EXPERIMENTAL STUDY OF DUAL EVAPORATOR LHP WITH TWO COMPENSATION CHAMBERS

EXPERIMENTAL STUDY OF DUAL EVAPORATOR LHP WITH TWO COMPENSATION CHAMBERS Donatas Mishkinis 2, Igors Ušakovs 1 Dmitry Nasibulin 2 1 Allatherm SIA Pulka 3, k.7, Riga LV-1007 Latvia Tel +371 256 73 677; E-mail info@allatherm.com 2 Allatherm S.L. Plaza del Sol, 17 L.Dcha, Torres de la Alameda, Spain, 28813 Tel. +34 640 787 027 Abstract This paper describes a novel design of a LHP Dual Evaporator Block which consists of two ALTOM evaporator modules integrated with two parallel compensation chambers. This thermal architecture is advantageous in comparison with typical one-wick evaporators with one compensation chamber due to (1) the possibility to distribute the evaporator modules over a heat source which leads to an increase in the LHP performance (thermal conductance and maximum heat transfer capacity) and (2) the insensitivity of the LHP operation to the Evaporator Block orientation, resulting in better performance in body force fields. The experimental results demonstrate that such evaporators can transfer up to 900W and that the LHP operates reliably and steadily in any orientation. KEYWORDS Dual Evaporator, Modular Evaporator, Loop Heat Pipe, Heat Loop, Power Electronics Thermal Management, Two-Phase Heat Transfer System, Altom INTRODUCTION The global trend towards a continuous increase of environmentally sound technologies in the production of electricity, with reduction in greenhouse gas emissions, has resulted in the development of environmentally friendly electric traction transportation as a main green and efficient alternative to vehicles using gasoline or other hydrocarbon fuels. Along with famous players such as Tesla, which announced expected car sales of at least 1 million per year by 2020, most of the world's auto giants joined the race, with expected sales of 1-2 million electric vehicles per year by 2025: Volvo - 1 million by 2025, Renault - Nissan from 1.5 million by 2020 [1]. According to the International Energy Agency's prognosis, the European fleet of electric vehicles alone will have at least 200 million units by 2030 [2]. The rejection of internal combustion engines in favor of electric motors leads to radical changes in the approach to vehicle design. In essence, except for the cooling system, the majority of the electric car systems differ fundamentally from their predecessors, incorporating recent technological advances. Meanwhile, the cooling system has retained its conservative single-phase design. Circulating pumps, expansion tanks, radiators, hoses, valves, clamps etc. with some modifications have moved from classic vehicles to electric, retaining the whole set of their shortcomings. As an important technological breakthrough in the development of thermal management systems for electric vehicles, we foresee a transition to a two-phase thermal architecture using capillary (for example, LHP) and/or mechanically driven heat loops with an evaporation-condensation cycle for cooling the electric vehicle equipment. This will bring a number of advantages inherent to two-phase loops in comparison with single-phase loops. Capillary driven heat loops are fully passive, hermetically sealed devices (versus the one-phase loops that need a mechanical pump) and, consequently: (1) No external power source is necessary for fluid circulation (2) No accompanying electronics are needed for control of the 2-phase loop

(3) No moving mechanical elements (4) Better heat transfer performance (5) No maintenance during its lifetime (6) More than 3-times longer lifetime expectations (7) No noise (8) Less mass and volume characteristics (9) No plumbing! For a comparative analysis and illustration of the advantages of a two-phase heat loop over a one-phase cooling system, a special demonstration test setup has been designed and manufactured by Allatherm (Fig.1.). Fig. 1. Demonstration stand (Test setup) for comparative analysis of advanced two-phase cooling loop with capillary pumps (on the right) and one-phase cooling loop with mechanical pump (on the left). The setup consists of two heat loops with the same dimensions. To ensure equal conditions, the same cartridge heaters are used in both, and the same heat sinks (condenser for two-phase loop and heat exchanger for one-phase loop) are placed into the glass jar where chilled water is circulating. Heat load is applied to same sized cold plates. A standard cold plate is used in the case of the one-phase heat loop and a novel design of the cold plate, with two embedded parallel evaporator modules (two Altoms) is used in the twophase loop. Tests are conforming better thermal performance of the two-phase loop in comparison with the one-phase system. The use of two-phase heat loops for electric motor cooling requires their deep integration at the motor design level. However, the thermal management of motor control power electronics, such as inverters, used to convert direct current electricity from a battery into alternating current to power a motor, is normally performed by cold plates. A typical EV inverter consists of several insulated-gate bipolar transistors and

power diodes. Considering that the efficiency of modern converters is in the range of 90-97%, the generated heat during a car operation can vary in the range of 0.5-5 kw [3]. The novel ALTOM [4] modular technology, developed by Allatherm, can be applied directly for invertor cooling. This technology provides the possibility to integrate several evaporator modules (called Altoms) into one or several cold plates within the same heat loop, thus increasing its heat transfer capabilities and improving overall thermal conductance, while remaining practically in the same dimensions as the cooled device. An experimental study of the parameters of the two-phase heat loop, where the heat source is placed on a cold plate with two evaporator modules built-in by ALTOM technology and connected by two parallel compensation chambers has been performed. The tests were conducted for different spatial positions of the heat loop. The results of this study are presented below. 1. LHP DESIGN 1.1. LHP main parameters and layout The studied two-phase heat loop by the principle of action is a Loop Heat Pipe (LHP) but has significant differences in the design of the evaporator. In our case, instead of a single evaporator with a compensation chamber, a dual evaporator block is used in the form of a cold plate with two evaporators integrated into it. The dimensions of the evaporator block L x S x H are 200 mm x 100mm x 20 mm. Fig. 2. LHP layout. Evaporator1, Evaporator2, Cold Plate, Compensation Chamber1 and Compensation Chamber2 constitute the Evaporator Block Dual Evaporator LHP with two Compensation Chambers LHP Main Parameters LHP Element ID, OD, Width, Height, Length, mm mm mm mm mm Material Evaporator (1&2) 13.8 15.4 - - 160 AISI 316 Compensation Chamber (1&2) 14-16 18 100 AISI 304 Cold Plate - - 100 20 135 AL 6061 Heater Block - - 50 30 96 AL 6061 Wick 6.5 13.8 155 AISI 304 Vapor Line 2.5 3 2000 AISI 304 Liquid Line 1.5 2 2000 AISI 304 Condenser 2 2.5 2000 AISI 304 Table 1.

The design of the evaporator block is discussed in more detail in paragraph 1.2. The vapor and liquid lines are made of stainless steel tubes of 2000 mm length. The condenser is a coiled tubing with a length of 2000 mm and a revolution diameter of 60 mm, placed in a tank with running water circulating through the chiller. The distance between the evaporator block and the condenser is 500 mm. Ammonia was used as the working fluid. The layout of LHP is presented in Fig.2. and Fig.3., and the main parameters are given in Table 1. b) a) c) Fig. 3. Photos and main dimensions of LHP (a), Evaporator with heater Block (b) and Condenser inside the chilled water jar (c). 1.2. Evaporator Block with ALTOM14 modules The evaporator block is manufactured using the modular ALTOM technology, which is presented in detail in our recent publication [4]. The Block photo is shown in Fig. 3b. The two Altom14 evaporators are located along the plate at 15 mm from the edges of the plate. The marking 14 means that the outer diameter of the evaporator wick is 14 mm. The working part of each of the evaporators corresponds to the length of the plate and is equal to 135 mm. The evaporators inside the plate are placed as close as possible to the heating zone, considering the location of the mounting holes for the fixation of cooled devices. The particularity of the evaporator block design is that the evaporators have through liquid channels in the wicks and two common compensation chambers, one on each side of the evaporators. Compensation chambers are located at about 15 mm from the cold plate to avoid thermal contact with it and are connected to each other through the liquid channels of the evaporators by secondary wicks. The geometric dimensions of the compensation chambers are chosen so that they do not exceed the dimensions of the cold plate.

The liquid is fed through the bayonets inside both wicks at the same time and is further distributed by means of the secondary wick. The vapor through vapor channels of the wicks is moved in a collector located on the side of the plate. The symmetrical arrangement of the compensation chambers ensures the stability of operation of the heat loop in the presence of fields of acceleration of different directions that inevitably arise during motion. This is important issue for the application of this type of cold plates on transport. 2. TEST SETUP The experimental study of the LHP in various spatial orientations is a way to test its parameters under the action of acceleration fields of different directions. In this case, gravity plays the role of the acceleration field. For the convenience of rotation, we placed the heat loop on a moving flat platform. A heater block was installed on the surface of the saddle-cold plate. It was a dummy of a power diode. The temperature was monitored by 12 T-type thermocouples. The layout of the thermocouples is shown in Fig. 4., and their designations and description of control points are given in Table 2. LHP was not thermally insulated during the tests but heat losses to ambient due to natural convection was taken into account (3-7 % of total applied power). Fig. 4. Points of thermocouple locations (red dots) on the LHP Table 2 Designation and description of thermocouples TC_HB Heater block TC_CP1 Cold plate. Point 1. TC_CP2 Cold plate. Point 2. TC_EV1 Evaporator 1 TC_EV2 Evaporator 2 TC_CC1 Compensation chamber 1 TC_CC2 Compensation chamber 2 TC_LLout Liquid line out TC_LLin Liquid line in TC_VLin Vapor line in TC_Vout Vapor line out AMB Ambient temperature Some additional thermocouples were used on the cold plate and condenser during the first stage of the test campaign (not shown in picture). The temperature measurements where performed by ADAM dataloggers and were collected and processed by Allatherm computer code developed in the graphical programming language LabVIEW. Sampling rate of data was 0.25 Hz with a resolution of 16-bit. All figures in the following paragraph (Test Results) are screenshots from the computer during the test elaborations. LHP orientations during the test campaign are shown in the Fig.5

Horizontal Position of LHP g0_lhp Horizontal Position with compensation chamber 1 above Evaporator Block: g0_cc1up Horizontal Position with compensation chamber 2 above Evaporator Block: g0_cc2up Position with Evaporator Blok 0.5m below condenser (Gravity assisted LHP operation): g+_cdup Position with Evaporator Blok 0.5m above condenser (LHP operation against gravity): g _EVup Fig. 5. LHP orientation in gravity field during the testing. 3. TEST RESULTS The test campaign consisted of two stages: maximum power (stage 1) and thermal performance in different orientations (stage 2). During the first stage, the maximum heat transfer capacity of the LHP was tested in the most unfavorable orientation (g _EVup). The LHP is capable of transferring more than 900W without dryout, with 900W being the limit of the heating system for this setup. However, degradation of thermal conductance characteristics has been observed above the level of 650W. This effect has been accomplished by high-power hysteresis. As it can observed from the Fig. 6, at the power step down (690W), after power increase of up to 810W, the values of the LHP cold plate temperatures (CP1, CP2, CP3) are significantly greater than for the power step up (from 500W to 680W). This high-power hysteresis can be associated with the effect of evaporation front deepening into the wick (heat exchange in evaporation zone) and an increased heat leak into the compensation chamber. High-power hysteresis in LHPs is well known and a detailed analysis of the operational mode of such LHP can be found in [5]. Experimentally it was established that the front starts to move inside the wick at heat input levels of above 650W (see Fig.7, where no high-power hysteresis was observed). To recover the LHP from the degraded mode of operation, it is necessary to reduce the power below the hysteresis limit. This was performed in the first experiment (Fig. 6): the heat input has been reduced to 300W from power step 690W and no hysteresis was observed in the following power profile 300W-500W-300W.

Fig. 6. LHP Temperature profile for g _EVup orientation as a function of input power: high power hysteresis. Chiller T=20 C; Position g-evup. Thermocouple numbers: 1-CC1; 2-CC2; 3-CP1; 4-CP2; 5-LLout; 6-VLout;7-EV1;8-EV2;9- VLout; 10-LLin; 11-CDin; 12-CP3. Tests of stage 2 were performed 4 months after the stage 1 for different environmental conditions: ambient temperature was 10-12 C higher (up to 30-32 C) and chilled water temperatures were also significantly higher. Fig.8 demonstrates the stable operation of the LHP in g0_lhp position (see Fig.5) for the power input profile 350W-175W-350W-650W-350W-500W-100W. Overall, the LHP conductance (Power divided on the difference between the cold plate average temperature and the coldest LHP temperature liquid line inlet) is around 30-35W/K. Fig. 7. LHP Temperature profile for g _EVup orientation as a function of input power: no high-power hysteresis. Chiller T=12 C Position g-evup. Thermocouple numbers: 1-CC1; 2-CC2; 3-CP1; 5-LLout; 6-CP2; 7-VLout; 8-EV1;9-VLout; 10-LLin; 11-CDin; 12-CP4. The results of the test for a 350W power input, five different orientations and two chilled water temperatures (18 and 26 C) are shown in Fig.9. It is clear that the orientation of the LHP in the gravity field has minor influence on its performance. Additionally, the LHP operates properly without any changes in temperature profiles during manipulations (changing of the LHP position). The same type of the test has been conducted for 650W. The results are presented in Fig. 10. It confirms the statement that the ALTOM design LHP is a gravity force independent heat transfer device for the given specification of the thermal management system (high and moderate power levels, 0.5m between heat sink and heat source).

Fig. 8. LHP Temperature profile for g0-lhp orientation as a function of input power: Chiller T=26 C. Thermocouple numbers: 2-CP1;3-CP2;4-VLin; 5-VLout; 6-CC2; 7-CC1; 8-LLin; 9-LLout 10-EV1; 11-EV2; 12-AMB a) b) Fig. 9. LHP Temperature profile for different orientations and constant power input 350W: Chiller T=18 C (a) T=26 C (b). Thermocouple numbers: 1-HB; 2-CP1;3-CP2;4-VLin; 5-VLout; 6-CC2; 7-CC1; 8-LLin; 9- LLout 10-EV1; 11-EV2; 12-AMB

Some difference can be observed in the temperatures of the compensation chambers (see positions g0_cc2up and g0_cc1up), which has no impact on the temperatures of the cooled unit (HB) and the cold plate (CP1 and CP2). The vapor phase is always present in the compensation chamber during the LHP operation. As soon as one of the two compensation chambers is in the upper position over the second chamber, the vapor bubble will migrate to this chamber due to buoyancy (small temperature increase). Fig. 10. LHP Temperature profile for different orientations and constant power input 350W: Chiller T=18 C (a) T=26 C (b). Thermocouple numbers: 1-HB; 2-CP1;3-CP2;4-VLin; 5-VLout; 6-CC2; 7-CC1; 8-LLin; 9- LLout 10-EV1; 11-EV2; 12-AMB 4. CONCLUSION In this paper we have presented a novel thermal management system with a dual evaporator, integrated into the cold plate and based on the modular approach with elementary building units called Altoms. The Altom technology provides the opportunity to design two-phase cold plates for power electronic thermal management (for instance, electric vehicle invertor cooling) with a different number of evaporators, with various heat transfer capacities (up to tens of kw), geometrical dimensions, thermal interface surfaces, volume(s) of compensation chamber(s), etc. easily and quickly. The experimental results demonstrate that the dual evaporator can transfer up to 900W with overall LHP conductance of more than 30 W/K. The LHP performance is insensitive to the orientation in gravity. Thus, robust operation of the LHP in the fields of acceleration in altered directions is expected. The modular Altom method for design and development of modern advanced two-phase thermal management systems is the way for achieving cost reduction and opening new terrestrial areas of LHP/CPL applications: from space to earth. Acknowledgement This project has received funding from the European Union s Horizon 2020 SME Instrument programme under the Grant Agreement No.815494-ALTOM References 1. World Energy Outlook 2017 / Global Energy Trends, IEA (International Energy Agency) 2017. 2. Electric Vehicles, a Global Business Report, Global Industry Analyst Inc., 2017. 3. Drobnik J., Jain P. Electric and Hybrid Vehicle Power Electronics Efficiency, Testing and Reliability // World Electric Vehicle Journal. 2013. Vol. 6 (ISSN 2032-6653). Pp. 719 730. 4. Mishkinis D., Ušakovs I., Nasibulin D., Novel Modular Evaporator Architecture for Electronics Cooling Applications, // Joint 19th IHPC and 13th IHPS, Pisa, Italy, June 10-14, 2018. 5. Vershinin S.V., Maydanik Y.F. Hysteresis phenomena in loop heat pipes // Applied Thermal Engineering. 2007. Vol. 27. Issues 5-6, P. 962 968.