Review of Related Literatures and Studies This chapter mainly focuses on the related concepts, principles and studies about the operation of converting waste thermal energy into a usable electric signal. Information was cited in various sources and references including books, newspapers, magazines, journals and internet. Foreign Literature The basis of the studies about thermocouples was first established by Thomas Johann Seebeck in 1821 when he discovered that a conductor generates a voltage when subjected to a temperature gradient (Cavicchi, Thomas, 1993).
A circuit made from two dissimilar metals with junctions exposed at different temperature was able to deflect a compass magnet. Due to that phenomenon, he first concluded that the outcome was due to magnetism induced by the temperature difference. However, after further studies, it has been proven that it was the current which made the compass be deflected with respect to the change in temperature (Cook, N. 1997) According to Robert G Seippel in his book Fundamentals of Electricity (1974), this phenomenon was in line with the Law of Conservation of Mass created by a French chemist Antoine Laurent Lavoiser.
According to him, “Energy cannot be created nor destroyed; it can only be transferred from one form to another”. In relation with this, waste heat can also be converted to a usable microvoltage output through the principle of thermocoupling. To measure this voltage, one must use a second conductor material which generates a different voltage under the same temperature level. Otherwise, if the same material was used for the measurement, the voltage generated by the two materials can then be measured and related to the corresponding temperature gradient.
It is thus clear that, based on Seebeck’s principle; thermocouples can only measure temperature differences and need a known reference temperature to yield the absolute readings. (Cook, 1997) Grolier Encyclopedia of Knowledge (1995) explained the three major effects involved in a thermocouple circuit namely Seebeck, Peltier and Thomson Effect. If a homogenous material having charges has temperature T1 at one end and T2 at the other end while it is in an open circuit, then a difference in electric voltage will occur between the two ends. This voltage is directly roportional to the temperature difference (T1 – T2). If the material is homogenous, the voltage will depend only on T1 and T2 and will be independent on the detailed temperature conditions between the two ends. This existence of a voltage difference was called Seebeck Effect and was first reported to the Prussian Academy of Sciences by Thomas Seebeck in 1822. Unlike the Seebeck effect, which occurs in a single material in the presence of a temperature difference without an electric current, the Peltier effect only occurs at the junction of two dissimilar materials when electric current flows.
Heat, called the Peltier heat, is either emitted or absorbed at the junction, depending on the direction of current flow. This effect was discovered by the French Physicist Jean C. A. Peltier in 1834. Meanwhile, in 1854, William Thomson used thermodynamic arguments to relate the Peltier and Seebeck effects. In the process he predicted a third effect – namely, that an electric current flowing through a homogenous material that also has a temperature difference will cause the emission or absorption of heat in the body of the material.
The direction of the electric current relative to the sense of the temperature difference (that is, flowing toward a higher or lower temperature) determines whether heat is emitted or absorbed. This effect was subsequently discovered and called the Thomson effect. ThomasNet. com published last February 8, 2012 that because different combinations of metals will produce different temperatures, and these different metals have different durability and strength levels, researchers have produced standardized combinations to exploit maximum outcome potential in a standardized set of combinations.
The following table explains the difference on each combination and configuration of a thermocouple. Thermocouple Type| Composition| Temperature Range| B| Platinum 30% Rhodium (+)| 2500-3100 degrees F| | Platinum 6% Rhodium (-)| 1370-1700 degrees C| C| W5Re Tungsten 5% Rhenium (+)| 3000-4200 degrees F| | W26Re Tungsten 26% Rhenium (-)| 1650-2315 degrees C| E| Chromel (+)| 200-1650 degrees F| | Constantan (-)| 95-900 degrees C| J| Iron (+)| 200-1400 degrees F| | Constantan (-)| 95-760 degrees C|
K| Chromel (+)| 200-2300 degrees F| | Alumel (-)| 95-1260 degrees C| M| Nickel (+)| 32-2250 degrees F| | Nickel (-)| 0-1287 degrees C| N| Nicrosil (+)| 1200-2300 degrees F| | Nisil (-)| 650 -1260 degrees C| R| Platinum 13% Rhodium (+)| 1600-2640 degrees F| | Platinum (-)| 870-1450 degrees C| S| Platinum 10% Rhodium (+)| 1800-2640 degrees F| | Platinum (-)| 980-1450 degrees C| T| Copper (+)| negative 330-660 degrees F| | Constantan (-)| negative 200-350 degrees C| Table 1.
Type, compositions and temperature ranges of thermocouples Due to its basic applications brought about by its great advantages such as inexpensiveness, durability, and consistency if used appropriately, many inventions were developed (Newman, Thomase, 1995). This leads to the development of concepts which directed into the idea of using it as an effective transducer to provide an alternative source of energy. Improvised thermoelectric generators (TEG) which utilizes very small voltage such as motor-operated inventions were materialized by mini-scoped technological researches (Maniktala, Sanjaya, 2005).
However, the need for thermocouples in order to obtain temperature feedbacks remains one of the most difficult aspects of administering its limitations such as invasiveness, limited number of temperature measurement points, uncertainty in sensor locations, and self-heating antifacts (Webster, John G. ,1996). Unless new semiconductor materials can be developed which are capable of operating at high temperatures, thermoelectric converters are likely to be used as “bottomers” rather than “toppers” in binary cycles.
They are especially suitable for waste heat recovery, perhaps using the exhaust gases of gas turbines as a source of heat, or the heat generated by a radioactive source as in the American Snap III generator designed for space applications. Thermoelectric generators using paraffin heating have been used in the USSR as a power source for radio receivers in rural areas. Thermoelectric devices can also be run in reverse: by supplying a current from an external source they act as refrigerators. The coefficient of performance is quite low, about 0. , but the simplicity of the device makes it useful for some applications (Kasap, S. O, 2006) With recent advancements in thermoelectric material performance, thermoelectric generators have become a viable alternative for power generation using small temperature differentials with benefits that cannot be found in other energy conversion methods. The power generated by a thermoelectric generator, using a small ?T, is characterized by a relatively high current (-?5 A), but a relatively low voltage (;0. 3 V), which is often not suited for many practical applications.
In order to make use of the thermoelectric generated power in applications requiring a higher voltage, a DC-DC step up converter that can handle low input voltage is needed. Commercial available DC step up converters require an input voltage of at least 0. 7 volts, which is the minimal voltage required for operating a bipolar junction switch. Several novel approaches for low input voltage DC-DC converter concepts have been studied and proved to be feasible. (Shen, B. Henry R. , Cancheevaram, J. , Watkins, C. , Mantini, M. , Venkatasubramanian, 2005)
Foreign Studies The principle of using thermoelectrically converted heat energy for powering portable electronic equipment or charging its battery has been investigated for a laptop computer. This study appears on the Proceedings of EMPD International Conference on Energy Management and Power Delivery, 1995. The thermoelectric battery charger developed, consists of a thermoelectric converter system, powered from butane gas, and a DC-to-DC boost up converter. Both the sub-assemblies are designed using computer models and constructed.
The unit produces an output power of 5 watts which doubles the life of the laptop computer’s internal batteries. (Rahman, Shuttleworth, 1995) According to a study conducted by Kishi, Nemoto, Hamao, Yamamoto, Sudou, Mandai and Yamamoto of Seiko Instrum Inc. that appeared on Eighteenth International Conference on Thermoelectric on 1999, they presented the fabrication and the properties of the micro-TE module, as well as the TE powered wristwatch. Microthermoelectric (TE) coolers are currently used in high power electronic devices such as laser diodes to stabilize temperature.
With the aim of miniaturizing this technology, they have developed a fabrication technique to create micro-TE modules with a cross-section element size of about 100 ?m?100 ?m and a height of several hundred mm. They have previously reported the cooling properties of a module fabricated by this technique. With the recent advances in micro-electric technology, which has decreased the energy consumption in an electric wristwatch to about 1 ?W, there has been increased discussion about utilizing these micro-TE modules to generate energy of several microwatts (?W) to power a wristwatch.
A thermoelectric power generator in silicon technology is used for the energy supply of low power systems. An application is described generating an electrical power of 1. 5 mW with a temperature difference of 108C. With the generated electrical power it is possible to operate a small preamplifier and a sensor control system. For complexer applications a generator with a power in the region of 20 mW would be desirable. (Elsevier, 1999) The rising interest in low temperature heat energy conversion encourages the application of thermoelectric devices.
However, conventional thermoelectric devices used in the Seebeck mode as thermoelectric generators have several shortcomings and thus are inefficient when used as a generator. Additionally, the high cost–power ratio of these modules anticipates the commercial success on a broad basis. One way to achieve better suited products is provided by miniaturization of thermoelectric devices in order to enable the use of mass production methods. But in small devices the contact effects become dominant and reduce the efficiency and power density considerably.
We show that using pn-junctions with thermal generation of free carriers offers the possibility to achieve better contact properties and thus higher efficiencies and power densities. (Holmgren, 2007) A micro-thermoelectric generator shown in Fig. 1(a) has been proposed, across which the heat input from top surface is confined passing through in-plane thermolegs. Thermal isolation cavity design beneath the thermolegs is to eliminate heat leakage and facilitate better thermoelectric conversion. The design is based on co-planer thermocouples as illustrated in Fig. 1(b). The stacked design proposed in this work is shown in Fig. 1(c).
A plurality of n-polysilicon on p-polysilicon thermoelectric layers overlies the silicon substrate to form a thermocouple in stack structure, where the two thermolegs are separated by an electrical insulation oxide layer. The stacked thermolegs are interconnected electrically to generate a voltage by the temperature difference between the cold and hot junctions. Similar to the co-planar design, a thermal insulation cavity is also formed beneath the thermocouples. Fig. 1(d) shows the top view and the cross sections of the stacked polysilicon thermocouples with the thermoleg length (Lg), width (Wg), thickness (tpand tn), to be used in analysis. Yang, 2009) Figure 1. (a) A ?TEG with the conventional in-plane and cross-plane designs and a thermal isolation cavity to prevent heat loss, (b) the ?TEG with the thermocouple of co-planar thermolegs, (c) the ?TEG design proposed in this work with a stacked polysilicon thermocouple, and (d) the top view and the cross sections of the stacked polysilicon thermocouple indicating the design parameters. Stepper Technology provides sales and service of photolithography steppers for the Microelectronics, MEMS and Nano Tech industries introduced a micromachined thermoelectric energy harvester with 6 ?m high thermocouple.
Micromachined thermocouples are considered a cost-effective breakthrough solution for energy harvesters working at low thermal gradients and weak heat flows, typical for e. g. human body as well as machine-related waste heat. The thermoelectric generators will be used for autonomous wireless sensor nodes in a body area Network. Temperature differences in/on artificial objects (machinery, buildings, transport, pipelines) and on the skin of animals and man can be used to power autonomous devices.
For example, the first wearable wireless sensors and medical devices (Electroencephalograph (EEG) system, an Electrocardiography System in a Shirt) fully powered by thermoelectric generators (TEG) on man have been recently demonstrated. (Goedbloed, 2009) Ying, Luming, and Wei, students from School of Electrical Engineering, Southeast University, Nanjing, China (2010) tackles about thermoelectric generators (TEGs) that can directly convert heat energy to electrical energy and have been employed in the automobile industry to recover waste heat energy.
Meanwhile, solar energy can also be used in Electric Vehicles. A thermoelectric-photovoltaic hybrid energy system composed of two TEGs and one photovoltaic generator is proposed for hybrid electric vehicles. A Cuk-Cuk-Cuk multiple-input DC-DC converter (MIC) is adopted to draw power from different energy sources independently. The topology of this MIC is analyzed, including the basic units and synthesizing approach. The maximum power point tracking and asynchronous trigger control strategy is adopted. The simulation results are given to verify the theoretical analysis.
Local Literature According to Gerald E. Williams from his book ”The Basic of Electricity and Electronics” (1997) ,a good thermoelectric material ought to be a semiconductor with very special properties and its thermal resistance must be as high as possible as current must flow through it easily. In semiconductors the rate of diffusion is much greater, and furthermore the effect in each limb of the pair can be additive for the following reason. In some types of semiconductors the charge carriers are electrons as in ordinary metals; these are referred to as n-type materials.
But in others, p-type materials, the charge carriers are positive can be regarded as ‘holes’ vacated by electrons. The current produce in a pair of n and p-type materials is in effect equal to the sum of the rates of diffusion of negative and positive charge carriers. Moreover, a high thermoelectric power alpha is not the only criterion of importance in the choice of materials for a thermoelectric converter. It should be clear that a low thermal conductivity k is necessary to keep the heat flow by conduction across the elements to a minimum, and a low electrical resistivity p is required to reduce the internal ohmic losses.
Semiconductors are also good in these respects, because they are essentially materials of low thermal and electrical conductivity with the latter alone improve by the addition of impurities alpha, k and p are temperature dependent, and when arriving at what is called the figure of merit m for a pair of n and p –type materials, it has been found convenient to include the temperature range which it is intended to employ (Higgins, 1985).
With this, the output voltage and current are limited by the temperatures, materials, and geometry of the device. The generators which are usually connected electrically in series and thermally in parallel, with the hot terminals surrounding the heat source in a radical array and staging the generators in series and/or parallel, can substantially change the output voltage and current. (Durling, 1969). .