Direct Thermal – Electric Conversion Device
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1 Introducing the device and objectives. This paper proposes a device aimed at thermal energy conversion (from any source) into electrical energy. It is expected that once developed device has a conversion efficiency greater than 30%. Furthermore would have other features. • • •
Reliable device. (Its working principle of isolation from the outside gives it an important capability not subject to wear and tear items). Device without moving parts. The device would require a complete developed compressor but this rotor can be of magnetic type to increase its reliability. Device with low maintenance requirements
The device is aimed at working temperatures below 170 ° C. The development of this device would provide an important opportunity for people to have a quality of energy from a variety of sources. Currently the mechanisms of thermal energy conversion (exposed to temperature) are nonexistent or are very expensive there. The development of this device should provide people with a useful tool to solve many problems related to energy. This document sets out the principles of operation, construction technologies for their study and references to assist in their development. This document is intended for college students, specialists and those who are interested in developing a device of this kind.
2 Introduction. Fuel Cells. To give background on the device we want to expose is necessary to talk about fuel cells, however this device is not a fuel cell. Is currently undertaking a major effort to develop the hydrogen economy. This economy is based on hydrogen as an energy carrier. Especially expected use in the transportation sector. The hydrogen economy has resulted in an important development in fuel cells. Página nº 1 de 13
Direct Thermal – Electric Conversion Device
Fuel cells are aimed to replace traditional combustion engine Otto cycle or Diesel cycle. In favor of fuel cells can exhibit achievable efficiences. The efficience of diesel engines around 35% for small engines and up to 50% for large motors. The efficience of fuel cells ranging from 40% to 60% depending on the type of technology However the technology of fuel cells is not yet mature to compete with internal combustion engines due to the half life of the fuel cells. Currently the average life of a fuel cell designed to fulfill the tasks of a diesel engine just reaches 20% on the average life of a diesel engine. The device that is exposed below is possible to build thanks to advances made in this field.
3 Introduction This paper proposes to build a device that converts thermal energy from any source into electrical energy directly. This device would have the following characteristics: No moving parts. It is robust and compact. Work with temperatures below 200 ° C. It is a simple system that is easy to construct and has a light maintenance. This system allows has features like: It can be attached to the exhaust pipe of an internal combustion engine to recover energy from the flue gases. Can be coupled to any heat source such as a cogeneration system. You can build efficient systems for electrical energy conversion from natural gas burning in isolated systems for pipelines, oil pipelines, etc.. This device is based on electrical production processes of a fuel cell.
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4 Description of the thermal electric conversion:
Cold H2 H+
Internal Zone Cold H2
* H+ *
Cold H2
Cold point
Hot point Thermal Energy into
e-
External Zone
Electrical Energy out
Thermal Energy out
* Nickel porous elements. The central element, yellow, is a cation transport membrane "Nafion". El funcionamiento es el siguiente: 1. Hydrogen (H2) enters into a heated chamber where temperature and pressure is acquired. 2. The hot hydrogen in contact with nickel. Hydrogen is adhered to the wall of nickel in the form of H +. Hydrogen electrons are located in the electronic network of nickel. 3. The pressure and temperature of the hot chamber to push the H + that are attached to the wall of the nickel to the Nafion membrane. 4. The Nafion membrane only allows the passage of H +, the electrons have to go through an external circuit. (Here is where we get the electricity in the form of electrical current) 5. Driven by the pressure and temperature of the hot reservoir H + reach the element nickel is in the cold. 6. Due to the drop in temperature and the thrust of the H + coming from the hot the H + that has cooled recovers its electrons and becomes gaseous H2. 7. The H2 gas is used again by a small compressor to start the cycle again.
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4.1 ¿What is the expected performance of this device? The true performance of the device is expected maximum of 60%. However it is expected that at least one performance obtained at least 30% of the available heat.
4.2 Considerations on the development of this device: 1. The technology needed to build the device is available. 2. It is a compact and robust. The inner zone is differentiated from the outside so that no wear or aging. 3. It is a simple device so the budget for development is low. 4. It is a modular device with great capacity to adapt to different industrial requirements.
5 Study of the thermal irreversibilities This chapter attempts to assess the potential of studying device efficiency heat losses and thermal irreversibilities are responsible for the downturn in the performance of any heat engine. Suppose we start from hydrogen gas with the following conditions: Presión 1 bar Temperatura 25ºC Volumen: It can take any reference volume since this first study to estimate the start and end cameras have the same volume. Assumption important for the study: It is assumed that the heat input by the camera is hot and the heat output by the cold. The rest of the system is assumed to be isolated and therefore suffers no heat loss through radiation, convention or conduction. Características del Hidrógeno. (H2) Propiedades del gas Peso Molecular •
Peso Molecular: 2.016 g/mol
Fase Sólida • •
Punto de fusión: -259 °C Calor latente de fusión (1,013 bar, en el punto triple) : 58.158 kJ/kg
Fase líquida • • • •
Densidad del líquido (1.013 bar en el punto de ebullición) : 70.973 kg/m3 Equivalente Líquido/Gas (1.013 bar y 15 °C (59 °F)) : 844 vol/vol Punto de ebullición (1.013 bar) : -252.8 °C Calor latente de vaporización (1.013 bar en el punto de ebullición) : 454.3 kJ/kg
Punto Crítico • Temperatura Crítica : -240 °C Página nº 4 de 13
Direct Thermal – Electric Conversion Device • •
Presión Crítica: 12.98 bar Densidad Crítica : 30.09 kg/m3
Punto triple • •
Temperatura del punto triple: -259.3 °C Presión del punto triple : 0.072 bar
Fase gaseosa • • • • • • • • • •
Densidad del gas (1.013 bar en el punto de ebullición) : 1.312 kg/m3 Densidad del Gas (1.013 bar y 15 °C (59 °F)) : 0.085 kg/m3 Factor de Compresibilidad (Z) (1.013 bar y 15 °C (59 °F)) : 1.001 Gravedad específica (aire = 1) (1.013 bar y 21 °C (70 °F)) : 0.0696 Volumen Específico (1.013 bar y 21 °C (70 °F)) : 11.986 m3/kg Capacidad calorífica a presión constante (Cp) (1 bar y 25 °C (77 °F)) : 0.029 kJ/(mol.K) Capacidad calorífica a volumen constante (Cv) (1 bar y 25 °C (77 °F)) : 0.021 kJ/(mol.K) Razón de calores específicos (Gama:Cp/Cv) (1 bar y 25 °C (77 °F)) : 1.384259 Viscosidad (1.013 bar y 15 °C (59 °F)) : 0.0000865 Poise Conductividad Térmica (1.013 bar y 0 °C (32 °F)) : 168.35 mW/(m.K)
5.1 Hot Chamber When gas enters the camera begins to warm. As the gas begins to make contact with the walls of the anode (nickel compound) begins desorption reactions occur. This causes the entry of hydrogen into the crystal lattice of the nickel compound. The desorption reaction is exothermic. Therefore this reaction releases heat and causes the temperature increase of the chamber. At this point the heat in the chamber will be heated due to the heat input from the outside plus heat from the exothermic reaction of desorption. (Aprox. 30 kJulios/mol)
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figu. 2 Diagram of Van’t Hoff for some nickel metal compounds[Züttel et al. 2005]
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This table shows at the same temperature as the absorption requires a higher pressure than the desorption. Thus in the device that raises the hot chamber pressure must be greater than the pressure in the cold. This was already indicated by the statement and the proposed device. Thermal irreversibilities in the hot chamber. In the hot chamber is supposed to be a heat inflow is not expected to have much heat losses estimated in this study than in the hot chamber no energy losses. In the hot chamber will establish a balance on the surface of the nickel compound and the temperature of the hot chamber of hydrogen desorption and absorption. This balance will be clearly absorption since there is a flow of hydrogen through the crystal lattice of the nickel compound to PĂĄgina nÂş 7 de 13
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the proton conducting membrane (for example Nafion membrane). To that hydrogen will move through the crystal structure is necessary energy consumption. Thermal Irreversibility 1: Here is the first planned energy loss. This energy loss should be minimized by this layer of material as thin as possible. When the hydrogen reaches the metal-hydrogen membrane energy consumption need to switch to metal ion H + to ion H + free (stabilized in the polymer network). This process is not spontaneous energy required since there is a separation between the proton H + and electrons. But this energy is useful because it is at this point that we get the pressure energy conversion to electricity. Once the ion H + is within the polymeric network has to move to the new interface membrane metal. Thermal Irreversibility 2: Here is the second planned energy irreversibility. The ion energy need to advance to the next stage. This is called membrane resistance. This resistance must be minimized using a thin membrane or thin as possible.
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Once the H + ion reaches the interface membrane - metal ion passes into the crystalline structure of the metal spontaneously because at this point is again proton with electrons and thus is part of the electrical circuit. Therefore not expected to have an irreversibility temperature here. Hydrogen once again reached the nickel compound must advance by the crystal structure to the new metal-gas interface. Irreversibility Like 3 (same as 1): Here is the third planned energy loss. This energy loss should be minimized by this campaign material as thin as possible. Once the gas is in the gas-metal interface will cause an absorption-desorption equilibrium which is distinctly surface desorption pressure as this chamber is maintained at a lower pressure than the hot chamber. Hydrogen gas that passes will phase at a high temperature (for the whole membrane metal + Nickel + nickel metal will be at a high temperature). However desorption reaction is endothermic, and this will help cool the metal-gas interface of the cold chamber. However even the released gas will be very hot and must be cooled by contact with a heat exchanger at room temperature. Thermal irreversibility # 4. The hot gas must be cooled to lower blood pressure and reuse it in the PĂĄgina nÂş 9 de 13
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cycle. This heat loss can not be minimized but is linked to the mass of hydrogen is used during the process.
In the hot chamber there is a separation between the nickel element is hot and the cold block of stainless steel. This separation is to minimize or eliminate heat loss by conduction. However apart from the heat irreversibility number 4 there is a small energy loss by another convention and radiation. Thermal irreversibility No. 5. Energy losses in the hot chamber between the nickel element and the heat exchanger by convection and radiation. These losses can be minimized or eliminated making sure that there is a laminar flow of hydrogen gas from nickel element and heat exchanger.
5.2 Estimation in watts * hours of energy losses, and efficiences useful work. The calculations will be done by assuming that the hydrogen mass flow rate is 1 kg / hour. Thermal irreversibilities 1 associated to friction of hydrogen atoms by the crystal structure. As we know the friction converts kinetic energy into heat. The heat generated by the friction returns to the system (the system is isolated). Consequently no energy losses occur because the heat returned to the system. Obviously this will depend on the constructive solution. And while the above statement is not perfect if that applies to these estimates. Thermal irreversibility, = 0 (watios hora). The thermal irreversibilities 2 and 3 is associated with the friction of the hydrogen atoms in the crystal structure. As we know the friction converts kinetic energy into heat. In this case the heat generated to the system returns but is directed to the cold. The heat generated in these locations does not reverse at the point at which the electric energy is generated by heat. It is passed in the metal-membrane. Thermal Irreversibility 2 = 0,1 ohmios * 0,5² Amperios = 0,025 watios (for each cm²). It means 1340 watios hour in losses. Thermal irreversibility 3 = 1340 watios (It is not known this energy losses but it sure is less than the thermal irreversibility # 2.) The heat lost by cooling hydrogen (1 kg) of 150 ° C to 25 ° C is 500 watts * hours (approximately). Therefore this would be heat irreversibility No. 4. irreversibilidad térmica nº 4 = 500 watios hora thermal irreversibility No. 5 depends entirely on the constructive solution. So let's assume that the Página nº 10 de 13
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constructive solution is perfect and for this study we assume that energy losses in this case are zero. Irreversibilidad térmica nº 5 = 0 watios hora.
5.3 Useful work: 1 kg of hydrogen hour provides an intensity of about 26,800 amperes per second (during 3600 seconds). But we do not know the potential that can be obtained with the proposed device. In the fuel cell is linked to the potential variation of enthalpy between the beginning of the chemical reaction and the end. (enthalpy variation between the reactants and products). However in our case is reverse conversion. What we are doing is converting thermal energy into electricity. To study this case we can rely on the science of membrane potentials in solutions. theory of Theorell, Meyer and Sievers (TMS theory) This theory can be found in the book explained: Y. Tanaka, Ion exchange membranes: Fundamentals and Applications, Elsevier Science, 2007.
In these cases the membranes are immersed in a solution where an all-enveloping liquid (water) and some ions which are in solution. This membrane system in solution is very similar to our case, because H + ions are involved in a solid matrix rather than liquid. If we do this extrapolation we can expect that potential obtained in each cell is 120 mV maximum. (When the ion enters the membrane has a potential saving of 60 mV, to make this happen is to absorb energy into potential energy converting system. After ion must leave the membrane into the crystal lattice. Spontaneous process is a potential of 60 mV. So for this case the total potential Página nº 11 de 13
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available is 60 +60 = 120 mV). 26800 amperios * 0,12 V = 3216 watios hora = useful work maximum (Hydrogen mass flow of 1 kg/hour). The 120 mV is for the case where the ion mobility is maximized. In our case it is obvious that the ion mobility is limited. Due to the small size of the hydrogen proton is expected that the yield is high. So assuming a mobility of 0.5 on a scale of 0-1 is expected at least that useful work is 1500 watt hours. Expected work = 1500 watios hora Calor total puesto en juego = 500 watios hora + 1500 watios hora. + 1340 + 1340 = 2180
6 Efficiences Estimated efficiency as conservative criteria for the device. = 32%. Maximum theoretical Efficiency = 86% (if there were ohmic losses) Actual maximum efficiency = 63% (including ohmic losses and other technological limitations).
6.1 Anotaciones importantes: This performance is an estimate as it depends on a value (mobility of H + ions) unknowns. We have evaluated compromise estimate = 0.5 (the higher the temperature increases the mobility). The yield can be increased if we reduce the ohmic losses in the membrane. This can be done by the larger device. Eventually you will reach a compromise between size and performance. The study was done following conservative criteria. It is therefore expected that the attainable efficiency is higher. The performance of low powered devices electrochemical processes often exceed 60%.
7 Bibliographic references Although here are some references the reader can find all the information in books and scientific publications related to the following topics: Electrochemical processes associated with ion exchange membranes. Absorption and desorption processes of hydrogen gas in solid matrices. Metal hydrides. Construction and technology of fuel cells and low temperature polymer. 1) Y. Tanaka, Ion exchange membranes: Fundamentals and Applications, Elsevier Science, 2007. 2) Nafion and modified-Nafion membranes for polymer electrolyte fuel cells: An overview A K SAHU†, S PITCHUMANI†, P SRIDHAR† and A K SHUKLA* Página nº 12 de 13
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Solid State and Structural Chemistry Unit, Indian Institute of Science, Bangalore 560 012, India Central Electrochemical Research Institute, Karaikudi 630 006, India 3 ) Ionic Conductivity of an Extruded Nafion 1100 EW Series of Membranes S. Slade,a,d S. A. Campbell,a T. R. Ralph,b,* and F. C. Walshc,*,z 4 ) Universidad Carlos III de Madrid . Escuela Politécnica Superior . Departamento de Ingeniería Eléctrica PROYECTO FIN DE CARRERA Descripción y Modelado de una Pila de Combustible de Membrana de Intercambio Protónico Autor: Antonio Mayandía Aguirre Directora: Lucía Gauchía Babé
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