1. Introduction
There exists a great risk of heat-related diseases when working in hot outdoor working conditions, such as at COVID-19 quarantine sites where wearing protective clothing is mandatory [
1]. This is because protective clothing acts as a barrier to prevent the convection of inner air and body heat to the ambient environment. Therefore, a technical need for portable cooling devices suitable for protective clothing has emerged to enhance comfort while preventing the risk of heat-related illness, such as heat syncope, in hot weather. Furthermore, such portable cooling systems should be lightweight, compact, and low-power-consuming for long-lasting serviceability without interfering with quarantine tasks and charging during shift hours.
Depending on the energy source, protective clothing cooling devices can be classified into passive systems that use cold heat sources, such as ice packs, and active systems that use refrigeration cycles or thermoelectric cooling. Passive systems bear disadvantages such as difficulties regarding temperature control and air circulation. In addition, there is a risk of frostbite and poor comfort due to dew formation from cooling sources at very cold temperatures compared to body temperature [
2,
3]. The active system can control the temperature and flow rate of air during its operation. However, there may be the disadvantage of poor portability because high-density components such as batteries and motors must be equipped. In particular, refrigeration cycle-based coolers are unsuitable for portable use in protective clothing since they must be equipped with pumps, compressors, and evaporators for refrigerant circulation, evaporation, and condensation [
4,
5]. On the other hand, thermoelectric cooling does not require complex refrigerant processes, thereby achieving miniaturization and weight reduction of the portable cooler [
6,
7].
Portability significantly differs depending on the cooling range and capacity, even in the same thermoelectric cooling method. A full-body cooling system has been developed to allow cooled air to circulate through the upper chest and back areas of the protective suits [
6]. This cooling device also has a dehumidification function against moisture caused by sweat inside the protective clothing. Despite adopting such advanced technology, the system is heavy weighing 1.2 kg and equipped with two 80 mm-diameter circulation fans, making it difficult for workers to wear it for long periods of time. A half-body cooling system that supplies cold air only to the upper body has also been developed. The weight of the system, excluding the blower fans and batteries, is 450 g, showing a dramatic reduction in weight [
8]. This paper introduces the design and performance of an even more lightweight Peltier cooler than the two devices mentioned above. The cooler, with a compact structure and weighing only 279 g, can blow cooled air into the protective clothing without the requirement of additional fans in the other parts of the clothing.
Experimental studies, system modeling studies, and numerical analysis studies have been conducted complementarily to design and evaluate the performance of portable Peltier coolers. Experimental studies aim to measure cooler prototypes’ performance, identify technical issues, and draw design improvement plans where necessary. The testing apparatus for Peltier coolers has been designed and constructed to measure air flow rate and temperature, surface temperature distribution, fan rotation speed, and power consumption by using various sensors and data acquisition devices such as anemometers, thermocouples, thermographic cameras, electrical power sensors, motor encoders, and so forth [
9,
10,
11,
12]. For example, experimental studies have been conducted to determine the performance characteristics of the cooler by measuring the coefficient of performance (COP), a widely used figure of merit for thermoelectric cooling systems, against the blower fan speed or by measuring the electric consumption versus COP relation [
11,
12].
The system modeling approach can derive an initial design that fulfills the performance requirements from the experimental models or rules of thumb of various components constituting the Peltier cooler. For example, the performance curve of the blower fan [
13], the heat pumping characteristic of the Peltier element [
14], and the heat exchange model of the cooling fin [
13] can be utilized in this method. Model-based design strategy is beneficial at the initial design and prototyping stages before the detailed development process through numerical and computational approaches. This can help component selection and system construction by predicting the device’s cooling performance approximately [
15].
When developing thermoelectric cooling devices, computer-aided engineering (CAE) methods have been widely exploited complementarily with experimental or system modeling approaches because computer simulations can verify the design rapidly and reliably before the prototyping and testing stages. The Peltier cooler operates inherently by coupling thermoelectric effects, fluid flow, and heat transfer. Therefore, a numerical method for it should have multiphysics characteristics [
16,
17]. Numerical modeling and analyses of Peltier coolers have been generally performed three-dimensionally rather than simplifying to lower dimensions due to the complex geometries of thermoelectric elements, heat exchanging fins, and fan blades. In terms of time dependence, both steady-state and transient analyses have been of great interest in this simulation field. This is because the prediction of both steady-state response and initial dynamic behavior is vital for the performance evaluation of a Peltier cooler, which takes a relatively long time of over 200 s to reach the steady state [
18,
19]. In the airflow analysis using computational fluid dynamics (CFD), turbulence models have been popularly used instead of laminar models because the associated Reynolds number is very high due to fast impeller speed and narrow cooling fin channels [
8,
9,
13]. Comparing three widely used turbulent models, i.e., the standard
model, the realizable
model, and the shear stress transport (SST)
model, the SST
model showed the highest accuracy in a three-dimensional steady-state CFD analysis for a home refrigerator with an integrated Peltier cooling unit [
19]. As a material constitutive relation, linear thermoelectric equations have been most widely used [
20,
21]. Their material coefficients are usually treated as constants, but temperature-dependent nonlinearity becomes non-negligible if the operating temperature difference is significant [
22,
23].
Three-dimensional steady-state CFD analysis of thermoelectric air cooling chamber with a liquid heat exchanger on the high-temperature side was carried out based on the realizable
turbulent model [
24]. Design verification of a portable Peltier air cooler integrated with the work jacket was performed with CFD simulations [
8]. Transient CFD and thermoelectric analyses were conducted in a one-way coupling fashion [
25]. While performing transient CFD analysis based on the
turbulent model and transient thermoelectric analysis in order, the average wall temperatures on both TEC sides obtained from the CFD analysis were reflected in the thermoelectric model computation as boundary conditions. Although the blower fan geometry was included in the CFD analysis in the above-mentioned simulation studies, any heat transfer through the blower surface was neglected by treating it as an insulation boundary condition. To the best of the author’s knowledge, computational research on the effects of fan motor heat on the performance degradation of thermoelectric coolers (TECs) for protective clothing has yet to be published, even though the amount of heat generation is considerable.
This paper introduces a numerical study on the effect of heat from a blower motor on the performance of a portable Peltier cooler for protective clothing. A three-dimensional, steady-state, coupled-field analysis based on an incompressible turbulent model and linear thermoelectric constitutive relations is performed for multiphysics phenomena in which turbulent airflow, thermoelectric cooling, and heat transfer through conduction and convection co-occur. The simulation results for the case incorporating the motor heat were verified by comparing them with the experimental results. In addition, a simulation without the internal heat source was carried out to quantify the improvement of cooling capacity and efficiency.
The rest of this paper is structured as follows. The consecutive
Section 2 and
Section 3 introduce the design characteristics of the proposed Peltier cooler and its performance testing results, respectively.
Section 4 presents the basic numerical models and analysis setups for the simulation of the Peltier cooler.
Section 5 shows the simulation results with experimental observations and discusses the effect of an internal heat source on the overall cooling performance. Lastly,
Section 6 presents the conclusions of this computational study.
2. Peltier Cooler Design
According to the opinions of the medical staff at COVID-19 quarantine sites, the main technical specifications of the cooler required for outdoor tasks in hot weather conditions can be summarized into four main items. First, the cooler for protective clothing should prevent potential heat diseases by supplying conditioned air at least 5 °C lower than the outside temperature in hot weather. Second, the cooler should operate continuously for at least two hours before the work shift without battery replacement or charging. Third, the cooler structure should be simple and compact, less than 10 cm in thickness, to facilitate its attachment and detachment to and from protective clothing as well as to avoid any interferences with tasks during operation. Fourth, the system’s total weight, excluding the battery, should be less than 300 g so as to mitigate muscle fatigue in outdoor workers.
A portable Peltier cooler was designed to satisfy the four requirements mentioned above, and its prototype was built for immediate use in the field.
Figure 1 shows the CAD image of the Peltier cooler (Cyro™ designed and manufactured by NK Innovation, Inc., Sejong, Korea) used in this computational study. The overall dimension of the cooler is 172 mm × 49 mm × 61 mm, and its weight is 279 g. The power consumption of the cooler is 30 W, which can be used for more than 2 h with a 11.1 V 8700 mAh rechargeable battery supplying 96.57 Wh at maximum. Structural materials should facilitate rapid prototyping and enable effective thermal insulation between cold and hot air ducts. Therefore, a 3-D printable photoactive polymer resin (Formlabs™ White Resin) with low thermal conductivity and lightweight was used as the primary structural material. The thermoelectric element adopted for the cooling system is TEM TB-127-1.4-2.0 manufactured by Kyrotherm. Heat sinks were attached to both sides of the Peltier element, and the insulation wall between them separated cold and hot air ducts. Heat sinks are made of aluminum alloy with lightness and high thermal conductivity. Due to its high thermal conductivity and excellent adhesion characteristics, TSE3941 silicone adhesive from Momentive Performance Materials Inc. was used for heat sink attachment. In addition, each flow channel is equipped with a blower fan to ventilate air and induce forced convection around the heat sink.
Figure 2 shows the constructed prototype and an example of the protective clothing with the Peltier cooler installed. The Peltier cooler design in this study is similar to that of [
9] in that the cooling and heating parts are separated for insulation but different in that the airflows are in the same direction, not in the opposite direction. As shown in
Figure 2a, the device frame consists of three 3D-printed parts: the middle substrate and two duct covers with the air inlet and outlet.
Figure 2b shows an example of the Peltier cooler installed inside a protective suit. The two holes are the hot air duct’s inlet and outlet. The cooler and the battery pack are fixed by an inner chest harness, which accomplishes structural and installation simplicity.
The blowers in the air ducts have an integrated structure of the motor and the impeller. They inhale stagnant outer air in the direction of the impeller’s axis of rotation and blow it toward the heat sink for heat exchange. In this process, the heat from the motor is unwantedly transferred to the air, causing a preheating effect before reaching the heat sink. As a result, the blower improves heat-exchanging efficiency through forced convection at the low-temperature heat sink while also lowering cooling performance by the air-preheating effect. As the motor output boosts to enhance blowing capacity, the degradation in cooling efficiency due to preheating becomes more significant. Similarly, the preheating effect from the hot side blower also deteriorates the heat dissipation capability at the high-temperature heat sink. Therefore, this paper analyzes the impact of motor heat on the thermoelectric cooling performance using multiphysics simulations and predicts the potential performance improvement by removing such internal heat sources.
3. Performance Test Results
An experimental study was conducted to evaluate the performance of the Peltier cooler prototype and obtain experimental data to validate the simulation results.
Figure 3 shows the experimental apparatus for testing the cooler’s performance.
Figure 3a,b present an experimental setup for the cold and the other side duct, respectively. The apparatus comprises a structural frame, various sensors, and a data collection board with a flat-panel display. The main frame for fixing the cooling device and the sensing components is the assembled structure of aluminum extrusion profiles. A pair of thermal anemometers measured the velocity and temperature of the air in both the inlet and outlet. Although not shown in
Figure 3, the non-contact thermal camera module (Letpton 3.5 manufactured by Teledyne FLIR LLC, Wilsonville, OR, USA) and a portable thermal camera (GTC-600C supplied by Robert Bosch GmbH, Gerlingen, Germany) monitored the surface temperature of the subject device. The opaque plastic cover was partially cut and replaced with a transparent film, as shown in
Figure 3a, to capture the surface temperature of the heat sink and the blower inside the cold air duct with thermal cameras. Power digital wattmeters (Model SEN0291 by Zhiwei Robotics Corp., Shanghai, China) were used to measure power consumption by sensing the shunt voltage and the electric current for electric components such as the thermoelectric element and the blower motors. A single-board computer (Raspberry Pi 4 Model B made by Raspberry Pi Foundation, Cambridge, UK) processed the wattmeter data to be displayed on the monitoring panel.
Coefficient of performance (COP) is a widely used performance figure for Peltier coolers [
11,
12,
26]. COP is defined by the ratio of cooling capacity
to total power consumption
and can be expressed as
where
and
are the power consumption of the Peltier element and the blowers, respectively.
For the Peltier cooler of the design shown in
Figure 1, the cooling capacity
represents the net thermal energy reduction rate in the cold air duct [
24], so it can be expressed as
where
is the mass flow rate,
is the heat capacity at constant pressure, and
T is the absolute temperature.
Figure 4 shows the temperature distribution plots for the portable Peltier cooler in operation obtained by the infrared thermal imaging camera. The minimum temperature on the upper surface of the heat sink in the cold air duct was measured as 20.8 °C. Meanwhile, the maximum temperature on the blower surface was 44.7 °C and 47.8 °C in the cold and hot air ducts, respectively. These measured surface temperatures were used as the essential boundary conditions imposed on the blower surfaces for heat transfer analysis.
Table 1 shows the experimental results and the calculated performance indices. Each measured value in this table is the mean of three data points in a steady state with a low data fluctuation. The COP of the cooler, including the internal heat source, was evaluated as 10%. The air temperature could be reduced by 1.9 °C at most, falling short of the design target of 5 °C.
5. Simulation Results and Discussion
Numerical simulations to evaluate and improve the thermoelectric cooling performance of the portable Peltier cooler shown in
Figure 2 were conducted in a coupled fashion using the commercial software package COMSOL Multiphysics with its extension modules, including CFD Module, AC/DC Module, and Heat Transfer Module.
According to the presence of heat sources in the air ducts due to the fan motor operation, numerical simulation was conducted in two cases: one incorporating the internal heat and the other suppressing the motor heat. Each simulation was performed through three stages: turbulent CFD analysis, electric field analysis, and conjugate heat transfer analysis accompanying thermoelectric cooling and Joule heating effects. As a result, such a multiphysics simulation yielded the flow and pressure fields of the air, the potential and current density fields of the electric parts, and temperature fields throughout the entire computational domain.
Figure 6 shows the fluid velocity fields visualized as vector arrows in the cold and hot air ducts. The cold duct has a maximum speed of 27.32 m/s in the entire domain and an average normal velocity of 6.28 m/s through the middle cross-section of the heat sink. Meanwhile, the highest fluid speed in the hot air duct and the average cross-sectional velocity in the heat sink was calculated as 19.63 m/s and 7.60 m/s, respectively.
Figure 7 shows the simulation results in the electric domains, i.e., the copper wiring layers and semiconductor segments.
Figure 7a displays the computed electric potential distribution by applying 10.38 V to the input port. Since the junctions of p-type and n-type semiconductors interconnected by copper conductors occur repeatedly in space, a uniform voltage drop appears across each junction.
Figure 7b illustrates the electrical field intensity distribution with a color map and the current density with vector arrows at 30,000 different Gauss quadrature points. The length of each arrow was set in proportion to the vector magnitude of the corresponding current density. Due to the inherent characteristics, highly conducting copper plates possess nearly zero electric field intensity but relatively high current density. On the other hand, semiconducting bismuth telluride blocks have a higher electric field intensity of 23.2 V/m and a relatively lower current density than conductors.
Figure 8 illustrates the post-processed temperature distribution plots for the two simulation cases. The outer surfaces were set to be translucent to reveal the internal wall temperature features clearly.
To verify the reliability of these simulation results,
Figure 9 compares the numerically computed temperature distribution with an experimentally obtained thermographic image. It should be noted that the thermal image in
Figure 9b displays the external wall temperature, except for the internal wall temperature of selected portions replaced with the transparent film that can be observed in
Figure 3a. On the other hand,
Figure 9a shows the temperature distribution on the fluid boundaries with top surfaces virtually hidden. In other words, in
Figure 9, it is only valid to compare the temperature distribution in two boundaries, i.e., the blower’s circular top plane and the heat sink surface. Direct comparison in temperature of the other surfaces is not compatible. The minimum temperature on the heat sink surface was 20.8 °C in the experiment, while it was computed as 4.8% lower at 19.7 °C in the simulation. Therefore, it was confirmed that the simulation results predict the experimental results precisely within a 5% error.
Figure 10 depicts a bar chart comparing the average temperatures computed at three locations of interest from the temperature fields numerically obtained for the two simulation cases shown in
Figure 8. The minimum heat sink surface temperature, the cold duct mean outlet temperature, and the hot duct average outlet temperature were 19.7 °C, 24.26 °C, and 30.04 °C, respectively, in the simulation accounting for the blower heat generation. In the other simulation with no account for the heat generation effect, meanwhile, the minimum heat sink surface temperature, the cold duct mean outlet temperature, and the hot duct average outlet temperature were 17.6 °C, 22.86 °C, and 35.4 °C, respectively, which were all lower than the simulation results considering blower heat.
Figure 11 depicts the color-contoured temperature distribution only in the electric domain and the thermal energy transfer represented by vector arrows at 5000 different Gauss quadrature points. The length of each arrow was set in proportion to the vector magnitude of the heat transfer. The simulation results illustrate that the Peltier effect pumps the thermal energy from the top low-temperature region to the bottom high-temperature region so that the air can refrigerate in the upper chamber. In addition, this numerical analysis confirmed that the Peltier effect becomes more active due to the removal of the motor heat, which leads to further lower temperatures in the low-temperature side of the thermoelectric cooling module.
Comprehensively summarizing the above-described simulation results,
Table 6 lists the experimental results and the calculated performance figures of merit. In this table, the total power consumption
is obtained as in the denominator of Equation (
1) by adding Peltier power consumption
obtained from the simulation and blower power consumption
measured from the experiment.
Figure 12 comparatively shows the cooling capacity and COP derived from the performance test and two simulation cases as a bar chart. Even considering that the simulation results slightly underestimate the experimental results, it can be concluded from these thermo-electro-fluidic simulations that removing the internal heat source can significantly improve the cooling capacity by 80.6% from 4.68 W to 8.45 W, and accordingly, the COP by 10.6% from 13.0% to 23.6%.
Internal heat sources can be technically removed in a way that the electric motors located outside the air ducts transmit the rotational power inward through the shafts. Cyro™, the Peltier cooling device for protective clothing shown in
Figure 2a, satisfies three of the four technical requirements specified in
Section 2, i.e., operating time, size, and weight, except for temperature reduction by 5 °C. The next version of Cryo™, with improved cooling performance through design optimization and heat removal inside the air duct, is currently under development through industry–academia cooperation.