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INTRODUCTION
1.PROBLEM STATEMENT
The use of thermoelectric power generators for electrical power generation out of low temperature heat sources is the general topic of this work. This chapter describes the actual situation, future trends and required actions. Thermoelectric power generation seems to be an interesting possibility for the direct heat to power conversion.
1.1. INTRODUCTION
Automotive Thermoelectric Generators (ATEG) recovers heat that escapes from a vehicle powered by an internal combustion engine, and generate electricity with the heat. This leads to the significant increase of demand for electric power in the vehicle which has to be generated by the alternator. It is predicted that if only 6% of the heat contained in the exhaust gases was changed into electric power, it would allow to lower fuel consumption by 10% due to the decreased waste resulting from the resistance of the alternator drive. Power generation system using the thermoelectric generator should generally consist of the following components: heat exchanger, thermoelectric module, cooling system and DC/DC voltage converter.
Recovery of heat from exhaust gases. Through the application of thermoelectric generators (TEG), in the future a proportion of the energy carried by the exhaust gases that would previously have been lost will be recovered as electrical energy.
The German Aerospace Center (DLR) has developed TEG for use in vehicles for doing this and resulting from collaboration with the BMW Group, 200 W of electrical power in a test vehicle was achieved in a demonstration. Given the use of future materials, performances of up to 600 W can be expected, yielding a usage potential of 5 percentage.
Automotive thermoelectric generator (ATEG) allows the automobile to generate
electricity from the engine's thermal energy rather than using mechanical energy to power an
electric generator or alternator [3, 4]. Therefore, the automobile releases less heat and fewer emissions, and therefore lowers global warming and pollution [2, 5, 6]. The ATEG’s ability to generate electricity without moving parts is an advantage over mechanical electric generators or alternators [1].
2: Principles and Analysis of ATEG
As is illustrated in Figure 1a (i), when heat is absorbed on one side of an ATEG (red arrow) the movable charge carriers begin to diffuse, resulting in a uniform concentration distribution in the ATEG along the temperature gradient, and producing the difference in the electrical potential on both sides of the ATEG (Seebeck’s generator). To maximize the power generation output, p-bars and n-bars (see circles) are connected together in a cell. Due to the thermoelectric effect, electrons flow through the n-type element to the cold side while in the p-type elements, the positive charge carriers flow to the cold side. This illustrates how connecting the p-bar and the n-bar augments the voltage of each bar and the voltage of each unit cell.
WORKING OF TEG
When two different conductors are placed in contact, electrons flow from one to the other if the energy levels of the electrons are different in the two materials. The higher energy electrons cross the junction until the energy levels are the same on both sides. The thermoelectric module is made from two conductors whose energy levels change at different rates when the temperature changes. If the junctions are not at the same temperature, there are unequal differences in energy levels across the junctions. Thus, unequal numbers of electrons have to cross the junctions and unequal voltages are established. Since there is a net voltage around the loop, a current will flow.
1.3 LOCATION OF TEG IN AUTOMOTIVE ENGINES
The location of the thermoelectric generator is an important factor, decisive of its operability. The TEG generator can be installed on the exhaust pipe immediately between the collector and the catalytic converter or behind the catalytic converter. The heat is absorbed from the exhaust pipe and later converted into electricity by using TEG generator.
1.4 LOCATION OF TEG IN AUTOMOTIVE ENGINES
The testing of the temperature distribution was performed at different engine speeds: 2300 and 3300 rpm. The temperatures received at lower engine speed are higher, even though the difference of temperature of the exhaust gases measured for the both engine speeds in front of and behind the heat exchanger is at every engine load point higher for the 3300 rpm by 50°C on average. However, in the first case the coolant flow of 21 l/h was used.
1.4.1 Low temperature heat sources
Two of the main issues with respect to a sustainable energy supply system are the more efficient usage of energy at all stages along the energy supply chain and the intended use of renewable resources (e.g. geothermal energy). Despite this, we have been wasting enormous amounts of heat from various sources, such as factories, transportation systems and even private houses or public buildings. The waste heat is difficult to use due to its nature of low temperature and low energy density, although the total energy amount is very large. Especially the utilisation of heat which is at too low temperature to drive a turbo generator should be a major task for future energy conversion research and development.
1.4.2 Actual thermal to electrical energy conversion
So far today’s electrical energy production is mostly affected by generators based on electromagnetic induction. Reciprocating steam engines, internal combustion engines, and steam and gas turbines have been coupled with such generators in utilizing chemical heat sources such as oil, coal and natural gas and nuclear heat for the production of electrical energy. Renewable energy sources like geothermal energy, solar energy and biomass energy are also being added to the list of heat sources used in modern electric power plants. Furthermore, solar energy provides hydropower indirectly.
The steam-Rankine cycle is the principle exploited for producing electric power from high temperature fluid streams. Gas and steam cogeneration and combined heat and power technologies (CHP) help to improve the electrical and total efficiencies of modern power plants from 35% to about 60%.
For the conversion of low temperature heat (below 150°C) e.g. out of geothermal sources, into power, modifications of the steam-Rankine cycle like the Organic–Rankine cycle (ORC) [40] or the Kalina - process [40] are well known possibilities although also with quite limited potential and high costs. For making efficient use of the low temperature waste heat (< 80°C) generated by prime movers such as micro-turbines, internal combustion engines, fuel cells and other electricity and/or heat producing technologies, the energy content of the waste heat must be sufficient to operate equipment found in cogeneration and trigeneration power and energy systems such as absorption chillers, refrigeration applications, heat amplifiers, dehumidifiers, heat pumps for hot water, turbine inlet air cooling and other similar devices.
1.4.3 Direct Heat to Electricity Conversion
Efficient Direct Heat to Electricity Conversion (DHEC) has been sought for decades. To date work has focused on two primary directions as mentioned above. There are the thermionic converters, which are only capable of high power densities and efficiency only at temperature in excess of 1000°C, and thermoelectric converters, which operate at low temperature but suffer from low efficiency. The challenge for the future is to create a direct heat to electricity converter of high power and acceptable efficiency, but which is also capable of generating power over a relatively wide range of temperatures. The performance of heat engines and direct conversion devices is limited by the same laws of thermodynamics, and the real application environment therefore plays an important role in determining actual and acceptable performance.
Thermoelectric devices, on which this work is focused, allow the direct conversion of heat from sources like geothermal energy, solar energy or waste heat into electrical power. The main advantages are the low maintenance requirement, the high modularity and the possibility of utilising heat sources over a wide temperature range. Efficient solid state energy conversion based on the Seebeck effect calls for materials with high electrical conductivity σ, high Seebeck coefficient α and low thermal conductivity λ. These properties can be summarized in themaximum ZTs of about at most unity. This gives efficiencies of about 10T −Tto 15 % of the particular Carnot efficiency ηCarnot = HTH C containing the temperatures TH of the heat source and TC of the sink.
For commercial electricity generation it is necessary to seriously consider generation costs. Thermoelectrics suffer from low efficiency, however, when the heat source is nearly free of charge the low generating cost will offset the capital cost of the thermoelectric generator over its lifetime. To minimise the investment cost one would choose to keep the number of thermoelectric modules per kWe low and limit the amount of expensive material used by reducing the thermoelement leg length, which leads in general to higher power output, however, to lower efficiency
working
1.Peltier Effect: This effect introduce power to the module with a resultant
cooling of one side and heating of the other these type of modules are low
amp typically in the 6 amp range and are designed for low temperature exposure of NO MORE THAN 100°C to 110°C hot side. Higher temperature exposure will cause the module to either break apart or the soldered couples to melt from high heat making them poor choices for power generation!
2.Seebeck Effect: This effect is created by a temperature differential across the TEG module from heating one side (hot side) and cooling the other side ((cold side) heat removal side) by moving the heat flux away from the modules face cold side as fast as it moves through the module you will produce the most power. LIQUID IS THE ONLY TRUE method to do this all other forms of heat removal will lower overall TEG power generation.
Thermoelectric Generators using the Seebeck Effect work on a temperature differentials. The greater the differential (DT) of the hot side less the cold side, the greater the amount of power (Watts) will be produced. Two critical factors dictate power output :
1. The amount of heat flux(FLOW) that can successfully move through each TEG module.
2. The temperature of the hot side less the temperature of the cold side Delta Temperature (DT).
3. To understand how difficult it is to maintain a large DT go to What’s news page and read the blog pages. They will explain the best ways to get the most efficient designs, which will allow you to lower your cost per watt produced.
Great effort must be placed on the heat input design and especially the heat removal design (cold side). The better the TEG Generator construction is at moving heat from the hot side to the cold side and dissipating that heat as it moves thru the module array to the cold side the more power will be generated. Unlike solar PV which use large surfaces to generate power. Thermoelectric Seebeck effect modules are designed for very high power densities, on the order of 50 times greater than Solar PV!.
Thermoelectric Seebeck Generators using liquid on the cold side perform significantly better then any other method of cooling and produce significantly more net additional power than the pump consumes. As the system size increases so does the ability to produce a more efficient Thermoelectric Generator (TEG).
For any thermoelectric power generator (TEG), the voltage(V) generated by the TEG is directly proportional to the number of couples (N) and the temperature difference (Delta T) between the top and bottom sides of the TE generator and the Seebeck coefficients of the n and p- type materials. When you look at our TEG modules you will see a 126 or 70, 32, 25 in the part number. This is a reference to the amount of couples is series. The greater the couple count the greater the resultant voltage produced given everything else being equal.
The standard universal material we work with is BiTe. The best efficiency that can be achieved with this material is approximately 5%. But once the material is placed into a constructed module the efficiency drops to 3 to 4% depending on DT because of thermal and electrical impedance! Other material for different temperatures zone are also available. Such as CMO modules with temperatures up to 800°C . The standard BiTe hot side up to 320°C, Hybrid BiTe- PbTe up to 360°C, SnSe – PbSnTe up to 600°C, Calcium Manganese (CMO) up to 800°C, and CMO cascade with BiTe stacked up to 600°C . Soon we will be adding a new Cascade that works up to 750°C.
No other semiconductor material can perform as well as BiTe as far as efficiency is concerned at temperatures below 250°C.
Other material like PbTe are used but are far less efficient at lower temperatures, and must be used at significantly higher temperatures in the 400°C-600°C hot side range and CMO Calcium Manganese in the 450°C to 800°C to be efficient but are expensive to make and volume is low so cost is high!
Power output based on (DT) is very predictable and well documented, but access to this information is difficult to find. With power generation the thinner the length or thickness of the module the greater the amp output or rating.
Our low temperature modules (TEG2) are high amp modules with contacts that are soldered using AgTn solder on both sides. Although, the temperature of the solder has a 240°C melting point the solder begins to degrade at about 190-200°C . Therefore we recommend the hot side stay below 190C to allow for small temperature variations.
Our High Temperature Modules (TEG1 up to 320°C) use flame spraying high temperature metal Aluminum on the hot side and can withstand much higher temperatures in the range of 300°C hot side and have considerably larger tolerances when it comes to incidental higher temperature over shoots. So, much so that you can expose the hot side to 320°C intermittently with very little module degradation. This technique is much more expensive to implement and therefore the cost is reflected .
Temperature of the hot side is probably the most critical component when considering Thermoelectric Generators. (DT) Delta T needs to be in the 100°C range to get a viable power output from each module.
The electric load of a vehicle is increasing due to improvements in driving performance and comfort .In order to satisfy the increasing demands of electricity in modern vehicles, bigger and heavy alternators are coupled to engines. These alternators of vehicle. [6]
One potential solution is the usage of the exhaust waste heat of combustion engines. This is possible by the waste heat recovery using thermoelectric generator. A thermoelectric generator converts the temperature gradient into useful voltage that can used for providing power for auxiliary systems such as air conditioner and minor car electronics.
TEG MATERIAL SELECTION
The driving principle behind thermoelectric generation is the known as the Seebeck effect. Whenever a temperature gradient is applied to a thermoelectric material, specifically metals or semiconductors, the heat passing through is conducted by the same particles that carry charge. The movement of charge produces a voltage. The junctions of the different conductors are kept at different temperatures which cause an open circuit electromotive.
Lead Acid Batteries Work
Lead Acid batteries have changed little since the 1880's although improvements in materials and manufacturing methods continue to bring improvements in energy density, life and reliability. All lead acid batteries consist of flat lead plates immersed in a pool of electrolyte. Regular water addition is required for most types of lead acid batteries although low-maintenance types come with excess electrolyte calculated to compensate for water loss during a normal lifetime.
Battery Construction
Lead acid batteries used in the RV and Marine Industries usually consist of two 6-volt batteries in series, or a single 12-volt battery. These batteries are constructed of several single cells connected in series each cell produces approximately 2.1 volts. A six-volt battery has three single cells, which when fully charged produce an output voltage of 6.3 volts. A twelve-volt battery has six single cells in series producing a fully charged output voltage of 12.6 volts.
In order for lead acid cell to produce a voltage, it must first receive a (forming) charge voltage of at least 2.1-volts/cell from a charger. Lead acid batteries do not generate voltage on their own; they only store a charge from another source. This is the reason lead acid batteries are called storage batteries, because they only store a charge. The size of the battery plates and amount of electrolyte determines the amount of charge lead acid batteries can store. The size of this storage capacity is described as the amp hour (AH) rating of a battery. A typical 12-volt battery used in a RV or marine craft has a rating 125 AH, which means it can supply 10 amps of current for 12.5 hours or 20-amps of current for a period of 6.25 hours. Lead acid batteries can be connected in parallel to increase the total AH capacity.
Lead Acid Battery Recharge Cycle
The most important thing to understand about recharging lead acid batteries is that a converter/charger with a single fixed output voltage will not properly recharge or maintain your battery.
Proper recharging and maintenance requires an intelligent charging system that can vary the charging voltage based on the state of charge and use of your RV or Marine battery. Progressive Dynamics has developed intelligent charging systems that solve battery problems and reduce battery maintenance.
The discharged battery shown in figure # 6 on the next page is connected to a converter/charger with its output voltage set at 13.6-volts. In order to recharge a 12-volt lead acid battery with a fully charged terminal voltage of 12.6-volts, the charger voltage must be set at a higher voltage. Most converter/chargers on the market are set at approximately 13.6-volts. During the battery recharge cycle lead sulfate (sulfation) begins to reconvert to lead and sulfuric acid.
During the recharging process as electricity flows through the water portion of the electrolyte and water, (H2O) is converted into its original elements, hydrogen and oxygen. These gasses are very flammable and the reason your RV or Marine batteries must be vented outside. Gassing causes water loss and therefore lead acid batteries need to have water added periodically. Sealed lead acid batteries contain most of these gasses allowing them to recombine into the electrolyte. If the battery is overcharged pressure from these gasses will cause relief caps to open and vent, resulting in some water loss. Most sealed batteries have extra electrolyte added during the manufacturing process to compensate for some water loss.