1. INTRODUCTION:

Seeback thermoelectric phenomenon is the conversion of heat into electricity with the advent of thermocouples. Where as a thermocouple is an assembly of two different materials, generally metals; joined at the two ends called junctions. When a temperature gradient is established at the two junctions there is the generation of thermo-emf due to the contact potential which depends on electron density. The general equation of thermoelectricity to explain the generation of thermo emf, is E = where α and β are the Seebeck constants in µV/⁰C and µV/⁰C² respectively and T is the temperature gradient (temperature difference between two junctions). Thermo-power, the rate of change of magnitude of thermo-emf w. r. t. the temperature gradient, is given as:.

Hence, it is clear that the thermo power increases with increase in temperature gradient because “α” and “β” are the constants for a given material. Finally, the equation of thermo-emf generation is generally taken as dE/dT = α ^[1] because β is very small as compared to α. Thermoelectric generation of electric power is also beneficial due to its pollution free nature, no moving parts and no complex designing. With such advantages it can play an important role to overcome the energy crisis and environmental degradation. This has always motivated the researchers for advancements of this field to look for increase in thermo-emf generation with classical or advanced thermoelectric materials as well as to study the effect of other operating parameters ^[2-4].

Waste heat is an integral part of all the industrial applications/processes in general and of combustion in particular. Advancements in the utilization of waste heat by thermoelectric materials are of great interest. With the improvements in the technologies: the efficiency, modeling, designing and selection of materials can vary but the wastage of energy (mostly in the form of heat) can’t be completely eliminated. It is not available only in the domestic areas like in the kitchens but also in the industries i.e., generators, electric motors, computers and in the furnaces also. Sometimes, waste heat in significant amount also originates from the data centers, rubbing processes, welding technologies and in the heating cooling systems. This waste heat can be utilized by converting it into electricity with the advent of thermocouples i.e. thermo-generators ^[5-9]. The power generated by thermoelectric techniques can be recycled or stored for the use of same devices. So, a thermo-generator making use of efficient and cost effective thermocouples is always sought to recover waste heat by converting it into useful thermo power. Along with the conversion aspects, the thermoelements are also much advantageous in cooling applications ^[10]. The prospective use of low cost and easily available classical thermocouple materials in thermo-generator is the basic approach of present research work with an aim to investigate the enhancement of thermo-emf generation. Thus, we have selected the classical thermoelectric materials (copper, iron, nichrome and constantan). The elemental characterization of these market available thermoelectric materials was also done in order to find out their composition for quality comparison. The nichrome material is an alloy of copper and Fe materials with a very low composition of nickel and chromium.

The effect of magnetic field on the performance of thermocouples has been reported ^[11-17] for its role in significant enhancement of the thermo-emf generation under different conditions and materials. Availability of waste heat can be accompanied by the presence of magnetic field or can also be applied from outside. Presently, measurements of the change in Seebeck voltage were carried out with full length thermocouple in the magnetic field for parallel and anti-parallel modes and applying magnetic field at the centre of thermocouple in perpendicular modes with temperature. Magnetic field strength dependence was investigated for its three lower values of 260, 360 and 460 Gauss. Being an energy dependent parameter, the magnetic field strength was selected in its lower range so that the ratio of energy produced to energy used should remain greater than one. It was found that the generation of thermo-emf can be enhanced considerably for an optimum value of applied magnetic field which highlights towards better efficiency of thermo-emf generation from waste heat with cheap and easily available thermocouples under the effect of applied magnetic field.

2. EXPERIMENTAL:

2.1 Measurement of Physical Parameter

The physical parameters like electrical conductivity of thermocouple wires and thermo-emf generation were measured with the help of a standard digital multimeter (make HP 34401A) with an accuracy of six decimal places. The measured physical parameters of different wires used to make thermocouples are given in Table 1.

2.2 Characterization of Thermoelectric Materials

The thermoelectric materials selected in present investigations were characterized to find out their composition for the sake of performance comparison with the other versions of these thermoelectric materials available in the market. The characterization was carried out using the XRF technique at the Tata Institute of Fundamental Research (TIFR) Bombay (India). The characterization graphs are given in Fig. 1

Fig. 1: Characterization of all thermoelectric materials, 1(a) Copper, 1(b) Nichrome, 1(c) Iron and 1(d) Constantan.

2.3 Experimental Set-Up

The temperature gradient is established at the two junctions of the thermocouples by the heating and cooling arrangements. The generated thermo-emf measurements in the temperature range of 30⁰C to 350⁰C were made with digital multimeter HP34401A. The electromagnets were used to provide the required magnetic field strength. The magnetic field in two orientations i.e. parallel and perpendicular is applied on the each thermocouple by electromagnets. The heating and cooling arrangements are same as of the normal mode and the magnetic field is maximum at the centre of the thermocouple for the perpendicular mode; whereas for the parallel mode the strength of magnetic field is minimum at the centre of thermocouple. The distance between two poles of the electromagnet is only 8Cm in the perpendicular orientation but in the parallel orientation this distance increases to 50Cm. This variation of strength of magnetic field with the length of thermocouple is given in Fig. 2.

Fig. 2: Variation of Strength of magnetic field with the length of thermocouple, 2(a) Perpendicular Mode, 2(b) Parallel Mode.

3. RESULT AND DISCUSSION:

3.1 Normal Mode and Parallel Magnetic Field Mode

The graphical comparison of thermo-emf generation as a function of temperature gradient for all selected classical thermocouples in various modes is shown in Fig. 3. Figure 3a shows the results in the normal mode (i.e., without any applied magnetic field). From Fig. 3a, it is very clear that Fe-constantan thermocouple generates maximum thermo-emf whereas Cu-Fe generates minimum. The values of generated thermo-emf at the maximum temperature gradient of 330⁰C are 1.8mV and 0.1mV respectively for these thermocouples. With parallel mode of applied magnetic field, thermo-emf generation in comparison to normal mode for all the thermocouples with temperature gradient not only enhances but also shows more generation stability. Figure 3b, c and d show the thermo-emf generation with temperature gradient for the applied magnetic field strengths of 260, 360 and 460 Gauss in parallel mode. Thermo-emf generation is more stable as compared to that in normal mode and it increases linearly with increase in temperature difference. The maximum values of thermo-emf generated with 260, 360 and 460 Gauss applied magnetic field strength in parallel mode are 2.3mV, 4.2mV and 2.7mV respectively at the maximum temperature difference. From Fig. 3b,c and d, it is very clear that Fe-constantan and constantan-nichrome thermocouples turn up to be better thermoelectric materials. Besides, it is also found that thermo-emf generation under similar conditions for same thermocouples for parallel mode is a function of magnetic field strength and a value of 360 Gauss magnetic field strength gives the best thermo-emf generation results as compared to 260 and 460 Gauss.

Fig.3: Thermo emf generation with the variation of temperature gradient: 3(a) Normal Mode, 3(b) Parallel Magnetic Field Mode of 260Gauss, 3(c) Parallel Magnetic Field Mode of 360Gauss, 3(d) Parallel Magnetic Field Mode of 460Gauss.

3.2 Perpendicular Mode

Figure 4a, b and c give the graphical representation of thermo-emf generation as a function of temperature gradient for all selected classical thermocouples in perpendicular mode of applied magnetic field. The perpendicular mode was found to generate higher thermo-emf under same temperature gradient and applied magnetic field strength with more stability as compared to even parallel mode of applied magnetic field. In this mode for a magnetic field strength of 260 Gauss, constantan-nichrome thermocouple generates about 3.4mV thermo-emf at the maximum temperature difference of 330⁰C. Whereas, for 360 Gauss and 460 Gauss the maximum thermo-emf generated at the maximum temperature difference was 3.7mv for nichrome-constantan and 10.2mV for Cu-Fe thermocouple respectively.

Fig.4: Thermo emf generation with the variation of temperature gradient: 4(a) Perpendicular Magnetic Field of 260Gauss, 4(b) Perpendicular Magnetic Field of 360Gauss, 4(c) Perpendicular Magnetic Field Mode of 460Gauss.

Investigations indicate that in perpendicular mode, higher magnetic field strength gives better results in terms of thermo-emf generation. But generation of higher magnetic field will again need more energy; therefore, its optimum value has to be chosen. The lower value of magnetic field i.e. 260 Gauss is not that effective either in parallel or in the perpendicular orientations, however some thermocouples do generate slightly more thermo-emf at some temperature differences for this value of magnetic field also as compared to the normal mode. A comparison of thermo-emf generation at the minimum and maximum temperature difference in the normal, parallel and perpendicular modes of applied magnetic field for all presently investigated classical thermocouples are given in Table 2 for their suitability as thermo-generator elements for waste heat utilization.

Table 1: Experimental parameters of the selected thermoelectric materials

S. No.	Parameter	Copper	Iron	Constantan	Nichrome
1.	Resistance (Ohm)	0.1918	0.7062	0.5174	1.6874
2.	Area of Cross-Section (m²)	1.51x10^-6	9.5x10^-7	1.112x10^-6	9.7x10^-7
3.	Length (m)	48x10^-2	48x10^-2	48x10^-2	48x10^-2
4.	Resistivity ρ ( Ohm-m)	6x10^-6	1.4x10^-6	1.2x10^-6	3.41x10^-6
5.	Electrical Conductivity σ (Sm^-1)	1.67x10⁶	7.143x10⁵	8.33x10⁵	2.933x10⁵

Table 2: A comparison of thermo-emf generation with temperature gradient and magnetic field

Sr. No.	Mode	Temp. Diff (⁰C )	Cu-Fe (mV)	Fe-constantan (mV)	Cu-nichrome (mV)	nichrome-constantan (mV)	Fe-nichrome (mV)
1.	Normal	30	0.0076	0.1819	0.0046	0.0191	0.0423
1.	Normal	350	0.1201	1.7906	0.11305	0.6349	0.2591
2.	Perpendicular (460 Gauss)	30	4.3293	1.5839	0.0112	0.4139	0.1023
2.	Perpendicular (460 Gauss)	350	6.9161	8.9185	0.2791	3.6865	0.5567
3.	Parallel (360 Gauss)	30	0.1651	0.1975	0.0309	0.137	0.0058
3.	Parallel (360 Gauss)	350	0.6935	4.2771	0.4867	2.4798	0.2742

4. CONCLUSION:

It was found that the generation of thermo-emf not only enhances considerably with increasing temperature gradient under the applied magnetic field but makes the generation a more stable process which highlights towards better efficiency of thermo-emf generation from waste heat with cheap and easily available thermocouples under the effect of applied magnetic field. The paper concludes that the thermo-emf generation enhanced in both the parallel and perpendicular modes of applied magnetic field than the normal mode and higher the value of applied magnetic field is better especially in perpendicular mode where as in parallel mode there is an optimum value of magnetic field. From the present experimental investigations, Fe-constantan and nichrome-constantan thermocouples emerged as better choices as thermo-elements of a thermo-generator to convert waste heat into electricity under all the modes of applied magnetic field and other operating parameters in the high temperature range.

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Received on 16.06.2011 Accepted on 30.09.2011

Research J. Engineering and Tech. 2(4):Oct.-Dec. 2011 page207-212