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Design & Fabrication of Air Cooler using Peltier Modules

: The project aims to design & fabricate a cooling system using Peltier modules. Peltier modules are thermoelectric devices capable of producing cooling & heating effect with help of electrical power, thus capable to cool or heat surrounding air. This project focuses on designing a duct which would draw fresh outdoor air & cool the air with the help of Peltier modules. This air would then be given to the blades of ceiling fan which would further circulate it within the room & provide cool air to the occupants under the fan. The Peltier modules mounted inside a ducting system, which would cool the air flowing through the duct. The surface area is increased by providing fins on the cooling side & the hot junction is cooled by recirculation of coolant. The heat exchanger in the circuit helps to lower the temperature of the coolant. The duct contains blowers to help flow of air through the ducts. The system also provides ventilation by constantly drawing fresh air from the outdoor environment. The inlet of the duct has a desiccant dehumidifier which absorbs the moisture content from the inlet air thus dehumidifying the air. The exhaust duct has a heating coil which helps in regeneration of the desiccant dehumidifier by carrying away the moisture into the outdoor environment. The enthalpy wheel is kept in constant rotation at a low rpm with a help of a 12V dc motor. Air filter provided at the inlet of the duct to refrains dirt & dust particles from entering the system. Two DC fans provided in the exhaust duct to carry heat from the hot side of the module to outdoor environment. When a DC voltage is applied to terminals Tl & T2 an electrical current (I) will flow in the circuit. As a result of the current flow, cooling effect (Qc) will occur at thermocouple junction A where heat is absorbed & a heating effect (Qh) will occur at junction B where heat is expelled. [10]


INTRODUCTION A. Need & Background
Mostly due to the high costs & high-power consumption it is difficult for majority of population in India to afford air conditioners. This brings out the need to develop low cost alternatives which use lesser power making it an affordable alternative to use. [2] The first important discovery relating to thermoelectricity occurred in 1821 when a German scientist, Thomas Seebeck, found that an electric current would flow continuously in a closed circuit made up of two dissimilar metals provided that the junctions of the metals were maintained at two different temperatures. In 1834, a French watchmaker and part time physicist, Jean Peltier, while investigating the "Seebeck Effect," found that there was an opposite phenomenon whereby thermal energy could be absorbed at one dissimilar metal junction and discharged at the other junction when an electric current flowed within the closed circuit. [9] A compressor-based dehumidifier dehumidifies air by cooling the evaporator coils & as the humid air passed over the evaporator coil the moisture condenses & the air gets dehumidified. Compressor-based systems require high power & are costly. Whereas a desiccant dehumidifier uses a chemical called as desiccant which absorbs moisture from the air but regeneration is essential to avoid saturation of desiccant hence making use of a heating coil is imperative to use such dehumidifiers for commercial purposes. Peltier effect is defined as the production of heat at one junction & the absorption of heat at the other junction of a thermocouple when a current is passed around the thermocouple circuit.

B. Peltier Effect
When a DC voltage is applied to terminals Tl & T2 an electrical current (I) will flow in the circuit. As a result of the current flow, cooling effect (Qc) will occur at thermocouple junction A where heat is absorbed & a heating effect (Qh) will occur at junction B where heat is expelled. [ [13] Thermoelectric module or Peltier module, is a semiconductor based solid-state active heat pump which transfers heat from the cold side to its hot side. By application of a DC power source, the heat is moved through the module from cold side to hot side. The reversal of this phenomenon can also be done by a simple change to the applied DC polarity. Two semiconductors of different material, one n-type & one p-type, are used because they need to have different electron densities. The semiconductors are placed thermally in parallel to each other & electrically in series & joined to a thermally conducting plate on each side. When a DC voltage is applied at the ends of the two semiconductors there is a flow of DC current across the junctions of the semiconductors causing a temperature difference. The cooling side absorbs heat & pumps to the hot side of the device. [10] 2) Thermoelectric Materials: There are several semiconductor materials such as Bismuth Telluride (Bi2Te3), Lead Telluride (PbTe), Silicon Germanium (SiGe), & Bismuth-Antimony (Bi-Sb) alloys that may be used for making Peltier devices. But the most common material used for Peltier devices is an alloy of bismuth telluride that has been suitably doped to provide individual elements having distinct "N" & "P" characteristics. The selection of material is based on the figure of merit or relative performance for various materials over a range of temperatures. This can be illustrated from the graph shown. As the working range for the system is below 2000 C, Bismuth Telluride (Bi2Te3) would be the most suitable material. [ 3) Working Principle of Desiccant Dehumidifier: Desiccant Dehumidification Process can be achieved by using a hygroscopic material, which absorbs or adsorbs the water vapor from the moisture. The dehumidifier has two zones the "process zone" & the "recharge zone". The humid air that needs to be dehumidified enters through the process zone of the enthalpy wheel. Here, the air gets dehumidified by the desiccant as it absorbs the moisture of the air passing over it. The absorption of moisture from the air leads to exothermic reaction thus raising the temperature of the incoming air. The exhaust air passes through the recharge zone where a heater covers the recharge zone. The exhaust air is heated by the heating coil which liberates moisture from the desiccant. Thus, moisture is transferred from desiccant to the air into the atmosphere. Thus, this cycle continues as the wheel rotates & the desiccant is reused again & again.

II. DESIGN OF AIR COOLER COMPONENTS A. Need & Background
The design of the system is created based on the objectives decided based on the need of the project. The CAD model is created & simulated for various parameters such as air flow, temperatures at various points of the system. Based on the data obtained the models are optimized until satisfactory results are obtained.

B. Design Of Duct 1)
The design is based on Velocity reduction method.
) mm of water [7] 3) Where V: velocity of flow 4) Pressure loss due to friction (Pf) is given by [7]

F. Design of Radiator
The coolant coming out of the cooling coil is of considerable high temperature which must be cooled down to some temperature so that it can be re circulated into the circuit again for cooling. This coolant is cooled using a radiator. Design of radiator For calculation purpose, assuming that heat lose by the coolant = heat gained by air Therefore, 3) Tco = 59.08 0C …….outlet temperature of coolant 4) Now, Heat capacity ratio (R) is given by [8]

5)
The effectiveness of heat exchanger is given by [8] E = = ( -) ( -) = 0.7 6) Now, the number of transfer units can be calculated as follows [8] ln [1 + 0.801 × ln (1 -0.7)] = 4.12 7) Now, the surface area required for heat transfer (S) is given by But the optimum sized available heat exchanger is of area …………., thus incorporating a reservoir of coolant in order to increase the surface area & maximize the heat transfer.

G. Design of cooling fin
The area available at the Peltier module is insufficient as well as very inefficient for the getting the desired cooling effect as the surface area for heat transfer is very less, hence an aluminum heat sink is used. The heat sink designed is of rectangular fins & has an array on fins over the rectangular block. The heat transfer rate is increased by using a heat sink as the surface area increases. When fins are attached on a base the surface area is increased the design & performance is explained below: 2) the non-dimensional fin parameter is calculated as μ = mL = 10.95 × 7 × 10 -2 = 0.7665 3) fin efficiency is then calculated as [8] 4) The breadth of the base is given by [8] B = 2nt + (n -1) S = 0.203 5) The total exposed area for convection is calculated as [8] At = 2LWn + SW (n -1) = (2 × 7 × 10-2 × 0.4 × 33) + 0.005 × 0.4 × (33 -1) = 1.9282 m2 6) Area of fin is calculated as [8] Af = 2LWn = 2 × 7 × 10-2 × 0.4 × 33 = 1.848 m 2 7) Area of parent surface is calculated as [8] Ap = BW = 0.8471 m 2 8) The exposed parent area is calculated as [8] Ae = (n -1) SW = 32 × 0.005 × 0.4 = 0.064 m 2 9) The effectiveness of the fin is given by [8]

H. CAD Model of Duct using Solid Works
According to the design, a rectangular duct shape is used. Rectangular duct shape is easier to work with in relation to finish surface of room, but it has slightly higher friction compare with circular ducts. The model is divided into three sections to ensure easy installation & maintenance. All the modeling has been done on solid works software. Based on the Design Calculations & some assumption the CAD model is made. 1) Model of Duct: In model 1, the fresh air from environmental is sucked by blower fan & that air will cool using thermoelectric modules. The cooled air passes through the duct & given the blade of fan using bend duct. The fan will blow the cool mixed air to the occupants & the hot air is thrown out from the duct as shown in fig 3. The system is mounted on the ceiling of the room. The room dimension is 10 × 10 × 10ft. The ceiling fan is mounted at the center of the ceiling as shown in figure.

10) Heater:
Heater is an electrical device that converts electrical energy into heat. A coil type heater is used in the system in the exhaust duct of the system. The main reason for employing the heater is to heat the exhaust air which would carry away the moisture from the desiccant into the atmosphere. Fig. 3.11. Heater

11) Coolant:
Ethylene glycol is used as coolant in the system for carrying away the heat generated by the Peltier modules by recirculating the coolant through the system. The coolant is mixed in the ratio of 6 parts of water to 1 part of coolant. The coolant has better heat transfer coefficient than water alone hence helps in better heat dissipation through the perforated block. It has a working temperature range of -29 ℃ to 129 ℃.  In order to use the Peltier at its peak performance its hotter side needs to be cooled hence water cooling was opted. This cooling circuit includes perforated cooling block, coolant, hoses, radiator & blowers. & to maximize the cooling effect the surface area of the modules needs to be increased which is done by using an aluminum fin. There are two processes happening simultaneously in upper & lower duct. Which are explained below: a) Inlet Duct: The fresh air from atmosphere is sucked into the duct. It is conditioned & then passed into the room. The air is first made to pass over the enthalpy wheel which dehumidifies the air. This is done by the silica gel which is a good absorbent of moisture hence dehumidification is done. Dehumidification is also necessary so that the condensation of water can be avoided on the aluminum fins. Once the air is dehumidified it is made to pass over the fins hence the air gets cooled. This cooled or conditioned air is then passed into the room which is thrown over the ceiling fan. b) Exhaust Duct: Upper duct handles the unconditioned air. The components inside the upper ducts are perforated cooling block, radiator, blowers & enthalpy wheel. The air inside the room is sucked in using a blower & this air is used to cool the coolant using a radiator. Then this air is thrown out of the room. c) Reason for Rotation of Enthalpy Wheel: When air is sucked into the lower duct the air is humid. It is dehumidified using the enthalpy wheel, where silica gel absorbs the moisture. After some time there is a possibility of saturation of the silica gel it is necessary to remove the moisture from the gel. Thus, the wheel is rotated & when the silica gel plated reaches the upper duct hotter air is blown using a heater which results in evaporation of water from the gel. The wheel is rotated at a very low rpm i.e. 3.5rpm

E. Conclusion
The components have been fabricated & assembled properly as per designs & the electrical connections have been made for the assembly.

B. Results of Experimental Setup
The experimental setup was tested in a room with ambient temperature as 33 0 C DBT & 27.5 0 C WBT. The relative humidity of the room was 68.5%.
1) The temperature of fin has been found to be 15 0 C. However, the fin took around 8.5 minutes to reach 15 0 C from room temperature.

2)
The Coolant temperature at outlet of perforated block has been found to be 55 0 C.
3) The inlet conditions of the air into the room has been measured using a sling Psychrometer & the conditions have been found to be 26 0 C DBT & 21 0 C WBT. A dehumidification of 2% has been achieved. The outlet conditions have been plotted on a psychrometric chart as shown in the figure. 2) The Auxiliary items such as blowers, pump, motor combined consume 60W of electrical power.
3) The heater mounted consumes 100 W of electrical power to remove moisture from the solid desiccant. However, it has to be used intermittently at intervals to 5 minutes. When the heater is used continuously, it leads to heating up the desiccant thereby raising the inlet temperature of air.

V. CONCLUSION
The air cooler using Peltier modules is a device which uses the principle of Peltier effect combined with mechanical systems to cool the air flowing through the system to give the desired cooling effect. It is a flexible system which produces cooling as well as heating with a simple polarity switch. This is an alternative system for air conditioners which eliminates the use of compressors & refrigerants. As well as being portable & compact device, the electrical power through the system is accurately controlled through a switch mode power supply. This enables the system to produce the required cooling on the fins to cool the air in turn flowing through it. An innovative concept for air cooling was presented. After a short overview of today's electronic systems using Peltier modules for cooling electronic cabinets. With the proposed design it is possible to produce cooling effect. However, with the use of proper controllers it is possible to control temperature of air accurately. Finally, the feasibility of system was demonstrated using the blower for air flow through the duct. The project work & testing show that this setup solves the problems such as global warming potential, ozone depletion potential thereby exhibiting a good integrated result. This system requires low maintenance, does not require skilled labour, has longer life span & requires less capital investment. It has low running cost & can be implemented to small office cubicles. The entire system can be automated which gives better control & ease of operation.

VI.
FUTURE SCOPE A. Design of a shell & coil type of heat exchanger to cool the recirculating coolant more efficiently & thus dissipate the heat from the Peltier B. Design of a feedback controller device which would give the real time information about the performance of the Peltier modules as well as of other auxiliary components C. Temperature regulation of the outlet air using a wireless remote control. D. To run the setup using solar panel as all the components run on DC supply. E. Using the setup for heating purpose by reversing the polarity of the modules.