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Waste heat which has been of some concern for its environmental impact is today a valuable resource, on an average for every unit of chemical energy converted to electrical or mechanical energy, two units are rejected to the environment. In the recent years research for advanced technologies in high efficient engines is increasing. As the energy demands worldwide are increasing at a rapid rate, the oil resources are depleting day by day. With rapid increase in demand for energy, research is in progress to identify an alternative source, at the same time present day equipment's are being developed to give maximum output to conserve resources till an alternative is developed.
Internal combustion SI engines thermal efficiency ranges from 27% to 29%, energy lost in coolant and lubricant oil is about 30% , about 30%-40% of heat is lost in exhaust and remaining unaccounted (radiation and frictional losses). The potential of this waste heat is phenomenal, and can be utilized to produce some useful energy improving the overall efficiency of a given engine.
Various ways of extracting thermal energy from vehicle exhaust:
Heat is rejected through exhaust gases at high temperature when compared to heat rejected through coolant and lubricating oil. To recover the waste heat, various methods are being adopted. These include Organic Rankine cycle (ORC), Turbo compound technology and Thermoelectric generators (TEG).
Organic Rankine cycle (ORC):
Rankine cycle is a closed loop power cycle, where heat transfer to the working fluid is converted to power. The working cycle includes heating (isentropic), boiling (isobaric), superheating (isobaric), expanding (isentropic), condensation (isobaric) and compression (isentropic). The working fluid absorbs heat from the exhaust and vaporizes. The vaporized fluid is made is made to expand over a turbine to derive power. The vapor is then condensed to liquid form in the condenser using an external fluid. The condensed liquid is further compressed to increase pressure and cycle is repeated.
Working fluid has a major impact on the efficiency of the cycle, suitable working fluid for the Rankine cycle depends on operating temperature and latent heat of evaporation for the fluid is a good indicator of choosing the working fluid. Organic fluids have highest efficiency, but because of the flammability limit, use of organic fluids is limited.
compounding uses turbine to recover energy, from the exhaust system. There are 2 forms; mechanicala turbo compounding and electrical turbo compounding. In mechanical turbo compounding, the energy recovered from exhaust gases is converted to kinetic energy ans fed back to the engine. In electrical turbo compounding heat is converted into electrical energy.The turbo charger must be highly efficient to prevent excessive back pressure in nthe exhaust system.
The turbo compounding is a very heavy and occupies a lot of space, it also consists of a lot of moving components there by friction losses. The turbine requires continious cooling and lubrication. Turbine increases back pressure affecting the performance of the engine, at lower engine speed the pressure of the exhaust gases is low to run the turbine. Turbine operates efficiently only above certain engine speed .
Thermo Electric Generators (TEG):
TEG is a heat generator which converts heat energy to electrical energy, it works on the principle of Seebek effect. TEG are highly reliable operates quietly and environmentally friendly. TEG uses semiconductor materials in conjunction with copper inter-connecting pads, which offers one of the best Seebek coefficients, electrical resistivity and thermal conductivity. The thermo electrical model has a cold side and a hot side, at the hot side heat is absorbed by the electrons and pass from high energy level (n type semiconductor) to a low energy level (p type semiconductor). At the cold side heat is rejected to a cold sink to maintain a temperature gradient. A thermo electric material with high Seebek coefficient is used to create a temperature difference across the faces of the model. A single thermocouple produces low voltage, thereby to obtain higher voltage, a number of thermocouples are connected electrically in series and thermally in parallel. The model is heated at one side and temperature gradient is maintained by cooling the other side.
Selection of Model:
TEG in a vehicle is a low maintenance device for power generation. The thermoelectric generator will be located in the exhaust system and will make use of the temperature difference between hot exhaust gases and the low temperature on the other side (sink), If energy from the exhaust can be tapped and converted to a useful form the overall efficiency of the system can be improved.
TEG has been identified suitable for application on SI engines for power generation. TEG has many advantages compared to other methods. No moving parts, little vibrations, east to maintain, environmentally friendly. Heat energy from exhaust is directly converted to electricity, which can be stored and utilized easily.
TEG unit Packaging:
The TEG unit is a bolt on, and easy to install to an existing exhaust system with small modifications. The TEG has very little effect on the working on the exhaust system. The TEG unit is of similar size as the catalytic converter and weighs about 55 kgs, it is installed before the catalytic converter and right after the exhaust manifold to extract heat and still maintain the temperature of the exhaust gases high enough for the catalytic converter to work efficiently. The power produced from the TEG unit is stored in a battery placed under the front hood. Coolant is supplied to the cold side of the TEG to maintain a temperature gradient in the system, a separate pump and coolant pipes is required to circulate the coolant to the unit.
A valve controls the flow of the exhaust gases for different operating conditions.
Material selection &Efficiency of the system:
Thermo-electric materials fall into two categories, n-type and p-type whose behavior differs in the form of the electrical conduction. By combining two TE materials with two junctions across a temperature difference a potential difference is developed. The efficiency of the system depends on the semiconductor material properties, a semiconductor material with highest ZT is chosen in the working temperature range.
The TEG would work at an average temperature of 800K-1000k so Si1-xGex is used as the semiconductor for the TEG and the average ZT is assumed to be about 0.8.
A thermo electric generator converts heat energy to electrical energy with efficiency (Î·).
P = Î·Ã-Q
Power (P), Efficiency (Î·) and heat (Q)
The amount of heat that can be utilized by the thermo electric material depends on the size of the exchanger.
The efficiency of the TEG depends on the temperature difference (Î”T = Th - Tc) and properties of the thermoelectric material (ZT). The efficiency of the TEG cannot be greater than the carnot cycle efficiency (Î”T/ Th).
The efficiency is given as:
Temperature on the hot side (Th), Temperature on the cold side (Tc)
zT = Î±2T
Seebeck coefficient (Î±), electrical resistivity (Ï), and thermal conductivity (Îº) and temperature dependent materials properties (T)
Design of the system:
A plate module type module is used with number of previously designed thermo-electric couple, connected in series electrically via thin copper strips. Thin layer of electrically insulated surface such as ceramic is applied to each face to reduce conduction. A thin copper layer I used on the hot side (source)
And aluminum is used on the thin side (sink). Direct contact between the 2 metals must be avoided to reduce the heat transfer between the hot and cold side metal, ceramic material is used to avoid contact.
The cold side is circulated with the coolant to maintain the temperature difference and to carry away excessive heat.
A separate pump and coolant lines are required for the circulation of the coolant on the cold side.
A control valve is used to bypass the exhaust gases from the TEG unit at low exhaust temperatures, the exhaust temperature is maintained high enough for the catalytic converter to work efficiently.
Thermo electric materials have been considered too inefficient to be cost effective in automotive applications. The recent interest in thermoelectric material, with theoretic predictions suggest that thermoelectric efficiency could be significantly increased through Nano structure engineering (thin film technology) which will lead to cost reduction in bulk production of thermoelectric material. The recent developments in high temperature thermoelectric materials have shown the increased application of thermoelectric generators in automotive applications.
It is possible to replace passenger car alternators with TEGs, but the comparatively low mileage compared to buses makes current manufacturing costs uneconomical. To bring the payback within 10 years, the modules used would have to have 1/10th of the current unit cost with current TE materials. Increased TE figure-of-merits would further reduce the payback period in passenger car application. With the expected development of new technologies typified by quantum well, wire and dot construction modules this goal looks reachable.