EXHAUST GAS HEAT RECOVERY POWER GENERATION SYSTEM
EXHAUST GAS HEAT.doc (Size: 1.08 MB / Downloads: 183)
Even a highly efficient combustion engine converts only about one-third of the energy in the fuel into mechanical power serving to actually drive the car. The rest is lost through heat discharged into the surroundings or, quite simply, leaves the vehicle as “waste heat”. Clearly, this offers a great potential for the further reduction of CO2 emissions which the BMW Group’s engineers are seeking to use through new concepts and solutions.
The generation of electric power in the motor vehicle is a process chain subject to significant losses. Quite simply because the chemical energy contained in the fuel is first converted into mechanical energy and then, via an generator, into electric power. Now the BMW Group’s engineers are working on a technology able to convert the thermal energy contained in the exhaust gas gas directly into electric power. This thermoelectric process of recovering energy and generating power by means of semi-conductor elements has already been used for decades by NASA, the US Space Agency, in space probes flying into outer space.
WASTE HEAT FROM EXHAUST GAS GASES GENERATED FROM AUTOMOBILE APPLICATIONS
The utilization of waste heat energy from exhaust gas gases in reciprocating internal combustion engines (e.g. automobiles) is another novel application of electricity generation using thermoelectric power generators. Although reciprocating piston engine converts the chemical energy available in fossil fuels efficiently into mechanical work substantial amount of thermal energy is dissipated to the environment through exhaust gas gas, radiation, cooling water and lubricating oils. For example, in a gasoline powered engine, approximately 30% of the primary gasoline fuel energy is dissipated as waste heat energy in the exhaust gas gases; waste heat energy discharged in the exhaust gas gases from a typical passenger car travelling at a regular speed is 20-30 Kw. A comprehensive theoretical study concluded that a thermoelectric generator powered by exhaust gas heat could meet the electrical requirements of a medium sized automobile. It was reported in that among the established thermoelectric materials, those modules based onPbTetechnologies were the most suitable for converting waste heat energy from automobiles into electrical power.
INDUSTRIAL WASTE HEAT APPLICATIONS:
Most of the recent research activities on applications of thermoelectric power generation have been directed towardsutilisation of industrial waste heat. Vast amounts of heat are rejected from industry, manufacturing plants and power utilities as gases or liquids at temperature which are too low to be used in conventional generating units (<450 K).
In this large-scale application, thermoelectric power generators offer a potential alternative of electricity generation powered by waste heat energy that would contribute to solving the worldwide energy crisis, and the same time help reduce environmental global warming. In particular, the replacement of by-heat boiler and gas turbine by thermoelectric power generators makes it capable of largely reducing capital cost, increasing stability, saving energy source, and protecting environment. In this application, the thermoelectric device used the temperature difference between hot and cold legs of a glycol natural gas dehydrator cycle.
Basically the generator consists of an array of modules sandwiched between hot and cold water-carrying channels. Some of the heat flux which is established by the hot and cold temperature difference between the hot and cold water flows is directly converted into electrical power. When operated using hot water at a temperature of approximately 90 0C and cold flow at ambient.
CURRENT & FUTURE DEVELOPMENTS:
Recently, an increasing concern of environmental issues of emissions, in particular global warming and the constraints on energy sources has resulted in extensive research into innovative technologies of generating electrical power and thermoelectric power generation has emerged as a promising alternative green technology. In addition, vast quantities of waste heat are discharged into the earth’s environment much of it at temperatures which are too low (i.e. low-grade thermal energy) to recover using conventional electrical power generators.
Thermoelectric power generation offers a promising technology in the direct conversion of waste-heat energy, into electrical power. In this paper, a background on the basic concepts of thermoelectric power generation is presented and recent patents of thermoelectric power generation with their important and relevant applications to waste-heat energy are reviewed and discussed. Currently, waste heat powered thermoelectric generators are utilized in a number of useful applications due to their distinct advantages. These applications can be categorized as micro- and macro-scale applications depending on the potential amount of heat waste energy available for direct conversion into electrical power using thermoelectric generators. Micro-scale applications included those involved in powering electronic devices, such as microchips. Since the scale at which these devices can be fabricated from thermoelectric materials and applied depends on the scale of the miniature technology available.
DESCRIPTION OF THE EQUIPMENT
In isolated systems away from the grid, batteries are used for storage of excess solar energy converted into electrical energy. The only exceptions are isolated sunshine load such as irrigation pumps or drinking water supplies for storage. In fact for small units with output less than one kilowatt. Batteries seem to be the only technically and economically available storage means. Since both the photo-voltaic system and batteries are high in capital costs. It is necessary that the overall system be optimized with respect to available energy and local demand pattern. To be economically attractive the storage of solar electricity requires a battery with a particular combination of properties:
(1) Low cost
(2) Long life
(3) High reliability
(4) High overall efficiency
(5) Low discharge
(6) Minimum maintenance
(A) Ampere hour efficiency
(B) Watt hour efficiency
Inside a lead-acid battery, the positive and negative electrodes consist of a group of plates welded to a connecting strap. The plates are immersed in the electrolyte, consisting of 8 parts of water to 3 parts of concentrated sulfuric acid. Each plate is a grid or framework, made of a lead-antimony alloy. This construction enables the active material, which is lead oxide, to be pasted into the grid. In manufacture of the cell, a forming charge produces the positive and negative electrodes. In the forming process, the active material in the positive plate is changed to lead peroxide (pbo₂). The negative electrode is spongy lead (pb).
Sulfuric acid is a combination of hydrogen and sulfate ions. When the cell discharges, lead peroxide from the positive electrode combines with hydrogen ions to form water and with sulfate ions to form lead sulfate. Combining lead on the negative plate with sulfate ions also produces he sulfate. There fore, the net result of discharge is to produce more water, which dilutes the electrolyte, and to form lead sulfate on the plates.
As the discharge continues, the sulfate fills the pores of the grids, retarding circulation of acid in the active material. Lead sulfate is the powder often seen on the outside terminals of old batteries. When the combination of weak electrolyte and sulfating on the plate lowers the output of the battery, charging is necessary.
On charge, the external D.C. source reverses the current in the battery. The reversed direction of ions flows in the electrolyte result in a reversal of the chemical reactions. Now the lead sulfates on the positive plate reactive with the water and sulfate ions to produce lead peroxide and sulfuric acid. This action re-forms the positive plates and makes the electrolyte stronger by adding sulfuric acid.
Electricity is no longer a luxury; it has become a necessity in our everyday lives. Have you ever had to live without electricity for an extended period of time? If so then we know what it is like to lose all the food in your refrigerator and/or chest freezer and shivering in the cold because we have no heat. Every year thousands, even millions have been in this position when a winter storm knocked out power over large areas. Not to mention rapidly rising energy costs and an uncertain economic future. Still many people have become complacent about their electrical energy needs. Solar panels are a great alternative energy source, but they only produce electricity during daylight hours. In addition their daily output is significantly reduced during winter months and cloudy days. Now, using a TEG in conjunction with solar and wind, their combined output can provide all off your home’s energy needs and depending on what state you live in, you will be getting a check from the electric company instead off a bill!
CHARGE CARRIER DIFFUSION:
Charge carriers in the materials (electrons in metals, electrons and holes in semiconductors, ions in ionic conductors) will diffuse when one end of a conductor is at a different temperature than the other. Hot carriers diffuse from the hot end to the cold end, since there is a lower density of hot carriers at the cold end of the conductor. Cold carriers diffuse from the cold end to the hot end for the same reason.
If the conductor were left to reach equilibrium, this process would result in heat being distributed evenly throughout the conductor (see heat transfer). The movement of heat (in the form of hot charge carriers) from one end to the other is called a heat current. As charge carriers are moving, it is also an electrical current.
In a system where both ends are kept at a constant temperature relative to each other (a constant heat current flows from one end to the other), there is a constant diffusion of carriers. If the rate of diffusion of hot and cold carriers were equal, there would be no net change in charge. However, the diffusing charges are scattered by impurities, imperfections, and lattice vibrations (phonons). If the scattering is energy dependent, the hot and cold carriers will diffuse at different rates. This will create a higher density of carriers at one end of the material, and the distance between the positive and negative charges produces a potential difference; an electrostatic voltage.