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Design and experimental investigation of a novel thermoelectric water dispenser unit
Accepted Manuscript Design and experimental investigation of a novel thermoelectric water dispenser unit Ahmet Çağlar PII: DOI: Reference:
S1359-4311(18)31073-1 https://doi.org/10.1016/j.applthermaleng.2018.11.028 ATE 12909
To appear in:
Applied Thermal Engineering
Received Date: Revised Date: Accepted Date:
16 February 2018 7 October 2018 7 November 2018
Please cite this article as: A. Çağlar, Design and experimental investigation of a novel thermoelectric water dispenser unit, Applied Thermal Engineering (2018), doi: https://doi.org/10.1016/j.applthermaleng.2018.11.028
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Design and experimental investigation of a novel thermoelectric water dispenser unit Ahmet Çağlar Department of Mechanical Engineering, Akdeniz University, 07058 Antalya, Turkey Pbx:+(90) (242) 3106342, Fax: +(90) (242) 3106306 E-mail: [email protected]
Abstract: In this study, a novel thermoelectric water dispenser unit is proposed to supply cold and hot drinking water simultaneously. For this purpose, the heat sinks attached to the cold and hot surfaces of a Peltier module are placed into the cold and hot water tanks of the thermoelectric water dispenser. Supplying power to the thermoelectric module, cold water tank is cooled while the hot water tank is heated at the same time. The cooling and heating performances of the system are examined for three cases: the tanks made of glass walls without insulation; the tanks made of polyethylene walls without insulation: and the tanks made of polyethylene walls with insulation. Results show that water tanks with polyethylene wall have better thermal performance than those with glass wall. Furthermore, insulation of the tanks has a signifant enhancement in COP especially on the heating side. Results of this study also indicate that TE water dispenser can compete with conventional types, with the advantages of being environment-friendly, smaller, silent and operable with renewable energy sources.
Keywords: Thermoelectric effect; Peltier element; Heating-cooling; Water dispenser
1. Introduction Researches on alternative energy technologies and conversion of waste heat into useful work have accelerated to reduce fossil fuel consumption. One of the research topics that has become increasingly interesting in recent years is thermoelectric (TE) elements. TE effect is a phenomenon in which a temperature difference or heat transfer occurs due to the passage of electric current through a junction between two different semiconductors, and vice versa. This effect is defined as the Seebeck, Peltier or Thomson effect depending on the conversion type of the energy. TE elements are widely used for cooling purposes but also for heating purposes. TE elements have the advantages of being environment-friendly, compact, less space need, long life, no moving parts, silent operation and no maintenance requirement. However, their applications are limited due to their lower energy conversion efficiencies than those for the other well-known methods. In 1822, German physicist Seebeck [1] discovered a magnetic field, which could turn a compass needle, and correspondingly an electric field occurred between two different metals
held at different temperatures. Peltier [1] observed that this effect was also in the opposite direction, that is, when a voltage is applied to different metals, a temperature difference occurs. Altenkirch [2] described thermoelectric figure of merit and stated that ideal thermoelectric materials should have high electrical conductivity and low thermal conductivity. There are many studies on TE elements both theoretically and experimentally in the literature [3-5]. Takahashi et al. [6] used a multi-layered TE pipe made of BST / Ni material as the tube of a shell and tube type heat exchanger and obtained both a more efficient heat exchanger and high power generation by the TE device. Zebarjadi [7] proposed the use of TE materials with high thermal conductivity and power factor instead of traditional high-efficient TE materials in electronic cooling. He et al. [8] developed a PV-assisted system that could make space heating in winter and space cooling and water heating in summer using a thermoelectric device. The cooling performance of TE cooler has been investigated by several authors in different modes experimentally and theoretically [10,11]. Chen et al. [12] investigated the optimization of heat transfer area allocation of four heat exchangers for maximizing the cooling load and the coefficient of performance (COP) of a combined TE generatorrefrigerator device. Modified pulse operation of TE cooler for building cooling was proposed by Manikandan et al. [13] and it could provide a COP value of 1.01, which was higher than the normal mode of operation. Soprani et al. [14] used a topology optimization model to optimize the design of a TE cooler for different operating conditions and to de-fine the optimal working conditions of the system and they applied the new methodology on an electronics unit. Dişlitaş et al. [15] developed a parabola algorithm to determine maximum current, voltage and thermo emf, and hence the performance of TE modules, based on measurements of temperature, current and voltage. Tipsaenporm et al. [16] performed an application of a direct evaporative cooling system for improving the performance of a compact TE air conditioner, they achieved 40.6% increase in the cooling capacity, and 20.9% increase in the COP. Tan et al. [17] proposed a TE cooling system integrated with phase change material (PCM) for space cooling such that PCM stored cold thermal energy at night and functioned as a heat sink to reduce hot side temperature of TE modules during day-time cooling period and thus increased the COP of the system from 0.5 to 0.78. An experimental performance analysis of mini-channel water cooled-TE refrigerator for different voltages and flow rates was presented by Gökçek et al. [18]. In another study on water-cooled TE cooler, effect of water mass flow rate on performance is investigated using temperature control
strategies under severe environment for prevent condensation and saving energy [19]. Heat dissipation mechanisms employed to remove waste heat from the cold side of a TE device were reviewed and summarized by Sajid et al. [20]. Joshi et al. [21] developed a TE fresh water generator for the people in coastal and humid regions with relative humidity above 60% having scarcity of drinking water. An efficient thermoelectric distillation system was designed and constructed for production of drinkable water [22]. Yıldırım et al. investigated experimentally a portable desalination unit using a TE cooler [23]. Commercial TE devices are generally operated in the longitudinal mode such that the heat flow is parallel to the electrical current. Transverse TE devices, on the other hand, can produce thermoelectric effects in which the electrical and thermal flows are perpendicular to one another [24-26]. Qian et al. [27] examined cooling performance of a hypothetical transverse TE device and concluded that transverse refrigerators might offer higher cooling capacity with some sacrifice in COP when compared to their longitudinal counterparts. Reitmaier et al. [28] compared the performances of transverse and conventional longitudinal thermoelectric cooler (TEC) and they have pointed out that the performance of transverse TEC made of metal semiconductors as multilayered is lower than that of conventional longitudinal TEC made of n- and p-type semiconductors, but transverse TEC devices are more compact, smaller and have a more easily accessible hot/cold sample surface. In this study, Peltier element is applied on a water dispenser unit that is used frequently for hot and cold drinking water supply in public places. Peltier elements are generally used for a single purpose: cooling or heating. Cooling applications can be reviewed as follows. A nanofluid cooled multiport minichannel heat exchanger coupled with thermoelectric cooler is designed for cooling the electronic devices [29]. Results showed that cooling of the TE module by nanofluid gived lower water cabin temperatures and higher COP values than that by pure water. To improve the energy conversion efficiency of TE cooling system, TE module is combined with water-cooling to reject heat from the hot side effectively [30]. It was reported that minimum water temperature could be obtained by increasing the electrical current input and decreasing the heat sink thermal resistance for a TE cooler using water as refrigerated object [31]. Heating applications can be reviewed as follows. Zang et al. [32] developed a solar TE cogenerator that can perform both heating and electricity generation. Allouhi et al. [33] performed a theoretical analysis of a TE heating system for an office room and obtained 64% reduction in energy use as compared to the conventional electric heater. An open-type TE space heating system with multiple channels is developed by Liu et al. [34] and they observed 1.3 heating COP. There are few works on both cooling and heating by TE
modules. 5.0 and 1.0 heating and cooling COPs were achieved respectively in the performance analysis of a TE air heating/cooling unit at different TEC voltages and fan speeds [35]. The potential application of TE heating/cooling system for residential buildings is studied using 16 TE modules [36]. TE radiant heating/cooling panel is used for airconditioning systems [37]. PV TE cooling/heating system is proposed for gaining space cooling and hot water simultaneously [38]. A TE module is operated in the combination of water cooling and heating modes [39]. Hot and cold streams are used to circulate water through the heat sinks clamped together both sides of TE module and passes through the hot and cold water tanks respectively, so a combined thermoelectric heater and cooler is achieved for the first time. The total COP (heating + cooling COP) of 4.5 is obtained and drawback of high temperature difference between module surfaces is discussed in the study. As seen in the literature, only cooling or only heating applications are generally performed and a few studies are carried out on simultaneous heating/cooling application. Furthermore, most of these applications are generally based on air conditioning. Simultaneous water heating/cooling application is encountered only in Ref. [39]. In that study, water is circulated through the surfaces of TE module and rejects/takes heat to/from the tanks using HXs. It did not cover water tanks contacting directly with TE module as in case of a water dispenser unit. This study is unique as it covers simultaneous water heating/cooling in separate tanks supplying conditioned drinking water as a water dispenser. In the study, a novel water dispenser unit is proposed by applying a Peltier element on the hot and cold sides of a portable water dispenser. In conventional water dispenser systems, a compressor and resistance heater are used for cooling and heating of the water inside the cold and hot water tanks, respectively. In this study, however, both cooling and heating of water are performed simultaneously by using the cold and hot surfaces of a single TE device. Since TE element is more compact and smaller, a system that is smaller, lighter and therefore more portable than a traditional water dispenser system is obtained. TE water dispenser is also a cleaner production due to having no any refrigerant. Furthermore, since the surface temperature of a thermoelectric device is directly proportional to the input of electricity, the temperature of the system can be controlled easily and precisely. In addition, since water dispensers require low powers, it is also possible to develop a PV-assisted TE water dispenser unit, which can be driven by the electricity generated from PV installed on the top and sides of the system. As a clean energy technology without air and noise pollution, and being different from conventional types, the novel TE water dispenser enables the production of hot and cold drinking water and will find widespread utilization in public spaces. So, the current study can be a first step in
implementing TE modules in new thermal applications like instant water conditioning and can lead to new designs and investigations in this area. In the study, variations of temperatures of cold and hot water tanks, power consumption of Peltier element and cooling/heating performances of the system have been investigated for three cases. Case I, II and III define the use of glass walls without insulation, polyethylene walls without insulation and polyethylene walls with insulation respectively for the prototype. The water temperatures and performances for each of the cases are evaluated and compared with each other.
2. Material and method
The main components of the TE water dispenser unit are the Peltier cooler, cold and hot water reservoirs and power supply that supplies electricity at the appropriate voltage (12 V). Since the rest of the water dispenser design (package, taps, water supply etc.) is only concerned with configuration, safety and control, the other components are out of scope of the study. The schematic of the system is shown in Fig. 1. A prototype is designed and manufactured to test the system for each of the three different cases: the reservoirs made of glass walls without insulation; the reservoirs made of polyethylene walls without insulation: and the reservoirs made of polyethylene walls with insulation. A series of experiments are conducted for each cases, and measurements are recorded. The main components and assembled situation of the system are explained in the following sections in detail.
Fig. 1. Schematic of TE water dispenser unit.
2.1. Peltier element
In the system, 12 V (max 6 A) 4x4 cm Peltier element is used. The view and technical specifications of the Peltier element used are given in Fig. 2. When electricity is applied, one surface of the Peltier element heats while the other surface cools depending on the phase connection. Thus, a temperature difference between the surfaces occur depending on the physical properties of two thermoelectric materials in the Peltier element. GW Instek GPS3030DD DC power supply is used to provide constant voltage for the TE device.
Technical Specifications:
Model Dimensions Feed Voltage Vmax (V) Imax (A) Max. Power (W) Feed cable length Weigth Color
: TEC1-12706 : 4 x 4 x 0.38 cm : 12V : 15V : 4-6 A : 72 W : 30 cm : 22 g : White
Fig. 2. View and technical specifications of the Peltier element.
2.2. Heat sinks (Fins)
Heat sinks are mounted on both sides of the Peltier element to increase the amount of heat transferred from the surfaces to the water. The 6x6 cm fins made of aluminum material are placed on the surfaces of the Peltier element using thermal paste, so the contact resistance between the fins and the surfaces is therefore minimized. The views of the heat sink and thermal paste are given in Fig. 3. The thermal paste used has a thermal conductivity of 5.6 W/(mK).
(a) (b) Fig. 3. (a) Heat sink; (b) Thermal paste used in the system.
2.3. Hot and cold water tanks
Top and bottom walls and three sides of the water tanks are made of glass in the first prototype. The other wall on which the Peltier element is mounted is made of polyethylene. Polyethylene plate has the advantages of -50 – 90 °C temperature resistance, no microorganism formation, impermeable, low thermal conductivity.
Using separate
polyethylene plates, the hot and cold water tanks were isolated from each other, thus preventing heat transfer between the tanks. The cold and hot sides of the Peltier element are embedded in the middle of the polyethylene plates. The heat sinks, however, remain in contact with water in the tanks. Thus, heat is transferred from the hot surface of the Peltier element to the fin and then from the fin to the water on the heating side, while heat is transferred from the water to the fin and then from the fin to the cold surface of the Peltier element on the cooling side. In the second and third prototypes, all walls of the water tanks are made of polyethylene plates. The size of the tanks is 10x15x15 cm3.
2.4. Liquid seal and silicone
Silicone is used to assemble the glass or polyethylene walls to each other, thus sealing is achieved. Heat-resistant silicone is chosen due to its steady properties at high temperatures. As the polyethylene plates are combined with the Peltier element, liquid seal with high adhesive power and high freezing rate is used, so a good heat insulation as well as sealing are obtained.
2.5. Measurement devices
Type T Copper-Constantan thermocouples are used to measure the water temperatures in the experiments. These thermocouples can measure temperatures between -200 °C and 300 °C with ± 0.5 °C accuracy. Temperatures at five different points (see Figure 1) inside the tanks are measured. Temperature measurement points are determined such a way that the mean of
the measurements represent the mean of the water temperature inside the tanks. For this purpose, each tank volume is considered to have five sections and each section is represented by the following points: the center of the tank, the midpoint of the water cross section adjacent to the bottom surface of the tank, near the outlet of the water (near taps), the midpoint of the water cross section adjacent to one of the other parallel sides of the tank and near to the heat sink. The average of the five temperature readings is taken as the temperature of the corresponding water tank. In addition, the surface temperature of the heat sinks are checked for maintaining the Peltier element at appropriate operating conditions. All temperature measurements are collected and stored by Elimko E-680 data logger seen in Fig. 4. Voltage and current supplied to the Peltier during the experiments are measured by a digital oscilloscope. While the voltage is measured directly, the current is measured by using a shunt resistor. As known, input current changes depending on the voltage applied to the Peltier element and temperature difference between the hot and cold surfaces. For an accurate measurement, instantenous changes are determined by the oscilloscope. The average power consumption by the Peltier element is calculated using the accurate measurements for the time intervals selected. The views of DC power supply, thermocouples, digital oscilloscope and data logger are given in Fig. 4. a
b
c
d
Fig. 4. (a) Power supply; (b) Type T thermocouple; (c) Digital oscilloscope; (d) Data logger.
3. Results and discussion
The view of the first prototype of the system proposed is given in Fig. 5. The Peltier element is placed between the hot and cold water tanks. The fins are attached to both surfaces of the Peltier element. Tests are performed for three different cases. In Case I, all the walls of the tanks are made of glass except for the walls where the Peltier element is mounted. No insulation is applied to the walls. In Case II, all the walls are made of polyethylene plates and not insulated. In Case III, the walls are made of polyethylene plates and insulated. During the
experiments, the temperatures of hot and cold water tanks and ambient temperature are measured and their changes over time were recorded. The maximum hot water temperature and minimum cold water temperature obtained from the system are determined for each case.
Fig. 5. First prototype.
A series of experiments were conducted with the experimental setup. Temperatures (from 5 points for each tank) and total power consumption, calculated by accurate readings of instantaneous voltage and current values, are recorded with 5-minute time intervals during 1 hour. Initial water temperature for each cases is taken as 20 oC which is the temperature of tap water. The volume of the water put into the tanks is 2 liters for each. The data obtained for the prototype made of uninsulated glass walls is given in Fig. 6 for Case I. As seen in Fig. 6, the temperature of the hot water storage tank increases linearly at the beginning. The increase in the temperature slows down after a certain time. After 60 minutes, the hot water tank reaches 47.2 oC. The temperature of the cold water tank decreases slowly with time and temperature gradients are smaller than that in the hot water tank. The cold water tank temperature decreases to 5.6 oC after 60 minutes. It can be concluded that the hot side of Peltier element is performing more efficiently than the cold side for the specified conditions and simultaneous heating/cooling processes. On the other hand, the power consumption of the Peltier element decreases with time. During the experiments, it is observed that the amount of current drawn by the Peltier element decreases with increasing temperature difference between its surfaces when a constant voltage is applied. For this reason, the power consumption decreases with increasing temperature difference. The power consumption varies from 56.7 W to 36.6 W while the temperature difference between the hot and cold water tanks varies from 0 to 41.6 oC. The reason for this is the fact that both heating and cooling capacities decreases with the increasing temperature difference of the Peltier sides.
55 50
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0
Power Consumption (W)
50
Temperature (oC)
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Cold tank temp. Hot tank temp. Ambient temp. Power Consumption
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25 0
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Time (minutes) Fig. 6. Variation of temperatures and power consumption with time for glass-wall prototype without insulation (Case I).
In Case II, the walls of the tanks are all made of polyethylene plates. The building material of the model, which is a design condition, has been changed keeping the tanks’ volumes and Peltier element the same and maintaining the system uninsulated. Using the same initial temperature (20 oC), variation of temperatures inside the water tanks and the power consumption of the Peltier element is investigated. The observation results are shown in Fig. 7.
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25 0
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Pwoer Consumption (W)
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Temperature (oC)
60
Cold tank temp. Hot tank temp. Ambient temp. Power Consumption
60
60
Time (minutes) Fig. 7. Variation of temperatures and power consumption with time for polyethylene-wall prototype without insulation (Case II).
As seen Fig. 7, temperature gradients for both heating and cooling sides are higher than in Case I. Using polyethylene plates instead of glass, the temperature of the cold water decreases down to 4.1 oC in the cooling side, while the temperature of the hot water increases up to 50.1 o
C in the heating side in the period of 60 minutes. In the both sides, so the cooling and heating
sides, the temperature gradients are higher at beginning of the processes, then the gradient decreases with time. This is actually due to the decrease in heating/cooling capacities and COP of the Peltier element with increasing temperature difference between the hot and cold sides. When comparing with Case I, in Case II the cold water temperature is reduced by 1.5 o
C more than in Case I, while the hot water temperature is increased by 2.9 oC more than in
Case I. On the other hand, the power consumption of the Peltier element varies from 48.7 W to 32.7 W during 60-minute period. Power consumptions seem to be lower than in Case I depending on the higher temperature differences in Case II since more temperature difference between the hot and cold surfaces results in less power consumption. Consequently, less power consumptions with higher cooling/heating rates yields higher COPC/COPH values. This situation is confirmed in Section 3.1 related with the cooling/heating performance of the system. As a result, it can be stated that the model with polyethylene wall shows better thermal performance than that with glass wall. In Case III, the water tanks made of polyethylene plates are insulated by 1-cm thick aluminum foil insulation sheet (see Fig. 8). In this case, the effect of the insulation on the temperatures and power consumption is investigated. The results obtained are shown in Fig. 9.
Fig. 8. Prototype made of polyethylene walls with insulation.
60 60
Temperature (oC)
Cold tank temp. Hot tank temp. Ambient temp. Power Consumption
40 30
50 45 40
20
35
10
30
0
Power Consumption (W)
55 50
25 0
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Time (minutes) Fig. 9. Variation of temperatures and power consumption with time for polyethylene-wall prototype with insulation (Case III).
In the insulated prototype, the minimum cold water temperature is obtained as 1.3 °C while the maximum hot water temperature reaches to 61.5 °C. Comparing with Case II, the temperature of the cold water tank after 60 minutes is 2.8 °C lower although initial temperatures are the same. On the other hand, the temperature of the hot water tank is 11.4 °C higher than that obtained in Case II after 60 minutes. Among the three cases, the highest temperature in the heating side and the lowest temperature in the cooling side are both obtained in Case III. The variations of water temperatures have the same behavior for all cases. Temperature difference is not allowed for exceeding a certain value to prevent the Peltier module from a possible damage. Furthermore, the hot side of the Peltier element should be well cooled for maintaining the cooling effect of the cold side. Otherwise, the cold side cannot absorb heat and its temperature starts to increase.
3.1. Cooling and heating rates of Peltier element
Simplified energy equilibrium model which is validated by many researchers [30,40,41] is generally used to determine heat transfer rates. According to this model, the heat absorbed and released for a TE cooler can be calculated by QC o ITc Ko (Th Tc ) 0.5Ro I 2
(1)
QH o ITh Ko (Th Tc ) 0.5Ro I 2
(2)
where αo, Ko and Ro are the total Seebeck coefficient, total thermal conductance and total electric resistance, respectively, I is the electrical current, Tc and Th are the cold and hot side temperatures, respectively. Since the cold and hot water temperatures are measured in this study, and surface temperatures of TE element are not measured, the rate of cooling and heating is found by
QC Qa
QH Qr
where
(mc p T )CW t (mc p T ) HW
and
t
(3)
(4)
are the heat adsorbed by the cold water and the heat rejected to the hot water
(W), respectively. Subscripts CW and HW denote the cold and hot water, respectively. m, cp and ΔT are water mass (kg), specific heat (kJ/kg K) and temperature difference (K). Δt is the time (s) elapsed for the corresponding temperature difference. The changes of cooling and heating rates are shown in Fig. 10. The rates are calculated for 5-minute time periods. Both the cooling and heating rates decrease with time. The reason for that is the relationship between the temperature difference and heat transfer rates. Heat transfer rates decrease with increasing temperature difference between the cold and hot sides. Because the temperature difference increases with time, the transfer rates at both sides decreases during the experiment. The highest rates are obtained in Case III while the lowest rates are obtained in Case I. In Case III, the maximum and minimum cooling rates are 142.1 W and 11.8 W while the maximum and minimum heating rates are 193.7 W and 33.7 W. The heating rates seem to be higher than the cooling rates. This situation indicates that TE water dispenser performs more efficiently as a heater than as a cooler.
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120 (filled markers)
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Heating Rate (W)
Cooling Rate (W)
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(unfilled markers)
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Time (minutes) Fig. 10. Variation of the cooling and heating rates of Peltier device with time.
3.2. Cooling and heating COP of Peltier element
To evaluate the performances of the Peltier element in cooling and heating, the cooling and heating COP (coefficient of performance) can be calculated by the following equations, respectively:
COPC
Qa (mc p T )CW / t WP I P .VP
(5)
COPH
Qr (mc p T ) HW / t WP I P .VP
(6)
In Eqn. (1), COPC and COPH represents the cooling and heating coefficient of performance respectively.
is the power consumption by the Peltier element (W). IP and VP
are the current (A) and voltage (V) drawn by the Peltier element, respectively. The variation of COPC of the system over time for each of the three cases is given in Fig. 11. The cooling COPs for the Cases I, II and III vary between 2.26-0.24, 2.46-0.28 and 2.900.42, respectively, during 60-minute period. The COP values decrease with time due to the lower cooling capacities at higher temperature differences. The best cooling performance is obtained in Case III. Case III has higher COP C values than the other cases except for t=15
min. In conclusion, the insulation of the system in Case III provides about 22% increase in the mean COPC compared to the uninsulated system in Case II.
3.00 2.50
Case I Case II
2.00
COPC
Case III
1.50 1.00 0.50
0.00 0
10
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Time (minutes) Fig. 11. Variation of the cooling COP of Peltier device with time.
The variation of COPH of the system over time for all cases is given in Fig. 12. The heating COPs for the Cases I, II and III vary between 2.41-0.32, 3.07-0.79 and 3.95-1.19, respectively, during 60-minute period. The COP values decrease with time due to the lower heating capacities at higher temperature differences between two sides of the Peltier element. The best heating performance is obtained in Case III. The positive effect of the insulation on the mean heating performance of the system occurs at about 45% level when the insulated case (Case III) is compared with the uninsulated case (Case II). On the other hand, the prototype with uninsulated glass wall has the lowest COP H values.
4.00 3.50
COP H
3.00 2.50 2.00 1.50 Case I
1.00
Case II
0.50
Case III
0.00 0
10
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Time (minutes) Fig. 12. Variation of the heating COP of Peltier device with time.
Performance values of Peltier devices can reach to 4-5 at ideal conditions, namely at zero temperature difference between two surfaces and for optimal values of voltage and current. Peltier elements are typically used for cooling (especially for electronic cooling), and high performance can be achieved if the hot side is cooled well. In this study, the use of Peltier element for both heating and cooling purpose causes lower performances than the ideal case.
4. Conclusions
In this study, a novel TE water dispenser is developed by using a Peltier module that can perform both cooling and heating of drinking water simultaneously. For this purpose, two water tanks are used as cold and hot water storage device for the TE water dispenser. One of the tanks is cooled by the cold side of the Peltier element while the other is heated by the hot side of it. This novel system enables to setup a water dispenser unit that provides conditioned drinking water at appropriate temperatures depending on climatic conditions. The TE water dispenser are investigated experimentally for three different cases and a number of tests are performed for each case. In Case I, the water tanks are made of glass walls without insulation. In Case II, the water tanks are made of polyethylene walls without insulation. In Case III, the tanks are made of polyethylene walls with insulation. Variation of water temperatures, power consumption of the Peltier element, and cooling/heating
performances (COPC/COPH) of the system are investigated. Results show that the Case III, i.e. prototype made of ployethylene walls with insulation, has the best thermal performance. The insulation of the system has a significant effect on the thermal performance of the system as expected.In Case III, minimum cold water temperature on the cooling side is obtained as 1.3 °C while maximum hot water temperature on the heating side is obtained as 61.5 °C at the end of 60-minute time period. The cooling and heating COPs for Case III are obatined as 2.90 and 3.95, respectively. When two uninsulated cases are compared with each other, the prototype with polyethylene walls has better thermal characteristics than that with glass walls. Results also show the novel TE water dispenser can be competitive with conventional types with its advantages such as smaller and compact design, environmentally friendly and noiseless operation and being operable with renewable energy sources like solar energy. Consequently, this is the first project to use a TE module in a water dispenser unit. The TE water dispenser unit is experimentally proven to achieve simultaneous water cooling and heating operation and meet the need for conditioned drinking water in this study. As a future work, a solar assisted TE water dispenser unit will be carried out by mounting PV panels on the roof and solar-facing sides of the unit to supply drinking water in open public spaces. PV modules can thus supply electricity power to the TE module. Total area of PV modules can be determined by the capacity of TE module. It should be noted that the capacity of TE module used in this study is 72-W. The larger the TE capacity, the larger the cooling and heating rates obtained. References [1] S. Twaha, J. Zhu, Y. Yan, B. Li, A Comprehensive review of thermoelectric technology: Materials, applications, modelling and performance improvement, Renew. Sust. Energy Rev. 65 (2016) 698–726. [2] C. Gayner, K.K. Kar, Recent advances in thermoelectric materials, Prog. Mater. Sci. 83 (2016) 330–382. [4] S.B. Riffat, X. Ma, Thermoelectrics: a review of present and potential applications, Appl. Therm. Eng. 23 (2003) 913–935. [5] D.M. Rowe, G. Min, Evaluation of thermoelectric modules for power generation, J. Power Sources, 73 (1998) 193–198. [6] B.J. Huang, C.J. Chin, C.L. Duang, A design method of thermoelectric cooler, Int. J. Refrig. 23 (2000) 208–218. [7] K. Takahashi, T. Kanno, A. Sakai, H. Tamaki, H. Kusada, Y. Yamada, Bifunctional thermoelectric tube made of tilted multilayer material as an alternative to standard heat exchangers, Sci. Rep., 3 (2013), 1501. [8] M. Zebarjadi, Electronic cooling using thermoelectric devices, Appl. Phys. Lett. 106 (2015) 203506. [9] W. He, J. Zhou, J. Hou, C. Chen, J. Ji, Theoretical and experimental investigation on a thermoelectric cooling and heating system driven by solar, Appl. Energy 107 (2013) 89–97.
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[27] B. Qian, F. Ren, Cooling performance of transverse thermoelectric devices, Int. J. Heat Mass Transf. 95 (2016) 787–794. [28] C. Reitmaier, F. Walther, H. Lengfellner, Transverse thermoelectric devices, Appl. Phys. A 99 (2010) 717– 722. [29] N. Ahammed, L.G. Asirvatham, S. Wongwises, Thermoelectric cooling of electronic devices with nanofluid in a multiport minichannel heat exchanger, Exp. Therm. Fluid Sci. 74 (2016) 81–90. [30] H.S. Huang, Y.C. Weng, Y.W. Chang, S.L. Chen, M.T. Ke, Thermoelectric water-cooling device applied to electronic equipment, Int. Commun. Heat Mass Transf. 37 (2010) 140–146. [31] R. Chein, Y. Chen, Performances of thermoelectric cooler integrated with microchannel heat sinks, Int. J. Refrig. 28 (2005) 828–839. [32] M. Zhang, L. Miao, Y.P. Kang, S. Tanemura, C.A.J. Fisher, G.Xu, C.X. Li, G.Z. Fan, Efficient, low-cost solar thermoelectric cogenerators comprising evacuated tubular solar collectors and thermoelectric modules, Appl. Energy 109 (2013) 51–59. [33] A. Allouhi, A. Boharb, T. Ratlamwala, T. Kousksou, M.B. Amine, A. Jamil, A.A. Msaad, Dynamic analysis of a thermoelectric heating system for space heating in a continuous-occupancy office room, Appl. Therm. Eng. 113 (2017) 150–159. [34] D. Liu, F.Y. Zhao, H. Yang, G.F. Tang, Theoretical and experimental investigations of thermoelectric heating system with multiple ventilation channels, Appl. Energy. 159 (2015) 458–468. [35] M.Z. Yilmazoglu, Experimental and numerical investigation of a prototype thermoelectric heating and cooling unit, Energy Build. 113 (2016) 51–60. [36] M. Ibanez-Puy, J. Bermejo-Busto, C. Martin-Gomez, M. Vidaurre-Arbizu, J.A. Sacristan-Fernandez, Thermoelectric cooling heating unit performance under real conditions, Appl. Energy 200 (2017) 303–314. [37] L. Shen, Z. Tu, Q. Hu, C. Tao, H. Chen, The optimization design and parametric study of thermoelectric radiant cooling and heating panel, Appl. Therm. Eng. 112 (2017) 688–697. [38] R. Daghigh, Y. Khaledian, Effective design, theoretical and experimental assessment of a solar thermoelectric cooling-heating system, Sol. Energy 162 (2018) 561–572. [39] M.A. Ahamat, R. Abidin, S.M. Abdullah, Performance of thermoelectric module as a water cooler and water heater, Int. J. Adv. Sci. Eng. Inf. Techno. 6 (2016) 524–528. [40] L. Shen, F. Xiao, H. Chen, S. Wang, Investigation of a novel thermoelectric radiant air-conditioning system, Energy Build. 59 (2013) 123–132. [41] Y. Zhou, J. Yu, Design optimization of thermoelectric cooling systems for applications in electronic devices, Int. J. Refrig. 35 (2012) 1139–1144.
Highlights
Novel TE water dispenser unit is used for water cooling/heating simultaneously Polyethylene wall for water tanks shows better thermal performance than glass wall Insulation of tanks has a significant enhancement in COP especially on heating side Less power consumption of TE module yields higher thermal performance values COP evaluation proves TE water dispenser to be competitive with conventional types
S1359-4311(18)31073-1 https://doi.org/10.1016/j.applthermaleng.2018.11.028 ATE 12909
To appear in:
Applied Thermal Engineering
Received Date: Revised Date: Accepted Date:
16 February 2018 7 October 2018 7 November 2018
Please cite this article as: A. Çağlar, Design and experimental investigation of a novel thermoelectric water dispenser unit, Applied Thermal Engineering (2018), doi: https://doi.org/10.1016/j.applthermaleng.2018.11.028
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Design and experimental investigation of a novel thermoelectric water dispenser unit Ahmet Çağlar Department of Mechanical Engineering, Akdeniz University, 07058 Antalya, Turkey Pbx:+(90) (242) 3106342, Fax: +(90) (242) 3106306 E-mail: [email protected]
Abstract: In this study, a novel thermoelectric water dispenser unit is proposed to supply cold and hot drinking water simultaneously. For this purpose, the heat sinks attached to the cold and hot surfaces of a Peltier module are placed into the cold and hot water tanks of the thermoelectric water dispenser. Supplying power to the thermoelectric module, cold water tank is cooled while the hot water tank is heated at the same time. The cooling and heating performances of the system are examined for three cases: the tanks made of glass walls without insulation; the tanks made of polyethylene walls without insulation: and the tanks made of polyethylene walls with insulation. Results show that water tanks with polyethylene wall have better thermal performance than those with glass wall. Furthermore, insulation of the tanks has a signifant enhancement in COP especially on the heating side. Results of this study also indicate that TE water dispenser can compete with conventional types, with the advantages of being environment-friendly, smaller, silent and operable with renewable energy sources.
Keywords: Thermoelectric effect; Peltier element; Heating-cooling; Water dispenser
1. Introduction Researches on alternative energy technologies and conversion of waste heat into useful work have accelerated to reduce fossil fuel consumption. One of the research topics that has become increasingly interesting in recent years is thermoelectric (TE) elements. TE effect is a phenomenon in which a temperature difference or heat transfer occurs due to the passage of electric current through a junction between two different semiconductors, and vice versa. This effect is defined as the Seebeck, Peltier or Thomson effect depending on the conversion type of the energy. TE elements are widely used for cooling purposes but also for heating purposes. TE elements have the advantages of being environment-friendly, compact, less space need, long life, no moving parts, silent operation and no maintenance requirement. However, their applications are limited due to their lower energy conversion efficiencies than those for the other well-known methods. In 1822, German physicist Seebeck [1] discovered a magnetic field, which could turn a compass needle, and correspondingly an electric field occurred between two different metals
held at different temperatures. Peltier [1] observed that this effect was also in the opposite direction, that is, when a voltage is applied to different metals, a temperature difference occurs. Altenkirch [2] described thermoelectric figure of merit and stated that ideal thermoelectric materials should have high electrical conductivity and low thermal conductivity. There are many studies on TE elements both theoretically and experimentally in the literature [3-5]. Takahashi et al. [6] used a multi-layered TE pipe made of BST / Ni material as the tube of a shell and tube type heat exchanger and obtained both a more efficient heat exchanger and high power generation by the TE device. Zebarjadi [7] proposed the use of TE materials with high thermal conductivity and power factor instead of traditional high-efficient TE materials in electronic cooling. He et al. [8] developed a PV-assisted system that could make space heating in winter and space cooling and water heating in summer using a thermoelectric device. The cooling performance of TE cooler has been investigated by several authors in different modes experimentally and theoretically [10,11]. Chen et al. [12] investigated the optimization of heat transfer area allocation of four heat exchangers for maximizing the cooling load and the coefficient of performance (COP) of a combined TE generatorrefrigerator device. Modified pulse operation of TE cooler for building cooling was proposed by Manikandan et al. [13] and it could provide a COP value of 1.01, which was higher than the normal mode of operation. Soprani et al. [14] used a topology optimization model to optimize the design of a TE cooler for different operating conditions and to de-fine the optimal working conditions of the system and they applied the new methodology on an electronics unit. Dişlitaş et al. [15] developed a parabola algorithm to determine maximum current, voltage and thermo emf, and hence the performance of TE modules, based on measurements of temperature, current and voltage. Tipsaenporm et al. [16] performed an application of a direct evaporative cooling system for improving the performance of a compact TE air conditioner, they achieved 40.6% increase in the cooling capacity, and 20.9% increase in the COP. Tan et al. [17] proposed a TE cooling system integrated with phase change material (PCM) for space cooling such that PCM stored cold thermal energy at night and functioned as a heat sink to reduce hot side temperature of TE modules during day-time cooling period and thus increased the COP of the system from 0.5 to 0.78. An experimental performance analysis of mini-channel water cooled-TE refrigerator for different voltages and flow rates was presented by Gökçek et al. [18]. In another study on water-cooled TE cooler, effect of water mass flow rate on performance is investigated using temperature control
strategies under severe environment for prevent condensation and saving energy [19]. Heat dissipation mechanisms employed to remove waste heat from the cold side of a TE device were reviewed and summarized by Sajid et al. [20]. Joshi et al. [21] developed a TE fresh water generator for the people in coastal and humid regions with relative humidity above 60% having scarcity of drinking water. An efficient thermoelectric distillation system was designed and constructed for production of drinkable water [22]. Yıldırım et al. investigated experimentally a portable desalination unit using a TE cooler [23]. Commercial TE devices are generally operated in the longitudinal mode such that the heat flow is parallel to the electrical current. Transverse TE devices, on the other hand, can produce thermoelectric effects in which the electrical and thermal flows are perpendicular to one another [24-26]. Qian et al. [27] examined cooling performance of a hypothetical transverse TE device and concluded that transverse refrigerators might offer higher cooling capacity with some sacrifice in COP when compared to their longitudinal counterparts. Reitmaier et al. [28] compared the performances of transverse and conventional longitudinal thermoelectric cooler (TEC) and they have pointed out that the performance of transverse TEC made of metal semiconductors as multilayered is lower than that of conventional longitudinal TEC made of n- and p-type semiconductors, but transverse TEC devices are more compact, smaller and have a more easily accessible hot/cold sample surface. In this study, Peltier element is applied on a water dispenser unit that is used frequently for hot and cold drinking water supply in public places. Peltier elements are generally used for a single purpose: cooling or heating. Cooling applications can be reviewed as follows. A nanofluid cooled multiport minichannel heat exchanger coupled with thermoelectric cooler is designed for cooling the electronic devices [29]. Results showed that cooling of the TE module by nanofluid gived lower water cabin temperatures and higher COP values than that by pure water. To improve the energy conversion efficiency of TE cooling system, TE module is combined with water-cooling to reject heat from the hot side effectively [30]. It was reported that minimum water temperature could be obtained by increasing the electrical current input and decreasing the heat sink thermal resistance for a TE cooler using water as refrigerated object [31]. Heating applications can be reviewed as follows. Zang et al. [32] developed a solar TE cogenerator that can perform both heating and electricity generation. Allouhi et al. [33] performed a theoretical analysis of a TE heating system for an office room and obtained 64% reduction in energy use as compared to the conventional electric heater. An open-type TE space heating system with multiple channels is developed by Liu et al. [34] and they observed 1.3 heating COP. There are few works on both cooling and heating by TE
modules. 5.0 and 1.0 heating and cooling COPs were achieved respectively in the performance analysis of a TE air heating/cooling unit at different TEC voltages and fan speeds [35]. The potential application of TE heating/cooling system for residential buildings is studied using 16 TE modules [36]. TE radiant heating/cooling panel is used for airconditioning systems [37]. PV TE cooling/heating system is proposed for gaining space cooling and hot water simultaneously [38]. A TE module is operated in the combination of water cooling and heating modes [39]. Hot and cold streams are used to circulate water through the heat sinks clamped together both sides of TE module and passes through the hot and cold water tanks respectively, so a combined thermoelectric heater and cooler is achieved for the first time. The total COP (heating + cooling COP) of 4.5 is obtained and drawback of high temperature difference between module surfaces is discussed in the study. As seen in the literature, only cooling or only heating applications are generally performed and a few studies are carried out on simultaneous heating/cooling application. Furthermore, most of these applications are generally based on air conditioning. Simultaneous water heating/cooling application is encountered only in Ref. [39]. In that study, water is circulated through the surfaces of TE module and rejects/takes heat to/from the tanks using HXs. It did not cover water tanks contacting directly with TE module as in case of a water dispenser unit. This study is unique as it covers simultaneous water heating/cooling in separate tanks supplying conditioned drinking water as a water dispenser. In the study, a novel water dispenser unit is proposed by applying a Peltier element on the hot and cold sides of a portable water dispenser. In conventional water dispenser systems, a compressor and resistance heater are used for cooling and heating of the water inside the cold and hot water tanks, respectively. In this study, however, both cooling and heating of water are performed simultaneously by using the cold and hot surfaces of a single TE device. Since TE element is more compact and smaller, a system that is smaller, lighter and therefore more portable than a traditional water dispenser system is obtained. TE water dispenser is also a cleaner production due to having no any refrigerant. Furthermore, since the surface temperature of a thermoelectric device is directly proportional to the input of electricity, the temperature of the system can be controlled easily and precisely. In addition, since water dispensers require low powers, it is also possible to develop a PV-assisted TE water dispenser unit, which can be driven by the electricity generated from PV installed on the top and sides of the system. As a clean energy technology without air and noise pollution, and being different from conventional types, the novel TE water dispenser enables the production of hot and cold drinking water and will find widespread utilization in public spaces. So, the current study can be a first step in
implementing TE modules in new thermal applications like instant water conditioning and can lead to new designs and investigations in this area. In the study, variations of temperatures of cold and hot water tanks, power consumption of Peltier element and cooling/heating performances of the system have been investigated for three cases. Case I, II and III define the use of glass walls without insulation, polyethylene walls without insulation and polyethylene walls with insulation respectively for the prototype. The water temperatures and performances for each of the cases are evaluated and compared with each other.
2. Material and method
The main components of the TE water dispenser unit are the Peltier cooler, cold and hot water reservoirs and power supply that supplies electricity at the appropriate voltage (12 V). Since the rest of the water dispenser design (package, taps, water supply etc.) is only concerned with configuration, safety and control, the other components are out of scope of the study. The schematic of the system is shown in Fig. 1. A prototype is designed and manufactured to test the system for each of the three different cases: the reservoirs made of glass walls without insulation; the reservoirs made of polyethylene walls without insulation: and the reservoirs made of polyethylene walls with insulation. A series of experiments are conducted for each cases, and measurements are recorded. The main components and assembled situation of the system are explained in the following sections in detail.
Fig. 1. Schematic of TE water dispenser unit.
2.1. Peltier element
In the system, 12 V (max 6 A) 4x4 cm Peltier element is used. The view and technical specifications of the Peltier element used are given in Fig. 2. When electricity is applied, one surface of the Peltier element heats while the other surface cools depending on the phase connection. Thus, a temperature difference between the surfaces occur depending on the physical properties of two thermoelectric materials in the Peltier element. GW Instek GPS3030DD DC power supply is used to provide constant voltage for the TE device.
Technical Specifications:
Model Dimensions Feed Voltage Vmax (V) Imax (A) Max. Power (W) Feed cable length Weigth Color
: TEC1-12706 : 4 x 4 x 0.38 cm : 12V : 15V : 4-6 A : 72 W : 30 cm : 22 g : White
Fig. 2. View and technical specifications of the Peltier element.
2.2. Heat sinks (Fins)
Heat sinks are mounted on both sides of the Peltier element to increase the amount of heat transferred from the surfaces to the water. The 6x6 cm fins made of aluminum material are placed on the surfaces of the Peltier element using thermal paste, so the contact resistance between the fins and the surfaces is therefore minimized. The views of the heat sink and thermal paste are given in Fig. 3. The thermal paste used has a thermal conductivity of 5.6 W/(mK).
(a) (b) Fig. 3. (a) Heat sink; (b) Thermal paste used in the system.
2.3. Hot and cold water tanks
Top and bottom walls and three sides of the water tanks are made of glass in the first prototype. The other wall on which the Peltier element is mounted is made of polyethylene. Polyethylene plate has the advantages of -50 – 90 °C temperature resistance, no microorganism formation, impermeable, low thermal conductivity.
Using separate
polyethylene plates, the hot and cold water tanks were isolated from each other, thus preventing heat transfer between the tanks. The cold and hot sides of the Peltier element are embedded in the middle of the polyethylene plates. The heat sinks, however, remain in contact with water in the tanks. Thus, heat is transferred from the hot surface of the Peltier element to the fin and then from the fin to the water on the heating side, while heat is transferred from the water to the fin and then from the fin to the cold surface of the Peltier element on the cooling side. In the second and third prototypes, all walls of the water tanks are made of polyethylene plates. The size of the tanks is 10x15x15 cm3.
2.4. Liquid seal and silicone
Silicone is used to assemble the glass or polyethylene walls to each other, thus sealing is achieved. Heat-resistant silicone is chosen due to its steady properties at high temperatures. As the polyethylene plates are combined with the Peltier element, liquid seal with high adhesive power and high freezing rate is used, so a good heat insulation as well as sealing are obtained.
2.5. Measurement devices
Type T Copper-Constantan thermocouples are used to measure the water temperatures in the experiments. These thermocouples can measure temperatures between -200 °C and 300 °C with ± 0.5 °C accuracy. Temperatures at five different points (see Figure 1) inside the tanks are measured. Temperature measurement points are determined such a way that the mean of
the measurements represent the mean of the water temperature inside the tanks. For this purpose, each tank volume is considered to have five sections and each section is represented by the following points: the center of the tank, the midpoint of the water cross section adjacent to the bottom surface of the tank, near the outlet of the water (near taps), the midpoint of the water cross section adjacent to one of the other parallel sides of the tank and near to the heat sink. The average of the five temperature readings is taken as the temperature of the corresponding water tank. In addition, the surface temperature of the heat sinks are checked for maintaining the Peltier element at appropriate operating conditions. All temperature measurements are collected and stored by Elimko E-680 data logger seen in Fig. 4. Voltage and current supplied to the Peltier during the experiments are measured by a digital oscilloscope. While the voltage is measured directly, the current is measured by using a shunt resistor. As known, input current changes depending on the voltage applied to the Peltier element and temperature difference between the hot and cold surfaces. For an accurate measurement, instantenous changes are determined by the oscilloscope. The average power consumption by the Peltier element is calculated using the accurate measurements for the time intervals selected. The views of DC power supply, thermocouples, digital oscilloscope and data logger are given in Fig. 4. a
b
c
d
Fig. 4. (a) Power supply; (b) Type T thermocouple; (c) Digital oscilloscope; (d) Data logger.
3. Results and discussion
The view of the first prototype of the system proposed is given in Fig. 5. The Peltier element is placed between the hot and cold water tanks. The fins are attached to both surfaces of the Peltier element. Tests are performed for three different cases. In Case I, all the walls of the tanks are made of glass except for the walls where the Peltier element is mounted. No insulation is applied to the walls. In Case II, all the walls are made of polyethylene plates and not insulated. In Case III, the walls are made of polyethylene plates and insulated. During the
experiments, the temperatures of hot and cold water tanks and ambient temperature are measured and their changes over time were recorded. The maximum hot water temperature and minimum cold water temperature obtained from the system are determined for each case.
Fig. 5. First prototype.
A series of experiments were conducted with the experimental setup. Temperatures (from 5 points for each tank) and total power consumption, calculated by accurate readings of instantaneous voltage and current values, are recorded with 5-minute time intervals during 1 hour. Initial water temperature for each cases is taken as 20 oC which is the temperature of tap water. The volume of the water put into the tanks is 2 liters for each. The data obtained for the prototype made of uninsulated glass walls is given in Fig. 6 for Case I. As seen in Fig. 6, the temperature of the hot water storage tank increases linearly at the beginning. The increase in the temperature slows down after a certain time. After 60 minutes, the hot water tank reaches 47.2 oC. The temperature of the cold water tank decreases slowly with time and temperature gradients are smaller than that in the hot water tank. The cold water tank temperature decreases to 5.6 oC after 60 minutes. It can be concluded that the hot side of Peltier element is performing more efficiently than the cold side for the specified conditions and simultaneous heating/cooling processes. On the other hand, the power consumption of the Peltier element decreases with time. During the experiments, it is observed that the amount of current drawn by the Peltier element decreases with increasing temperature difference between its surfaces when a constant voltage is applied. For this reason, the power consumption decreases with increasing temperature difference. The power consumption varies from 56.7 W to 36.6 W while the temperature difference between the hot and cold water tanks varies from 0 to 41.6 oC. The reason for this is the fact that both heating and cooling capacities decreases with the increasing temperature difference of the Peltier sides.
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Time (minutes) Fig. 6. Variation of temperatures and power consumption with time for glass-wall prototype without insulation (Case I).
In Case II, the walls of the tanks are all made of polyethylene plates. The building material of the model, which is a design condition, has been changed keeping the tanks’ volumes and Peltier element the same and maintaining the system uninsulated. Using the same initial temperature (20 oC), variation of temperatures inside the water tanks and the power consumption of the Peltier element is investigated. The observation results are shown in Fig. 7.
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Time (minutes) Fig. 7. Variation of temperatures and power consumption with time for polyethylene-wall prototype without insulation (Case II).
As seen Fig. 7, temperature gradients for both heating and cooling sides are higher than in Case I. Using polyethylene plates instead of glass, the temperature of the cold water decreases down to 4.1 oC in the cooling side, while the temperature of the hot water increases up to 50.1 o
C in the heating side in the period of 60 minutes. In the both sides, so the cooling and heating
sides, the temperature gradients are higher at beginning of the processes, then the gradient decreases with time. This is actually due to the decrease in heating/cooling capacities and COP of the Peltier element with increasing temperature difference between the hot and cold sides. When comparing with Case I, in Case II the cold water temperature is reduced by 1.5 o
C more than in Case I, while the hot water temperature is increased by 2.9 oC more than in
Case I. On the other hand, the power consumption of the Peltier element varies from 48.7 W to 32.7 W during 60-minute period. Power consumptions seem to be lower than in Case I depending on the higher temperature differences in Case II since more temperature difference between the hot and cold surfaces results in less power consumption. Consequently, less power consumptions with higher cooling/heating rates yields higher COPC/COPH values. This situation is confirmed in Section 3.1 related with the cooling/heating performance of the system. As a result, it can be stated that the model with polyethylene wall shows better thermal performance than that with glass wall. In Case III, the water tanks made of polyethylene plates are insulated by 1-cm thick aluminum foil insulation sheet (see Fig. 8). In this case, the effect of the insulation on the temperatures and power consumption is investigated. The results obtained are shown in Fig. 9.
Fig. 8. Prototype made of polyethylene walls with insulation.
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60
Time (minutes) Fig. 9. Variation of temperatures and power consumption with time for polyethylene-wall prototype with insulation (Case III).
In the insulated prototype, the minimum cold water temperature is obtained as 1.3 °C while the maximum hot water temperature reaches to 61.5 °C. Comparing with Case II, the temperature of the cold water tank after 60 minutes is 2.8 °C lower although initial temperatures are the same. On the other hand, the temperature of the hot water tank is 11.4 °C higher than that obtained in Case II after 60 minutes. Among the three cases, the highest temperature in the heating side and the lowest temperature in the cooling side are both obtained in Case III. The variations of water temperatures have the same behavior for all cases. Temperature difference is not allowed for exceeding a certain value to prevent the Peltier module from a possible damage. Furthermore, the hot side of the Peltier element should be well cooled for maintaining the cooling effect of the cold side. Otherwise, the cold side cannot absorb heat and its temperature starts to increase.
3.1. Cooling and heating rates of Peltier element
Simplified energy equilibrium model which is validated by many researchers [30,40,41] is generally used to determine heat transfer rates. According to this model, the heat absorbed and released for a TE cooler can be calculated by QC o ITc Ko (Th Tc ) 0.5Ro I 2
(1)
QH o ITh Ko (Th Tc ) 0.5Ro I 2
(2)
where αo, Ko and Ro are the total Seebeck coefficient, total thermal conductance and total electric resistance, respectively, I is the electrical current, Tc and Th are the cold and hot side temperatures, respectively. Since the cold and hot water temperatures are measured in this study, and surface temperatures of TE element are not measured, the rate of cooling and heating is found by
QC Qa
QH Qr
where
(mc p T )CW t (mc p T ) HW
and
t
(3)
(4)
are the heat adsorbed by the cold water and the heat rejected to the hot water
(W), respectively. Subscripts CW and HW denote the cold and hot water, respectively. m, cp and ΔT are water mass (kg), specific heat (kJ/kg K) and temperature difference (K). Δt is the time (s) elapsed for the corresponding temperature difference. The changes of cooling and heating rates are shown in Fig. 10. The rates are calculated for 5-minute time periods. Both the cooling and heating rates decrease with time. The reason for that is the relationship between the temperature difference and heat transfer rates. Heat transfer rates decrease with increasing temperature difference between the cold and hot sides. Because the temperature difference increases with time, the transfer rates at both sides decreases during the experiment. The highest rates are obtained in Case III while the lowest rates are obtained in Case I. In Case III, the maximum and minimum cooling rates are 142.1 W and 11.8 W while the maximum and minimum heating rates are 193.7 W and 33.7 W. The heating rates seem to be higher than the cooling rates. This situation indicates that TE water dispenser performs more efficiently as a heater than as a cooler.
200
180
180
160
160
140
140
120
120 (filled markers)
100
100
80
80
60
60
40
40
20
Heating Rate (W)
Cooling Rate (W)
200
20
(unfilled markers)
0
0 0
10
20
30
40
50
60
Time (minutes) Fig. 10. Variation of the cooling and heating rates of Peltier device with time.
3.2. Cooling and heating COP of Peltier element
To evaluate the performances of the Peltier element in cooling and heating, the cooling and heating COP (coefficient of performance) can be calculated by the following equations, respectively:
COPC
Qa (mc p T )CW / t WP I P .VP
(5)
COPH
Qr (mc p T ) HW / t WP I P .VP
(6)
In Eqn. (1), COPC and COPH represents the cooling and heating coefficient of performance respectively.
is the power consumption by the Peltier element (W). IP and VP
are the current (A) and voltage (V) drawn by the Peltier element, respectively. The variation of COPC of the system over time for each of the three cases is given in Fig. 11. The cooling COPs for the Cases I, II and III vary between 2.26-0.24, 2.46-0.28 and 2.900.42, respectively, during 60-minute period. The COP values decrease with time due to the lower cooling capacities at higher temperature differences. The best cooling performance is obtained in Case III. Case III has higher COP C values than the other cases except for t=15
min. In conclusion, the insulation of the system in Case III provides about 22% increase in the mean COPC compared to the uninsulated system in Case II.
3.00 2.50
Case I Case II
2.00
COPC
Case III
1.50 1.00 0.50
0.00 0
10
20
30
40
50
60
Time (minutes) Fig. 11. Variation of the cooling COP of Peltier device with time.
The variation of COPH of the system over time for all cases is given in Fig. 12. The heating COPs for the Cases I, II and III vary between 2.41-0.32, 3.07-0.79 and 3.95-1.19, respectively, during 60-minute period. The COP values decrease with time due to the lower heating capacities at higher temperature differences between two sides of the Peltier element. The best heating performance is obtained in Case III. The positive effect of the insulation on the mean heating performance of the system occurs at about 45% level when the insulated case (Case III) is compared with the uninsulated case (Case II). On the other hand, the prototype with uninsulated glass wall has the lowest COP H values.
4.00 3.50
COP H
3.00 2.50 2.00 1.50 Case I
1.00
Case II
0.50
Case III
0.00 0
10
20
30
40
50
60
Time (minutes) Fig. 12. Variation of the heating COP of Peltier device with time.
Performance values of Peltier devices can reach to 4-5 at ideal conditions, namely at zero temperature difference between two surfaces and for optimal values of voltage and current. Peltier elements are typically used for cooling (especially for electronic cooling), and high performance can be achieved if the hot side is cooled well. In this study, the use of Peltier element for both heating and cooling purpose causes lower performances than the ideal case.
4. Conclusions
In this study, a novel TE water dispenser is developed by using a Peltier module that can perform both cooling and heating of drinking water simultaneously. For this purpose, two water tanks are used as cold and hot water storage device for the TE water dispenser. One of the tanks is cooled by the cold side of the Peltier element while the other is heated by the hot side of it. This novel system enables to setup a water dispenser unit that provides conditioned drinking water at appropriate temperatures depending on climatic conditions. The TE water dispenser are investigated experimentally for three different cases and a number of tests are performed for each case. In Case I, the water tanks are made of glass walls without insulation. In Case II, the water tanks are made of polyethylene walls without insulation. In Case III, the tanks are made of polyethylene walls with insulation. Variation of water temperatures, power consumption of the Peltier element, and cooling/heating
performances (COPC/COPH) of the system are investigated. Results show that the Case III, i.e. prototype made of ployethylene walls with insulation, has the best thermal performance. The insulation of the system has a significant effect on the thermal performance of the system as expected.In Case III, minimum cold water temperature on the cooling side is obtained as 1.3 °C while maximum hot water temperature on the heating side is obtained as 61.5 °C at the end of 60-minute time period. The cooling and heating COPs for Case III are obatined as 2.90 and 3.95, respectively. When two uninsulated cases are compared with each other, the prototype with polyethylene walls has better thermal characteristics than that with glass walls. Results also show the novel TE water dispenser can be competitive with conventional types with its advantages such as smaller and compact design, environmentally friendly and noiseless operation and being operable with renewable energy sources like solar energy. Consequently, this is the first project to use a TE module in a water dispenser unit. The TE water dispenser unit is experimentally proven to achieve simultaneous water cooling and heating operation and meet the need for conditioned drinking water in this study. As a future work, a solar assisted TE water dispenser unit will be carried out by mounting PV panels on the roof and solar-facing sides of the unit to supply drinking water in open public spaces. PV modules can thus supply electricity power to the TE module. Total area of PV modules can be determined by the capacity of TE module. It should be noted that the capacity of TE module used in this study is 72-W. The larger the TE capacity, the larger the cooling and heating rates obtained. References [1] S. Twaha, J. Zhu, Y. Yan, B. Li, A Comprehensive review of thermoelectric technology: Materials, applications, modelling and performance improvement, Renew. Sust. Energy Rev. 65 (2016) 698–726. [2] C. Gayner, K.K. Kar, Recent advances in thermoelectric materials, Prog. Mater. Sci. 83 (2016) 330–382. [4] S.B. Riffat, X. Ma, Thermoelectrics: a review of present and potential applications, Appl. Therm. Eng. 23 (2003) 913–935. [5] D.M. Rowe, G. Min, Evaluation of thermoelectric modules for power generation, J. Power Sources, 73 (1998) 193–198. [6] B.J. Huang, C.J. Chin, C.L. Duang, A design method of thermoelectric cooler, Int. J. Refrig. 23 (2000) 208–218. [7] K. Takahashi, T. Kanno, A. Sakai, H. Tamaki, H. Kusada, Y. Yamada, Bifunctional thermoelectric tube made of tilted multilayer material as an alternative to standard heat exchangers, Sci. Rep., 3 (2013), 1501. [8] M. Zebarjadi, Electronic cooling using thermoelectric devices, Appl. Phys. Lett. 106 (2015) 203506. [9] W. He, J. Zhou, J. Hou, C. Chen, J. Ji, Theoretical and experimental investigation on a thermoelectric cooling and heating system driven by solar, Appl. Energy 107 (2013) 89–97.
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Highlights
Novel TE water dispenser unit is used for water cooling/heating simultaneously Polyethylene wall for water tanks shows better thermal performance than glass wall Insulation of tanks has a significant enhancement in COP especially on heating side Less power consumption of TE module yields higher thermal performance values COP evaluation proves TE water dispenser to be competitive with conventional types