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ABSTRACT
In recent years, due to environmental difficulties like emissions, global warming, etc., are the limiting factor for the energy resources which resulting in massive research and novel technologies are required to generate electric power. Thermoelectric power generators have emerged as a promising another green technology due to their diverse advantage. The TEGs directly convert the thermal energy into the electrical energy. The thermoelectric (TE) technology is used in both the air–conditioning and the electrical energy generation. Also, the TE technology is eco–friendly as it has no greenhouse gas emissions, tough due to the absence of moving parts and is silent. However, the conversion efficiency of the thermoelectric modules (TEMs) used economically is less than about 10%. In this paper the structures of the TEGs have been reported.
INTRODUCTION
The electrical power used in automobiles is generated using part of the energy converted into a driving force with an alternator. The main trouble of this energy transformation is that only part of the energy flow supplied by the fuel is converted into brake power output and although the efficiency of the alternator is high, the ratio between the electric energy produced and the fuel depleted is very low. The efficiency of a modern internal combustion engine oscillates from 37 percentage in a normal passenger car spark ignition engine to more than 50 percentage in a low speed marine diesel engine. The energy dissipated is lost by transmission to the environment through exhaust gas, lubrication oil, cooling water and radiation. In a gasoline engine, about 30 percentage of the primary gasoline energy is released as waste heat in the exhaust gases. Furthermore the electric load of a vehicle is increasing due to comfort, driving performance and power transmission. In line with this tendency, the alternator size, load of engine power and engine weight is becoming larger. However, the engine room is becoming smaller in order to improve the aerodynamic characteristic and extend the passenger room. For this reason, the space for the alternator cannot be freely increased. If approximately 6 percentage of the exhaust heat could be converted into electrical power, more or less the same quantity of driving energy that demands the production of electrical power would be released and then, it would be possible to reduce the fuel consumption around 10 percentage. This is the reason why thermoelectric generators (TEGs) can be profitable in the automobile industry.
LITERATURE REVIEW
Birkholz, U. et al, in collaboration with Porsche proposed a structure with rectangular cross section. This thermoelectric generator was designed to be adjusted to the exhaust pipe of a 944 engine with a length of 500 mm and with a total maximum cross section of 300 x 300 mm2made of Hastelloy X (Ni 47, Cr 22, Fe 18, and Mo 9). The free section 0, 23 dm2 decreasing of the engine power. Some fin heat sinks were included in the interior part of the channel to improve the heat transmission through the thermoelectric pairs.
Serksnis, A. W. In proposed a design which used the proper exhaust shape of the exhaust pipe. The thermoelectric generator would have a length of 460 mm and an internal diameter of 76 mm. The material used was stainless steel. No system was included to improve the heat transmission from the exhaust gases to the support structure.
Takanose, E. and Tamakoshi, H. proposed in a different structure where the TE modules were mounted inside cylindrical structure with a diameter of 190 mm and a height of 180 mm. The total weight of the generator was 5.8 kg and 10 kg including accessories. Different diffusers were installed and tested inside this thermoelectric generator to improve the heat conduction creating turbulence in the exhaust gases. Experimental results show that a diffuser injecting the exhaust gas to inner wall of the TE modules performed better than systems which diffused exhaust gas spirally.
Yu and Chau has proposed and implemented an automotive thermo-electric waste heat recovery system by adopting a Cuk converter and a maximum power point tracker (MPPT) controller into its proposed system as tools for power conditioning and transfer. They reported that the power improvement is recorded from 7.5% to 9.4% when the hot-side temperature of the TEG is heated from1000 hot-side temperature of the TEG is fixed at 2500 improvement as much as 4.8% to 17.9% can be achieved. Conductor carrying a current generates heat at a rate proportional to the product of the resistance ® of the conductor and the square of the current (I). A circuit of this type is called a thermocouple; a number of thermocouples connected in series are called a thermopile.
Numerous cooling techniques are available for cooling electrical components; the selection of a cooling technique depends on various factors such as the shape and physical size of the component being cooled. Upon the review of the common cooling techniques, single-phase forced convection of liquids was the most appropriate cooling technique for designing the cold side heat exchanger prototypes for the TEG POWER project. Two cooling strategies were selected for designing the cold side heat exchanger prototypes. The first heat exchanger prototype utilizes single-phase forced convection in straight minichannels, while the second heat exchanger prototype employs submerged liquid jets impinging on a flat-plate.
Harms et al. conducted experiments to study single-phase convective heat transfer in deep rectangular micro channels fabricated in a 25 mm × 25 mm silicon substrate. Two micro channel configurations were tested. The first configuration had a single micro channel 25 mm wide × 1 mm deep (?ℎ= 1.923 mm), the second configuration utilized 68 micro channels 0.251 mm wide and 1.030 mm deep (?ℎ= 0.404 mm). The channel Reynolds number ranged from 173 to 12900. Results indicated that the thermal resistance of the multiple micro channels configuration was approximately 45% lower compared to the single micro channel configuration. However, local Nusselt numbers for a single micro channel was higher than the local Nusselt numbers for multiple microchannels. At high Reynolds number, good agreement was found between the experimental results and the local Nusselt numbers obtained using conventional flow correlations but for lower Reynolds values the agreement deteriorated. This behaviour was attributed to the manifold design, which was also deemed responsible for the early transition to from laminar flow conditions which occurred at ? = 1500.
Qu and Mudawar conducted an experimental and numerical investigation to study single-phase pressure drop and heat transfer in a microchannel heat sink. Twenty one rectangular microchannels 0.231 mm wide × 0.731 mm deep (?ℎ = 0.351 mm) and 44.8 mm long were machined in a copper substrate. Experimental heat transfer and pressure drop results were in a good agreement with numerical results obtained using Navier-Stokes and energy equations. However, their experimental data were not compared to conventional heat transfer and pressure drop correlations. Early transition from laminar flow conditions inside the microchannels was not observed for the tested Reynolds numbers which were between 139 – 1692.
Lee et al. studied single-phase heat transfer in microchannels experimentally and numerically. Ten microchannels were machined on a 25.4mm × 24.4 mm × 70 mm copper substrate. The microchannels width ranged from 0.194 mm to 0.534 mm. The microchannel depth was nominally five times the microchannel width. Reynolds numbers were in the range of 300 – 3500 which allowed for either simultaneously developing, or hydro dynamically developed thermally developing flow conditions in the microchannels. Experimental results indicated that the transition from laminar to turbulent flow conditions occurred at Reynolds numbers of 1500 – 2000. Significant deviations were observed between the experimentally obtained Nusselt numbers and the Nusselt numbers calculated using convectional flow correlations. The authors proposed that the deviations observed were due to boundary and inlet conditions mismatch between the convectional correlations and those of the test section. However, numerical simulations based on the Navier-Stokes equations were found to be in good agreement with experimental results which seems to confirm the conclusions of Qu and Mudawar . The authors concluded that Navier-Stokes can predict the performance of microchannels if the effects of the boundary and inlet conditions are carefully accounted for.
Shen et al. conducted experiments to study single-phase convective heat transfer and fluid flow in micro channels. The heat exchanger was made from copper (50 mm × 15.3 mm) on which 26 rectangular micro channels 0.3 mm wide × 0.8 mm deep (?ℎ = 0.43 mm) were fabricated. The micro channel’s length was 50 mm long, and the relative roughness was between 4-6%. Tested Reynolds numbers varied from 152 to 1257 and were in the laminar regime. Heat transfer results showed that the local and average Nusselt numbers were lower than local and average Nusselt numbers predicted by conventional correlations. The authors proposed that the deviations in the heat transfer results are attributed to the cross sectional area and surface roughness effects on the ‘near wall velocity’. However, surface roughness does not typically affect the heat transfer in the laminar regime. In fact, if roughness did actually affect the heat transfer in laminar flow, it would tend to increase in heat transfer instead of decreasing it. The transition from laminar flow was not reported in this testing range.
Peng and Peterson experimentally studied fluid flow and heat transfer in twelve different micro channels configurations machined on stainless steel substrate with micro channel’s ?ℎ = 0.133 – 0.367 mm. Tested Reynolds numbers were between 50 – 4000. Results showed that a micro channels’ geometric configuration, such as aspect ratio and hydraulic diameter to centre to centre spacing, affected the convective heat transfer and fluid flow behaviour. The authors proposed correlations for predicting Nusselt number for micro channels in the laminar the turbulent flow regimes.
Pfund et al. experimentally measured the measured the friction factors for water flowing in smooth and rough high aspect ratio rectangular micro channels with depth from 0.128 mm to 521 mm. Tested Reynolds number were in the range of 60 – 3450. Results indicated that the transition from laminar flow conditions occurred at Reynolds number of 2500, which is about the same for conventional size tubes which is 2300 [51]. Moreover, it was found that laminar friction factors were higher than what is predicted by conventional theory especially for the rough micro channels.
Jiang et al. Experimentally measured the friction factors for a micro channel heat exchanger. The channels were rectangular and had a hydraulic diameter of 0.300 mm, the relative micro channel roughness was 0.1%. The experimentally measured friction factors were larger than the values predicted by conventional theory. The discrepancy was attributed to the effects of the hydrodynamic entrance region present in the test section.
Shengqiang Bai et al. investigated six different heat exchanger (empty cavity, inclined plate, parallel plate, separate plate with holes, serial plate and pipe structure).The result shows serial plate structure forced the exhaust to flow back enhanced the heat transfer with the shell wall and had the maximum heat transfer rate of all the structures. The serial plate also had a maximum pressure drop 200% and 115% more than the parallel and separate plates, respectively. Under the maximum power output condition, only the inclined plate and the empty cavity structure had less pressure drops. But low heat transfer rate.
W.S.Wang et al. investigated three different heat exchanger(fishbone-shaped,acordion-shaped, scatter-shaped)made in brass and iron Modules on the brass exchanger have higher output power than those on the iron heat exchanger. The results show accordion shape in brass relatively high surface temperature and uniform temperature distribution to improve the efficiency of the TEG. It is obvious that the thermal performance of the brass is much better than that of the iron one.
Dongyi Zhou investigated cylindrical thermoelectric generator with fins and without fins. Fins in the exhaust channel are 15 mm high and 3-7 mm thick. It shows when the flow velocity is fixed to 60 m/s, there is a lesser pressure drop in the smooth exhaust channel(200 pa) than exhaust channel with fin(500 pa).Fins can enhance the heat transfer efficiency. When the exhaust moves at a low speed (40 m/s), the efficiency is enhanced greatly. When the exhaust moves at a high speed (60 m/s), the efficiency is enhanced slightly. Number of fins can enhance the heat transfer efficiency, but pressure drop of exhaust increases as well.
Jorge Vazquez et al. Studied on several main characteristics of the different structures proposed. It includes Birkholz, U. et al proposed a structure with rectangular cross section length 500mm and with a total maximum cross section of 300*300 mm^2.it has rectangular four fins on either side. Nissan research centre developed TEG for different temperature ranges shape similar to Birkholz. Heat exchanging fins with different area ratios along the length of exhaust flow. Result shows Nissan TEG has high efficiency. It indicates efficiency depends on fins size.