tE eFFect

The term "thermoelectric effect" encompasses three separately identified effects: the Seebeck effect, Peltier effect and Thomson effect. This separation derives from the independent discoveries of French physicist Jean Charles Athanase Peltier and balt-German physicist Thomas Johann Seebeck. Joule heating, the heat that is generated whenever a voltage is applied across a resistive material, is related though it is not generally termed a thermoelectric effect. The Peltier–Seebeck and Thomson effects are thermodynamically reversible, whereas Joule heating is not.

 

            The Seebeck effect is the conversion of temperature differences directly into electricity and is named for the balt-German physicist Thomas Johann Seebeck, who, in 1821 discovered that a compass needle would be deflected by a closed loop formed by two metals joined in two places, with a temperature difference between the junctions. This was because the metals responded differently to the temperature difference, creating a current loop and a magnetic field. Seebeck did not recognize there was an electric current involved, so he called the phenomenon the thermomagnetic effect. Danish physicist Hans Christian Ørsted rectified the mistake and coined the term "thermoelectricity". The voltage created by this effect is of the order of several microvolts per kelvin difference. One such combination, copper-constantan, has a Seebeck coefficient of 41 microvolts per kelvin at room temperature.

            The Peltier effect is the presence of heat at an electrified junction of two different metals and is named for French physicist Jean-Charles Peltier, who discovered it in 1834. When a current is made to flow through a junction composed of materials A and B, heat is generated at the upper junction at T₂, and absorbed at the lower junction at T₁.

            The Thomson effect was predicted and subsequently observed by Lord Kelvin in 1851. It describes the heating or cooling of a current-carrying conductor with a temperature gradient. Any current-carrying conductor (except for a superconductor) with a temperature difference between two points either absorbs or emits heat, depending on the material.

            In metals such as zinc and copper, whose temperature is directly proportional to their potential, when current moves from the hotter end to the colder end, there is a generation of heatand the positive Thomson effect occurs. Conversely, in metals such as cobalt, nickel, and iron, whose temperature is inversely proportional to their potential, when current moves from the hotter end to the colder end, there is an absorption of heat and the negative Thomson effect occurs. If the Thomson coefficient of a material is measured over a wide temperature range, it can be integrated using the Thomson relations to determine the absolute values for the Peltier and Seebeck coefficients. This needs to be done only for one material, since the other values can be determined by measuring pairwise Seebeck coefficients in thermocouples containing the reference material and then adding back the absolute thermopower of the reference material. Lead is commonly stated to have a Thomson coefficient of zero; in fact, it is non-zero, albeit being very small. In contrast, the thermoelectric coefficients of all known superconductors are zero.

 

            For monitoring the performance of photovoltaic and thermoelectric, LabView has been used. Basically each renewable energy plant or system plants needs the monitoring control systems. Same as hybrid system, where the performances and parameters must be closely monitored and controlled, thus allow adequate data acquisition system.  The data acquisition system requires large number of measured data where very frequent recording necessary needs to be automated to eliminate the probability of human error as well as to save time. This project is mainly about computer based real time monitoring system center which use LabView as Graphic User Interface (GUI) to provide graphical display output chart, graph or pie chart.

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Research

            Photovoltaics are best known as a method for generating electric power by using solar cells to convert energy from the sun into electricity because of the increasing efficiency and ease of use. PV modules come in a huge variety of types and sizes and are used as the power solution for many different applications including residential and commercial grid-tie solar power systems as well as off-grid and industrial systems.

            Made most commonly using the  Silicon Crystal , solar cells essentially create electricity by converting photons of light into electrons. The "photovoltaic effect" occurs when photons of light from the sun strikes these cells, a portion of the energy is absorbed into the silicon, displacing electrons which then begin to flow. In order to harness this flow, the electrons are drawn into a magnetic field generated by positively- and negatively-charged metal contacts on the top and bottom of the cell. producing direct current, or DC, electricity. Using a DC to AC inverter, the DC current is converted to alternating current, or AC, which can then be used to power electrical appliances.

            Solar cells (SC) produce direct current electricity from sunlight, which can be used to power equipment or to recharge a battery.  Nowadays, the majority of photovoltaic modules are used for grid connected power generation. In this case an inverter is required to convert the DC to AC.

            A photovoltaic system consists of multiple components, including cells, mechanical and electrical connection and mountings and means of regulating and modifying the electrical output. Due to the low voltage of an individual solar cell typically 0.5V, several cell are combined into photovoltaic modules, which are in turn connected together into an array.

            Photovoltaic power systems are generally classified according to their functional and operational requirements, their components configurations and how the equipment is connected to other power sources and electrical loads. The three principal classifications are grid connected or utility interactive systems, photovoltaic hybrid system and PV stand alone systems. Photovoltaic system can be design to provide DC and AC power service, can operate interconnected with or independent of the utility grid and can be connected with other energy source and energy storage system. 

Advantages Photovoltaic

• No pollution and totally silent in process energy compare wind and water based from turbine and very noisy
•  Low cost maintenance and have a long lifetime
•  Not required large space to build. The solar panel can put on the roof top
•   Appropriate to use in Malaysia

The figure above show the solar panel which cell connected in series and parallel

           

 Thermoelectric is a device that converts heat into electricity and it is a two-way process. It can refer either to the way a temperature difference between one side of a material and the other can produce electricity, or to the reverse: the way applying an electric current through a material can create a temperature difference between its two sides, which can be used to heat or cool things without combustion or moving parts.

The thermoelectric effect is the direct conversion of temperature differences to electric voltage and vice-versa. A thermoelectric device creates a voltage when there is a different temperature on each side. Conversely, when a voltage is applied to it, it creates a temperature difference. At the atomic scale, an applied temperature gradient causes charge carriers in the material to diffuse from the hot side to the cold side.

            Thermoelectric modules are solid-state heat pumps that operate on the Peltier effect. A thermoelectric module consists of an array of p-type and n-type semiconductor elements heavily doped with electrical carriers. The array of elements is soldered so that it is electrically connected in series and thermally connected in parallel. This array is then affixed to two ceramic substrates, one on each side of the elements. Electrons can travel freely in the copper conductors but not so freely in the semiconductor. As the electrons leave the copper and enter the hot side of the p-type, they must fill a "hole" in order to move through the p-type. When the electrons fill a hole, they drop down to a lower energy level and release heat in the process. Essentially the holes in the p-type are moving from the cold side to the hot side. Then, as the electrons move from the p-type into the copper conductor on the cold side, the electrons are bumped back to a higher energy level and absorb heat in the process. Next, the electrons move freely through the copper until they reach the cold side of the n-type semiconductor. When the electrons move into the n-type, they must bump up an level in order to move through the semiconductor. Heat is absorb when this occurs. Finally, when the electrons leave the hot side of the n-type, then can move freely in the copper. They drop down to a lower energy level and release heat in the process.

           

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            The photovoltaic is a device that that convert solar radiation into electricity. However, during the operation of the PV cell, only around 15% of solar radiation is converted to electricity with the rest converted to heat. The electrical efficiency will decrease when the operating temperature of the PV module increases. So, for getting the highest efficiency, the optimum temperature must be 25^oC.

            In Malaysia, the ambient temperature is  about 34^oC. Due to the increasing temperature, the efficiency will decreased and the output will be low. It is not worth it with the high cost. So, to reach the 25^oC in Malaysia is impossible but by using an active cooling, the temperature can be adjust to the lowest as possible, so the efficiency will be increased.

            There are many types of photovoltaic materials that can be used such as crystalline silicon, amorphous and etc. The commonly types that being used in industry is crystalline silicon. The crystalline silicon (c-Si) has been used as the light-absorbing semiconductor in most solar cells, even though it is a relatively poor absorber of light and requires a considerable thickness (several hundred microns) of material. Nevertheless, it has proved convenient because it yields stable solar cells with good efficiencies (15-17%, half to two-thirds of the theoretical maximum).

            The performance of photovoltaic and thermoelectric is being monitored and measured by using LabView that provide graphical display output chart, graph or pie chart. LabView also will become a problem because inefficient in using it. To overcome, LabView will be studied in more detail.

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            In recent years, renewable energy is widely advocated by many countries. Photovoltaic cell is one of the most popular renewable energy products. It is a technology which generates electricity from sunlight and it works only with certain spectrum of the white light component of the sun. In Malaysia, the abundance of sunlight makes solar photovoltaic a very viable form for generating electricity. Since the sun is the fuel source of photovoltaic electricity, this form of electricity generation is said to be renewable.  Solar photovoltaic (PV) is not only renewable; since there is no CO2 emission in the process of electricity generation, solar photovoltaic is also considered a clean form of electricity generation or can be described as friendly environment.

            However, during the operation of the PV cell, only around 15% of solar radiation is converted to electricity with the rest converted to heat. The electrical efficiency will decrease when the operating temperature of the PV module increases.

That is because the optimum temperature of photovoltaic is 25^oC but in Malaysia, the ambient temperature is 34^oC.So, to overcome this problem, the thermoelectric is being used because it can moved the heat from one side to the other side. From this heat, it can convert into electricity because thermoelectric is a device that creates voltage when there is a different temperature on each side. By this, the efficiency will increased.

            However, it is unrealistic to expect the efficiency of both continuously. There must be some time the efficiency will drop. So, to overcome this, an active cooling have been used. Fan has been found to be the best methods to cooling the thermoelectric that can lower the temperature of thermoelectric and by this, it will raise the temperature differences between photovoltaic and thermoelectric. The highest temperature differences between the photovoltaic and thermoelectric, the higher the efficiency and the performance of PV-TE will be improved.