Review on Thermo-electric cooling for Atmospheric water extraction

Transcription

1 Review on Thermo-electric cooling for Atmospheric water extraction Shubham Kasbe, Onkar Khade, Prof. C.P. Waykole Mechanical Engineering, MITCOE, Savitribai Phule Pune University Abstract - In many countries like India it is difficult to obtain water for irrigation or other purposes, especially in the dry regions. The problem of water shortage is also observed in other places of the world due to lack of rainfall. However, in highly humid areas such as places close to the sea, water can be obtained by condensing the water vapour present in air. Here, the paper presents the method to develop a water condensation system based on thermoelectric cooler. Water Generator is a device that can convert atmospheric moisture directly into usable and even drinkable water. It is such a device which uses the principle of latent heat to convert molecules of water vapour into water droplets. It has been introduced a bit before, though it is not very common in India and some other countries. It has a great application standing on such age of technology where we all are running behind renewable sources. I. INTRODUCTION Atmosphere contains large amount of water in the form of vapour, moisture etc. Within those amounts almost 30% of water is wasted. This amount of water can be used by implementing an equipment like Atmospheric Water Generator [1]. This device is capable of converting atmospheric moisture directly into usable and even drinking water. The device uses the principle of latent heat to convert water vapour molecules into water droplets. In many countries like India, there are many places which are situated in mild region; there are desert, rain forest areas and even flooded areas where atmospheric humidity is outstanding. But resources of water are limited. In the past few years some projects have already been done to establish the concept of air condensation as well as generation of water with the help of Peltier devices, such as harvesting water for trees using Peltier plates that are powered by photovoltaic solar energy [2], etc. According to previous knowledge, we know that the temperature require to condense water is known as dew point temperature. Here, the goal is to obtain that specific temperature practically or experimentally to condense water with the help of some electronics devices. This device consists of a thermoelectric Peltier (TEC) couple [3], which is used to create the environment of water condensing temperature or dew point, however conventional compressor and evaporator system could also be used to condense water by simply exchanging the latent heat of coolant inside the evaporator. The condensed water will be collected to use for drinking purpose and various other uses. II. PELTIER EFFECT The Peltier effect is the presence of heating or cooling at an electrified junction of two different conductors and is named after French physicist Jean Charles Athanase Peltier, who discovered it in Figure 1 shows that the 44

2 thermocouple circuit has been modified to obtain a different configuration that illustrates the Peltier Effect, a phenomenon opposite that of the Seebeck Effect. If a voltage (Ein) is applied to terminals T1 and T2, an electrical current (I) will flow in the circuit. As a result of this, as light cooling effect (QC) will occur at thermocouple junction A (where heat is absorbed), and a heating effect (QH) will occur at junction B (where heat is expelled). This effect may be reversed when a change in the direction of electric current flow will reverse the direction of heat flow. Joule heating, having a magnitude of I 2 R (where R is the electrical resistance), also occurs in the conductors as a result of current flow. This Joule heating effect acts in opposition to the Peltier Effect and causes a net reduction of the available cooling. The Peltier effect can be expressed mathematically as: QC or QH = β x I = (α T) x I β is the differential Peltier coefficient between the two materials A and B in volts. I is the electric current flow in amperes. QC and QH are the rates of cooling and heating, respectively, in watts. Case 1: When High energy electrons move from right to left, thermal current and electric current flow in opposite directions and β < 0 i.e. negative Peltier coefficient. Case 2: When High energy electrons move from left to right, Thermal current and electric current flow in same directions and β > 0 i.e. positive Peltier coefficient. Fig. 1 Peltier effect III. THERMOELECTRIC PRINCIPLE OF OPERATION Figure 2 shows that the thermoelectric module has been made by using two thin ceramic wafers with a series of P and N doped bismuth-telluride semiconductor material sandwiched between them. The ceramic material has been there on both sides of thermoelectric which adds rigidity and necessary electrical insulation. The N type material has an excess of electrons, while the P type material has a deficit of electrons. One P and one N make up a couple, as shown in Figure 2. The thermoelectric couples are electrically in series and thermally in parallel. A thermoelectric module can contain one to several hundred couples. As the electrons move from the P type material to the N type material through an electrical connector, the electrons jump to a higher energy state absorbing thermal energy (cold side). Continuing through the lattice of material; the electrons flow from the N type material to the P type material 45

3 through an electrical connector dropping to a lower energy state and releasing energy as heat to the heat sink (hot side).thermoelectric can be used to heat and to cool, depending on the direction of the current. In an application requiring both heating and cooling, the design should focus on the cooling mode. Using a thermoelectric in the heating mode is very efficient because all the internal heating (Joulian heat) and the load from the cold side is pumped to the hot side. This reduces the power needed to achieve the desired heating. Figure 2 TEC principle of operation IV. PARAMETERS REQUIRED FOR DEVICE SELECTION In practical couples are combined in a module where they are connected electrically in series and thermally in parallel. Modules are available in a great variety of sizes, shapes, operating currents, operating voltages and ranges of heat pumping capacity. The present trend, however, is toward a larger number of couples operating at lower currents. Three specific system parameters must be determined before device selection can begin. These are: TC: Cold Surface Temperature TH: Hot Surface Temperature QC: The amount of heat to be absorbed at the Cold surface of TE. Generally, if the object to be cooled is in direct contact with the cold surface of the thermoelectric, the desired temperature of the object can be considered the temperature of the cold surface of the TEC (T C). The hot surface temperature TH is defined by two major parameters: 1) The temperature of the ambient environment to which the heat is being rejected. 2) The efficiency of the heat exchanger that is between the hot surface of the TEC and ambient. These two temperatures (TC and TH) and the difference between them (T) are very important parameters and therefore must be accurately determined if the design is to operate as desired. One additional criteria that is often used to pick the "best" module(s) is the product of the performance (COP) which is the heat absorbed at the cold junction, divided by the input power (QC / P). The maximum COP case has the advantages of minimum input power and therefore, minimum total heat to be rejected by the heat exchanger (QH = QC + P). It naturally follows that the major advantage of the minimum COP case is the lowest initial cost. Single stage thermoelectric devices are capable of producing a "no load" temperature differential of approximately 67 C. Temperature differentials greater than this 46

4 can be achieved by stacking one thermoelectric on top of another. This practice is often referred to as cascading. The design of a cascaded device is much more complex than that of a single stage device [2]. Another important two parameters for TE devices are the maximum allowed electrical current Imax through the device (exceeding the current will damage the TEC) and the geometry factor (G). The number of thermocouples and the geometry factor help to describe the size of the device; more thermocouples means more pathways to pump heat. One thing about G is that it is related to the density of thermocouples per square area and it is also related to the thickness of the TEC, improving the intrinsic energy-conversion efficiency of the materials but also implementing recent advancements in system architecture [3]. Using nanotechnology, the researchers at BC and MIT produced a big increase in the thermoelectric efficiency of bismuth antimony telluride in bulk form [4]. Specifically, the team realized a 40 percent increase in the alloy's figure of merit.the achievement marks the first such gain in a half-century using the costeffective material that functions at room temperatures and up to 250 C. The success using the relatively inexpensive and environmentally friendly alloy means the discovery can quickly be applied to a range of uses, leading to higher cooling and power generation efficiency. Power supply and temperature controller are additional items that must be considered for a successful TE system. Regardless of method, the easiest device parameter to detect and measure is the temperature. Therefore, the cold junction is used as a basis of control. The controlled temperature is compared to some reference temperature, usually the ambient or opposite face of the TE. The various control circuits are numerous, complex and constantly being upgraded [5]. Suffice it to say that the degree of control and consequent cost, varies considerably with the application. V. COMPONENT SELECTED Peltier Module is selected considering following parameters: 1. Temperature difference between hot side and cold side 2. Cost 3. Power required The required temperature difference is between 14 to 20 C. TEC is selected whose specifications are as follows: TABLE I SPECIFICATIONS OF TEC MODULE Module No Size Maximum Current Resistance TEC *40*3.9 mm 6.4 Amp 1.98 Ω Temperature difference 68 0 C Weight 27 gm No of couples 127 For this module, we have selected battery with following specifications: Lithium Ion Battery 12V, 2500 mah. 47

5 VI. SAMPLE MODEL Figure 3 Sample model VII. REFERENCES [1] Angrist S.W., Direct Energy Conversion. Allyn and Bacon, Inc., Boston, [2] Lindler K.W., Use of multi-stage cascades to improve performance of thermoelectric heat pumps. Energy Conversion and Management. 39: H. Poor, An Introduction to Signal Detection and Estimation. New York: Springer-Verlag, 1985, Ch. 4. [3] Bell, L.E., Cooling, Heating, Generating Power and Recovering Waste Heat with Thermoelectric Systems. Sci., 12(321): [4] Science, Nanotech advance heralds new era in heating, cooling and power generation. At the Science express web site, on Thursday, March 20. See [5] M. Rowe and C. M. Bhandari, Modern Thermoelectrics, Hot Technology, 1983 [6] Vain and D. Astrain, Development of a heat exchanger for the cold side of a thermoelectric module, Applied Thermal Engineering, vol. 28, no , pp , August [7] Manoj Kumar Rawat (2013) et.al, A review on developments of thermoelectric Refrigeration and air conditioning systems: a novel Potential green refrigeration and air conditioning Technology. International Journal of Emerging Technology and Advanced Engineering Volume 3, Special Issue 3: ICERTSD, pages [8] Abdul-Wahab, S.A., A. Elkamel, A.M. Al-Damkhi, I.A. Al-Habsi, H. Al-Rubai'ey, A. Al-Battashi, A. Al-Tamimi, K. Al-Mamari and M. Chutani, Omani Bedouins' readiness to accept solar thermoelectric refrigeration systems. International J. Energy Technology and Policy, 7: