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An exhaustive experimental study of a novel air-water based thermoelectric cooling unit
Applied Energy 181 (2016) 357–366
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Applied Energy journal homepage: www.elsevier.com/locate/apenergy
An exhaustive experimental study of a novel air-water based thermoelectric cooling unit Hamed Sadighi Dizaji ⇑, Samad Jafarmadar, Shahram Khalilarya, Amin Moosavi Faculty of Mechanical Engineering, Urmia University, Urmia, Iran
h i g h l i g h t s A novel air-water based thermoelectric cooling unit is experimentally investigated. Different climate conditions are simulated using of different air flow rates and temperatures. Various parameters are evaluated to find optimum condition. COP/COPmax is studied as a new consideration and memorable behavior was observed.
a r t i c l e
i n f o
Article history: Received 21 May 2016 Received in revised form 27 July 2016 Accepted 13 August 2016
Keywords: Air conditioning system Thermoelectric Cooling COP/COPmax Air flow Water flow
a b s t r a c t In this paper, the cooling feasibility of air flow via a novel air-water based TEC system (as an alternative air cooling unit) is experimentally investigated for different climate conditions. Contrary to previous studies, thermoelectric hot side temperature was adjusted by a low constant water flow rate (and not by an air fan) which significantly increased the cold side performance of TEM. Ten parameters including _ a; m _ w and DC voltage and DC current were directly recorded by Ta,inlet, Tw,inlet, Ta,outlet, Tw,outlet, Tc, Ta, m measurement instruments during the experiments. Six other parameters including qc, qh, COPc, COPmax, COPc/COPmax and qair were evaluated by formulas and correlations using of aforesaid measured _ w, _ a; m data. Five numbers of aforementioned parameters were variant parameters. Indeed, the effect of m DC voltage/current and Ta,inlet (variant parameters) on other impressionable parameters were investigated in present study. Optimum working condition was evaluated from a new point of view. Indeed, in this paper, it was accidently found out that, despite the descending behavior of both COPc and COPmax (due to changing of variants), the ratio of said parameters (COPc/COPmax) creates a peak point (ascending and then descending) in all cases. Said peak point can be considered as an appropriate working condition of thermoelectric units. Findings showed that, the cold side of thermoelectric system can act as an applicable air cooling system especially when the hot side of thermoelectric is cooled by a current liquid such as water. Ó 2016 Elsevier Ltd. All rights reserved.
1. Introduction Contrary to conventional cooling units (compression refrigeration cycle) which comprise moving parts such as compressor, thermoelectric cooling units do not require any moving part. To that reason, TEC systems are recently considered as one of the most popular cooling units by researchers. Indeed, according to Peltier effect, applying a DC voltage between two electrodes of TEM causes heating generation on one side of TEM and cooling generation on the other side of TEM. Cold surface of TEM can be employed ⇑ Corresponding author. E-mail addresses: [email protected], [email protected] (H. Sadighi Dizaji). http://dx.doi.org/10.1016/j.apenergy.2016.08.074 0306-2619/Ó 2016 Elsevier Ltd. All rights reserved.
as a cooling unit in other applications such as refrigerators, electronic components, air conditioning systems, photovoltaic equipment and so on. Thermoelectric cooling system can be considered as one of the most affordable cooling units by utilization of renewable methods such as photovoltaic modules as a required energy provider of TEM. According to Seebeck effect, thermoelectric modules can be used vice versa in order to produce electrical energy. Indeed, electricity generation is another application of thermoelectric which can be obtained by applying a temperature difference between two sides of TEM. The main studies on thermoelectric cooling systems in ten recent years are chronologically summarized as below. Lineykin and Ben-Yaakov [1] proposed a graphical method for calculating the steady state operational point of a thermoelectric
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Nomenclature C COP I Imax
Km LPM _ m P q qa qph qpc qj qcon qc qh Q Rm T Th Tc TEC TEM
specific heat, J/kg K coefficient of performance electrical current, A data sheet parameter. The current that provides a temperature difference of DTmax under a specific Th and heat flux qc = 0, A thermal conductivity of TEC, W/m K liter per minute mass flow rate, kg/s input electrical power, W heat transfer rate, W heat transfer rate between air fluid and cold surface of TEM Peltier heating, W Peltier cooling, W Joule heating, W Fourier heating, W cooling power of TEM, W heating power of TEM, W volumetric air flow rate, LPM electrical resistance of TEC, X temperature, K temperature of the hot side of the TEC, K temperature of the cold side of the TEC, K thermoelectric cooler thermoelectric module
cooler. The method could help designers to examine and choose a thermoelectric module from catalogs to meet a specific cooling problem. Cosnier et al. [2] performed an experimental and numerical study on a thermoelectric module that cools or warms an air flow. The experimental results confirmed the feasibility of cooling or heating air through thermoelectric modules. The application of nanofluids as the working fluid on a heat pipe liquid-block combined with thermoelectric cooling was investigated by Putra et al. [3]. The higher thermal performance heat pipe liquid-block and thermoelectric cooled system with nanofluids proved its potential as a working fluid in said study. Calise et al. [4] investigated on optimal thermoeconomic configuration of Solar Heating and Cooling systems. Chen et al. [5] numerically studied on the performance of miniature thermoelectric cooler affected by Thomson effect. The obtained results suggested that the cooling power of a thermoelectric cooling module with Thomson effect can be improved by a factor of 5–7%. Zhou and Yu [6] presented a theoretical model for the optimization of a thermoelectric cooling system in which the thermal conductances from the hot and cold sides of the system are taken into account. The analysis results showed that the maximum coefficient of performance (COP) and the maximum cooling capacity of the TEC system can be obtained when the finite total thermal conductance is optimally allocated. Zhu et al. [7] focused on the optimal heat exchanger configuration of a TEC system. The effects of total heat transfer area allocation ratio, thermal conductance of the TEC hot and cold side and TEM element material properties on the cooling performance of the TEC were investigated in detailed based on the developed mathematical model. Their results revealed that the heat transfer area allocation ratio is an applicable characteristic of optimum design for TEC systems. Feasibility study of a green energy powered thermoelectric chip based air conditioner for electric vehicles were performed by Miranda et al. [8]. It was seen that the TEC air conditioning cooling system can be switched to a heating pump with simple current reversal at the p and n junctions of the TEC module. He et al. [9]
V Vmax
W X
voltage, V data sheet parameter. The voltage drop across the TECs’ terminals, corresponding to current Imax and the temperature difference DTmax, V total uncertainty in the measurement independent variable
Greece symbols am seebeck coefficient (V/K) of TEC DT temperature difference, K DTmax data sheet parameter. The largest temperature differential that can be obtained between the hot and cold ceramic plates of a TEC for the given level of Th and qc = 0, K Subscripts a air c cold con Fourier heating j Joule heating h hot max maximum ph Peltier heating pc Peltier cooling w water
theoretically and experimentally investigated on a thermoelectric cooling and heating system driven by solar. The formulation of the classical basic equations for a thermoelectric cooler from the Thomson relations to the non-linear differential equation with Onsager’s reciprocal relations was performed by Lee [10] to basically study the Thomson effect in conjunction with the ideal equation. The comparison between the exact solutions and the ideal equation on the cooling power and the coefficient of performance over a wide range of temperature differences showed close agreement. In conclusion, the Thomson effect was small for typical commercial thermoelectric coolers and the ideal equation effectively predicts the performance. Russel et al. [11] examined the performance of a thermoelectric cooler (TEC) based thermal management system for an electronic packaging design that operates under a range of ambient conditions and system loads using a standard model for the TEC and a thermal resistance network for the other components. The results showed that there is a tradeoff between the extent of off peak heat fluxes and ambient temperatures when the system can be operated at a low power penalty region and the maximum capacity of the system. A prototype thermoelectric system integrated with PCM (phase change material) heat storage unit for space cooling has been introduced by Zhao and Tan [12]. An experimental evaluation of a solar thermoelectric cooled ceiling combined with displacement ventilation system was conducted by Liu et al. [13]. The results show that the total heat flux and COP of the panel are strongly influenced by operating voltage, ambient temperature and indoor temperature. Yildirim et al. [14] experimentally investigated on a portable desalination unit configured by a thermoelectric cooler. Geometric effect on cooling power and performance of an integrated thermoelectric generation-cooling system was studied by Chen et al. [15]. When the lengths of TEG and TEC vary, the maximum reduction percentages of system performance were 12.45% and 18.67%, respectively. Optimization performance analysis of a thermoelectric refrigerator with external heat transfer was performed by Ding et al. [16].
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Hu et al. [17] experimentally studied on water-cooled thermoelectric cooler for central processing unit (CPU) under severe environment. Results showed that, water-cooled TEC using temperature control can prevent dew and over heat under variable operating condition as well as save energy. Wang et al. [18] examined the geometric parameter sensitivity and optimized the cooling performance of two-stage thermoelectric cooler. The geometric structure parameters included total thermoelectric element number, crosssectional area ratio of the p-type leg to the leg pair, height ratio of the cold stage leg to the two stages legs, and base area ratio of semiconductor legs to the TEC. The optimal values of said parameters were evaluated in that paper. Irshad et al. [19] researched on performance of a thermoelectric air duct for energy-efficient buildings. Performance analysis of a thermoelectric cooler with a corrugated architecture was studied by Owoyele et al. [20]. They showed that parasitic heat transfer through the substrate in the corrugated thermoelectric module is a source of performance losses, but can be minimized with proper thickness selection. Koh et al. [21] studied on performance and mass optimization of thermoelectric micro-coolers. Liu et al. [22] theoretically and experimentally investigated on thermoelectric heating system with multiple ventilation channels. The results showed that the average heating coefficient of the thermoelectric heating system could reach to 1.3, which is greater than that of a typical electric heater with heating coefficient of less than one. Ahammed et al. [23] studied on thermoelectric cooling of electronic devices with nanofluid in a multiport minichannel heat exchanger. The result showed 40% enhancement in the coefficient of performance (COP) of thermoelectric module for 0.2% of nanoparticle volume concentration. Liu et al. [24] investigated on thermoelectric mini cooler coupled with micro thermosiphon for CPU cooling system. Experimental results indicated that the cooling production increases with promotion of thermoelectric operating voltage. Surface temperature of CPU heat source linearly increased with increasing of power input, and its maximum value reached 70 °C as the prototype CPU power input was equivalent to 84 W. Luo et al. [25] presented a novel and promising active building integrated photovoltaic thermoelectric wall system. He et al. [26] focused mainly on the thermoelectric performance study under four different types of cooling methods with temperature gradient modeling. Lv et al. [27] suggested a new design concept which combines two-stage design with transient supercooling effect is proposed to enhance the maximum temperature drop across thermoelectric coolers. Attar and Lee [28] discussed on designing and testing the optimum design of automotive air-to-air thermoelectric air conditioner (TEAC) system. It was concluded that the technique of combining the dimensional analysis method with the thermal isolation can provide a simple and reliable method to design and optimize a TEAC whole system from given inputs. The performance of a thermoelectric power generation (TEPG) module and a device designed to convert engine exhaust heat directly into electricity was studied under different operating conditions using a proposed thermoelectric (TE) model by Zhang [29]. It was concluded that for the given heat exchangers and operating parameters, there exists an optimal fill factor such that the device performance is optimal under different mass flow rates. In this paper, attempts are made to reduce the air flow temperature using of a prototype thermoelectric system as an air conditioning cooling system. Contrary to previous studies, hot side of thermoelectric was cooled by a low constant flow rate of water fluid (and not by an air fan) which significantly increased the cold side performance of TEM. Various air inlet temperature and air flow rates were experimented as simulation of different climate conditions. The effect of air flow rate, air inlet temperature, DC voltage value on various parameters such as thermoelectric surface temperatures, air fluid outlet temperature, and COPc, cooling
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power of thermoelectric system, are experimentally investigated. Optimum working condition was evaluated from a new point of view (COPc/COPmax) which has not been performed in former researches. Indeed, in this paper, it was found that, despite the descending behavior of both COPc and COPmax (for instance with changing of DC voltage or changing of air inlet temperature), the ratio of said parameters (COPc/COPmax) creates a peak point (ascending and then descending) in all cases. Said peak point can be considered as an appropriate working condition of thermoelectric units. Finally, findings showed that, the cold side of thermoelectric system can act as an applicable air cooling system especially when the hot side of thermoelectric is cooled by a current liquid such as water.
2. Experiments 2.1. Experimental set-up and test section A general view of experimental set-up is shown in Fig. 1 and the detail of test section is illustrated in Fig. 2. Water fluid is pumped from a water tank into the hot side of thermoelectric and then is collected inside another tank. Air fluid is flowed from a compressor toward the cold side of TEM and then is released to the surrounding. The amounts of air and water flow rates were measured by means of air-Rotameter and water-Rotameter respectively. Air/ water inlet temperature (Ta,inlet, Tw,inlet), air/water outlet temperature (Ta,outlet, Tw,outlet), TEM cold surface temperature (Tc) and TEM hot surface temperature (Th) were recorded by K type thermocouples and Lotron TM-941 Data logger. The resolution of temperature recording system is 0.1 °C. It is noted that, the thermocouples which was employed to measure the TEM surfaces temperatures were placed between thermoelectric and heat sinks which contains no fluid. Required electric current was supplied by a DC power supply. The test section was made of plexiglass and it was completely insulated during the experiments. As seen in Fig. 1(b), two heat sinks are attached on two sides of TEM.
2.2. Experiments procedure Ten parameters including Ta,inlet, Tw,inlet, Ta,outlet, Tw,outlet, Tc, Ta, _ w and DC voltage and DC current were directly recorded by _ a; m m measurement instruments during the experiments. Six other parameters including qc, qh, COPc, COPmax, COPc/COPmax and qair (which are explained in Section 2.3.2) are evaluated by formulas and correlations using of aforesaid measured data. Four numbers _ a , DC voltage/ of aforementioned parameters including Ta,inlet, m current are variable parameters. Other ten parameters (Ta,outlet, Tw,outlet, Tc, Th, qc, qh, qa, COP, COPmax, COP/COPmax) are impressionable parameters. Indeed, variable parameters are changed (as described below) and their effects on impressionable parameters are investigated in this paper. Two numbers of parame_ w ) were kept constant during the all experiments. ters (Tw,inlet, m Experiments procedure is classified into two main modes as seen in Table 1. In first mode, the amount of Ta,inlet was kept at a _ a and DC voltage/current on aforefixed value and the effects of m said impressionable parameters are studied. According to Table 1, eight different amounts of DC voltages were tested. Five different air flow rate (2–6 LPM) were employed for each DC voltage. In sec_ a were kept constant at 6 V ond mode, DC voltage/current and m and 6 LPM respectively and Ta,inlet was varied. The effect of Ta,inlet on impressionable parameters are probed in second mode. Eight different values of air inlet temperatures between 20 °C and 90 °C were considered. Indeed Different air flow rates and inlet temperatures were applied to simulate various climate conditions
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Fig. 1. Experimental test set-up: 1 – Water tank, 2 – Water pump, 3 – Valves, 4 – Water Rotameter, 5 – Test section, 6 – Air Rotameter, 7 – Compressor, 8 – Water tank, 9 – DC voltage supplier, 10 – Data logger, 11 – Computer.
Fig. 2. Detail of test section: 1 – Frame, 2 – Thermoelectric, 3 – Inlet or outlet, 4 – Heat sink, 5 – Test section, 6 – Actual photograph of test section.
or applications. All measured values were recorded at steady state condition. 2.3. Calculations method 2.3.1. Uncertainty of measurements Uncertainty of measurements is calculated using of the method which was employed in Sadighi Dizaji et al. [30,31] study (Moffat [32] method). As described in [30,31], the uncertainties in
calculating a result (W+R) can be evaluated by following equation. Where, x1, x2, etc., are independent variables of a defined result.
W Rþ ¼
@Rþ w1 @X1
2
þ 2 þ 2 !12 @R @R þ w2 þ þ wn @X2 @Xn
ð1Þ
The obtained uncertainty for each parameter is shown in Table 2. As seen in Table 2, the amounts of uncertainty are in reasonable ranges.
H. Sadighi Dizaji et al. / Applied Energy 181 (2016) 357–366 Table 1 Different tested conditions. DC voltage (V)
Air flow rate (LPM)
Air inlet temperature (°C)
Water inlet temperature (°C)
First mode 2 4 6 8 10 12 14 16
2, 2, 2, 2, 2, 2, 2, 2,
15 15 15 15 15 15 15 15
Around Around Around Around Around Around Around Around
20–90
Around 14
3, 3, 3, 3, 3, 3, 3, 3,
4, 4, 4, 4, 4, 4, 4, 4,
5, 5, 5, 5, 5, 5, 5, 5,
6 6 6 6 6 6 6 6
Second mode 6 6
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qj ¼ Rm I2
ð7Þ
qcon ¼ Km ðTh Tc Þ
ð8Þ
The cooling power (qc) and heating power (qh) of TEM can be found by using of energy balance of four aforesaid equations. In the evaluation of cooling and heating power it is assumed that half of Joule heat passes from the cold side and the other part passes from the hot side. Hence qc and qh can be calculated by Eqs. (9) and (10).
14 14 14 14 14 14 14 14
1 1 qc ¼ qpc qj qcon ¼ am ITc Rm I2 Km ðTh Tc Þ 2 2
ð9Þ
1 1 qh ¼ qph þ qj qcon ¼ am ITh þ Rm I2 Km ðTh Tc Þ 2 2
ð10Þ
The input electrical power to TEM is calculated from Eq. (11).
Table 2 Maximum uncertainty of measurement. Parameter
Unit
Uncertainty in the temperature measurement Air fluid inlet temperature Air fluid outlet temperature Water fluid inlet temperature Water fluid outlet temperature Uncertainty in the measurement of volume flow rate Water Air Uncertainty in reading values of table (q, k, . . .)
°C °C °C °C °C m3 h1 m3 h1 m3 h1 %
Comment
P ¼ am ðTh Tc Þ þ Rm I2
ð11Þ
TEM coefficient of performance for cooling (COPc) is evaluated from Eq. (12).
±0.5 ±0.5 ±0.5 ±0.5
COP c ¼
±0.02 ±0.01 ±0.1–0.2
2.3.2. Calculating of cooling and heating characteristics Commercial TEC1-12706 thermoelectric was employed in present study. The specifications of TEC1-12706 are shown in Table 3 which can be found in datasheet of TEC1-12706. Seebeck coefficient of TEM (am), electrical resistance (Rm) and thermal conductivity (Km) of TEM are evaluated using of Table 1 as shown below [1]
am ¼
V max Th
ð2Þ
Rm ¼
V max ðT h DT max Þ Th Imax
ð3Þ
qc p
ð12Þ
The COPc values mainly depend on the temperatures at the two sides of the thermoelectric element [33]. This fact is well indicated starting from the definition of the (ideal) Carnot COP, here indicated as COPmax that considers the temperatures of the hot source Th and of the cold source Tc [33]:
COP max ¼
Tc Th Tc
ð13Þ
Moreover, heat transfer rate between air fluid and cold surface can be calculated from Eq. (14).
_ a cp ðT a;inlet T a;outlet Þ qair ¼ m
ð14Þ
3. Results and discussions
1 DT max 2T h ¼ K m Imax V max ðT h DT max Þ
ð4Þ
Four kinds of heat transfer can appear in thermal analysis of TEM. Peltier heating (qph), Peltier cooling (qpc), Joule heating (qj) and Fourier heating (qcon) are said four heating effects which can be evaluated from Eqs. (5)–(8). Joule heat appears when an electrical current passes along an electric resistance. Fourier heat is the heat transfer from a hot surface toward a cold surface due to the conduction.
qpc ¼ am ITc
ð5Þ
aph ¼ am ITh
ð6Þ
Table 3 Specifications of TEC1-12706. Type
Couples N
Imax (A)
Vmax (V)
Qmax (W)
DTmax (K)
Reference temperature at hot-end (K)
TEC1-12706
127
6
15.2
56.5
68
300
_ a ) and voltage 3.1. The effect of air flow rate (m In this section the amounts of air and water inlet temperatures are kept constant and the effects of air flow rate and DC voltage on Tc (thermoelectric cold side surface temperature), Th (thermoelectric hot side surface temperature), Ta,outlet (air outlet temperature), DTa (Ta,inlet Ta,outlet), cooling power (qc), heat transfer rate (qa), COP c are investigated. Main findings from present section COPc and COP max are described below. 3.1.1. The effect of air flow rate and voltage on Tc, Th, Ta,outlet and DTa (a) The effect of air flow rate and DC voltage on Tc, Ta,outlet are presented in Figs. 3 and 4 respectively. Figs. 3 and 4 should be considered simultaneously in order to exactly and conveniently clarify their behavior. Firstly it is noted that, achieving lower amounts of Ta,outlet is considered as more beneficial operation of TEM system. A general explanation and the reasons of curves behavior in Figs. 3 and 4 are schematically illustrated in Fig. 5 (for a constant air flow rate and for a constant DC voltage). Increment of DC voltage or air flow rate reduces the amount of Ta,outlet with two different mechanisms as summarized in Fig. 5 and as descried below. As seen in Figs. 3 or 5, increment of DC voltage reduces the thermoelectric cold surface temperature (Tc) which obviously causes reduction
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Fig. 3. The effect of DC voltage and air flow rate on Tc.
Fig. 4. The effect of DC voltage and air flow rate on Ta,outlet.
of Ta,outlet in Fig. 4. As illustrated in Fig. 5, for a constant DC voltage (for example 16 V), increment of air flow rate increases the amount of Tc (warmer). But it (increment of air flow rate) reduces the air outlet temperature (Ta,outlet). Indeed, in a fixed DC voltage, increment of air flow rate causes higher fluid Reynolds number and then higher heat transfer rate between air fluid flow and cold surface temperature which causes reduction of air fluid outlet temperature and consequently enhancement of thermoelectric cold surface temperature (Tc). It should be noted that, said phenomenon will not continue for any arbitrary extra air flow rate. Because each amount of DC voltage, can produce a specific-limited cold surface heat cooling capacity (qc). For instance in 2 V DC voltage (Fig. 4), the positive effect of the increment of air flow rate on the reduction of Ta,outlet reduces step by step. And the difference between Ta,outlet in 5 LPM and 6 LPM is very less than the difference between Ta,outlet in 1 LPM and 2 LPM. This means that, if the air flow rate is increased more again, the amount of Ta,outlet may not be reduced again (but it does not mean the amount of qa is reduced because _ a in addition to the Ta,outlet). according to Eq. (14), qa depends on m However maximum heat transfer rate between air and cold surface (qa) is always smaller than the (qc). The differences between qa and qc are clarified in Section 3.1.2. (b) The effect of air flow rate on Ta,outlet in higher amounts of DC voltages is more tangible than the lower amounts of DC voltages. Indeed in higher DC voltages, more amount of air flow rate can be supplied which reduces the Ta,outlet yet. Besides, in higher amounts of air flow rate, reduction of Ta,outlet due to increment of DC voltage occur with a sharp slope. (c) According to Fig. 6, DTa = Ta,inlet Ta,outlet increases with the increase of DC voltage or air flow rate. Indeed, reduction of Ta,outlet (which explained above) increases the amount of DTa. Maximum DTa was obtained in DC voltage of 16 V and air flow rate of 6 LPM. Minimum cold surface temperature was observed at minimum air flow rate (2 LPM) and maximum DC voltage supply (16 V) too.
Fig. 5. Graphical explanation for the curves behavior of Figs. 3 and 4.
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Fig. 6. The effect of DC voltage and air flow rate on DTa.
(d) Fig. 7 shows the thermoelectric hot surface temperature (Th). The scale of Figs. 7 and 3 were chosen the same in order to compare the changes of Tc and Th. As seen in Fig. 7, the changes of hot side temperature are lower than the changes of cold side temperature. It is pertain to the naturally characteristics of water fluid which has higher thermal capacity and density. Said characteristics can remain the thermoelectric hot side temperature in a lower amount which causes more effectiveness of cold side. Air flow rate almost does not effect on hot side temperature (in our tested range). It is mentioned that the used water flow rate in this study was 1 LPM. 3.1.2. The effect of air flow rate and voltage on qc and qa Firstly the differences between qc and qa should be clarified. qc and qa are not the same. Indeed, qc is the total cooling power of TEM. Some of qc can be transmitted to air flow which we termed it qa. On the other hand, qc is the cooling ability of thermoelectric. However it does not mean that, all of qc is transmitted into the air flow. Indeed, this cooling power is expected (is spent) for two things. 1: reduction and then remaining of cold surface of TEM on a low temperature (for example 5 °C) and 2: reduction of the air flow temperature. As an example, imagine that, the amount of air flow rate is reduced until 0 LPM (no air flow rate). In this case, all amount of qc is spent for reduction of cold surface temperate for
Fig. 7. The effect of DC voltage and air flow rate on Th.
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example until 10 °C and keeps the cold surface temperature on 10 °C. And now if the amount of air flow rate is increased to 1 LPM, some of qc (not all of qc) causes reduction of air outlet temperature; and the temperature of cold surface is enhanced form 10 °C to for example 5 °C. Hence, by increment of air flow rate (or other method such as using turbulator) the amount of qc which is spent for reduction of air temperature is increased. Hence, the temperature of cold surface of TEM in enhanced too. The effect of air flow rate and DC voltage on qc and qa are presented in Figs. 8 and 9 respectively. As seen in Figs. 8 and 9, both qc and qa increase with the increase of air flow rate or DC voltage. Maximum qc and qa were obtained at air flow rate of 6 LPM and DC voltage of 16 V. For lower amounts of DC voltage, the effect of air flow rate on qc and qa is minor in comparison with the higher amounts of DC voltage. The slope of enhancement of air flow rate due to increment of DC voltage augments with the increase of air flow rate. For instance, increment of DC voltage at air flow rate of 2 LPM almost does not effect on the amount of qa. The amounts of qa are less than the amounts of qc for all cases. It means that, a high capacity of thermoelectric cold surface (qc) has not been used yet and higher amounts of qa can be achieved compared to the present values for a constant air flow rate and constant DC voltage via other methods. Indeed, other designing of heat sink such as finned heat sink, may be employed in order to get higher value of qair from qc.
Fig. 8. The effect of DC voltage and air flow rate on qc.
Fig. 9. The effect of DC voltage and air flow rate on qair.
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COP 3.1.3. The effect of air flow rate and voltage on COPc, COPmax and COP max COPc is a parameter which considers both thermoelectric production power (qc) and thermoelectric input electrical power (P). On the other words, although that the increment of DC voltage increases the amount of qc, it increases the input electrical power of thermoelectric too. Hence a simultaneous comparison between qc and P should be considered in order to make the best decision. In these cases, COPc is evaluated for said purpose which is the ratio of qc to P. The effects of air flow rate and DC voltage on COPc and COPmax are shown in Fig. 10. As seen in Fig. 10, increment of DC voltage causes reduction of both COPc and COPmax. It means that, despite the enhancement of qc duo to the increment of DC voltage, the amount of P (input electrical power) is increased with a sharper slope compared to the qc. Hence, the amount of COPc is reduced. It is noted that, augmentation of air flow rate has a positive effect on COPc yet. Indeed, increment of air flow rate increases the amount of qc however it does not have any effect on thermoelectric input electrical power; hence, numerator of COPc increases while the value of denominator is constant which causes enhancement of COPc. Air flow rate does not have significant effect on COPmax. Maximum COPc was obtained at DC voltage of 2 and air flow rate of 6 LPM. Fig. 11 shows the variations of COPc/COPmax with air flow rate and DC voltage. As seen in Fig. 11, a peak point is appeared at DC voltage of 6 V for all tested air flow rates. Despite the reduction of COPc in DC voltage of 2–6, the amount of COPc/COPmax is increased in said range. After DC voltage of 6 V, the values of COPc/ COPmax are reduced again. DC voltage of 6 V creates a peak point in this figure. The reason of this phenomenon can be clarified by analyzing and comparing the slopes of COPc and COPmax. Actually, by increment of DC voltage up to 6 V, the reduction slope of COPmax is more than reduction slope of COPc. But by increasing DC voltage more than 6 V, the reduction slope of COPc and COPmax inversed and COPc decrease faster than the COPmax. This peak point can be demonstrated as an optimum condition for proposed TEC system. It is noted that, inlet temperature of air flow creates a peak point too, which is described in next sections. Indeed, COPc/COPmax could propose as a new parameter in the optimization of TEC systems which should be consider to approach maximum performance of TEC systems. Generally, COP/COPmax mentions this important that, decrement of COP (for example in Fig. 10) may not always considered as a negative point; because as seen in Fig. 11, despite the decrement of COP (from 1 V to 3 V), the amount of COP/COPmax is increased and creates a peak point. It means that the value of
Fig. 11. The effect of DC voltage and air flow rate on COPc/COPmax.
COP has the minimum difference with its maximum working condition (COP max) in said voltage (6 V). 3.1.4. The effect of air inlet temperature on Tc, Ta,outlet, qa, qc, COPc and COP COPmax
_ a and DC voltage were kept conIn this section, the amount of m stant in 6 LPM and 6 V respectively and air inlet temperature was changed between 20 and 90 °C. Indeed, the air inlet temperature was varied via an electrical heater. The heater was placed inside a box which has one inlet and one outlet. The power of electrical heater was adjust by a dimmer. The effect of air inlet temperature on Tc and Ta,outlet is shown In Fig. 12. Variations of qa and qc due to the change of air inlet temperature are illustrated in Fig. 13. And COP c are finally the effects of air inlet temperature on COPc and COP max shown in Figs. 14 and 15 respectively. According to Fig. 12 both Tc and Ta,outlet increase with the increase of air inlet temperature. The slop of this enhancement is almost identical for both of them. Increment of air inlet temperature influences the cold surface temperature and obviously causes increment of thermoelectric surface temperature and consequently the value of Ta,outlet is reduced. Nonetheless, the amount of Ta,outlet is significantly reduced until in high values of Ta,inlet such as 90 °C. As seen in Fig. 13, increment of air inlet temperature augments both qc and qa. The slope of enhancement of qa is more than the slope of enhancement of qc. On the other words, the difference between qa and qc increases with increment of air inlet temperature. The amount of DTa = Ta,inlet Ta,outlet is sharply intensified
Fig. 10. The effect of DC voltage and air flow rate on COPc and COPmax.
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Fig. 12. The effect air inlet temperature on Tc and Ta,outlet.
Fig. 15. The effect air inlet temperature on COPc/COPmax.
Fig. 13. The effect air inlet temperature on qc and qair.
Indeed, qc and qa are couple with each other and the variation of one of them means the variation of another. Fig. 14 reveals that, the amount of COPc increases with enhancement of air inlet temperature. As described above, enhancement of air inlet temperature increases the qc. On the other hand, qc is the numerator of Eq. (12). Hence, obviously the increment of qc enhances the COPc. Same as Fig. 11, COPc/COPmax creates a peak point again (see Fig. 15). Two more extra temperatures (100 and 110 °C) is experimented again for this section in order to ascertain the existence of a peak point in around air inlet temperature of 70 °C. However, the amount of this peak point may be changed for other values of air flow rates or DC voltages. Nonetheless, the creation of a peak point in COPc/COPmax is seems as rule. From Fig. 15, it can be concluded that, at the inlet temperature of around 70 °C the thermoelectric unit has the best operating condition from the point of COPc/ COPmax. As a general result, it seems that, COPc/COPmax is a promising parameter for optimizing of thermoelectric units from a new point of view. And extra investigations should be conducted to clarify the curve behavior of COPc/COPmax.
4. Conclusion
Fig. 14. The effect air inlet temperature on COPc.
by enhancement of air inlet temperature. And according to Eq. (14) it causes enhancement of qa. Besides, enhancement of air inlet temperature causes augmentation of heat transfer rate between air flow and thermoelectric cold surface which increases the amount of Tc. Subsequently, increment of Tc increases the qc.
In this paper, a novel air-water based TEC system was employed as an alternative air cooling unit. Indeed, different air flow rate and different air inlet temperature simulated the various climate conditions. Contrary to previous studies, thermoelectric hot side temperature was adjusted by a low constant water flow rate (and not by an air fan) which significantly increased the cold side performance of TEM. Different climate conditions (air inlet temperature and air flow rate) were tested. Fourteen various parameters includ_ a , DC voltage/current), ten ing four variable parameters (Ta,inlet, m impressionable parameters (Ta,outlet, Tw,outlet, Tc, Th, qc, qh, qa, COP, COPmax, COP/COPmax) are evaluated. Indeed, the effects of variable parameters on impressionable parameters were investigated. Two _ w ) were kept constant during the numbers of parameters (Tw,inlet, m experiments. It was accidently found out that, despite the descending behavior of both COPc and COPmax (with changing of DC voltage), the ratio COPc to COPmax (COPc/COPmax) creates a peak point (ascending and then descending) in all cases. Said peak point can be considered as an appropriate working condition of thermoelectric units. COPc/COPmax was investigated for different air flow inlet
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temperatures too and a peak point was observed again. Air flow inlet temperature is a key parameter to attain maximum efficiency in TEC systems. Generally, the findings showed that, if thermoelectric hot side temperature is adjust by a water flow rate, the cold side of TEM can be act as an appropriate air cooling unit even for hot climate conditions. References [1] Lineykin S, Ben-Yaakov S. User-friendly and intuitive graphical approach to the design of thermoelectric cooling systems. Int J Refrig 2007;30:798–804. [2] Cosnier M, Fraisse G, Luo L. An experimental and numerical study of a thermoelectric air-cooling and air-heating system. Int J Refrig 2008;31:1051–62. [3] Putra N, Yanuar, Iskandar FN. Application of nanofluids to a heat pipe liquidblock and the thermoelectric cooling of electronic equipment. Exp Therm Fluid Sci 2011;35:1274–81. [4] Calise F, D’Accadia MD, Vanoli L. Thermoeconomic optimization of solar heating and cooling systems. Energy Convers Manage 2011;52:1562–73. [5] Chen WH, Liao CY, Hung CI. A numerical study on the performance of miniature thermoelectric cooler affected by Thomson effect. Appl Energy 2012;89:464–73. [6] Zhou Y, Yu J. Design optimization of thermoelectric cooling systems for applications in electronic devices. Int J Refrig 2012;35:1139–44. [7] Zhu L, Tan H, Yu J. Analysis on optimal heat exchanger size of thermoelectric cooler for electronic cooling applications. Energy Convers Manage 2013;76:685–90. [8] Miranda AG, Chen TS, Hong CW. Feasibility study of a green energy powered thermoelectric chip based air conditioner for electric vehicles. Energy 2013;59:633–41. [9] He W, Zhou J, Hou J, Chen C, Ji J. Theoretical and experimental investigation on a thermoelectric cooling and heating system driven by solar. Appl Energy 2013;107:89–97. [10] Lee HS. The Thomson effect and the ideal equation on thermoelectric coolers. Energy 2013;56:61–9. [11] Russel MK, Ewing D, Ching CY. Characterization of a thermoelectric cooler based thermal management system under different operating conditions. Appl Therm Eng 2013;50:652–9. [12] Zhao D, Tan G. Experimental evaluation of a prototype thermoelectric system integrated with PCM (phase change material) for space cooling. Energy 2014;68:658–66. [13] Liu ZB, Zhang L, Gong GC. Experimental evaluation of a solar thermoelectric cooled ceiling combined with displacement ventilation system. Energy Convers Manage 2014;87:559–85. [14] Yıldırım C, Soylu SK, Atmaca I, Solmus I. Experimental investigation of a portable desalination unit configured by a thermoelectric cooler. Energy Convers Manage 2014;85:140–5.
[15] Chen WH, Wang CC, Hung CI. Geometric effect on cooling power and performance of an integrated thermoelectric generation-cooling system. Energy Convers Manage 2014;87:566–75. [16] Ding ZM, Chen LG, Sun FR. Optimum performance analysis of a combined thermionic thermoelectric refrigerator with external heat transfer. J Energy Inst 2014:1–12. [17] Hu HM, Ge TS, Dai YJ, Wang RZ. Experimental study on water-cooled thermoelectric cooler for CPU under severe environment. Int J Refrig 2015. [18] Wang TH, Wang QH, Leng C, Wang XD. Parameter analysis and optimal design for two-stage thermoelectric cooler. Appl Energy 2015;154:1–12. [19] Irshad K, Habib K, Thirumalaiswamy N, Saha BB. Performance analysis of a thermoelectric air duct system for energy-efficient buildings. Energy 2015;91:1009–17. [20] Owoyele O, Ferguson S, O’Connor BT. Performance analysis of a thermoelectric cooler with a corrugated architecture. Appl Energy 2015;147:184–91. [21] Koh YR, Yazawa K, Shakouri A. Performance and mass optimization of thermoelectric microcoolers. Int J Therm Sci 2015;97:143–51. [22] Liu D, Zhao FY, Yang H, Tang GF. Theoretical and experimental investigations of thermoelectric heating system with multiple ventilation channels. Appl Energy 2015;159:458–68. [23] Ahammed N, Asirvatham LG, Wongwises S. Thermoelectric cooling of electronic devices with nanofluid in a multiport minichannel heat exchanger. Exp Therm Fluid Sci 2015;74:81–90. [24] Liu D, Zhao FY, Yang HX, Tang GF. Thermoelectric mini cooler coupled with micro thermosiphon for CPU cooling system. Energy 2015;83:29–36. [25] Luo Y, Zhang L, Liu Z, Wang Y, Meng F, Wu J. Thermal performance evaluation of an active building integrated photovoltaic thermoelectric wall system. Appl Energy 2016;171:25–39. [26] He W, Wang S, Lu C, Zhang X, Li Y. Influence of different cooling methods on thermoelectric performance of an engine exhaust gas waste heat recovery system. Appl Energy 2016;162:1251–8. [27] Lv Hao, Wang X, Meng J, Wang T, Yan W. Enhancement of maximum temperature drop across thermoelectric cooler through two-stage design and transient supercooling effect. Appl Energy 2016;175:285–92. [28] Attar A, Lee HS. Designing and testing the optimum design of automotive airto-air thermoelectric air conditioner (TEAC) system. Energy Convers Manage 2016;112:328–36. [29] Zhang T. Design and optimization considerations for thermoelectric devices. Energy Convers Manage 2016;112:404–12. [30] Sadighi Dizaji H, Jafarmadar S, Mobadersani F. Experimental studies on heat transfer and pressure drop characteristics for new arrangements of corrugated tubes in a double pipe heat exchanger. Int J Therm Sci 2015;96:211–20. [31] Sadighi Dizaji H, Jafarmadar S, Abbasalizadeh M, Khorasani S. Experiments on air bubbles injection into a vertical shell and coiled tube heat exchanger; exergy and NTU analysis. Energy Convers Manage 2015;103:973–80. [32] Moffat RJ. Describing the uncertainties in experimental results. Exp Therm Fluid Sci 1988;1:3–17. [33] Enescu D, Virjoghe EO. A review on thermoelectric cooling parameters and performance. Renew Sustain Energy Rev 2014;38:903–16.
Contents lists available at ScienceDirect
Applied Energy journal homepage: www.elsevier.com/locate/apenergy
An exhaustive experimental study of a novel air-water based thermoelectric cooling unit Hamed Sadighi Dizaji ⇑, Samad Jafarmadar, Shahram Khalilarya, Amin Moosavi Faculty of Mechanical Engineering, Urmia University, Urmia, Iran
h i g h l i g h t s A novel air-water based thermoelectric cooling unit is experimentally investigated. Different climate conditions are simulated using of different air flow rates and temperatures. Various parameters are evaluated to find optimum condition. COP/COPmax is studied as a new consideration and memorable behavior was observed.
a r t i c l e
i n f o
Article history: Received 21 May 2016 Received in revised form 27 July 2016 Accepted 13 August 2016
Keywords: Air conditioning system Thermoelectric Cooling COP/COPmax Air flow Water flow
a b s t r a c t In this paper, the cooling feasibility of air flow via a novel air-water based TEC system (as an alternative air cooling unit) is experimentally investigated for different climate conditions. Contrary to previous studies, thermoelectric hot side temperature was adjusted by a low constant water flow rate (and not by an air fan) which significantly increased the cold side performance of TEM. Ten parameters including _ a; m _ w and DC voltage and DC current were directly recorded by Ta,inlet, Tw,inlet, Ta,outlet, Tw,outlet, Tc, Ta, m measurement instruments during the experiments. Six other parameters including qc, qh, COPc, COPmax, COPc/COPmax and qair were evaluated by formulas and correlations using of aforesaid measured _ w, _ a; m data. Five numbers of aforementioned parameters were variant parameters. Indeed, the effect of m DC voltage/current and Ta,inlet (variant parameters) on other impressionable parameters were investigated in present study. Optimum working condition was evaluated from a new point of view. Indeed, in this paper, it was accidently found out that, despite the descending behavior of both COPc and COPmax (due to changing of variants), the ratio of said parameters (COPc/COPmax) creates a peak point (ascending and then descending) in all cases. Said peak point can be considered as an appropriate working condition of thermoelectric units. Findings showed that, the cold side of thermoelectric system can act as an applicable air cooling system especially when the hot side of thermoelectric is cooled by a current liquid such as water. Ó 2016 Elsevier Ltd. All rights reserved.
1. Introduction Contrary to conventional cooling units (compression refrigeration cycle) which comprise moving parts such as compressor, thermoelectric cooling units do not require any moving part. To that reason, TEC systems are recently considered as one of the most popular cooling units by researchers. Indeed, according to Peltier effect, applying a DC voltage between two electrodes of TEM causes heating generation on one side of TEM and cooling generation on the other side of TEM. Cold surface of TEM can be employed ⇑ Corresponding author. E-mail addresses: [email protected], [email protected] (H. Sadighi Dizaji). http://dx.doi.org/10.1016/j.apenergy.2016.08.074 0306-2619/Ó 2016 Elsevier Ltd. All rights reserved.
as a cooling unit in other applications such as refrigerators, electronic components, air conditioning systems, photovoltaic equipment and so on. Thermoelectric cooling system can be considered as one of the most affordable cooling units by utilization of renewable methods such as photovoltaic modules as a required energy provider of TEM. According to Seebeck effect, thermoelectric modules can be used vice versa in order to produce electrical energy. Indeed, electricity generation is another application of thermoelectric which can be obtained by applying a temperature difference between two sides of TEM. The main studies on thermoelectric cooling systems in ten recent years are chronologically summarized as below. Lineykin and Ben-Yaakov [1] proposed a graphical method for calculating the steady state operational point of a thermoelectric
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Nomenclature C COP I Imax
Km LPM _ m P q qa qph qpc qj qcon qc qh Q Rm T Th Tc TEC TEM
specific heat, J/kg K coefficient of performance electrical current, A data sheet parameter. The current that provides a temperature difference of DTmax under a specific Th and heat flux qc = 0, A thermal conductivity of TEC, W/m K liter per minute mass flow rate, kg/s input electrical power, W heat transfer rate, W heat transfer rate between air fluid and cold surface of TEM Peltier heating, W Peltier cooling, W Joule heating, W Fourier heating, W cooling power of TEM, W heating power of TEM, W volumetric air flow rate, LPM electrical resistance of TEC, X temperature, K temperature of the hot side of the TEC, K temperature of the cold side of the TEC, K thermoelectric cooler thermoelectric module
cooler. The method could help designers to examine and choose a thermoelectric module from catalogs to meet a specific cooling problem. Cosnier et al. [2] performed an experimental and numerical study on a thermoelectric module that cools or warms an air flow. The experimental results confirmed the feasibility of cooling or heating air through thermoelectric modules. The application of nanofluids as the working fluid on a heat pipe liquid-block combined with thermoelectric cooling was investigated by Putra et al. [3]. The higher thermal performance heat pipe liquid-block and thermoelectric cooled system with nanofluids proved its potential as a working fluid in said study. Calise et al. [4] investigated on optimal thermoeconomic configuration of Solar Heating and Cooling systems. Chen et al. [5] numerically studied on the performance of miniature thermoelectric cooler affected by Thomson effect. The obtained results suggested that the cooling power of a thermoelectric cooling module with Thomson effect can be improved by a factor of 5–7%. Zhou and Yu [6] presented a theoretical model for the optimization of a thermoelectric cooling system in which the thermal conductances from the hot and cold sides of the system are taken into account. The analysis results showed that the maximum coefficient of performance (COP) and the maximum cooling capacity of the TEC system can be obtained when the finite total thermal conductance is optimally allocated. Zhu et al. [7] focused on the optimal heat exchanger configuration of a TEC system. The effects of total heat transfer area allocation ratio, thermal conductance of the TEC hot and cold side and TEM element material properties on the cooling performance of the TEC were investigated in detailed based on the developed mathematical model. Their results revealed that the heat transfer area allocation ratio is an applicable characteristic of optimum design for TEC systems. Feasibility study of a green energy powered thermoelectric chip based air conditioner for electric vehicles were performed by Miranda et al. [8]. It was seen that the TEC air conditioning cooling system can be switched to a heating pump with simple current reversal at the p and n junctions of the TEC module. He et al. [9]
V Vmax
W X
voltage, V data sheet parameter. The voltage drop across the TECs’ terminals, corresponding to current Imax and the temperature difference DTmax, V total uncertainty in the measurement independent variable
Greece symbols am seebeck coefficient (V/K) of TEC DT temperature difference, K DTmax data sheet parameter. The largest temperature differential that can be obtained between the hot and cold ceramic plates of a TEC for the given level of Th and qc = 0, K Subscripts a air c cold con Fourier heating j Joule heating h hot max maximum ph Peltier heating pc Peltier cooling w water
theoretically and experimentally investigated on a thermoelectric cooling and heating system driven by solar. The formulation of the classical basic equations for a thermoelectric cooler from the Thomson relations to the non-linear differential equation with Onsager’s reciprocal relations was performed by Lee [10] to basically study the Thomson effect in conjunction with the ideal equation. The comparison between the exact solutions and the ideal equation on the cooling power and the coefficient of performance over a wide range of temperature differences showed close agreement. In conclusion, the Thomson effect was small for typical commercial thermoelectric coolers and the ideal equation effectively predicts the performance. Russel et al. [11] examined the performance of a thermoelectric cooler (TEC) based thermal management system for an electronic packaging design that operates under a range of ambient conditions and system loads using a standard model for the TEC and a thermal resistance network for the other components. The results showed that there is a tradeoff between the extent of off peak heat fluxes and ambient temperatures when the system can be operated at a low power penalty region and the maximum capacity of the system. A prototype thermoelectric system integrated with PCM (phase change material) heat storage unit for space cooling has been introduced by Zhao and Tan [12]. An experimental evaluation of a solar thermoelectric cooled ceiling combined with displacement ventilation system was conducted by Liu et al. [13]. The results show that the total heat flux and COP of the panel are strongly influenced by operating voltage, ambient temperature and indoor temperature. Yildirim et al. [14] experimentally investigated on a portable desalination unit configured by a thermoelectric cooler. Geometric effect on cooling power and performance of an integrated thermoelectric generation-cooling system was studied by Chen et al. [15]. When the lengths of TEG and TEC vary, the maximum reduction percentages of system performance were 12.45% and 18.67%, respectively. Optimization performance analysis of a thermoelectric refrigerator with external heat transfer was performed by Ding et al. [16].
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Hu et al. [17] experimentally studied on water-cooled thermoelectric cooler for central processing unit (CPU) under severe environment. Results showed that, water-cooled TEC using temperature control can prevent dew and over heat under variable operating condition as well as save energy. Wang et al. [18] examined the geometric parameter sensitivity and optimized the cooling performance of two-stage thermoelectric cooler. The geometric structure parameters included total thermoelectric element number, crosssectional area ratio of the p-type leg to the leg pair, height ratio of the cold stage leg to the two stages legs, and base area ratio of semiconductor legs to the TEC. The optimal values of said parameters were evaluated in that paper. Irshad et al. [19] researched on performance of a thermoelectric air duct for energy-efficient buildings. Performance analysis of a thermoelectric cooler with a corrugated architecture was studied by Owoyele et al. [20]. They showed that parasitic heat transfer through the substrate in the corrugated thermoelectric module is a source of performance losses, but can be minimized with proper thickness selection. Koh et al. [21] studied on performance and mass optimization of thermoelectric micro-coolers. Liu et al. [22] theoretically and experimentally investigated on thermoelectric heating system with multiple ventilation channels. The results showed that the average heating coefficient of the thermoelectric heating system could reach to 1.3, which is greater than that of a typical electric heater with heating coefficient of less than one. Ahammed et al. [23] studied on thermoelectric cooling of electronic devices with nanofluid in a multiport minichannel heat exchanger. The result showed 40% enhancement in the coefficient of performance (COP) of thermoelectric module for 0.2% of nanoparticle volume concentration. Liu et al. [24] investigated on thermoelectric mini cooler coupled with micro thermosiphon for CPU cooling system. Experimental results indicated that the cooling production increases with promotion of thermoelectric operating voltage. Surface temperature of CPU heat source linearly increased with increasing of power input, and its maximum value reached 70 °C as the prototype CPU power input was equivalent to 84 W. Luo et al. [25] presented a novel and promising active building integrated photovoltaic thermoelectric wall system. He et al. [26] focused mainly on the thermoelectric performance study under four different types of cooling methods with temperature gradient modeling. Lv et al. [27] suggested a new design concept which combines two-stage design with transient supercooling effect is proposed to enhance the maximum temperature drop across thermoelectric coolers. Attar and Lee [28] discussed on designing and testing the optimum design of automotive air-to-air thermoelectric air conditioner (TEAC) system. It was concluded that the technique of combining the dimensional analysis method with the thermal isolation can provide a simple and reliable method to design and optimize a TEAC whole system from given inputs. The performance of a thermoelectric power generation (TEPG) module and a device designed to convert engine exhaust heat directly into electricity was studied under different operating conditions using a proposed thermoelectric (TE) model by Zhang [29]. It was concluded that for the given heat exchangers and operating parameters, there exists an optimal fill factor such that the device performance is optimal under different mass flow rates. In this paper, attempts are made to reduce the air flow temperature using of a prototype thermoelectric system as an air conditioning cooling system. Contrary to previous studies, hot side of thermoelectric was cooled by a low constant flow rate of water fluid (and not by an air fan) which significantly increased the cold side performance of TEM. Various air inlet temperature and air flow rates were experimented as simulation of different climate conditions. The effect of air flow rate, air inlet temperature, DC voltage value on various parameters such as thermoelectric surface temperatures, air fluid outlet temperature, and COPc, cooling
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power of thermoelectric system, are experimentally investigated. Optimum working condition was evaluated from a new point of view (COPc/COPmax) which has not been performed in former researches. Indeed, in this paper, it was found that, despite the descending behavior of both COPc and COPmax (for instance with changing of DC voltage or changing of air inlet temperature), the ratio of said parameters (COPc/COPmax) creates a peak point (ascending and then descending) in all cases. Said peak point can be considered as an appropriate working condition of thermoelectric units. Finally, findings showed that, the cold side of thermoelectric system can act as an applicable air cooling system especially when the hot side of thermoelectric is cooled by a current liquid such as water.
2. Experiments 2.1. Experimental set-up and test section A general view of experimental set-up is shown in Fig. 1 and the detail of test section is illustrated in Fig. 2. Water fluid is pumped from a water tank into the hot side of thermoelectric and then is collected inside another tank. Air fluid is flowed from a compressor toward the cold side of TEM and then is released to the surrounding. The amounts of air and water flow rates were measured by means of air-Rotameter and water-Rotameter respectively. Air/ water inlet temperature (Ta,inlet, Tw,inlet), air/water outlet temperature (Ta,outlet, Tw,outlet), TEM cold surface temperature (Tc) and TEM hot surface temperature (Th) were recorded by K type thermocouples and Lotron TM-941 Data logger. The resolution of temperature recording system is 0.1 °C. It is noted that, the thermocouples which was employed to measure the TEM surfaces temperatures were placed between thermoelectric and heat sinks which contains no fluid. Required electric current was supplied by a DC power supply. The test section was made of plexiglass and it was completely insulated during the experiments. As seen in Fig. 1(b), two heat sinks are attached on two sides of TEM.
2.2. Experiments procedure Ten parameters including Ta,inlet, Tw,inlet, Ta,outlet, Tw,outlet, Tc, Ta, _ w and DC voltage and DC current were directly recorded by _ a; m m measurement instruments during the experiments. Six other parameters including qc, qh, COPc, COPmax, COPc/COPmax and qair (which are explained in Section 2.3.2) are evaluated by formulas and correlations using of aforesaid measured data. Four numbers _ a , DC voltage/ of aforementioned parameters including Ta,inlet, m current are variable parameters. Other ten parameters (Ta,outlet, Tw,outlet, Tc, Th, qc, qh, qa, COP, COPmax, COP/COPmax) are impressionable parameters. Indeed, variable parameters are changed (as described below) and their effects on impressionable parameters are investigated in this paper. Two numbers of parame_ w ) were kept constant during the all experiments. ters (Tw,inlet, m Experiments procedure is classified into two main modes as seen in Table 1. In first mode, the amount of Ta,inlet was kept at a _ a and DC voltage/current on aforefixed value and the effects of m said impressionable parameters are studied. According to Table 1, eight different amounts of DC voltages were tested. Five different air flow rate (2–6 LPM) were employed for each DC voltage. In sec_ a were kept constant at 6 V ond mode, DC voltage/current and m and 6 LPM respectively and Ta,inlet was varied. The effect of Ta,inlet on impressionable parameters are probed in second mode. Eight different values of air inlet temperatures between 20 °C and 90 °C were considered. Indeed Different air flow rates and inlet temperatures were applied to simulate various climate conditions
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Fig. 1. Experimental test set-up: 1 – Water tank, 2 – Water pump, 3 – Valves, 4 – Water Rotameter, 5 – Test section, 6 – Air Rotameter, 7 – Compressor, 8 – Water tank, 9 – DC voltage supplier, 10 – Data logger, 11 – Computer.
Fig. 2. Detail of test section: 1 – Frame, 2 – Thermoelectric, 3 – Inlet or outlet, 4 – Heat sink, 5 – Test section, 6 – Actual photograph of test section.
or applications. All measured values were recorded at steady state condition. 2.3. Calculations method 2.3.1. Uncertainty of measurements Uncertainty of measurements is calculated using of the method which was employed in Sadighi Dizaji et al. [30,31] study (Moffat [32] method). As described in [30,31], the uncertainties in
calculating a result (W+R) can be evaluated by following equation. Where, x1, x2, etc., are independent variables of a defined result.
W Rþ ¼
@Rþ w1 @X1
2
þ 2 þ 2 !12 @R @R þ w2 þ þ wn @X2 @Xn
ð1Þ
The obtained uncertainty for each parameter is shown in Table 2. As seen in Table 2, the amounts of uncertainty are in reasonable ranges.
H. Sadighi Dizaji et al. / Applied Energy 181 (2016) 357–366 Table 1 Different tested conditions. DC voltage (V)
Air flow rate (LPM)
Air inlet temperature (°C)
Water inlet temperature (°C)
First mode 2 4 6 8 10 12 14 16
2, 2, 2, 2, 2, 2, 2, 2,
15 15 15 15 15 15 15 15
Around Around Around Around Around Around Around Around
20–90
Around 14
3, 3, 3, 3, 3, 3, 3, 3,
4, 4, 4, 4, 4, 4, 4, 4,
5, 5, 5, 5, 5, 5, 5, 5,
6 6 6 6 6 6 6 6
Second mode 6 6
361
qj ¼ Rm I2
ð7Þ
qcon ¼ Km ðTh Tc Þ
ð8Þ
The cooling power (qc) and heating power (qh) of TEM can be found by using of energy balance of four aforesaid equations. In the evaluation of cooling and heating power it is assumed that half of Joule heat passes from the cold side and the other part passes from the hot side. Hence qc and qh can be calculated by Eqs. (9) and (10).
14 14 14 14 14 14 14 14
1 1 qc ¼ qpc qj qcon ¼ am ITc Rm I2 Km ðTh Tc Þ 2 2
ð9Þ
1 1 qh ¼ qph þ qj qcon ¼ am ITh þ Rm I2 Km ðTh Tc Þ 2 2
ð10Þ
The input electrical power to TEM is calculated from Eq. (11).
Table 2 Maximum uncertainty of measurement. Parameter
Unit
Uncertainty in the temperature measurement Air fluid inlet temperature Air fluid outlet temperature Water fluid inlet temperature Water fluid outlet temperature Uncertainty in the measurement of volume flow rate Water Air Uncertainty in reading values of table (q, k, . . .)
°C °C °C °C °C m3 h1 m3 h1 m3 h1 %
Comment
P ¼ am ðTh Tc Þ þ Rm I2
ð11Þ
TEM coefficient of performance for cooling (COPc) is evaluated from Eq. (12).
±0.5 ±0.5 ±0.5 ±0.5
COP c ¼
±0.02 ±0.01 ±0.1–0.2
2.3.2. Calculating of cooling and heating characteristics Commercial TEC1-12706 thermoelectric was employed in present study. The specifications of TEC1-12706 are shown in Table 3 which can be found in datasheet of TEC1-12706. Seebeck coefficient of TEM (am), electrical resistance (Rm) and thermal conductivity (Km) of TEM are evaluated using of Table 1 as shown below [1]
am ¼
V max Th
ð2Þ
Rm ¼
V max ðT h DT max Þ Th Imax
ð3Þ
qc p
ð12Þ
The COPc values mainly depend on the temperatures at the two sides of the thermoelectric element [33]. This fact is well indicated starting from the definition of the (ideal) Carnot COP, here indicated as COPmax that considers the temperatures of the hot source Th and of the cold source Tc [33]:
COP max ¼
Tc Th Tc
ð13Þ
Moreover, heat transfer rate between air fluid and cold surface can be calculated from Eq. (14).
_ a cp ðT a;inlet T a;outlet Þ qair ¼ m
ð14Þ
3. Results and discussions
1 DT max 2T h ¼ K m Imax V max ðT h DT max Þ
ð4Þ
Four kinds of heat transfer can appear in thermal analysis of TEM. Peltier heating (qph), Peltier cooling (qpc), Joule heating (qj) and Fourier heating (qcon) are said four heating effects which can be evaluated from Eqs. (5)–(8). Joule heat appears when an electrical current passes along an electric resistance. Fourier heat is the heat transfer from a hot surface toward a cold surface due to the conduction.
qpc ¼ am ITc
ð5Þ
aph ¼ am ITh
ð6Þ
Table 3 Specifications of TEC1-12706. Type
Couples N
Imax (A)
Vmax (V)
Qmax (W)
DTmax (K)
Reference temperature at hot-end (K)
TEC1-12706
127
6
15.2
56.5
68
300
_ a ) and voltage 3.1. The effect of air flow rate (m In this section the amounts of air and water inlet temperatures are kept constant and the effects of air flow rate and DC voltage on Tc (thermoelectric cold side surface temperature), Th (thermoelectric hot side surface temperature), Ta,outlet (air outlet temperature), DTa (Ta,inlet Ta,outlet), cooling power (qc), heat transfer rate (qa), COP c are investigated. Main findings from present section COPc and COP max are described below. 3.1.1. The effect of air flow rate and voltage on Tc, Th, Ta,outlet and DTa (a) The effect of air flow rate and DC voltage on Tc, Ta,outlet are presented in Figs. 3 and 4 respectively. Figs. 3 and 4 should be considered simultaneously in order to exactly and conveniently clarify their behavior. Firstly it is noted that, achieving lower amounts of Ta,outlet is considered as more beneficial operation of TEM system. A general explanation and the reasons of curves behavior in Figs. 3 and 4 are schematically illustrated in Fig. 5 (for a constant air flow rate and for a constant DC voltage). Increment of DC voltage or air flow rate reduces the amount of Ta,outlet with two different mechanisms as summarized in Fig. 5 and as descried below. As seen in Figs. 3 or 5, increment of DC voltage reduces the thermoelectric cold surface temperature (Tc) which obviously causes reduction
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Fig. 3. The effect of DC voltage and air flow rate on Tc.
Fig. 4. The effect of DC voltage and air flow rate on Ta,outlet.
of Ta,outlet in Fig. 4. As illustrated in Fig. 5, for a constant DC voltage (for example 16 V), increment of air flow rate increases the amount of Tc (warmer). But it (increment of air flow rate) reduces the air outlet temperature (Ta,outlet). Indeed, in a fixed DC voltage, increment of air flow rate causes higher fluid Reynolds number and then higher heat transfer rate between air fluid flow and cold surface temperature which causes reduction of air fluid outlet temperature and consequently enhancement of thermoelectric cold surface temperature (Tc). It should be noted that, said phenomenon will not continue for any arbitrary extra air flow rate. Because each amount of DC voltage, can produce a specific-limited cold surface heat cooling capacity (qc). For instance in 2 V DC voltage (Fig. 4), the positive effect of the increment of air flow rate on the reduction of Ta,outlet reduces step by step. And the difference between Ta,outlet in 5 LPM and 6 LPM is very less than the difference between Ta,outlet in 1 LPM and 2 LPM. This means that, if the air flow rate is increased more again, the amount of Ta,outlet may not be reduced again (but it does not mean the amount of qa is reduced because _ a in addition to the Ta,outlet). according to Eq. (14), qa depends on m However maximum heat transfer rate between air and cold surface (qa) is always smaller than the (qc). The differences between qa and qc are clarified in Section 3.1.2. (b) The effect of air flow rate on Ta,outlet in higher amounts of DC voltages is more tangible than the lower amounts of DC voltages. Indeed in higher DC voltages, more amount of air flow rate can be supplied which reduces the Ta,outlet yet. Besides, in higher amounts of air flow rate, reduction of Ta,outlet due to increment of DC voltage occur with a sharp slope. (c) According to Fig. 6, DTa = Ta,inlet Ta,outlet increases with the increase of DC voltage or air flow rate. Indeed, reduction of Ta,outlet (which explained above) increases the amount of DTa. Maximum DTa was obtained in DC voltage of 16 V and air flow rate of 6 LPM. Minimum cold surface temperature was observed at minimum air flow rate (2 LPM) and maximum DC voltage supply (16 V) too.
Fig. 5. Graphical explanation for the curves behavior of Figs. 3 and 4.
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Fig. 6. The effect of DC voltage and air flow rate on DTa.
(d) Fig. 7 shows the thermoelectric hot surface temperature (Th). The scale of Figs. 7 and 3 were chosen the same in order to compare the changes of Tc and Th. As seen in Fig. 7, the changes of hot side temperature are lower than the changes of cold side temperature. It is pertain to the naturally characteristics of water fluid which has higher thermal capacity and density. Said characteristics can remain the thermoelectric hot side temperature in a lower amount which causes more effectiveness of cold side. Air flow rate almost does not effect on hot side temperature (in our tested range). It is mentioned that the used water flow rate in this study was 1 LPM. 3.1.2. The effect of air flow rate and voltage on qc and qa Firstly the differences between qc and qa should be clarified. qc and qa are not the same. Indeed, qc is the total cooling power of TEM. Some of qc can be transmitted to air flow which we termed it qa. On the other hand, qc is the cooling ability of thermoelectric. However it does not mean that, all of qc is transmitted into the air flow. Indeed, this cooling power is expected (is spent) for two things. 1: reduction and then remaining of cold surface of TEM on a low temperature (for example 5 °C) and 2: reduction of the air flow temperature. As an example, imagine that, the amount of air flow rate is reduced until 0 LPM (no air flow rate). In this case, all amount of qc is spent for reduction of cold surface temperate for
Fig. 7. The effect of DC voltage and air flow rate on Th.
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example until 10 °C and keeps the cold surface temperature on 10 °C. And now if the amount of air flow rate is increased to 1 LPM, some of qc (not all of qc) causes reduction of air outlet temperature; and the temperature of cold surface is enhanced form 10 °C to for example 5 °C. Hence, by increment of air flow rate (or other method such as using turbulator) the amount of qc which is spent for reduction of air temperature is increased. Hence, the temperature of cold surface of TEM in enhanced too. The effect of air flow rate and DC voltage on qc and qa are presented in Figs. 8 and 9 respectively. As seen in Figs. 8 and 9, both qc and qa increase with the increase of air flow rate or DC voltage. Maximum qc and qa were obtained at air flow rate of 6 LPM and DC voltage of 16 V. For lower amounts of DC voltage, the effect of air flow rate on qc and qa is minor in comparison with the higher amounts of DC voltage. The slope of enhancement of air flow rate due to increment of DC voltage augments with the increase of air flow rate. For instance, increment of DC voltage at air flow rate of 2 LPM almost does not effect on the amount of qa. The amounts of qa are less than the amounts of qc for all cases. It means that, a high capacity of thermoelectric cold surface (qc) has not been used yet and higher amounts of qa can be achieved compared to the present values for a constant air flow rate and constant DC voltage via other methods. Indeed, other designing of heat sink such as finned heat sink, may be employed in order to get higher value of qair from qc.
Fig. 8. The effect of DC voltage and air flow rate on qc.
Fig. 9. The effect of DC voltage and air flow rate on qair.
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COP 3.1.3. The effect of air flow rate and voltage on COPc, COPmax and COP max COPc is a parameter which considers both thermoelectric production power (qc) and thermoelectric input electrical power (P). On the other words, although that the increment of DC voltage increases the amount of qc, it increases the input electrical power of thermoelectric too. Hence a simultaneous comparison between qc and P should be considered in order to make the best decision. In these cases, COPc is evaluated for said purpose which is the ratio of qc to P. The effects of air flow rate and DC voltage on COPc and COPmax are shown in Fig. 10. As seen in Fig. 10, increment of DC voltage causes reduction of both COPc and COPmax. It means that, despite the enhancement of qc duo to the increment of DC voltage, the amount of P (input electrical power) is increased with a sharper slope compared to the qc. Hence, the amount of COPc is reduced. It is noted that, augmentation of air flow rate has a positive effect on COPc yet. Indeed, increment of air flow rate increases the amount of qc however it does not have any effect on thermoelectric input electrical power; hence, numerator of COPc increases while the value of denominator is constant which causes enhancement of COPc. Air flow rate does not have significant effect on COPmax. Maximum COPc was obtained at DC voltage of 2 and air flow rate of 6 LPM. Fig. 11 shows the variations of COPc/COPmax with air flow rate and DC voltage. As seen in Fig. 11, a peak point is appeared at DC voltage of 6 V for all tested air flow rates. Despite the reduction of COPc in DC voltage of 2–6, the amount of COPc/COPmax is increased in said range. After DC voltage of 6 V, the values of COPc/ COPmax are reduced again. DC voltage of 6 V creates a peak point in this figure. The reason of this phenomenon can be clarified by analyzing and comparing the slopes of COPc and COPmax. Actually, by increment of DC voltage up to 6 V, the reduction slope of COPmax is more than reduction slope of COPc. But by increasing DC voltage more than 6 V, the reduction slope of COPc and COPmax inversed and COPc decrease faster than the COPmax. This peak point can be demonstrated as an optimum condition for proposed TEC system. It is noted that, inlet temperature of air flow creates a peak point too, which is described in next sections. Indeed, COPc/COPmax could propose as a new parameter in the optimization of TEC systems which should be consider to approach maximum performance of TEC systems. Generally, COP/COPmax mentions this important that, decrement of COP (for example in Fig. 10) may not always considered as a negative point; because as seen in Fig. 11, despite the decrement of COP (from 1 V to 3 V), the amount of COP/COPmax is increased and creates a peak point. It means that the value of
Fig. 11. The effect of DC voltage and air flow rate on COPc/COPmax.
COP has the minimum difference with its maximum working condition (COP max) in said voltage (6 V). 3.1.4. The effect of air inlet temperature on Tc, Ta,outlet, qa, qc, COPc and COP COPmax
_ a and DC voltage were kept conIn this section, the amount of m stant in 6 LPM and 6 V respectively and air inlet temperature was changed between 20 and 90 °C. Indeed, the air inlet temperature was varied via an electrical heater. The heater was placed inside a box which has one inlet and one outlet. The power of electrical heater was adjust by a dimmer. The effect of air inlet temperature on Tc and Ta,outlet is shown In Fig. 12. Variations of qa and qc due to the change of air inlet temperature are illustrated in Fig. 13. And COP c are finally the effects of air inlet temperature on COPc and COP max shown in Figs. 14 and 15 respectively. According to Fig. 12 both Tc and Ta,outlet increase with the increase of air inlet temperature. The slop of this enhancement is almost identical for both of them. Increment of air inlet temperature influences the cold surface temperature and obviously causes increment of thermoelectric surface temperature and consequently the value of Ta,outlet is reduced. Nonetheless, the amount of Ta,outlet is significantly reduced until in high values of Ta,inlet such as 90 °C. As seen in Fig. 13, increment of air inlet temperature augments both qc and qa. The slope of enhancement of qa is more than the slope of enhancement of qc. On the other words, the difference between qa and qc increases with increment of air inlet temperature. The amount of DTa = Ta,inlet Ta,outlet is sharply intensified
Fig. 10. The effect of DC voltage and air flow rate on COPc and COPmax.
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Fig. 12. The effect air inlet temperature on Tc and Ta,outlet.
Fig. 15. The effect air inlet temperature on COPc/COPmax.
Fig. 13. The effect air inlet temperature on qc and qair.
Indeed, qc and qa are couple with each other and the variation of one of them means the variation of another. Fig. 14 reveals that, the amount of COPc increases with enhancement of air inlet temperature. As described above, enhancement of air inlet temperature increases the qc. On the other hand, qc is the numerator of Eq. (12). Hence, obviously the increment of qc enhances the COPc. Same as Fig. 11, COPc/COPmax creates a peak point again (see Fig. 15). Two more extra temperatures (100 and 110 °C) is experimented again for this section in order to ascertain the existence of a peak point in around air inlet temperature of 70 °C. However, the amount of this peak point may be changed for other values of air flow rates or DC voltages. Nonetheless, the creation of a peak point in COPc/COPmax is seems as rule. From Fig. 15, it can be concluded that, at the inlet temperature of around 70 °C the thermoelectric unit has the best operating condition from the point of COPc/ COPmax. As a general result, it seems that, COPc/COPmax is a promising parameter for optimizing of thermoelectric units from a new point of view. And extra investigations should be conducted to clarify the curve behavior of COPc/COPmax.
4. Conclusion
Fig. 14. The effect air inlet temperature on COPc.
by enhancement of air inlet temperature. And according to Eq. (14) it causes enhancement of qa. Besides, enhancement of air inlet temperature causes augmentation of heat transfer rate between air flow and thermoelectric cold surface which increases the amount of Tc. Subsequently, increment of Tc increases the qc.
In this paper, a novel air-water based TEC system was employed as an alternative air cooling unit. Indeed, different air flow rate and different air inlet temperature simulated the various climate conditions. Contrary to previous studies, thermoelectric hot side temperature was adjusted by a low constant water flow rate (and not by an air fan) which significantly increased the cold side performance of TEM. Different climate conditions (air inlet temperature and air flow rate) were tested. Fourteen various parameters includ_ a , DC voltage/current), ten ing four variable parameters (Ta,inlet, m impressionable parameters (Ta,outlet, Tw,outlet, Tc, Th, qc, qh, qa, COP, COPmax, COP/COPmax) are evaluated. Indeed, the effects of variable parameters on impressionable parameters were investigated. Two _ w ) were kept constant during the numbers of parameters (Tw,inlet, m experiments. It was accidently found out that, despite the descending behavior of both COPc and COPmax (with changing of DC voltage), the ratio COPc to COPmax (COPc/COPmax) creates a peak point (ascending and then descending) in all cases. Said peak point can be considered as an appropriate working condition of thermoelectric units. COPc/COPmax was investigated for different air flow inlet
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