on natural gas pricing and availability. Coal will still play a major role in fueling existing and under-development coal-fired plants. Ultimately, high tech renewable energy technologies, nuclear energy, and clean coal technologies can be utilized to generate low-carbon content fuels and eventually to generate hydrogen to fuel a sustainable global economy.
Over the past years, there has been a transition in the fuels used for heating and electricity production from wood to coal to oil and, lastly, to natural gas. As one moves down this path, it will be noticed that there has been a reduction in the amount of carbon per hydrogen atom in the fuels used.
This general trend from use of high carbon fuels to low carbon fuels is called decarbonization. Natural gas has the highest hydrogen to carbon atomic ratio and the lowest CO2 emissions of all fossil fuels, emitting approximately half as much CO2 as coal for the same amount of energy. However, as has been demonstrated in the past few years, the heavy reliance on natural gas for electricity generation without a concurrent expansion in its infrastructure has inflated its cost significantly. As a means of offsetting the expected increase of natural gas prices and avoiding the risk of price volatility due to the rapid increase in its usage, it has been proposed, among other measures, to increase the electricity generation from coal-fired plants.
The falling prices of coal and the abundance of the coal supply in the certain regions in the world have strengthened this case. However, until the environmental issues that are associated with burning coal and the handling of CO2 emissions are resolved, coal will remain a non-sustainable source of electricity. Natural gas offers a bridge to a non-fossil energy future that is consistent with decarbonization. As a fossil fuel, natural gas is a finite resource. Currently, there are no recoverable energy sources with higher hydrogen to carbon atomic ratios than natural gas.
In order to avoid moving in the wrong direction down the decarbonization pathway back towards coal and wood as natural gas reserves are depleted, non-fossil energy sources will need to be introduced in the primary energy mix. These non-fossil energy sources include wind, solar and other renewable energy sources. As the natural gas contribution to global energy mix peaks and subsequently declines, carbon-free sources of energy would take over. This would be the end of the decarbonization pathway. A key step towards this goal is the construction of an energy infrastructure that allows hydrogen to be distributed to all end users in much the same way natural gas and electricity are today.
In this way, hydrogen can be used to produce heat through combustion and electricity through the use of a combustion engine or fuel cell. For many years, renewable energy advocates have relentlessly argued that renewable energy has tremendous environmental benefits and can provide a solution to the global warming problem. In addition to and with the ongoing security concerns in the developed countries and the geopolitical problems in oil producing countries, it is becoming clear that there is a need for global as well as 6 1 The Role of Renewable Energy national comprehensive energy policies that are based on self-reliance with a significant role for renewable resources to ensure national energy security.
The cost of renewable energy resources is still high compared with conventional energy systems. The most important challenges for the wide spread adoption of renewable energy technologies can be divided in three categories: economic, technological and social. With regard to economic challenges, renewable energy needs to be generated, stored and utilized in a cost competitive manner. The focus of this study will be to improve the economics of electricity produced from renewable energy resources with hydrogen storage systems.
There are many incentives to improve the technologies associated with and reduce the cost of renewable energy systems. Additionally, no green house gases are emitted as a result of their operation. As a result of the large number of electricity production technologies, renewable energy is also viewed as a means to enhance energy security. Since electricity can be generated from a variety of renewable sources, any country with a large enough energy resource can increase energy security.
The infrastructure requires a large amount of resources and can be expensive. Often, large diesel generators are used to provide electricity in areas with no infrastructure, but this too can be expensive because of fuel requirements and maintenance costs. In these cases, if a significant renewable resource is available, renewable energy systems are a viable alternative. There are also many problems associated with renewable energy systems.
They are: 1. Because of the resources from which renewable energy is derived, demand loads cannot be met with a high degree of reliability. Energy storage becomes necessary to achieve a high degree of reliability and this storage can be very costly. If renewable energy is ever to compete with conventional, fossil fuel based electricity generation, this problem will need to be addressed. Studies by Paynter et al. Utilization of some form of energy storage will be necessary if higher grid penetration levels are going to be achieved.
In this way, energy can be stored when production levels are high and released to the grid later, thereby improving capacity utilization and the economics of the renewable energy system. As a result of the high initial capital costs of renewable energy systems, smart energy management must be employed to minimize the cost of electricity over the life of the system. References 1. Part 1. Publication no. Epidemiology — Bobak M Outdoor air pollution, low birth weight and prematurity. Epidemiology — Ritz B, Yu F The effect of ambient carbon monoxide on low birth weight among children born in southern California between and Epidemiology — 8 1 The Role of Renewable Energy Environ Health Perspect — Xu X, Ding H, Wang X Acute effects of total suspended particles and sulfur dioxides on preterm delivery: a community-based cohort study.
Arch Environ Health — Energy Information Association Annual energy outlook Department of Energy, pp — Department of Energy, pp 83 Galvin Library, Chicago Energy Research Unit, Rutherford Appleton Laboratory, September Chapter 2 Renewable Energy Sources and Energy Conversion Devices Renewable energy sources are those sources that are regenerative or can provide energy, for all practical purposes, indefinitely. These include solar, wind, geothermal, tidal, wave, hydropower and biomass.
The status of development, installed capacity, theoretical potential and other considerations are shown in Table 2. For a more comprehensive treatment of the subject of solar engineering and calculations, please refer to the above referenced book. The amount of energy received by the earth per unit time, based on the average distance between the sun and the earth over the period of a year, is known as the global solar constant, Gsc.
This value can be used to calculate several values of interest when performing solar calculations. The dependence of extraterrestrial radiation on time of year is given by Eq. The demand for energy is projected to increase to 30 TW by and 46 TW by . The wind energy resource can make a significant contribution to energy demand in the near term, is technologically mature and economically attractive. Of the other resources listed, they do not possess sufficient exploitable capacity to meet demand or they are not technologically mature enough for deployment in the short-term.
For these reasons, solar and wind energy are the primary focus of this book. Solar noon is time of day when the sun appears the highest in the sky compared to its positions during the rest of the day. To determine the daily solar radiation on a horizontal surface, H0, Eq. The result of this integration is Eq. The sunset hour angle can be determined from Eq.
The fraction of radiation reflected back into space is called the albedo and has an annual, latitude-longitude average of 0. X-rays and other very short-wave radiation of the solar spectrum are absorbed by nitrogen and oxygen. Ultraviolet radiation is mainly absorbed by ozone and infrared radiation is mainly absorbed by water vapor and carbon dioxide.
Scattering occurs as radiation passes through the atmosphere and interacts with air, water and particulates. The extent of scattering is a function of the degree of particle interactions and the particle size with respect to the wavelength of the radiation. Equation 2. N the month to calculate the declination angle, d.
The mean day for each month is shown in Table 2. Knowledge of these components is important for calculating the incident 14 2 Renewable Energy Sources and Energy Conversion Devices Table 2. It is also important to know the beam component of the total radiation to determine the long-term performance of concentrating solar collectors. Erbs et al. The solar radiation models for determining incident radiation on a sloped surface are based on measured global irradiance on a horizontal surface.
On a clear day, the diffuse radiation is composed of three parts: the isotropic component, which is received uniformly from the entire sky dome; the circumsolar diffuse 2. The differences among the solar radiation models are in the way they treat the three parts of the diffuse radiation. The total incident radiation on a sloped surface can be determined using Eq. Rb is ratio of beam radiation on a tilted plane to that on the plane of measurement and can be calculated using Eq.
Id,iso is the incident diffuse radiation due to isotropic part of the diffuse radiation, Fc-s is the view factor of the tilted surface to the sky, Id,hz is diffuse radiation due to horizon brightening, Fc-hz is the view factor of the tilted surface to the horizon, I is the total radiation incident on a horizontal surface, qg is the ground reflectance and Fc-g is the view factor of the tilted surface to the ground. Equations 2. It can be determined using Eq. The solar radiation on tilted surfaces can be determined with using several different solar radiation models using measured data for a horizontal surface.
The three models described in this section are: 1. Isotropic diffuse model I 2. Hay and Davies model 3. HDKR model 2. The third and fourth terms from Eq. Fc-s is given by 1? Therefore, the third term in Eq. The diffuse radiation on the tilted surface is given by Eq. It contains a horizon brightening term.
A picture of a solar cell is shown in Fig. The solar cell has a dark area, which is usually silicon, and thin silver areas. The silicon absorbs the sunlight and a voltage is generated between the front and back of the cell. The thin sliver areas are called front contact fingers and are used to create an electrical contact to the front of the cell.
The back of the cell is a solid metal layer that reflects light back up through the cell and provides an electrical contact on the back side. Commercially available cells have a 25—30 year lifetime.
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Solar cells are strung together in various serial or parallel configurations to achieve desired electrical characteristics and assembled into modules. A picture of PV module is shown in Fig. The module protects cells from mechanical damage from handling and the environmental and provides the end user with a robust package that can easily be connected to other modules.
A system of connected modules is called an array. The array, in conjunction with electrical conditioning equipment constitutes a complete photovoltaic system. The open-circuit voltage of the cell is the voltage 2. In this case no current is being drawn from the cell. If the load resistance is reduced to zero, the cell is short-circuited and the resultant current in this situation is the short-circuit current.
The short-circuit current is directly proportional to the light falling on the cell. The I—V curve is a plot of the current vs. An example of an I—V curve for various ambient temperatures and insolation levels is shown in Fig. The power being generated by the solar cell is the product of the voltage and current at any point on the curve. The point on the curve at which the product is greatest is called the maximum power point. Cell performance is determined under a specified set of standard conditions. Of the inorganic type, three sub-types are commercially 20 2 Renewable Energy Sources and Energy Conversion Devices available: mono-crystalline, multi-crystalline and amorphous.
Mono-crystalline cells are constructed from single crystal silicon ingots by slicing the ingots as is done in microchip fabrication, while multi-crystalline cells are constructed by evaporating coatings onto a substrate. Because grain boundaries exist between crystals in the multi-crystalline cells, electrons can cross at the boundaries which increase losses and reduce efficiency with respect to mono-crystalline cells. Cells using amorphous silicon a-Si are of interest because they can be manufactured on continuous lines and require less silicon than the other PV cell types described.
The main problem with a-Si cells currently is that they are less efficient than multi-crystalline and mono-crystalline silicon cells. An explanation of bandgap energy is useful for understanding photon absorption. The bandgap energy is the difference in energy between the top valence band lower energy and bottom conduction band higher energy , as shown in Fig.
The bands refer to the electron energy ranges for electrons in different electron orbitals. The electrons in the conduction band can be used to create an electrical current. When a photon hits a piece of silicon or another type of semiconductor, several events may occur. If the photon energy is lower than the bandgap energy of the silicon semiconductor, it will pass through the silicon. If the photon energy is greater than the bandgap energy, it will be absorbed. The energy given to the electron excites it and moves it into the conduction band. Now it is free to move around within the semiconductor.
The band that the electron was previously a part of now has one less electron. This is known as a hole. The presence of a missing covalent bond allows the bonded electrons of surrounding atoms to move into the hole leaving another hole behind. By this mechanism, a hole can move through the semiconductor opposite the direction that the electrons flow. As a result, moving electrons and holes are created and move to opposite sides of the cell structure. This freedom of movement that occurs as a result of the absorption of the photon is what allows a charge differential voltage to be created within the cell.
By connecting an external circuit to the two sides of the silicon cell, electrons can recombine with holes and this flow of electrons via this external circuit can be utilized to do work. To construct an actual cell, an n-type semiconductor one with excess electrons that can donate electrons and a p-type semiconductor one deficient in electrons that can accept electrons must be placed in physical contact with one another.
The semiconductors can be modified by adding impurities, known as dopants, to the Si material. Depending on the number of electrons possessed by the dopant, an n-type or p-type semiconductor can be formed. It has a different function than the inorganic p—n junctions. When electrons and holes are produced upon absorption of light, the electrons and holes become bound to one another to form electron—hole pairs called excitons.
The excitons have no net electrical charge and cannot carry current. They must be broken apart in order to produce the free electrons and holes required to generate a current. This is the function of the junction between the n- and p-type organic compounds. When the excitons diffuse to this region of the cell, they split apart and produce the required free electrons and holes.
Generation I cells include single-crystal and multi-crystal Si solar cells. Generation II cells include those that involve the use of several types of thin films including both inorganic and organic materials. How is this possible? There are two suggested ways. One assumption made in the original calculations of the above limit is that the energy of an absorbed photon above the bandgap energy becomes heat. This can only be accomplished if the material that absorbs the photon is able to utilize the excess photon energy.
The other way to accomplish this is through the use of multi-junction cells. Multi-junction cells utilize several materials together with different bandgaps that are able to utilize photons with different energy levels. These cells have been shown to operate at efficiencies higher than those of single-crystal Si cells. Figure 2. Dashed lines show the cost per Wp and the Shockley-Queisser limit and thermodynamic limits at 1 sun and 46, suns are also shown. The different colored lines and tick marks correspond to different PV technologies.
The names at each tick mark are the labs or companies that achieved the given efficiency. A solar concentrator focuses light on a solar cell at intensities several times higher than that provided by a single sun. Several types of concentrators exist including parabolic, Fresnel and Winston. The benefit of such an approach is that, since the solar cell current is proportional to the intensity of the light incident to the cell, the solar cell should provide a higher current.
By substituting inexpensive optics for expensive semiconductor material, system costs are reduced. First, because of the increased light intensity, the cell begins to overheat and some passive or active means of heat management is required. This will increase costs. Depending on the geographical location of the system, the use of concentrators may not make sense. Additionally, solar cell performance will decrease with an increase in cell temperature. Second, the typical material used to seal the PV cells between glass, EVA, will degrade at high temperatures.
More expensive encapsulation materials, such as silicon, will be required and this too will increase costs. Lastly, for concentrators to be most effective, a tracker system will be required. Two types of tracking systems exist: single axis and double axis. Single axis systems track the sun as it moves throughout the day azimuthal tracking. For a flat-plate collector, which a PV module is, the maximum average gain from using a two axis system less than a factor of 2 would rarely justify the extra cost of the tracking system, which currently costs more than simply doubling the collector area .
In addition to module costs, a PV system also has costs associated with the power conditioning and energy storage components of the system. There are two causes for this variation in pressure: 1 the heating of the atmosphere by the sun and 2 the rotation of the earth. The warming effect of the sun varies with latitude and with the time of day. Warmer air is less dense than cooler air and rises above it so the pressure above the equator is lower than the pressure above the poles.
As the earth spins on its axis, it drags the atmosphere with it. The air higher up in the atmosphere is less affected by this drag effect. Instead of traveling in a straight line, the path of the moving air veers to the right. The result of the phenomena described is that the wind circles in a clockwise direction towards the area of low pressure in the Northern hemisphere and counter-clockwise in the Southern hemisphere. A wind map is constructed using meteorological wind data, which is usually not sufficient to accurately site a large wind power project.
After preliminary identification, data collection equipment anemometer is used to record the wind speed and direction for several locations at a given site, usually for a year. A higher resolution wind map of the area of interest is then constructed to identify the best location for turbines. This process is known as micro-siting. A good location would have a near constant flow of non-turbulent wind throughout the year without too many sudden bursts of wind.
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Since each turbine extracts some of the energy of the wind, it is important to provide adequate spacing between turbines to prevent interference and reduced energy conversion. As a general rule, turbines are spaced three to five rotor diameters apart perpendicular to the prevailing wind and five to ten rotor diameters apart in the direction of the prevailing wind The power of the wind passing perpendicularly through a given area, shown in Fig.
These types of turbines typically have two or three blades that are evenly distributed around the axis of rotation. These blades are designed to rotate at a specific angular velocity and torque. The mechanical energy produced can then be further converted to other forms of energy depending on the application Fig. To covert the mechanical energy to electrical energy, a generator is coupled to the shaft of the propeller.
The generator can be designed to operate at a fixed or variable angular velocity. For those generators designed to work at a fixed angular velocity, a transmission will be used to change the angular velocity from that of 26 2 Renewable Energy Sources and Energy Conversion Devices Fig. This type of generator will output the electrical energy as an AC current with constant frequency usually that of the electrical grid to which it is connected.
Alternatively, the energy converter may operate in a variable frequency mode. The electrical energy of this type of system would be output as an AC current with varying frequency . First, the wind speed data must be corrected to compensate for the difference between the height of the wind anemometer that was used to collect the wind speed data and the hub height of the turbine being considered in the calculations. The hub height is the height of the turbine as measured from the ground where the turbine is installed to the center of rotation of the turbine blades.
The wind speed profile is shown in Fig. Wind speed researchers, however, have found that the exponent depends on temperature, season, terrain roughness, and several other factors. Second, a power curve as shown in Fig. The power curve is a plot of the power output for a turbine plotted against wind speed.
The turbine shown in this example is a WES18 80 kW turbine . Lastly, the power captured from the wind depends not only on the wind turbine and the wind speed, but also on the air density. Thus, the power output determined above in the second step is multiplied by the air density ratio, which is the air density at the location of interest divided by the air density at standard conditions.
This ratio corrects for the altitude of the location of interest and is calculated as shown below in Eq. Trends in Photovoltaic Applications.
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International Energy Agency 2. Wind Energy Association- Statistics. World Wind Energy Association. J Geophys Res. Carella R World energy council survey of energy resources. World Energy Council. Craig J World energy council survey of energy resources. Lafitte R World energy council survey of energy resources. World Energy Council 9 Jan ] 7. Nault RM comp. Basic Energy Sciences, U. Wiley, London, pp 1— 9. Digital image [Spectral Distribution of Sunlight].
London Metropolitan University. Sol Energy — Digital image [Picture of Siemen Cell]. Energy Center of Wisconsin. Digital image [Sharp PV Module]. Digital image [Electronic Band Diagram]. Sorensen B Renewable energy, 2nd edn. Academic Press, London, pp — Academic Press, London, p Digital image [Large Windmill]. Photolytic processes will not be covered here. When heavy oils or coal is used, the gasification process is commonly used.
The typical feedstock is natural gas, which comes in several varieties including dry, wet, sweet and sour gas. These designations refer to the composition of the gas. Dry gas is mostly methane, whereas wet gas contains higher hydrocarbons. Sweet gas has little hydrogen sulfide, whereas sour gas contains higher levels of hydrogen sulfide. The desulfurization process is an exothermic process that is typically carried out in a packed bed reactor. The sulfur compounds are adsorbed by the packed bed, which is usually ZnO. The process is endothermic for natural gas and exothermic for heavier feedstocks.
This process further removes any sulfur left from the previous step. First, an oxidant, either air or oxygen is supplied to the reaction environment.
The oxidant reacts with the incoming gas from the primary reformer in an exothermic reaction to form water and raise the temperature. In the second step, a nickel catalyst bed is used to catalyze the endothermic reforming reaction. The WGS reaction involves reacting 3. There are several benefits to this approach. The steam can be used to convert some of the carbon monoxide to carbon dioxide via the WGS reaction. It can also reduce risk of explosion and reduce carbon deposition on the catalyst.
When the proper mixture of fuel, air and steam are supplied, the partial oxidation reaction supplies all the heat required to drive the steam reforming reaction. In the case of oil, the process used is called partial oxidation. There are three steps in this process: 1 Steam is used to reduce the size of the hydrocarbon chains known as cracking ; 2 Substoichiometric amounts of oxygen oxidize the oil into syngas; 3 Carbon particulates react with CO2 and steam to form syngas.
Coal can also be used as a feedstock to produce syngas. The processes involved include pretreatment, primary gasification, secondary gasification and 34 3 Hydrogen Production, Storage and Fuel Cells Fig. During pretreatment, oxygen is introduced to remove compounds that would otherwise cause the coal to agglomerate in the gasifier. The char is further reacted with steam in the second gasifier to produce syngas. Lastly, the water gas shift reaction is used to determine the final ratio of CO2 to H2 Fig. This process operates by separating different species under pressure according to these species affinity for an adsorbent material.
The adsorptive material is used to separate gases, adsorbing the undesired gases at high pressure leaving a stream of hydrogen to pass through. The adsorbent material gradually becomes saturated with the waste gases. The adsorbent bed is depressurized allowing the adsorbed waste gas molecules to flow out of the bed and the process is repeated. The desorbed gas is sent back to the furnace for combustion. This process provides only a 3.
The cathode half-cell potential, determined by using Eq. The minimum voltage that will allow the reaction to proceed is called the decomposition potential of water and is equal to 1. When this potential is applied, hydrogen will be produced at the cathode and oxygen at the anode. The above example is a purely thermodynamic analysis. An electrolyte solution is usually used in real-world processes to increase the rate of reaction. In these systems, oxygen ions migrate through the electrolytic material, leaving hydrogen gas dissolved in the water stream.
This hydrogen is extracted from the water and directed into a separating tank. Oxygen gas remains behind in the water. Water is recirculated and oxygen accumulates in a separation tank. Transportation applications of hydrogen storage have thus far received the most attention by governments and companies who are trying to commercialize the technology. To make progress in this technology, the US Department of Energy issued a roadmap with milestones to be met by and , shown in Table 3.
In this section, we will cover the various hydrogen storage technologies currently available and some of those that are being researched. These include compression, liquefaction, metal hydride storage, chemical hydrides, carbon-based storage and liquid carrier-based storage. It is the most simple of existing storage technologies, requiring only a compressor and storage vessel capable of being pressurized. Typical tank materials include steel, aluminum wrapped in fiberglass and high molecular weight lined tanks with a carbon composite shell.
Steel tanks can be used where weight and volume are not a constraint on system requirements such as in stationary applications. The two other tank types mentioned are used when weight and volume are significant constraints on system design such as vehicular applications. No allowable performance degradation from C to 40C. Allowable degradation outside these limits is TBD e Equivalent to ,; ,; and , miles respectively current gasoline tank spec f All targets must be achieved at end of file g In the near term, the forceout should be capable of delivering 10, psi compressed hydrogen, liquid hydrogen, or chilled hydrogen 77 k at 5, psi.
These are subject to change. Note the some storage technologies may produce contaminants for which effects are unknown; these will be addressed as more information becomes available k Total hydrogen lost into the environment as H2; relates to hydrogen accumulation in enclosed spaces. This includes any coating or enclosure that incorporates the envelope of the storage system l Total hydrogen lost from the storage system, including leaked or vented hydrogen; relates to loss of range a Units Table 3. The gas can be further compressed to a higher storage pressure, but this results in higher capital and operating costs.
Significant increases in volumetric energy density are not achieved at higher pressures. As the storage pressure is increased, the work of compression increases. The adiabatic compression work required to compress a gas is given in Eq. Figure 3. Quantum Technologies has recently developed a high pressure hydrogen containers by validating the first bar hydrogen storage tank TriShieldTM. The carbon fiber represents the largest portion of all costs and so the goal is to reduce the amount of carbon fiber needed while maintaining equivalent levels of performance and safety [6, 7] Fig.
Because of the low critical temperature of hydrogen 33 K , the liquid can only be stored in open systems. Liquefaction of hydrogen is done by cooling gaseous hydrogen to form a liquid. The simplest liquefaction process is the JouleThompson expansion cycle. In this process, the gas is compressed, cooled in a heat exchanger, and passed through a throttling valve where it undergoes isenthalpic expansion, which results in the production of some liquid.
The liquid is removed and the gas is returned to the compressor via a heat exchanger. Once liquefied, the hydrogen must be stored in an insulated vessel. A major concern in liquid hydrogen storage is minimizing hydrogen losses from liquid boiloff. Heat transfer from the environment to the liquid causes some hydrogen to evaporate. The vessel can be refrigerated to prevent boil-off losses, but this consumes energy. If not refrigerated, the hydrogen gas can be vented or captured, liquefied and returned to the tank.
These losses are typically 0. The high energy needed to liquefy hydrogen and the boil-off losses dramatically increase the cost to store hydrogen through this method Fig. Heat is released when a hydrogen storage container is filled. When the hydrogen pressure is increased, the hydrogen dissolves in the metal and then begins to bond to the metal. Heat released during hydride formation must be removed to prevent the hydride from heating up. When hydrogen is released, the tank requires heat to be transferred to it at a rate proportional to the rate at which hydrogen is being released.
There are many different alloys that can be used and each alloy has different performance characteristics. Compounds containing hydrogen bonds are known for every metal and nonmetal expect for noble gases in the periodic table. They can be divided into three groups of hydrides. The first and second group forms a saline whereas the transition metals form mainly metallic compounds. The covalent hydrides can be found at the right of the transition metals.
They are also called interstitial hydrides as hydrogen often appears on the interstitial sites in the metal lattice. The intermetallic phases are of particular interest for hydrogen storage. The properties of these hydrides can be tailored because of the large variation of the elements of the intermetallic compound. The simplest case is the ternary system ABxH, where A is usually a rare earth or an alkaline metal and B a transition metal. Their light weight and high number of hydrogen atoms per metal atom is an advantage over metal hydrides described previously.
The main difference between the metallic and complex hydrides is the transition to an ionic or covalent bond. A large variety of these hydrides are formed by the light metals of group 1, 2 and 3 of the periodic table, i. Li, Mg, B and Al. Using complex metal hydrides for hydrogen storage is a very promising method, since they potentially have a high gravimetric and volumetric hydrogen density. The use of these complex hydrides is challenging because of the thermodynamic and kinetic limitations. Future work involves the improvement of thermodynamic and kinetic properties of existing compounds and exploring new compounds with better practical characteristics for hydrogen storage.
As the interaction is weak, high physisorption is observed at low temperatures. There has been extensive research in the late s, particularly on carbon nanotubes. Carbon nanotubes are cylindrical structures with either open ends or closed ends. The closed end variety has a hemispherical end cap at each end. There are two types of carbon nanotubes: multi-walled nanotubes and singlewalled nanotubes. MWNTs are composed of multiple, concentric cylindrical tubes. An example of each is shown in Fig. For instance, one mole of naphthalene can be converted to one mole of decalin to provide five moles of H2.
Advantages are that the organic liquids have low volatility, low toxicity and can be made from low-cost raw materials. A major problem of this method however, is the kinetics of both the hydrogenation and dehydrogenation reaction, which often requires a substantial amount of heat and a suitable catalyst for the reaction to occur .
Unlike batteries, in which the required reactants are self-contained, fuel cells require constant streams of fuel and oxidant provided 3. The following sections will describe the anatomy of the fuel cell, the functions of the individual components, explain the thermodynamics of fuel cells and describe the various components of complete systems. More detailed information about fuel cells and their applications can be found in the Fuel Cell Handbook published by US Department of Energy .
An electrolyte is a material that conducts ions, but prevents the flow of electrons. An electrode is a material that conducts electrons. The fuel gas enters the cell at the anode negative electrode and the oxidant gas enters at the cathode positive electrode. The electrode is a porous material, so the gas can diffuse through to the opposite side of the electrode where the catalyst is present. The electrode serves several functions. It must capable of transporting the gas stream to the catalyst layer and evenly distribute the gas across the reaction surface area.
The electrode must also be capable of diffusing the product gases away from the electrolyte. Lastly, the electrode must have good electrical conductance characteristics because it is responsible for conducting the electrons away from the interface as they are formed in the reaction process. Low temperature fuel cells require expensive catalysts, often platinum, to obtain the desired reaction rates. At higher temperatures, the reactants can achieve the desired reaction rates using the bulk electrode material. The electrolyte is also serves several important functions. It must be able to conduct ions from one electrode to the other, but prevent electrons from crossing.
It must also act as a barrier between the fuel and oxidant gases to prevent mixing. In fuel cells with liquid electrolytes, the reactant gases diffuse through an electrolyte film that wets portions of the porous electrode and react on the electrode surface. If the porous electrode contains excess electrolyte, the electrode may block the transport of gaseous reactants to the reaction sites, which results in a reduction in the performance of the fuel cell. Control systems play an important role in maintaining balance among the electrode, electrolyte, and gaseous phases in the porous electrode.
A three-phase interface is established among the reactants, electrolyte, and catalyst in the region of the porous electrode. Depending on the type of fuel cell, molecules at the anode or the cathode will be adsorbed by the catalyst allowing for the formation of ions and electrons.
The electrons are conducted away by the electrode to an external circuit which can be used to do work. The charge-carrying ions diffuse through the electrolyte to the opposite electrode and recombine with the electrons. This electrochemical reaction produces both heat and electricity. The ideal performance of the fuel cell will first be explained and losses due to the non-ideal conditions of real systems will then be explained to develop equations that describe real-world fuel cell performance. Free energy earns its name because it is energy that is available for conversion into usable work.
The unavailable energy is lost due to the increased disorder, or entropy, S, of the system. The maximum electrical work Wel obtainable in a fuel cell operating at constant temperature and pressure is given by the change in Gibbs free energy DG of the electrochemical reaction: 3. The maximum amount of electrical energy available is DG, as mentioned above, and the total thermal energy available is DH. The amount of heat that is produced by a fuel cell operating reversibly is TDS. It relates the ideal potential to the concentrations of reactants and products.
The ideal efficiency of a fuel cell, operating reversibly, is given by: 48 3 Hydrogen Production, Storage and Fuel Cells Fig. Polarization refers to the departure of the potential from equilibrium conditions due to the flow of current. Overpotential refers to the magnitude of this departure.
The plot shown in Fig. The horizontal line represents the ideal cell voltage obtained from the Nernst equation.
The actual cell voltage begins to decrease as current starts flowing in the circuit. This potential is called the activation overpotential and is denoted gs. The rate of reaction can be related to the activation overpotential by the Bulter-Volmer equation, shown as Eq. A negative value will lead to a cathodic current, which means that electrons will be transferred from the electrode to the reactants.
The variable i0 is called the exchange current density. A high exchange current density value suggests that good fuel cell performance is possible because high reaction rates are possible with smaller activation overpotential. This is known as ohmic polarization. This polarization loss increases nearly linear with the current. The ohmic polarization is described by the following equation. This is known as concentration polarization. Under short-circuit conditions, the system potential reaches zero and the limiting current, iL, is reached. The limiting current is the maximum rate at which the reactant can be supplied to an electrode.
The concentration polarization is given by the following equation. The theory underlying the operation of fuel cells has been given in the previous sections. This section will summarize the different types of fuel cells and differences among them. Nafion is a widely used membrane manufactured by DuPont. The electrodes are porous carbon electrodes with a platinum catalyst. The most common fuel is pure H2 and the most common oxidant is air. The hydrogen enters at the anode, where it is ionized with the help of a platinum catalyst. The hydrogen atoms are conducted across the membrane while the free electrons are conducted in an external circuit to the cathode where they meet with the hydrogen and air to form water.
The platinum is highly sensitive to carbon monoxide, so the use of fuels other than pure H2 is challenging and costly. Much research has been aimed at decreasing the amount of platinum that is used to reduce the price of the fuel cell. Water management is a critical issue because a lack of water will decrease the cell life and ionic conductivity, while excess water will decrease the power output. Besides water management, temperature and pressure also play important roles in determining fuel cell performance.
Increasing the temperature of PEM fuel cell is advantageous because of the reduction of ohmic resistance of the electrolyte at higher temperatures and because of reduced mass transfer limitations. The poisoning of the catalyst by CO is also reduced at higher operating temperatures. Another important performance variable is pressure. Higher oxygen pressure reduces cathode polarization, which improves the fuel cell efficiency.
Another advantage is that higher pressure decreases the membrane dehydration at a given temperature, so higher temperatures are possible. The performance improvement at high pressure must be balanced against the membrane stability and energy required in compressing oxygen. Operating pressures of 2—3 atmospheres lead to an optimal balance between cost and performance. In addition, the use of concentrated acid minimizes allows for much simpler water management. The PAFC can operate at a range of temperatures, pressures, gas utilization rates and current densities. The benefit of finding the most effective operating conditions must be balanced against the energy required to achieve these conditions in a given environment as well as the effect on the system life.
An increase in the operating temperature increases cell voltage, but this increase must be limited to avoid catalyst and component wear. Utilization of a higher percentage of the fuel is desirable from a cost standpoint, but results in lowered partial pressure, which leads to higher concentration polarization losses. Lastly, cell voltage, and thus cell efficiency, decreases with an increase in current density. They have the attractive combination of being able to work at high efficiencies with natural gas fuel, while not being hampered by the same corrosion concerns as solid oxide fuel cells.
The electrolyte in this fuel cell is a combination of alkali carbonates. At these higher operating temperatures, the nickel and nickel oxide electrode material are sufficient to catalyze the reaction. Also, due to the high operating temperatures, MCFCs generate a lot of waste heat that can easily be used in cogeneration applications. The disadvantages of operating at such high temperatures are increased corrosion and reduced cell component life. The high temperature also allows them to operate on a variety of fuels.
Units have been developed that operate on methane, propane, butane, gasified biomass, and paint fumes. SOFC technology is still in the early stages of development compared to the other fuel cell types, so the potential for rapid advancement is present. This high temperature fuel cell has several attractive characteristics with regard to central power generation. In addition, a very high temperature allows internal reforming of various fuels. Fuel Process Technol 42 2—3 — 2. Ind Eng Chem Res 42 8 — 3. Lee S Methane and its derivatives, 1st edn.
Marcel Dekker, Inc, New York 4. Appl Catal A Gen — 5. Towards a Hydrogen Economy. Research Reports International 6. Bossel U, Baldur E Energy and the hydrogen economy. Andrighetti J Quantum hydrogen storage systems. Digital image [Carbon nanotubes]. University of Karlsruhe Germany. Digital image [Mutli-walled carbon nanotube]. More A Modern production technology—ammonia, methanol, hydrogen, carbon monoxide—a review. CRU, London Digital image [PEM Mechanism].
Third Orbit Power Systems. GIF Fuel Cell Handbook. Department of Energy. Long-term simulations used for system sizing will also be discussed. Energy storage costs can be reduced in the same way as a result of the complementary characteristics of the storage devices used. Hybrid power systems are designed for the generation and use of electrical power. They may consist of an AC distribution system, a DC distribution system, multiple electricity generation that utilize several energy resources, energy storage, rectifiers, inverters, user loads, heating and dispatchable loads and a system controller.
An example hybrid power system shown in Fig. In the system under consideration, the renewable energy sources are wind and solar radiation. A wind turbine converts wind energy into electrical energy and photovoltaic PV panels convert solar energy, in the form of photons, into electrical energy. This energy is then directed to the primary load, a battery bank, electrolyzer, heating load or dispatchable load. A dispatchable load is one that can be met when energy is available, which differs from a primary load that must be met immediately when demanded.
In systems that are grid connected, the excess energy could be sold to the electrical grid. The system could be operated so that energy is sold to the grid at times when the utility is paying higher rates to electricity producers. Available renewable energy resources are determined by: 1 location, defined by latitude, longitude and altitude; 2 time, both short term daily fluctuations and long term over the course of the year ; and 3 surrounding land topology.
Lastly, system component specifications and costs determine how the various components can be sized and how they can be operated. Initial component costs, maintenance costs and component lifetime will affect which combination of components and what operating scheme should be chosen to minimize the system lifetime costs. It is because of the large number of variables and the interaction among the many system components that simulation software is often necessary to design a hybrid energy system. It uses hourly average wind speed, incident solar 4.
Hourly data is used for several reasons. First, finding data on the order of minutes or seconds is very difficult. Data available to carry out these simulations is almost always averaged over the hour. Second, carrying out a simulation using high frequency data over an entire year would significantly increase simulation time. Lastly, hourly data has been shown to be sufficient to determine long term performance of systems and perform economic analyses of these systems [1—3].
The software simulates the system using various combinations of component sizes and searches for the system that meets the user constraints at the lowest cost. Energy is therefore conserved throughout the entire simulation. The schematic, shown in Fig.
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At the beginning of the simulation for a given system configuration, the initial state of the system is set. The program reads the incident solar radiation data, wind speed data and demand data for the given hour from the user provided data files. However, the end use and generation locations have to be in close proximity.
Liquid hydrogen and methanol, despite also being alternative energy vectors, have lower RTE values [than ammonia] as estimated in previous studies. Further, the infrastructure required for liquid hydrogen transport is almost nonexistent and methanol is an emission producing fuel at the point of use; make these alternatives less attractive at this stage. Ammonia therefore provides an attractive option in terms of RTE, as well as being an emission-less energy carrier. Energy input required to produce ammonia The energy input required to synthesize ammonia from fossil fuels is given as 7.
The authors also give us a sense of long-term technological potential. While we may be able to make ammonia sustainably, from renewable power, it will always be an energy-intensive process. Likewise, the study considers hydrogen compression and storage technologies — after all, this RTE assessment of ammonia as a hydrogen carrier is only relevant in relation to the RTE of hydrogen itself; the study also considers methanol, as an alternative synthetic fuel. The round trip efficiency estimates are good to know, thank you. In the end, though, it is total cost that matters.
A high round trip efficiency process that has very high capital costs can end up costing more than something with lower round trip efficiency that is cheaper to build and maintain. Renewable ammonia is an interesting concept where both hydrogen and nitrogen are produced using green energy. The round trip energy efficiency for ammonia is an important decision making tool, but the price of CO2 will become more important in the future and as such renewable NH3 will become a high value commodity.