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Application of thermoelectric cooler in temperature control system of space science experiment
Applied Thermal Engineering 168 (2020) 114888
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Application of thermoelectric cooler in temperature control system of space science experiment ⁎
Dongcai Guoa, , Qiang Shenga, Xiangyu Doua, Ze Wanga, Liyao Xieb, Bo Yangc,
T
⁎
a
Key Laboratory of Space Utilization, Technology and Engineering Center for Space Utilization, Chinese Academy of Sciences, Beijing 100094, China Hebei University of Technology, Tianjin 300401, China c School of Aeronautic Science and Engineering, Beihang University, Beijing 100191, China b
H I GH L IG H T S
cooler (TEC) is suitable for spacecraft temperature control. • Thermoelectric system of TEC and heat exchanger is proposed. • Combined • The combined system performs better than TEC-alone system.
A R T I C LE I N FO
A B S T R A C T
Keywords: Space station Space science experiment Temperature control Thermoelectric cooler
As an important mission of space stations and scientific satellites, space science experiment usually requires effective temperature control measures. Integrating heating and cooling capacities, thermoelectric cooler (TEC) is competent for this job attributed to the low complexity and high reliability. In this paper, two TEC systems are introduced, including direct temperature control system (DTCS) and environment temperature control system (ETCS). DTCS directly deals with the temperature control of the target element surface. The effects of the current and the heat sink (i.e. working liquid) temperature are analyzed under heating and cooling conditions respectively. ETCS deals with the temperature control of the entire environment surrounding the element. The element is indirectly heated or cooled by the ambient air. Depending on whether an additional intermediate air-to-liquid heat exchanger is consisted or not, ETCS can be further classified as TEC-HX system and TEC-alone system. The heat exchanger may improve the system performances. The effects of the liquid flow rate and heat exchanger efficiency on system performances are analyzed, providing guides for the design of the practical system.
0. Introduction Covering multilateral disciplines such as physics, chemistry and biology, space science experiments aim at exploring new scientific phenomena under microgravity and other special space conditions [1]. So far, those magnificent space projects like International Space Station (ISS) have established many experiment platforms, on which various researches involving biology, medicine and fluid mechanics are carried out and excellent results have been obtained [2–5]. More and more space science experiments are required in spaceflight missions, especially when the spaceflight prevails under background of the explosive development of business space. Many of the space science experiments require constant environment temperature, and some experimental instruments need to be operated below specific temperature. In space station, the terminal
⁎
cooling devices such as radiators [6], sublimators [7] or membrane evaporators [8] provide the basic heat sink for all the cooling requirements inside. However, the temperature requirements vary greatly among different space experiments, which may exceed the temperature range of the basic heat sink. Therefore, it is necessary to design a general temperature control method to satisfy different temperature requirements with a heat sink working in a relatively narrow temperature range. Despite of excellent performance, many refrigeration technologies commonly applied on ground may not be suitable for the special conditions of space. Taking the most common vapor compressive refrigerator using Freon for example, the existence of high-speed rotary compressor and the potential leakage of high-pressure refrigerant are harmful to the reliability of the whole system, although its coefficient of performance (COP) is quite outstanding. R134a is considered as the
Corresponding authors. E-mail addresses: [email protected] (D. Guo), [email protected] (B. Yang).
https://doi.org/10.1016/j.applthermaleng.2019.114888 Received 10 June 2019; Received in revised form 1 December 2019; Accepted 29 December 2019 Available online 30 December 2019 1359-4311/ © 2019 Elsevier Ltd. All rights reserved.
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Nomenclature A COP COPc COPh cp d h I K k Lc Lh Lliq m Nu P Q
Qc Qh R Re T1in T1out T2in T2out Tc Th Toc Toh
heat transfer area (m2) coefficient of performance coefficient of performance under cooling mode coefficient of performance under heating mode specific heat capacity (J/ (kg ·K)) hydrodynamic diameter (m) surface heat transfer coefficient (W/(m2 K)) current intensity (A) heat conductivity coefficient of thermoelectric module (W/K) efficiency ratio thermal resistance of cold side (K/W) thermal resistance of hot side (K/W) thermal resistance between the wall and the liquid (K/W) minimum mass flow rate of EG solution or air (kg/s) Nusselt number total power of TEC (W) heat transfer capacity (W)
cooling capacity of thermoelectric module (W) heating capacity of thermoelectric module (W) resistance of thermoelectric module (Ω) Reynolds number temperature of hot side inlet (K) temperature of hot side outlet (K) temperature of cold side inlet (K) temperature of cold side outlet (K) temperature of cold side (K) temperature of hot side (K) temperature of cold medium (K) temperature of hot medium (K)
Greek symbols u α η λ ν
flow speed (m/s) Seebeck coefficient (V/K) heat transfer efficiency of heat exchanger thermal conductivity (W/m K) kinematic viscosity (m2/s)
also act as a heater when the hot side is employed for temperature control. In space station, the primary liquid coolant loop is connected to the terminal heat dissipation facilities (e.g. radiator, sublimator and membrane evaporator). Through the intermediate heat exchange interfaces, the secondary liquid coolant loops are allocated to different experiment cabinets. Comprised by 35%wt. pure water and 65%wt. ethylene glycol, the ethylene glycol solution (EGS) is employed as the secondary coolant in this paper, which is operated in a typical temperature range of 17–26 °C with the maximum flow rate of 170 L/h. The thermophysic properties of EGS in this temperature range vary little, whose values are listed in Table 1. Sometimes, the coolant itself can cope with the heat load if the heat load is not high enough. Or else, additional active thermal control measures are required. Here, TEC acts as the active thermal control by providing cooling or heating capacity according to requirement. According to the control objects (equipment surface or surrounding environment), there are two kinds of TEC application modes, including direct temperature control and environment temperature control, as shown in Fig. 2. In direct temperature control system (DTCS), the TEC module attached on the surface of the equipment releases/intakes the heat to/from the liquid. In environment temperature control system (ETCS), the circulated air in the cabinet chamber is cooled or heated, creating the required thermal ambience for the space experiment. ETCS is further classified as TEC-alone system and TEC-HX system, as shown in Fig. 2(b1) and (b2), respectively. In the former one, the liquid exchanges the heat with the air directly through the TEC surface. In the latter one, the intermediate liquid is precooled or preheated by TEC and then exchanges the heat with the air in HX, achieving the temperature control of the objective chamber. It is obvious that the heat transfer
most promising refrigerant used in spacecraft attributed to its relatively low working pressure [9], but the discharge pressure of compressor still exceeds 10 bar. Moreover, compared to terrestrial conditions, the twophase heat transfer intensity is deteriorated under microgravity as the development of thermal boundary layer is restrained [10,11]. As a typical space-borne mechanic cryogenic technology, Stirling cooler is originally designed for deep cryogenic requirement [12], which is redundant for normal temperature control of most space experiments. With complex and bulky structure, Stirling cooler can only provide tiny cooling capacity in the order of watt or even lower [13]. Similar problems exist in pulse tube cooler, sorption cooler, Brayton cooler, demagnetization cooler, etc. [14,15] Comparatively, thermoelectric cooler (TEC) is powered by electricity without any moving parts and refrigerant. Correspondingly, there is no risk of leakage which may cause pollution to the space station environment [16,17]. Besides, TEC has the merits of non-noise and high integration [18,19]. Moreover, it is rather convenient to regulate the thermal control capacity just by changing the input current, and the cooling and heating modes can be easily switched by changing the current direction. There are already extensive applications of TEC in space experiment like biologic sample separation [20,21]. TEC has also been successfully applied to the modular equipment, such as space telescope infrared camera [22], X-ray telescope [23], quartz crystal microbalance [24], membrane evaporator [25], etc. According to the temperature control target, this paper proposes two application modes of TEC including direct temperature control and environment temperature control. The environment temperature control system (ETCS) is further classified as TEC-alone system and combined TEC and heat exchanger (TEC-HX) system. The thermodynamic performances of these systems under typical working conditions of space station are analyzed by simulation. 1. Basic principle of thermoelectric cooling and its application modes Thermoelectric cooling is a refrigeration technology based on Seebeck effect, Peltier effect, Joule effect, Thomson effect and Fourier effect. The dominant effect is Peltier effect, followed by Joule effect and Fourier effect [26]. As shown in Fig. 1, the TEC module connects several p-type and n-type semiconductors by using metal plates as conductors. When this unit is electrified, heat will be transferred from the cold side to the hot side. Ceramics are arranged on both sides on metal plates as the electricity insulation layers. Named as cooler although, TEC can
Fig. 1. Structure of TEC. 2
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applied in various fields including aerospace due to its reliable performances and mature commercial applications. Its characteristic parameters are provided by manufacturer, where α equals 0.05343 V/ K, R equals 2.48 Ω, K equals 0.52 W/K. By calculation, Lc and Lh are both 2.5 K/W. Heat transfer capacity (cooling capacity for cooling mode and heating capacity for heating mode) and COP are important indices for evaluating the performance of TEC module. COPh and COPc correspond to heating mode and cooling mode, respectively, as
Table 1 Thermophysic properties of EGS. Parameter Density Specific heat capacity Thermal conductivity Viscosity
Unit
Value 3
kg/m J/kg K W/m K m2/s
1045 3650 0.453 2.6 × 10−6
area of TEC-alone system is limited even if the TEC surface is arranged with fins because the TEC surface area is rather limited. In TEC-HX system, the additional HX enlarges the heat transfer area remarkably. It cannot be neglected that, however, the additional HX also brings about additional heat transfer processes and correspondingly increases the heat transfer resistance. So a quantitative analysis from comprehensive perspective is necessary, which will be presented later. Without additional electric heater, TEC can be switched from cooling mode to heating mode just by changing the current direction with H-bridge circuit, as shown in Fig. 3(a). Alternatively, the cooling/ heating mode can be switched by mechanic approach with four-way valve, as shown in Fig. 3(b).
COPh = Qh/ P
(6)
COPc = Qc / P
(7)
A certain temperature gradient exists between the liquid and the TEC surface. For simplifying the calculation, the surface temperatures of both hot and cold sides are considered to be uniform. The convective thermal resistance Lliq (including Lh and Lc) between TEC surface and liquid can be calculated by Eqs. (8)–(12) [27].
Lliq = 1/ hA
(8)
where A is the heat transfer area, and h is the convective heat transfer coefficient. The convective heat transfer coefficient is related to fluid properties and hydrodynamic diameter, as
2. General model of heat transfer
h = Nuλliq / d
Powered by electricity, TEC transfers the heat inversely from cold side to hot side, as shown in Fig. 4. The transferred heats on the two sides are calculated as follows
(9)
where Nu is Nusselt number, λliq is thermal conductivity of liquid, and d is hydrodynamic diameter. Nusselt number is related to flow scheme, which can be regarded as a constant when the flow is fully developed laminar, as presented in Eq. (10).
Qc = (Toc − Tc )/ Lc
(1)
Qh = (Th − Th)/ Lh
(2)
Qc = αITc − 0.5I 2R − K (Th − Tc )
(3)
When the flow is turbulent (Re > 2300), Nusselt number is calculated by Eqs. (11) and (12).
Q h = αITh + 0.5I 2R − K (Th − Tc )
(4)
Nu = 0.023Re 0.8Pr 0.4
(11)
P = UI = αI (Th − Tc ) + I 2R
(5)
Re = ud/ ν
(12)
Nu = 3.61
(10)
where Re is Reynolds number, u is flow speed, and ν is kinematic viscosity. In TEC-HX system, the heat transfer rate of HX can be calculated by efficiency. For a given HX, its efficiency is determined by the flow rates. The heat transfer equations of HX are formulated as follows.
where Qc is the cooling capacity of TEC module, Qh is the heating capacity of TEC module, Tc is the temperature of cold side, Th is the temperature of hot side, K is heat conductivity coefficient of TEC module accounting for thermal conductivity, area and thickness comprehensively, Lc is convective thermal resistance of cold side, Lh is convective thermal resistance of hot side, Toc is the temperature of cold media, Toh is the temperature of hot media, α is Seebeck coefficient, R is electric resistance of TEC module, U is voltage and I is current. In Eqs. (3) and (4), the first part represents Peltier effect, the second part represents Joule effect, and the third part represents Fourier effect. The coefficient 0.5 in the term of Joule heat corresponds to the fact that the Joule heat is allocated to both sides equally. This paper chooses the model TEC1-12706, which is commonly
T1in − T1out = η (T1in − T2in)
(13)
T1in − T1out = Q/(mcp)
(14)
In the liquid loop of TEC-HX system, the inlet of HX is also the outlet of TEC. The following equations are presented for cooling and heating modes, respectively.
ηmcp (T1out − T2in ) = Q (1 − η)
Fig. 2. TEC application modes. 3
(15)
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Fig. 3. Cooling/heating switch approaches by (a) H-bridge and (b) four-way valve in ETCS.
Fig. 6. The cooling capacity and COPc both increase first and then decrease. Nonetheless, the peak values of COPc and cooling capacity appear at different currents, meaning that the TEC module is impossible to run with optimum efficiency and cooling capacity at the same time. As presented in Eq. (3), Joule effect increases faster than Peltier effect with the increase of the current, leading to the decrease of the slope of the cooling capacity curve in Fig. 6. The COPc and cooling capacity both increase with the decrease of liquid temperature. Multiple TEC modules can be integrated to satisfy different heating or cooling demands. As the heating capacity under the heating mode is larger than the cooling capacity under the cooling mode at the same current, the quantity of TEC module should be determined by the cooling mode.
Fig. 4. Heat transfer in TEC.
ηmcp (T1in − T2out ) = Q (1 − η)
(16)
where the subscript “in” and “out” denote the inlet and outlet of HX, respectively; the subscripts “1” and “2” denote hot side and cold side of HX, respectively; Q is the heat transfer rate; η is the efficiency of HX; m is the less one of the mass flow rates of liquid and air, and cp is the less specific heat capacity correspondingly.
3.2. Characteristics of environment temperature control system According to the specification of the experiment cabinet of future Chines space station, the circulated air flow rate is fixed at 0.018 kg/s in ETCS, with the heat load of 275 W under cooling mode and 180 W under heating load, respectively. The overall liquid flow rate entering the system is fixed at 100 L/h. Since the characteristics of TEC-alone system is similar with that of DTCS discussed above, the analysis here is concentrated on TEC-HX system. In a specific HX, the fluid with the less equivalent heat capacity (flow rate times specific heat capacity) can obtain the larger temperature change. In order to heat (under heating mode) or cool (under cooling mode) the air in HX as greatly as possible, the equivalent heat capacity of the liquid should be greater than that of the air. Since the air flow rate is fixed at 0.018 kg/s in this study, the designed liquid flow rate of the branch loop having HX needs to be higher than 20 L/h. With the flow rate of this order, the Reynolds number is small and therefore the flow type is laminar. So the convective thermal resistance at either side of TEC is considered to be constant (0.0317 K/W) calculated by the constant Nusselt number of 3.61 as presented in Eq. (10). According to
3. Results and discussions Variable-condition simulation is conducted here to analyze the effects of some key parameters such as current, liquid temperature and flow rate on the performance of TEC modules and system. The objective device or environment temperature is 30 °C under heating mode and 10 °C under cooling mode, respectively. The circulated air flow rate is fixed at 0.018 kg/s in ETCS. The liquid temperature is varied from 17 to 26 °C, while the liquid flow rate is varied from 20 to 50 L/h. The data listed in Table 2 are determined for future Chinese space station. 3.1. Characteristics of direct temperature control system The COPh and heating capacity of DTCS under heating mode are shown in Fig. 5. The heating capacity increases with the increase of current, while the COPh increases first and then decreases. Above the current of 0.8 A approximately, the COPh is always greater than 1. The COPh increases sharply till the peak in the range of 1.5–2 A and then decreases smoothly. It can be seen from the figure that the TEC module has no heating capacity when the current is below 0.5 A approximately. The COPh increases with the increase of the liquid temperature, whereas the heating capacity is affected by the liquid temperature slightly. The COPc and cooling capacity under cooling mode are shown in
Table 2 Operating conditions.
4
Parameter
Value/range
Target temperature under heating mode Target temperature under cooling mode Liquid temperature Air flow rate Liquid flow rate
30 °C 10 °C 17–26 °C 0.018 kg/s 20–50 L/h
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Fig. 5. Effects of current on COPh and heating capacity of DTCS at various liquid temperature under heating mode.
the characteristics of HX, the efficiency of HX increases with the fluid flow rate from 0.86 to 1 nearly, as shown in Fig. 8. Under cooling mode, the objective air temperature is 10 °C, and the liquid temperature is 17 °C, respectively. Based on the operating conditions above, the cooling mode is simulated first. The HX inlet liquid temperature (i.e. the TEC cold side outlet liquid temperature) and the cooling capacity of TEC are calculated at different cold-side liquid flow rates, as shown in Fig. 7. The increase of the liquid flow rate benefits the improvement of TEC cooling performance, directly leading to the increase of the cooling capacity. Although the promotion of the HX inlet liquid temperature accompanied by the increase of the liquid flow rate can bring about the decrease of the heat load to some degree, its negative effect is slighter than the positive effect of the inlet flow rate increasing. Consequently, the cooling capacity of TEC increases with the liquid flow rate after all. The quantity of TEC modules required under cooling mode with respect to different currents and liquid flow rates is shown in Fig. 8. The lower the liquid flow rate is, the fewer the TEC modules are required. As shown in Fig. 6 above, when the current is smaller than 1 A, although the COPc is greater, the cooling capacity of a single TEC module is smaller, so a larger number of TEC modules are required to cope with the total heat load. Therefore, for a single TEC module, the current achieving the largest cooling capacity rather than the highest COPc should be selected as the working point. Besides, the total power of TEC increases as the current increases, so it is necessary to reduce both of the working current and the quantity of TEC modules. For instance,
Fig. 7. Effects of liquid flow rate on HX inlet liquid temperature and TEC cooling capacity under cooling mode.
when the liquid flow rate is 20 L/h, the current of 3–3.5 A and the TEC quantity of 12–15 are appropriate. Under the heating mode, the objective air temperature is 30 °C which is higher than the maximum temperature of liquid heat sink. When the liquid temperature is 26 °C, the effects of the liquid flow rate on HX inlet liquid temperature and heating capacity of TEC are shown
Fig. 6. Effects of current on COPc and cooling capacity of DTCS at different liquid temperature under cooling mode. 5
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Fig. 8. Quantity and total power of TEC modules with respect to current at different liquid flow rates under cooling mode.
The contours of k with respect to the liquid temperature and flow rate under both heating and cooling modes are plotted in Fig. 13, where the objective air temperature is 10 °C under cooling mode and 30 °C under heating mode, respectively. The regions covered by rainbow colors represent the liquid working conditions that can realize k > 1. Under cooling mode, the liquid temperature should be lower than 20.7 °C, and the allowed liquid flow rate increases as the liquid temperature decreases. Under heating mode, the liquid temperature should be higher than 23.4 °C, and the allowed liquid flow rate increases as the liquid temperature increases. Overall, TEC-HX system exhibits more remarkable superiority at lower liquid temperature under cooling mode and at higher liquid temperature under heating mode, respectively, while the lower liquid flow rate is always favorable under whatever mode. As seen from the figure, at the liquid temperature of 26 °C and the liquid flow rate of 20 L/h under heating mode, the value of k can reach as high as 2 approximately.
in Fig. 9. As with the cooling mode, the heating capacity increases with the liquid flow rate. The quantity of TEC modules required under heating mode with respect to different currents and liquid flow rates are shown in Fig. 10. The lower the liquid flow rate is, the fewer TEC modules are required. The curve trend here under heating mode is different from that under cooling mode as shown in Fig. 8 above. The quantity of TEC modules decreases monotonically with the increase of the current, because the heating capacity of a single TEC module increases. The total power of TEC decreases first and then increases with the increase of the current, which has an opposite trend to COPh curve shown above in Fig. 5. The optimum working currents for the lowest power and the highest COPh respectively are not the same, meaning the TEC working performances are different between ETCS and the DTCS. It is obvious that the quantity of TEC modules under heating mode is less than that under cooling mode, and the difference of the required quantity between cooling and heating modes is enlarged as the liquid flow rate increases, so the designed quantity should follow the calculation results under cooling mode.
3.4. Effect of heat exchanger efficiency on performance of TEC-HX system The heat exchanger is an important component in TEC-HX system. The COPTEC-HX might be higher than COPTEC-alone under some operating conditions because of the highly efficient heat transfer in HX. Of course, if the HX efficiency is too low, the heat transfer between the liquid and air via HX indirectly may not be superior to the direct heat transfer via the TEC surface instead. As seen, the HX efficiency is a key parameter
3.3. Performance comparison between TEC-alone system and TEC-HX system For TEC-alone system, the characteristics of a single TEC module is also the system characteristics. However, they are different for TEC-HX system due to the existence of HX. The COP of TEC-HX system under cooling and heating modes can be defined similarly with the definition in Eqs. (6) and (7). The effects of the current on the COPc,TEC-alone and the COPc,TEC-HX under cooling mode at different liquid flow rates are shown in Fig. 11. It is seen that when the liquid flow rate is small (e.g. 20 and 30 L/h), the COPc,TEC-HX is higher than the COPc,TEC-alone. As the flow rate increases, the COPc,TEC-HX decreases, tending to be lower than the COPc,TEC-alone instead (e.g. 40 L/h and 50 L/h). The effects of the current are similar between the COPc,TEC-HX and COPc,TEC-alone, both of which increase first and then decrease with the increase of the current. The effects of the current on the COPh,TEC-HX and COPh,TEC-alone under heating mode at different flow rates are shown in Fig. 12. It can be seen that the COPh,TEC-HX is always higher than COPh,TEC-alone, and both of them decrease with the increase of the liquid flow rate. It can be concluded that TEC-HX system performs better than TECalone system as long as COPh,TEC-HX is higher than COPh,TEC-alone. The COP ratio, k, is defined in Eq. (17), whose value exceeding 1 means that TEC-HX system performs better.
k = COPTEC − HX / COPTEC − alone
Fig. 9. Effects of liquid flow rate on HX inlet liquid temperature and TEC heating capacity under heating mode.
(17) 6
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Fig. 10. Quantity and total power of TEC modules with respect to current at different liquid flow rates under heating mode.
envelope is significantly affected by the HX efficiency. The higher the HX efficiency is, the wider the applicable range of the operating conditions will be; that is, the TEC-HX system with a high-efficiency HX can easily run better than TEC-alone system under most operating conditions.
that determines whether the system performance can be improved by combining TEC and HX. Fig. 14 plots the HX efficiency curves of five individual heat exchangers with respect to the liquid flow rate, where the efficiency increases from HX1 to HX5. The envelope curves of the operating conditions (including liquid temperature and flow rate) for achieving k = 1 are plotted in Fig. 15, where the regions below these curves indicate that k > 1. As seen, the
Fig. 11. Effects of current on COPc,TEC-alone and COPc,TEC-HX at different liquid flow rates under cooling mode. 7
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Fig. 12. Effects of current on COPh and total efficiency at different liquid flow rates under heating mode.
(1) In DTCS, an optimal current can be found to obtain the highest COP of the individual TEC module under both cooling and heating modes. The maximum cooling capacity of the individual TEC module exists under cooling mode, while the heating capacity increases monotonically with the current under heating mode. (2) In either DTCS or ETCS, since both the cooling and heating capacities of the individual TEC module working on the COP-optimized state are too small, the COP optimization should not be the primary issue. Instead, the current should be operated at the value that can achieve larger cooling capacity. (3) The lower flow rate the liquid flows at through the heat exchanger, the less TEC modules and power are required in TEC-HX system. The smallest quantity of TEC modules is consistent with the largest cooling capacity versus the current. (4) In TEC-HX system, the system COP decreases with the increase of the liquid flow rate through heat exchanger. High system COP can be achieved at low liquid temperature under cooling mode and at high liquid temperature under heating mode. (5) High efficiency of heat exchanger can enlarge the applicable range of operating conditions for achieving the COP ratio larger than 1. In these ranges, the TEC-HX system has better performance than the TEC-alone system.
Fig. 13. Contours of COP ratio with respect to liquid temperature and flow rate.
4. Conclusion Thermoelectric cooler has advantages of high reliability free of moving parts, which is suitable for temperature control in space scientific experiments. In this paper, TEC is used for direct temperature control and environment temperature control respectively. The influences of current, heat sink temperature and flow rate are discussed by calculation. The system COP is also discussed. Key findings from the study are summarized below.
CRediT authorship contribution statement Dongcai Guo: Conceptualization, Methodology, Software, Formal analysis, Supervision, Writing - original draft, Writing - review & editing. Qiang Sheng: Investigation, Visualization. Xiangyu Dou: 8
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Licandro, M.D. Rodríguez Frías, The infrared camera prototype characterization for the JemEuso space mission, Nucl. Instrum. Methods Phys. Res., Sect. A Accel. Spectrom. Detect. Assoc. Equip. 749 (2014) 74–83. [23] G. Bale, A. Holland, P. Seller, B. Lowe, Cooled Cdznte detectors for X-ray astronomy, Nucl. Instrum. Methods Phys. Res., Sect. A 436 (1999) 150–154. [24] P. Cleveland, S. Glazer, E. Packard, Enhancements to the Nasa/Goddard Space Flight Center (Gsfc) Space Environment Simulator (Ses) facility to support cryogenic testing of the James Webb Space Telescope (Jwst) Integrated Science Instrument Module (Isim), 40th International Conference on Environmental Systems, American Institute of Aeronautics and Astronautics, 2010. [25] J.H. Miernik, B.H. Shah, C.F. Mcgriff, Waste water processing technology for space station freedom - comparative test data analysis, International Conference on Evolvable Systems, 1991, pp. 229–240. [26] D. Bugby, W. Zimbeck, E. Kroliczek, Thermal management architecture for future responsive spacecraft, AIP Conference Proceedings, AIP, 2009, pp. 30–38. [27] K.S. Ramesh, P.S. Dusan, Fundamentals of Heat Exchanger Design, China Machine Press, Beijing, 2010 (in Chinese).
Fig. 14. Efficiency curves of different HX.
Fig. 15. Envelope of operating conditions for achieving k = 1 under different HX efficiencies.
Resources. Ze Wang: Data curation. Liyao Xie: Writing - original draft. Bo Yang: Methodology, Formal analysis, Writing - original draft, Writing - review & editing. Declaration of Competing Interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. Appendix A. Supplementary material Supplementary data to this article can be found online at https:// doi.org/10.1016/j.applthermaleng.2019.114888. References [1] S.N. Evetts, Space life and biomedical sciences in support of the global exploration roadmap and societal development, Space Policy 30 (2014) 143–145.
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Applied Thermal Engineering journal homepage: www.elsevier.com/locate/apthermeng
Application of thermoelectric cooler in temperature control system of space science experiment ⁎
Dongcai Guoa, , Qiang Shenga, Xiangyu Doua, Ze Wanga, Liyao Xieb, Bo Yangc,
T
⁎
a
Key Laboratory of Space Utilization, Technology and Engineering Center for Space Utilization, Chinese Academy of Sciences, Beijing 100094, China Hebei University of Technology, Tianjin 300401, China c School of Aeronautic Science and Engineering, Beihang University, Beijing 100191, China b
H I GH L IG H T S
cooler (TEC) is suitable for spacecraft temperature control. • Thermoelectric system of TEC and heat exchanger is proposed. • Combined • The combined system performs better than TEC-alone system.
A R T I C LE I N FO
A B S T R A C T
Keywords: Space station Space science experiment Temperature control Thermoelectric cooler
As an important mission of space stations and scientific satellites, space science experiment usually requires effective temperature control measures. Integrating heating and cooling capacities, thermoelectric cooler (TEC) is competent for this job attributed to the low complexity and high reliability. In this paper, two TEC systems are introduced, including direct temperature control system (DTCS) and environment temperature control system (ETCS). DTCS directly deals with the temperature control of the target element surface. The effects of the current and the heat sink (i.e. working liquid) temperature are analyzed under heating and cooling conditions respectively. ETCS deals with the temperature control of the entire environment surrounding the element. The element is indirectly heated or cooled by the ambient air. Depending on whether an additional intermediate air-to-liquid heat exchanger is consisted or not, ETCS can be further classified as TEC-HX system and TEC-alone system. The heat exchanger may improve the system performances. The effects of the liquid flow rate and heat exchanger efficiency on system performances are analyzed, providing guides for the design of the practical system.
0. Introduction Covering multilateral disciplines such as physics, chemistry and biology, space science experiments aim at exploring new scientific phenomena under microgravity and other special space conditions [1]. So far, those magnificent space projects like International Space Station (ISS) have established many experiment platforms, on which various researches involving biology, medicine and fluid mechanics are carried out and excellent results have been obtained [2–5]. More and more space science experiments are required in spaceflight missions, especially when the spaceflight prevails under background of the explosive development of business space. Many of the space science experiments require constant environment temperature, and some experimental instruments need to be operated below specific temperature. In space station, the terminal
⁎
cooling devices such as radiators [6], sublimators [7] or membrane evaporators [8] provide the basic heat sink for all the cooling requirements inside. However, the temperature requirements vary greatly among different space experiments, which may exceed the temperature range of the basic heat sink. Therefore, it is necessary to design a general temperature control method to satisfy different temperature requirements with a heat sink working in a relatively narrow temperature range. Despite of excellent performance, many refrigeration technologies commonly applied on ground may not be suitable for the special conditions of space. Taking the most common vapor compressive refrigerator using Freon for example, the existence of high-speed rotary compressor and the potential leakage of high-pressure refrigerant are harmful to the reliability of the whole system, although its coefficient of performance (COP) is quite outstanding. R134a is considered as the
Corresponding authors. E-mail addresses: [email protected] (D. Guo), [email protected] (B. Yang).
https://doi.org/10.1016/j.applthermaleng.2019.114888 Received 10 June 2019; Received in revised form 1 December 2019; Accepted 29 December 2019 Available online 30 December 2019 1359-4311/ © 2019 Elsevier Ltd. All rights reserved.
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Nomenclature A COP COPc COPh cp d h I K k Lc Lh Lliq m Nu P Q
Qc Qh R Re T1in T1out T2in T2out Tc Th Toc Toh
heat transfer area (m2) coefficient of performance coefficient of performance under cooling mode coefficient of performance under heating mode specific heat capacity (J/ (kg ·K)) hydrodynamic diameter (m) surface heat transfer coefficient (W/(m2 K)) current intensity (A) heat conductivity coefficient of thermoelectric module (W/K) efficiency ratio thermal resistance of cold side (K/W) thermal resistance of hot side (K/W) thermal resistance between the wall and the liquid (K/W) minimum mass flow rate of EG solution or air (kg/s) Nusselt number total power of TEC (W) heat transfer capacity (W)
cooling capacity of thermoelectric module (W) heating capacity of thermoelectric module (W) resistance of thermoelectric module (Ω) Reynolds number temperature of hot side inlet (K) temperature of hot side outlet (K) temperature of cold side inlet (K) temperature of cold side outlet (K) temperature of cold side (K) temperature of hot side (K) temperature of cold medium (K) temperature of hot medium (K)
Greek symbols u α η λ ν
flow speed (m/s) Seebeck coefficient (V/K) heat transfer efficiency of heat exchanger thermal conductivity (W/m K) kinematic viscosity (m2/s)
also act as a heater when the hot side is employed for temperature control. In space station, the primary liquid coolant loop is connected to the terminal heat dissipation facilities (e.g. radiator, sublimator and membrane evaporator). Through the intermediate heat exchange interfaces, the secondary liquid coolant loops are allocated to different experiment cabinets. Comprised by 35%wt. pure water and 65%wt. ethylene glycol, the ethylene glycol solution (EGS) is employed as the secondary coolant in this paper, which is operated in a typical temperature range of 17–26 °C with the maximum flow rate of 170 L/h. The thermophysic properties of EGS in this temperature range vary little, whose values are listed in Table 1. Sometimes, the coolant itself can cope with the heat load if the heat load is not high enough. Or else, additional active thermal control measures are required. Here, TEC acts as the active thermal control by providing cooling or heating capacity according to requirement. According to the control objects (equipment surface or surrounding environment), there are two kinds of TEC application modes, including direct temperature control and environment temperature control, as shown in Fig. 2. In direct temperature control system (DTCS), the TEC module attached on the surface of the equipment releases/intakes the heat to/from the liquid. In environment temperature control system (ETCS), the circulated air in the cabinet chamber is cooled or heated, creating the required thermal ambience for the space experiment. ETCS is further classified as TEC-alone system and TEC-HX system, as shown in Fig. 2(b1) and (b2), respectively. In the former one, the liquid exchanges the heat with the air directly through the TEC surface. In the latter one, the intermediate liquid is precooled or preheated by TEC and then exchanges the heat with the air in HX, achieving the temperature control of the objective chamber. It is obvious that the heat transfer
most promising refrigerant used in spacecraft attributed to its relatively low working pressure [9], but the discharge pressure of compressor still exceeds 10 bar. Moreover, compared to terrestrial conditions, the twophase heat transfer intensity is deteriorated under microgravity as the development of thermal boundary layer is restrained [10,11]. As a typical space-borne mechanic cryogenic technology, Stirling cooler is originally designed for deep cryogenic requirement [12], which is redundant for normal temperature control of most space experiments. With complex and bulky structure, Stirling cooler can only provide tiny cooling capacity in the order of watt or even lower [13]. Similar problems exist in pulse tube cooler, sorption cooler, Brayton cooler, demagnetization cooler, etc. [14,15] Comparatively, thermoelectric cooler (TEC) is powered by electricity without any moving parts and refrigerant. Correspondingly, there is no risk of leakage which may cause pollution to the space station environment [16,17]. Besides, TEC has the merits of non-noise and high integration [18,19]. Moreover, it is rather convenient to regulate the thermal control capacity just by changing the input current, and the cooling and heating modes can be easily switched by changing the current direction. There are already extensive applications of TEC in space experiment like biologic sample separation [20,21]. TEC has also been successfully applied to the modular equipment, such as space telescope infrared camera [22], X-ray telescope [23], quartz crystal microbalance [24], membrane evaporator [25], etc. According to the temperature control target, this paper proposes two application modes of TEC including direct temperature control and environment temperature control. The environment temperature control system (ETCS) is further classified as TEC-alone system and combined TEC and heat exchanger (TEC-HX) system. The thermodynamic performances of these systems under typical working conditions of space station are analyzed by simulation. 1. Basic principle of thermoelectric cooling and its application modes Thermoelectric cooling is a refrigeration technology based on Seebeck effect, Peltier effect, Joule effect, Thomson effect and Fourier effect. The dominant effect is Peltier effect, followed by Joule effect and Fourier effect [26]. As shown in Fig. 1, the TEC module connects several p-type and n-type semiconductors by using metal plates as conductors. When this unit is electrified, heat will be transferred from the cold side to the hot side. Ceramics are arranged on both sides on metal plates as the electricity insulation layers. Named as cooler although, TEC can
Fig. 1. Structure of TEC. 2
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applied in various fields including aerospace due to its reliable performances and mature commercial applications. Its characteristic parameters are provided by manufacturer, where α equals 0.05343 V/ K, R equals 2.48 Ω, K equals 0.52 W/K. By calculation, Lc and Lh are both 2.5 K/W. Heat transfer capacity (cooling capacity for cooling mode and heating capacity for heating mode) and COP are important indices for evaluating the performance of TEC module. COPh and COPc correspond to heating mode and cooling mode, respectively, as
Table 1 Thermophysic properties of EGS. Parameter Density Specific heat capacity Thermal conductivity Viscosity
Unit
Value 3
kg/m J/kg K W/m K m2/s
1045 3650 0.453 2.6 × 10−6
area of TEC-alone system is limited even if the TEC surface is arranged with fins because the TEC surface area is rather limited. In TEC-HX system, the additional HX enlarges the heat transfer area remarkably. It cannot be neglected that, however, the additional HX also brings about additional heat transfer processes and correspondingly increases the heat transfer resistance. So a quantitative analysis from comprehensive perspective is necessary, which will be presented later. Without additional electric heater, TEC can be switched from cooling mode to heating mode just by changing the current direction with H-bridge circuit, as shown in Fig. 3(a). Alternatively, the cooling/ heating mode can be switched by mechanic approach with four-way valve, as shown in Fig. 3(b).
COPh = Qh/ P
(6)
COPc = Qc / P
(7)
A certain temperature gradient exists between the liquid and the TEC surface. For simplifying the calculation, the surface temperatures of both hot and cold sides are considered to be uniform. The convective thermal resistance Lliq (including Lh and Lc) between TEC surface and liquid can be calculated by Eqs. (8)–(12) [27].
Lliq = 1/ hA
(8)
where A is the heat transfer area, and h is the convective heat transfer coefficient. The convective heat transfer coefficient is related to fluid properties and hydrodynamic diameter, as
2. General model of heat transfer
h = Nuλliq / d
Powered by electricity, TEC transfers the heat inversely from cold side to hot side, as shown in Fig. 4. The transferred heats on the two sides are calculated as follows
(9)
where Nu is Nusselt number, λliq is thermal conductivity of liquid, and d is hydrodynamic diameter. Nusselt number is related to flow scheme, which can be regarded as a constant when the flow is fully developed laminar, as presented in Eq. (10).
Qc = (Toc − Tc )/ Lc
(1)
Qh = (Th − Th)/ Lh
(2)
Qc = αITc − 0.5I 2R − K (Th − Tc )
(3)
When the flow is turbulent (Re > 2300), Nusselt number is calculated by Eqs. (11) and (12).
Q h = αITh + 0.5I 2R − K (Th − Tc )
(4)
Nu = 0.023Re 0.8Pr 0.4
(11)
P = UI = αI (Th − Tc ) + I 2R
(5)
Re = ud/ ν
(12)
Nu = 3.61
(10)
where Re is Reynolds number, u is flow speed, and ν is kinematic viscosity. In TEC-HX system, the heat transfer rate of HX can be calculated by efficiency. For a given HX, its efficiency is determined by the flow rates. The heat transfer equations of HX are formulated as follows.
where Qc is the cooling capacity of TEC module, Qh is the heating capacity of TEC module, Tc is the temperature of cold side, Th is the temperature of hot side, K is heat conductivity coefficient of TEC module accounting for thermal conductivity, area and thickness comprehensively, Lc is convective thermal resistance of cold side, Lh is convective thermal resistance of hot side, Toc is the temperature of cold media, Toh is the temperature of hot media, α is Seebeck coefficient, R is electric resistance of TEC module, U is voltage and I is current. In Eqs. (3) and (4), the first part represents Peltier effect, the second part represents Joule effect, and the third part represents Fourier effect. The coefficient 0.5 in the term of Joule heat corresponds to the fact that the Joule heat is allocated to both sides equally. This paper chooses the model TEC1-12706, which is commonly
T1in − T1out = η (T1in − T2in)
(13)
T1in − T1out = Q/(mcp)
(14)
In the liquid loop of TEC-HX system, the inlet of HX is also the outlet of TEC. The following equations are presented for cooling and heating modes, respectively.
ηmcp (T1out − T2in ) = Q (1 − η)
Fig. 2. TEC application modes. 3
(15)
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Fig. 3. Cooling/heating switch approaches by (a) H-bridge and (b) four-way valve in ETCS.
Fig. 6. The cooling capacity and COPc both increase first and then decrease. Nonetheless, the peak values of COPc and cooling capacity appear at different currents, meaning that the TEC module is impossible to run with optimum efficiency and cooling capacity at the same time. As presented in Eq. (3), Joule effect increases faster than Peltier effect with the increase of the current, leading to the decrease of the slope of the cooling capacity curve in Fig. 6. The COPc and cooling capacity both increase with the decrease of liquid temperature. Multiple TEC modules can be integrated to satisfy different heating or cooling demands. As the heating capacity under the heating mode is larger than the cooling capacity under the cooling mode at the same current, the quantity of TEC module should be determined by the cooling mode.
Fig. 4. Heat transfer in TEC.
ηmcp (T1in − T2out ) = Q (1 − η)
(16)
where the subscript “in” and “out” denote the inlet and outlet of HX, respectively; the subscripts “1” and “2” denote hot side and cold side of HX, respectively; Q is the heat transfer rate; η is the efficiency of HX; m is the less one of the mass flow rates of liquid and air, and cp is the less specific heat capacity correspondingly.
3.2. Characteristics of environment temperature control system According to the specification of the experiment cabinet of future Chines space station, the circulated air flow rate is fixed at 0.018 kg/s in ETCS, with the heat load of 275 W under cooling mode and 180 W under heating load, respectively. The overall liquid flow rate entering the system is fixed at 100 L/h. Since the characteristics of TEC-alone system is similar with that of DTCS discussed above, the analysis here is concentrated on TEC-HX system. In a specific HX, the fluid with the less equivalent heat capacity (flow rate times specific heat capacity) can obtain the larger temperature change. In order to heat (under heating mode) or cool (under cooling mode) the air in HX as greatly as possible, the equivalent heat capacity of the liquid should be greater than that of the air. Since the air flow rate is fixed at 0.018 kg/s in this study, the designed liquid flow rate of the branch loop having HX needs to be higher than 20 L/h. With the flow rate of this order, the Reynolds number is small and therefore the flow type is laminar. So the convective thermal resistance at either side of TEC is considered to be constant (0.0317 K/W) calculated by the constant Nusselt number of 3.61 as presented in Eq. (10). According to
3. Results and discussions Variable-condition simulation is conducted here to analyze the effects of some key parameters such as current, liquid temperature and flow rate on the performance of TEC modules and system. The objective device or environment temperature is 30 °C under heating mode and 10 °C under cooling mode, respectively. The circulated air flow rate is fixed at 0.018 kg/s in ETCS. The liquid temperature is varied from 17 to 26 °C, while the liquid flow rate is varied from 20 to 50 L/h. The data listed in Table 2 are determined for future Chinese space station. 3.1. Characteristics of direct temperature control system The COPh and heating capacity of DTCS under heating mode are shown in Fig. 5. The heating capacity increases with the increase of current, while the COPh increases first and then decreases. Above the current of 0.8 A approximately, the COPh is always greater than 1. The COPh increases sharply till the peak in the range of 1.5–2 A and then decreases smoothly. It can be seen from the figure that the TEC module has no heating capacity when the current is below 0.5 A approximately. The COPh increases with the increase of the liquid temperature, whereas the heating capacity is affected by the liquid temperature slightly. The COPc and cooling capacity under cooling mode are shown in
Table 2 Operating conditions.
4
Parameter
Value/range
Target temperature under heating mode Target temperature under cooling mode Liquid temperature Air flow rate Liquid flow rate
30 °C 10 °C 17–26 °C 0.018 kg/s 20–50 L/h
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Fig. 5. Effects of current on COPh and heating capacity of DTCS at various liquid temperature under heating mode.
the characteristics of HX, the efficiency of HX increases with the fluid flow rate from 0.86 to 1 nearly, as shown in Fig. 8. Under cooling mode, the objective air temperature is 10 °C, and the liquid temperature is 17 °C, respectively. Based on the operating conditions above, the cooling mode is simulated first. The HX inlet liquid temperature (i.e. the TEC cold side outlet liquid temperature) and the cooling capacity of TEC are calculated at different cold-side liquid flow rates, as shown in Fig. 7. The increase of the liquid flow rate benefits the improvement of TEC cooling performance, directly leading to the increase of the cooling capacity. Although the promotion of the HX inlet liquid temperature accompanied by the increase of the liquid flow rate can bring about the decrease of the heat load to some degree, its negative effect is slighter than the positive effect of the inlet flow rate increasing. Consequently, the cooling capacity of TEC increases with the liquid flow rate after all. The quantity of TEC modules required under cooling mode with respect to different currents and liquid flow rates is shown in Fig. 8. The lower the liquid flow rate is, the fewer the TEC modules are required. As shown in Fig. 6 above, when the current is smaller than 1 A, although the COPc is greater, the cooling capacity of a single TEC module is smaller, so a larger number of TEC modules are required to cope with the total heat load. Therefore, for a single TEC module, the current achieving the largest cooling capacity rather than the highest COPc should be selected as the working point. Besides, the total power of TEC increases as the current increases, so it is necessary to reduce both of the working current and the quantity of TEC modules. For instance,
Fig. 7. Effects of liquid flow rate on HX inlet liquid temperature and TEC cooling capacity under cooling mode.
when the liquid flow rate is 20 L/h, the current of 3–3.5 A and the TEC quantity of 12–15 are appropriate. Under the heating mode, the objective air temperature is 30 °C which is higher than the maximum temperature of liquid heat sink. When the liquid temperature is 26 °C, the effects of the liquid flow rate on HX inlet liquid temperature and heating capacity of TEC are shown
Fig. 6. Effects of current on COPc and cooling capacity of DTCS at different liquid temperature under cooling mode. 5
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Fig. 8. Quantity and total power of TEC modules with respect to current at different liquid flow rates under cooling mode.
The contours of k with respect to the liquid temperature and flow rate under both heating and cooling modes are plotted in Fig. 13, where the objective air temperature is 10 °C under cooling mode and 30 °C under heating mode, respectively. The regions covered by rainbow colors represent the liquid working conditions that can realize k > 1. Under cooling mode, the liquid temperature should be lower than 20.7 °C, and the allowed liquid flow rate increases as the liquid temperature decreases. Under heating mode, the liquid temperature should be higher than 23.4 °C, and the allowed liquid flow rate increases as the liquid temperature increases. Overall, TEC-HX system exhibits more remarkable superiority at lower liquid temperature under cooling mode and at higher liquid temperature under heating mode, respectively, while the lower liquid flow rate is always favorable under whatever mode. As seen from the figure, at the liquid temperature of 26 °C and the liquid flow rate of 20 L/h under heating mode, the value of k can reach as high as 2 approximately.
in Fig. 9. As with the cooling mode, the heating capacity increases with the liquid flow rate. The quantity of TEC modules required under heating mode with respect to different currents and liquid flow rates are shown in Fig. 10. The lower the liquid flow rate is, the fewer TEC modules are required. The curve trend here under heating mode is different from that under cooling mode as shown in Fig. 8 above. The quantity of TEC modules decreases monotonically with the increase of the current, because the heating capacity of a single TEC module increases. The total power of TEC decreases first and then increases with the increase of the current, which has an opposite trend to COPh curve shown above in Fig. 5. The optimum working currents for the lowest power and the highest COPh respectively are not the same, meaning the TEC working performances are different between ETCS and the DTCS. It is obvious that the quantity of TEC modules under heating mode is less than that under cooling mode, and the difference of the required quantity between cooling and heating modes is enlarged as the liquid flow rate increases, so the designed quantity should follow the calculation results under cooling mode.
3.4. Effect of heat exchanger efficiency on performance of TEC-HX system The heat exchanger is an important component in TEC-HX system. The COPTEC-HX might be higher than COPTEC-alone under some operating conditions because of the highly efficient heat transfer in HX. Of course, if the HX efficiency is too low, the heat transfer between the liquid and air via HX indirectly may not be superior to the direct heat transfer via the TEC surface instead. As seen, the HX efficiency is a key parameter
3.3. Performance comparison between TEC-alone system and TEC-HX system For TEC-alone system, the characteristics of a single TEC module is also the system characteristics. However, they are different for TEC-HX system due to the existence of HX. The COP of TEC-HX system under cooling and heating modes can be defined similarly with the definition in Eqs. (6) and (7). The effects of the current on the COPc,TEC-alone and the COPc,TEC-HX under cooling mode at different liquid flow rates are shown in Fig. 11. It is seen that when the liquid flow rate is small (e.g. 20 and 30 L/h), the COPc,TEC-HX is higher than the COPc,TEC-alone. As the flow rate increases, the COPc,TEC-HX decreases, tending to be lower than the COPc,TEC-alone instead (e.g. 40 L/h and 50 L/h). The effects of the current are similar between the COPc,TEC-HX and COPc,TEC-alone, both of which increase first and then decrease with the increase of the current. The effects of the current on the COPh,TEC-HX and COPh,TEC-alone under heating mode at different flow rates are shown in Fig. 12. It can be seen that the COPh,TEC-HX is always higher than COPh,TEC-alone, and both of them decrease with the increase of the liquid flow rate. It can be concluded that TEC-HX system performs better than TECalone system as long as COPh,TEC-HX is higher than COPh,TEC-alone. The COP ratio, k, is defined in Eq. (17), whose value exceeding 1 means that TEC-HX system performs better.
k = COPTEC − HX / COPTEC − alone
Fig. 9. Effects of liquid flow rate on HX inlet liquid temperature and TEC heating capacity under heating mode.
(17) 6
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Fig. 10. Quantity and total power of TEC modules with respect to current at different liquid flow rates under heating mode.
envelope is significantly affected by the HX efficiency. The higher the HX efficiency is, the wider the applicable range of the operating conditions will be; that is, the TEC-HX system with a high-efficiency HX can easily run better than TEC-alone system under most operating conditions.
that determines whether the system performance can be improved by combining TEC and HX. Fig. 14 plots the HX efficiency curves of five individual heat exchangers with respect to the liquid flow rate, where the efficiency increases from HX1 to HX5. The envelope curves of the operating conditions (including liquid temperature and flow rate) for achieving k = 1 are plotted in Fig. 15, where the regions below these curves indicate that k > 1. As seen, the
Fig. 11. Effects of current on COPc,TEC-alone and COPc,TEC-HX at different liquid flow rates under cooling mode. 7
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Fig. 12. Effects of current on COPh and total efficiency at different liquid flow rates under heating mode.
(1) In DTCS, an optimal current can be found to obtain the highest COP of the individual TEC module under both cooling and heating modes. The maximum cooling capacity of the individual TEC module exists under cooling mode, while the heating capacity increases monotonically with the current under heating mode. (2) In either DTCS or ETCS, since both the cooling and heating capacities of the individual TEC module working on the COP-optimized state are too small, the COP optimization should not be the primary issue. Instead, the current should be operated at the value that can achieve larger cooling capacity. (3) The lower flow rate the liquid flows at through the heat exchanger, the less TEC modules and power are required in TEC-HX system. The smallest quantity of TEC modules is consistent with the largest cooling capacity versus the current. (4) In TEC-HX system, the system COP decreases with the increase of the liquid flow rate through heat exchanger. High system COP can be achieved at low liquid temperature under cooling mode and at high liquid temperature under heating mode. (5) High efficiency of heat exchanger can enlarge the applicable range of operating conditions for achieving the COP ratio larger than 1. In these ranges, the TEC-HX system has better performance than the TEC-alone system.
Fig. 13. Contours of COP ratio with respect to liquid temperature and flow rate.
4. Conclusion Thermoelectric cooler has advantages of high reliability free of moving parts, which is suitable for temperature control in space scientific experiments. In this paper, TEC is used for direct temperature control and environment temperature control respectively. The influences of current, heat sink temperature and flow rate are discussed by calculation. The system COP is also discussed. Key findings from the study are summarized below.
CRediT authorship contribution statement Dongcai Guo: Conceptualization, Methodology, Software, Formal analysis, Supervision, Writing - original draft, Writing - review & editing. Qiang Sheng: Investigation, Visualization. Xiangyu Dou: 8
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Fig. 14. Efficiency curves of different HX.
Fig. 15. Envelope of operating conditions for achieving k = 1 under different HX efficiencies.
Resources. Ze Wang: Data curation. Liyao Xie: Writing - original draft. Bo Yang: Methodology, Formal analysis, Writing - original draft, Writing - review & editing. Declaration of Competing Interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. Appendix A. Supplementary material Supplementary data to this article can be found online at https:// doi.org/10.1016/j.applthermaleng.2019.114888. References [1] S.N. Evetts, Space life and biomedical sciences in support of the global exploration roadmap and societal development, Space Policy 30 (2014) 143–145.
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