A low power, eco-friendly multipurpose thermoelectric refrigerator

Abstract

There has been an immense endeavor to mitigate global warming in spite of which it has only been worse. This paper presents the design and implementation of a low power and eco-friendly refrigeration system using the thermoelectric effect. The conventional refrigerators make use of complex mechanisms which involves synchronous operation of various units, namely the compressor, condensers, expansion valves, evaporator, refrigerant and so on. But a thermoelectric refrigerator exploits the principle of the Peltier effect, thus avoiding the utilization of these complex components. This even helps curb the release of harmful chlorofluorocarbons (CFCs) into the atmosphere which contributes to the increase in global temperature. Moreover, the temperature can be controlled and set to required values with the help of a microcontroller. Hence, this can be used both for domestic and commercial purposes. The unit does not eject any harmful gases. Therefore, the heat expelled from the unit can be tapped for heating utilities, making the use of this device versatile in its application. Thus this proposal aims not only at reducing the air pollutants by not contributing to it but also at reducing the power consumption.

Graphical abstract

Keywords

low power / eco-friendly / multipurpose / TEC-12706 / Peltier effect / microcontroller (P89V51RD2BN) / temperature control

1 Introduction

The temperature of the globe is soaring up year by year. With the rapid advancement of applied science and technology, there is a great demand for low power and user friendly appliances. Refrigeration is something which is quintessential for everyone. The conventional refrigerators, in general, consume 500 W to 750 W of power on an average. Moreover, with the inflation of prices, it is tough for common people and students to afford such refrigerators.

Thermoelectric refrigerators, on the other hand, replace the major parts of the conventional refrigerator and thus reduce the cost and power consumption of the unit. They work on the principle of Peltier effect that “when a voltage difference is applied to a dissimilar conductor or semi-conductor junction pair, one junction gets heated up while the other one cools down.”

The colder junction of a thermoelectric cooler (TEC) module gets cooler due to the absorption of the energy by the electrons as they move from one semiconductor to the other. But for this to happen, the hotter junction needs to be cooled continuously. This eliminates the routine of absorption of heat by a liquid refrigerant and dissipation of heat by the condenser fins, as it happens in a conventional refrigerator. Besides, the conventional refrigerator releases many harmful gases which cannot be used for domestic purposes. But the heat released from the proposed design does not eject any harmful gases, rather the heat released can be used for various purposes like warming up the drinking water or keeping any food items warm for a longer time etc.

Hence, a thermoelectric module uses an electron transport phenomenon to perform the designed function and is free from any mobile parts which are acoustically and mechanically unreliable. The only drawback due to which the idea finds difficult to penetrate into the commercial market is the efficiency over which there is a rigorous research going on.

Different range of cooling is required for different applications like domestic applications, medical applications and so on and so forth. Thus, here the device comes with a controllable temperature ranging from 0°C to 9°C. This makes the unit to have wide applications.

2 Previous works and advancements

Reference [1] presents a vivid study of the performance of the existing module, presents an optimal pellet size for the thermoelectric module, and accounts for the thermal boundary resistance and electrical contact resistance at the module interconnects. In the present paper, the low-power consumption of the module is focused on. Hence, the idea presented in Ref. [1] can also be added to the proposal in the present paper to further improve the performance.

References [2] and [3] proposes a similar idea, harvesting the renewable energy even to supply the input power to the thermoelectric module. The solar panel is used to extract the power and store the poser in a 12 V battery which would supply the input to the module. However, the system would become quite unreliable as it depends on the solar energy which may not be available during bad weathers or at night. Nonetheless, this idea can be incorporated in the present work along with the direct electrical supply which enhances the energy-conserving feature of the refrigerator.

Reference [4] presents a thorough study of the potential application of thermoelectric module in thermal refrigeration and R&AC technology. In the present paper, a working model is proposed of the implementation of what has been presented in the referred work with additional features added to it.

The idea presented in Ref. [5] uses an Atmega8 microcontroller to implement the same idea. This can accept even analog inputs. In the present paper, it is substituted with the conventional 8051 microcontroller and an ADC0809, which improves the coding efficiency of the programmer.

3 Block diagram of the proposed model

3.1 Working unit

Figure 1 depicts the entire proposed system. This unit consists of the insulated cabin with water filled in it. A temperature sensor LM-35 is mounted on the inner walls of the cabin. A small square hole, slightly greater than 40 mm × 40 mm in area is cut out from the lower side of the insulation box and a thin aluminum sheet of larger dimensions is sealed upon it. The TEC module is mounted upon this aluminum sheet which handles the function of cooling water inside the insulated cabin. An aquatic air pump is used to agitate the water at the junction of aluminum sheet, in order to prevent the water from freezing. The hotter side of the TEC module is mounted upon by a heat sink, and a cooling fan is used to drain this heat out of the sink. Thus the TEC module is prevented from getting over-heated. This heat dissipated by the module can be tapped and further used for domestic purposes if the user requires.

3.2 Control unit

The heart of the control unit is the microcontroller (P89V51RD2BN). The analog output of the LM-35 is converted to digital form by the ADC0809 which gets displayed on the 7-segment display through the microcontroller. The controller unit allows or withdraws the power supply to the TEC module through relay based on the present temperature as sensed by the LM-35. Besides, it can be used to change the range of temperature of operation.

4 Components used

4.1 8-bit microcontroller−P89V51RD2

P89V51RD2 is an 8051 microcontroller (Fig. 2) manufactured by NXP (Philips). This is an 8-bit microcontroller which means that the size of its accumulator is 8-bit. This is a 40-pin IC which is purely digital in nature. It comes with four input-output ports, 64 kB on-chip flash memory, 1024 bytes of data RAM and many other salient features like three 16-bit timers, eight interrupts etc. It operates at 5 V operating voltage from 0 to 40 MHz, with a clock frequency of 11.0592 MHz. It can withstand a temperature variation from 0°C to 70°C, hence is highly adaptable to domestic conditions.

4.2 LM-35 temperature sensor

LM-35 (Fig. 3) is a centigrade precision temperature sensor whose output voltage is linearly proportional to the Centigrade temperature. This make this sensor more advantageous in comparison to the linear temperature sensors calibrated in Kelvin, as the user is more comfortable with Centigrade than with Kelvin scale. The LM-35 has low output impedance, linear output and precise inherent calibration, thus making it easy to interface the read out voltage to a control circuitry. Another advantage of using this temperature sensor is that it draws very low current, like 60 µA from its supply and thus self heating is very low. This sensor is rated to operate between ‒55°C to 150°C. Another salient feature of this sensor is that it has a reasonable accuracy of 0.5°C at room temperature.

4.3 ADC0809

ADC0809 (Fig. 4) is a data acquisition component. As described in the data sheet [6], it is a monolithic CMOS device with an 8-channel multiplexing input and 8-bit analog-to-digital converter. The conversion technique used by the ADC0809 is the successive approximation method. Moreover, the 8-channel multiplexing feature makes it possible to directly access 8-single ended analog signals. The outputs of ADC0809 satisfies the TTL level and also, this does not require any zero of full-scale adjustments. The total unadjusted error of ADC0809 is±1LSB (least significant bit).

Owing to the design of ADC0809, it offers high speed (100 µs) and high accuracy, and consumes minimal power (15 mW). These features makes the device used in this paper more suited for current demand for high speed and low power applications.

4.4 TEC1-12706

Thermoelectric module is the device which works on the principle of Peltier effect. It creates, on either side of the module, a temperature difference which can, therefore, be used to heat up something or cool down something. The most important thing is that the heat generated from the module has to be removed continuously using a heat sink for efficient working of the module. The 127 in TEC1-12706 stands for the fact that 127 thermocouples are used while 06 corresponds to 6 A current ratings. The TEC-12706 module operates at a maximum voltage of 15.4 V and a max current of 6 A. The maximum heat output is 92 W. The working of TEC-12706 is shown in Fig. 5.

Two dissimilar semi-conductor materials, namely n- and p-types, have different electron densities. Exploiting this fact, it is used in the thermoelectric module. The n- and p-type materials are parallel in thermal sense and series in electrical sense. There is a heat conducting plate on either side of the module. When a potential difference is impressed onto the free ends of the two semiconductors a DC current flows across the junction, thereby causing a temperature difference. The side with the cooling plate absorbs heat expelled by the heat sink attached to the other end of the device.

4.5 Aquatic air pump

An aquqtic air pump is an air pumping mechanism in water in order to aerate and agitate the water body. In the module in this paper, this is done to prevent the water from getting condensed to ice owing to the cooling caused by the TEC module. Water is used as refrigerant in preference to air because, water retains the cooling for a longer duration than air and hence, it reduces the use of a TEC module, thus reducing the power consumption. When the temperature decreases, the water used in the insulated box has to be simultaneously ensured not to get condensed to ice. Therefore, an aquatic air pump is used to agitate the water continuously. Although very powerful air pumps are available, a pump which causes moderate agitation in a controlled fashion is chosen.

4.6 Seven segment display

A seven segment display (Fig. 6) is a form of electronic display which displays the arabic numerals and some of the English alphabets according to the respective LEDs that are made to glow. There are two kinds of these displays, common anode and common cathode. The difference between the two displays is that in common anode, the common pin is connected to the power supply in order to activate the display module while in common cathode, the common pin is given to the ground to activate it. Although both the modules perform equally effective, a common anode seven-segment display is chosen in the paper, whose function is shown in Table 1.

4.7 Heat sink and fan

The thermoelectric module gets cooled on one side and heated on the other side. To make its operation more efficient, the heat from the hotter side needs to be sunk and released. It is analogous to the working of a condenser. The fan used in this paper is a 12 V DC fan which is commonly used in the computer.

4.8 Insulated cabin

The insulated cabin is basically a storage unit with water filled to a small level within it. This insulated cabin is used because in general, water retains the cooling for a longer time than air. So, rather than directly using the air medium, water is used to cool the air. This helps reduce the duration of power supply to the circuit.

4.9 Switching and control module

The switching and control module consists of push buttons interfaced with the pins of microcontroller 8051. This module has three push buttons, one for incrementing the temperature, one for decrementing the temperature, while the last for switching the control between setting the lower temperature or higher temperature or to display the sensed temperature on the seven segment display couple. While in operation, if new boundaries have to be set to the temperature, this module can be made use of.

4.10 Relay (12 V)

The relay (Fig. 7) used in this paper is a 12 V relay which either supplies a voltage or withdraws for the TEC module. When the temperature of the module goes below the designed temperature, the supply to the TEC module is cut off by sending the appropriate instruction from the 8051 to the relay. When the temperature slowly increases beyond the designed value, the microcontroller again activates the relay to supply voltage to the TEC module again.

5 Working of the circuit

The working model of the proposed model is depicted in Fig. 8. The microcontroller 8051 is used to control the operations of the temperature sensor. This microcontroller reads the input from the LM35 via ADC0809 and verifies it with the range of temperatures set by the user. Some temperatures are set by default. If the temperature read is within the range, the microcontroller continues to give supply to the relay which drives the TEC module. But once the temperature goes below the range, that is, if the insulated cabin gets cooler than what it is designed for, the microcontroller withdraws the supply to the TEC module. Henceforth, the temperature of the insulated cabin increases again. As soon as the temperature inside the cabin crosses the maximum value set by the user, the microcontroller again starts rendering supply to the relay which would start giving supply to the TEC module.

In the circuit, there is a provision for adjusting the range of temperatures. Under regular conditions, the seven segment display shows the temperature sensed by the LM-35 inside the insulated cabin. When the temperature range needs to changed, the “switch” button in the switching unit is operated. Then the microcontroller projects the lower limit of temperature of the unit on the seven segment display. Using the “increase” or “decrease” buttons, the value may be changed. When the “switch” button is operated again, the control shifts to setting of the upper limit. This temperature range also can be modified using “increase” or “decrease” buttons. Upon pressing the “switch” button again, the normal operation of the module is set. This enables the user to set the desired temperature ranges.

6 Mathematical formulations and representations

The general governing equations pertaining to the working of the thermoelectric module are as follows [4].

The rate of heat absorbed by the cold junction (Q_c) is given by

(1)Q_{\mathrm{c}}={\alpha }_{\mathrm{p}\mathrm{m}}{T}_{\mathrm{c}}I-\mathrm{0.5}{I}^{\mathrm{2}}R-K(T_{\mathrm{h}}-T_{\mathrm{c}}).

The electrical energy (W) consumed by the module is given by

(2)W={\alpha }_{\mathrm{p}\mathrm{m}}I(T_{\mathrm{h}}-T_{\mathrm{c}})+I^{\mathrm{2}}R.

The coefficient of performance (COP) of the module is defined as

(3)\mathrm{C}\mathrm{O}\mathrm{P}=Q_{\mathrm{c}}/W.

Combining all the effects of all the parameters, the figure of merit (FOM) of the system is obtained which is

(4)\mathrm{F}\mathrm{O}\mathrm{M}={\alpha }^{\mathrm{2}}/\lambda \rho .

The mathematical model of the TEC module has been discussed [7]. It consists of the parallel thermal resistors and serial electrical resistors whose equivalent networks are shown in Figs. 9 and 10. Also, a thorough analysis of the thermoelectric module is proposed [8] taking TB-127-1.4-1.2 as its reference. Tables 2 and 3 summarize the electrical analog of each thermal parameter.

7 Performance analysis

The performance specifications of the TEC-12706 [9] are listed in Table 4.

The performance analysis yields the relationship between voltage V, heat pump capacity W, device current I_max with respect to ∆T_max (where ∆T_max stands for maximum difference in temperature when heat load is zero), cooling capacity corresponding to ∆T_max and terminal voltage at I_max. The relationship curves are shown in Figs. 11, 12, and 13.

The optimal performance plots depict the behavior of the TEC module in the maximum COP mode. Few examples of such characteristic curves are given Fig. 14 [10].

8 Conclusions

This paper presents a working model of a thermoelectric refrigerator using the Peltier module or thermoelectric module TEC-12706. A detailed account of implementation and enhancement of the efficiency of the refrigerator has been discussed with a greater emphasis on the multi-purpose functionality and the cost-effectiveness of the module. Both the cooler and the hotter ends of the module are utilized effectively. Moreover, this module is portable and owing to its low cost and low power consumption, it can be used as a refrigerator in hostels and other places where power and money are the major constraints.

9 Future enhancements

There is quite a reasonable scope for improvising the module. Using of the upcoming microcontrollers like MSP430 etc. would avoid the use of a separate temperature sensor and an ADC. Besides, a specialized enclosure can be designed for this purpose so that the temperature would be retained within the cabin for a longer time, thus helping to decrease the switching of the TEC module to on and off consecutively for large number of times. Reasonable modifications of the module by retaining the basic idea would help to even construct an air conditioning system.

References

[1] Pettes A M, Hodes M S, Goodson K E. Optimized Thermoelectric Refrigeration in the presence of thermal boundary resistance. IEEE Transactions on Advanced Packaging, 2009, 32(2): 423–430

[2] Surith Nivas M, Vishnu Vardhan D, Raam kumar P H, Sai Prasad S, Ramya K. Photovoltaic driven dual purpose thermoelectric refrigerator for rural India. International Journal of Advancements in Research & Technology, 2013, 2(6): 111–117

[3] Sangale U V, Jhavar P, Seloskar G R. Thermoelectric refrigeration by using solar energy for domestic appliance. International Journal of Research in Advent Technology, 2015, 3(1): 72–75

[4] Rawat M K, Chattopadhyay H, Neogi S. An experimental investigation on thermoelectric refrigeration system: a potential green refrigeration technology. Journal of Environmental Research and Development, 2012, 6(4): 1059–1065

[5] Ghewari S. Thermoelectric refrigerator. Electronics for You, 2014, 2(11): 92–94

[6] Texas Instruments. ADC0808/ADC0809, 8-Bit μP compatible A/D converters with 8-channel multiplexer. 2014-12-23

[7] Meng F, Chen L, Sun F. Performance prediction and irreversibility analysis of a thermoelectric refrigerator with finned heat exchanger. Acta Physica Polonica A, 2011, 120(3): 397–406

[8] Lineykin S, Ben-Yaakov S. Analysis of thermoelectric coolers by a spice-compatible equivalent-circuit model. IEEE Power Electronics Letters, 2005, 3(2): 63–66

[9] Hebei I T. (Shanghai) Co., Ltd. Datasheet of TEC-12706. 2014-12-23

[10] Gromov G. Thermoelectric cooling modules. 2015-01-06

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Higher Education Press and Springer-Verlag Berlin Heidelberg