IC ENGINE
An engine is a device which transforms chemical energy or heat energy into mechanical energy. However, while transforming energy from one form into another, efficiency of conversion plays an important role. IC engine transforms low grade energy into high grade energy; therefore, thermal efficiency of IC engine comes out to be in the range of 30-35 %. Large amount of heat energy goes out with unutilized i.e. the exhaust gases. The detail of heat energy utilization in an IC engine can be well understand by knowing its heat balance sheet, as has been described in the following section.
HEAT BALANCE SHEET OF AN IC ENGINE
Heat balance sheet signifies the total amount of heat energy supplied and heat energy utilized in various ways in an IC engine. Necessary information related to performance of engine is obtained from the heat balance sheet. The total heat energy supplied (Q_s) to an engine can be calculated from calorific value of the fuel as given below
Q_s= m ̇_f*C.V (kW) (1.1)
where m ̇_f the mass flow rate of fuel supplied (kg/s) , C.V is calorific value of the fuel (kJ/kg).
The various ways by which heat is utilized in the engine is given by
Heat equivalent of brake power.
Heat carried away by cooling water.
Heat carried away by exhaust gases.
Some part of heat energy loss remains unaccounted. It includes the heat loss by convection/ radiation, heat energy utilized to run the various accessories like lubricating pump, cam shaft and water circulating pump. Percentage wise heat energy utilization in an IC engine has been shown in the Fig. 1.1.
Fig. 1.1: Heat energy utilization in an IC engine.
The exhaust gas from an IC engine accounts approximately 40℅ of the heat of combustion. Rather than directly improving the efficiency of the engine, efforts can be made to improve the efficiency of the engine indirectly by utilizing exhaust gas heat of an IC engine. There are different ways to recover waste exhaust gas heat of an IC engine, as has been listed below.
Thermoelectric technology
Thermal heat storage
Desalination
The present thesis work is concerned with the use of thermoelectric technology; detail of the same has been discussed in the following section.
1.3 THERMOELECTRIC TECHNOLOGY
Thermoelectric Technology involves the application of thermoelectric generator, converting thermal energy i.e. heat energy into electrical energy. Thermoelectric technology is regarded as environmental friendly technology for recovering waste heat and directly converting the same into electricity. Further, electric energy can be directly used or stored in a battery.
1.3.1 WORKING PRINCIPLE OF THERMOELECTIC GENERATOR
Thermoelectric effect was first discovered in 1822 by Seebeck, who observed that voltage could be produced by a circuit made from two different conductors, when one of the junction was heated while another junction was kept cold. Fig.1.2 shows the arrangement, which may be used to have the Seebeck effect.
Fig. 1.2: Seebeck effect.
The relationship between voltage and temperature difference between hot and cold junction can be expressed;
V=α(T_h-T_c) (1.2)
where α is Seebeck coefficient, which is thermoelectric material property and T_h and T_c are temperature of hot and cold junction respectively. Efficiency of thermoelectric materials is expressed as the figure of merit Z, which is often expressed in its dimensionless form ZT, where T is absolute temperature. The relationship for figure of merit (Z) of the thermoelectric material has been expressed as;
Z=(α^2 σ)/λ= α^(2 )/ρλ (1.3)
where λ ,σ and ρ represent thermal conductivity, electrical conductivity and electrical resistivity respectively. The figure of merit is a key consideration for comparing efficiency of thermoelectric materials. Eq. (1.3) shows that the figure of merit is affected directly by the electrical conductivity and Seebeck coefficient. Fig 1.3 shows the relation between figure of merit and hot surface temperature for p-type and n-type junctions.
Fig. 1.3: Relation between figure of merit with hot surface temp
1.3.2 THERMOELECTRIC MATERIALS
The thermoelectric module is clamped between the hot side heat exchanger and cold side heat sink, which absorbs heat at high temperature and rejects heat at lower temperature in environment while generating electricity. Higher the temperature difference between the hot side and cold side results in higher efficiency and power output. Over the last few decades, bismuth-telluride (Bi2Te3), lead-telluride (PbTe) and silicon-germanium (SiGe) alloy based thermoelectric generators have been extensively studied. Thermoelectric material should possess: large Seebeck coefficient, high electrical conductivity and low thermal conductivity.
1.3.3 CONSTRUCTION AND WORKING OF THERMOELECTRIC GENERATOR
A thermoelectric generator consists number of alternate p and n type semiconductor elements, which are connected in series and sandwiched between ceramic substrates. The construction of a commonly used thermoelectric generator is shown in Fig.1.4. Generally, thermoelectric generator consists of the following components;
Regular matrix of thermoelectric elements known as pellets. Usually, these pellets are made of semiconductor material such as bismuth telluride, antimony telluride, lead telluride etc. are used. Semiconductors are the best among the known materials due to optimal thermoelectric performance.
Ceramic plates provide mechanical integrity of a thermoelectric module. The plates must have good thermal conductance in order to provide heat transfer with minimal resistance. The aluminium oxide (Al2O3) based ceramic plates are most widely used due to optimal cost.
Electric conductors provide electric contacting of pellets in series with each other and contacts to leading wires.
Solders provide assembling of thermoelectric modules. The solder must provide good assembling of the thermoelectric module. The melting point of a solder is the limiting factor of operation temperature of the module. For long life module, operation temperature must be lower than melting point of the solder.
Fig. 1.4: Construction of thermoelectric generator.
A thermoelectric generator consist two semiconductor (n-type and p-type) which is connected electrically in series and thermally in parallel. In n-type semiconductor, most charge carriers are negatively charged electrons, where as in P-type semiconductor most charge carriers are positively charged holes. When one end of thermoelectric generator subjected to heat energy, then electrons and holes present in semiconductors start to move to cold end. Movement of electrons and holes are responsible for generating electrical energy .The net output electrical energy is the sum of voltage generated by electron and holes both and drives electrical current through electrical load.