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Man-portable personal cooling garment based on vacuum desiccant cooling
Applied Thermal Engineering 47 (2012) 18e24
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Applied Thermal Engineering journal homepage: www.elsevier.com/locate/apthermeng
Man-portable personal cooling garment based on vacuum desiccant cooling Yifan Yang a, Jill Stapleton b, Barbara Thiané Diagne a, Glen P. Kenny b, Christopher Q. Lan a, * a b
Department of Chemical and Biological Engineering, University of Ottawa, 161 Louis Pasteur Private, Ottawa, Ontario, Canada K1N 6N5 Human and Environmental Physiology Research Unit, School of Human Kinetics, University of Ottawa, Ottawa, Ontario, Canada
a r t i c l e i n f o
a b s t r a c t
Article history: Received 30 January 2012 Accepted 5 April 2012 Available online 13 April 2012
A man-portable personal cooling garment based on the concept of vacuum desiccant cooling (VDC) was developed. It was demonstrated with cooling pads that a cooling capacity of 373.1 W/m2 could be achieved in an ambient environment of 37 C. Tests with human subjects wearing prototype cooling garments consisting of 12 VDC pads with an overall weight of 3.4 kg covering 0.4 m2 body surface indicate that the garment could maintain a core temperature substantially lower than the control when the workload was walking on a treadmill of 2% inclination at 3 mph. The exercise was carried out in an environment of 40 C and 50% relative humidity (RH) for 60 min. Tests also showed that the VDC garment could effectively reduce the metabolic heat accumulation in body with subject wearing heavily insulated nuclear, biological and chemical (NBC) suit working in the heat and allow the participant to work safely for 60 min, almost doubling the safe working time of the same participant when he wore NBC suit only. Ó 2012 Elsevier Ltd. All rights reserved.
Keywords: Evaporative cooling Vacuum cooling Absorption Membrane Personal cooling garment
1. Introduction Numerous occupations expose workers to hot environmental settings, where they are regularly subjected to excessive heat stress. In addition to reduced worker performance and productivity [1], heat stress may be an underlying cause for many workplace accidents and injuries, as it impairs mental alertness and concentration [2e4], motor dexterity and coordination [5], muscle fatigue [6,7]. The increased physical discomfort associated with elevated body temperature promote irritability, anger and other emotional states, which often cause workers to overlook safety procedures or to divert attention during hazardous tasks [8]. The tragic death of a United Kingdom soldier who died on duty when trying to dismantle an improvised explosive device in Afghanistan without wearing a heavily armoured bomb suit due to the extreme discomfort caused by heat stress in the high temperature highlighted the need of efficient personal cooling systems. Furthermore, exposure to excessive heat could cause severe symptoms that may threaten of the life of workers. In 2002, 1816 active American soldiers were reported injured from heat related causes [9]. From 1992 to 2006, 68 U.S. farm crop workers were reported to have passed away due to excessive heat stress [10]. Healthy humans regulate core temperature to maintain a near constant level (w37 C) regardless of environmental conditions. To
* Corresponding author. Tel.: þ1 613 562 5800x2050. E-mail address: [email protected] (C.Q. Lan). 1359-4311/$ e see front matter Ó 2012 Elsevier Ltd. All rights reserved. doi:10.1016/j.applthermaleng.2012.04.012
do this, a balance must be maintained between the heat produced within the body and the heat lost to the environment via a combination of dry heat exchange and evaporative heat loss. Ultimately, the ability to offset increases in core temperature during heat exposure depends on the ability of the body to dissipate heat through skin to the immediate microenvironment surrounding it. Many cooling technologies have been employed to manage heat stress in daily activities, which can be classified into space cooling and personal cooling according to the immediate subject of cooling. Space cooling is the cooling of the interior space of a building or a room and is implemented at a relatively large scale. In occasions space cooling is very costly, impractical, or even impossible. Such cases include the hot outdoor environments and large work spaces such as steel mills, foundries, mines [11] and metallurgy plants. In these occasions, personal cooling (PC), which is designed to cool the immediate surroundings of the wearer, is more practical and cost-effective. PC acts directly on the microenvironment of the individual and is also referred to as microclimate cooling (MC). In scenarios where impermeable protective clothing is required, personal cooling garment also serves as the most effectively means of heat stress management [12e14]. Three different types of PC garments have been developed in the last several decades: i) fluid cooled garments (FCGs), also known as refrigeration aided PC systems, ii) phase change material (PCM) cooling vests, and iii) evaporative PC devices, also known as evaporative cooled garments (ECGs). FCGs employ air, water or non-toxic aqueous solutions as the coolant and are generally recognized as the most efficient PC technologies at present [15].
Y. Yang et al. / Applied Thermal Engineering 47 (2012) 18e24
They can achieve continuous cooling at capacities ranging from 600 W to1000 W [16]. Nevertheless, they are limited by lack of portability, due to the requirement of refrigeration unit and continuous power supply. PCM systems employ phase change materials, which are substances having the ability to absorb substantial amount of latent heat when changing from solid to liquid state in a temperature range that is suitable for personal cooling [17]. PCM garments are portable but offer only very limited cooling capacities. Some commercial vests containing up to 5 lbs of PCMs have been reported to remain cool for about 2 h at a cooling capacity of up to 95 W [18,19], which is much lower than what is achieved with FCGs. ECGs harness the large latent heat of water evaporation for cooling [20e23]. The latent heat of water evaporation, i.e., around 2400 kJ/kg in the temperature range of 0e30 C, promises the potential of creating effective ECGs using a small amount of water. The main drawback of conventional ECGs is that they do not work well in highly humid environments due to small evaporation flux. The objective of this study is to evaluate the feasibility of a novel cooling technology, the vacuum desiccant cooling (VDC), for microclimate cooling. In contrast to conventional vacuum cooling that require continuous vacuuming throughout the cooling process, VDC requires vacuuming only for a short period of initialization and can operate detached from a vacuum pump for a prolonged period of time afterward. It was demonstrated in the study that a portable VDC garment could provide sufficient capacity for personal cooling. 2. Experimental methods 2.1. VDC pad The cross-sectional structure of a VDC pad is shown in Fig. 1A. In this study, cooling pads of two different sizes were fabricated. The larger ones (Type I) had the dimensions of 180 250 mm with an
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effective membrane area of 0.030 m2 (150 200 mm); and the smaller ones (Type II) had dimensions 140 190 mm with an active membrane area of 0.015 m2 (100 150 mm). The honeycomb spacers were 16 mm thick. A completed cooling pad was 25e30 mm thick, which included the additional thickness of a cooling core and an absorption core. As depicted in Fig. 1A, the VDC pad was composed of four major components, a cooling core, a spacer, an absorption core and an outer bag. The cooling core (Figs. 1B and 2A) was a hybrid bag containing an amount of water as specified in Table 1. The hybrid bag was made of a soft impermeable plastic water bag (Seal-aMealÒ, Sunbeam Product, Inc. Rye, NY, USA) with one side replaced with a semi-permeable Teflon (PTFE) membrane (Sterlitech, Kent, WA, US). A piece of soft cotton cloth (Towel III) was used to cover the membrane for protection from possible damage caused by the edges of honeycomb matrixes. A piece of perforated aluminum foil was used to cover the Towel III to minimize the radioactive heat transfer from the hot absorption core to the cold cooling core. The membrane was a microporous hydrophobic membrane (Teflon membrane) that had a pore size of 0.2 m, a thickness in the range of 0.061e0.081 mm, and a liquid entrance pressure of water (LEPw) of 4.1 bars. The perforated aluminum foil had approximately 20 holes per square centimetre (diameter of approximately 1 mm), was attached to the cloth. The spacer (Fig. 2B) was made of honeycomb, which served to separate the cooling core from the absorption core. The absorption core (Figs. 1B and 2C) was composed of two layers of cotton towel glued together with lithium chloride (LiCl) powder being sandwiched in the middle of them. And the outer bag (Figs. 1B and 2D) was made of a plastic bag connected to a piece of ¼" polyvinyl chloride (PVC) tubing of 150 mm in length. To assemble a functional VDC pad, the prepared cooling core and the spacer were put together with the spacer sitting on top of the cooling core. The pre-prepared absorption core, which was sealed
Fig. 1. A. Schematic diagram of the cross-sectional view of a VDC cooling pad. B. Schematic diagram of the cross-sectional view of absorption core and cooling core (drawing not in proportion).
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Fig. 2. Parts of a VDC pad. A) The cooling core (water bag); B) The spacer (two layers of honeycomb); C) The absorption core (desiccant holder); D) The outer bag with vacuum tubing; and E) The cooling core side of an AVMEC pad (food grade blue dye was added for detection of water leakage).
in a storage bag, was taken out of the bag and placed on top of the spacer. After the cooling core, spacer and absorption core were aligned with each other, an outer bag was used to envelop the three-layer structure and the opening of the outer bag was sealed air-tight quickly using a Seal-A-MealÒ VS107 sealer (Sunbeam Product, Inc. Rye, NY, USA). The cooling pad could then be initialized using a high performance vacuum pump to start the cooling process. The cooling core side of an initialized VDC pad is shown in Fig. 2E. Blue dye was added into water for the purpose of detecting water leakage. The dotted pattern was generated because the soft cooling core was pushed against the hard spacer by vacuum and therefore revealed the pattern of the honeycomb matrix.
Cole-Parmer, Montreal, QC, Canada), which could deliver a maximum vacuum degree of 29.0 In Hg (corresponding to an absolute pressure of 3.048 kPa) and a free air capacity of 32.5 L/min at room temperature. The vacuum pump was turned on for 5 min to achieve the highest vacuum degree, after which the vacuum tubing of the pad was sealed and disconnected from the pump. The disconnected pad was then put in an incubator (MaxQ 5000, GENEQ Inc, Montreal, QC, Canada) maintained at a constant temperature of 37 C. Temperature of the surface of the absorption core and that of the surface of the cooling core were monitored by taping the probe of an Oakton Temp-300 dual-input type K thermocouple (Cole-Parmer, Montreal, QC, Canada) onto each surface. The mean evaporation flux was estimated by the following equation:
2.2. Cooling (VDC) pad experimental setup and procedures
J ¼
Type I cooling pad (Table 1) was used in cooling pad experiments to evaluate their performance. The cooling pad was first connected to a high performance vacuum pump (WZ-07061-11,
where Mi is the initial water mass (g), Mf the final water mass at the end of test (g); A the area for evaporation, i.e., the effective membrane area of the cooling core (m2) and t the testing time (h).
Mi Mf
. At
(1)
Table 1 Summary of some key features of structural components for VDC pad Type I and Type II. Pad
Component
Description
Dimensions (mm)
Weight (g)
Type I
Cooling Core Absorption Core
Hybrid bag containing 100 g water Two layers of cotton towels sealed into a bag with 30 g LiCl powder spreading evenly inside Polypropylene honeycomb (two-layer) Multi-layered PA/PE (Polyamide/Polyethylene) bag
180 250 Same as above
121 1.0 95 1.0
150 200 16 240 300
54 32 332 62.5
Type II
Spacer Outer Bag Subtotal Cooling Core Absorption Core
Spacer Outer bag Subtotal
Hybrid bag containing 50 g distilled water Two layers of cotton towels sealed into a bag with 15 g LiCl powder spreading evenly inside Polypropylene honeycomb (two-layer) Multi-layered PA/PE (Polyamide/Polyethylene) bag
140 190 Same as above
100 150 16 185 p 540
1.0 1.0 1.0 g 1.0
55 1.0
30 1.0 42 1.0 210 1.0 g
Y. Yang et al. / Applied Thermal Engineering 47 (2012) 18e24
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Assume that the sensible heat change of water and the internal heat exchange between the cooling core and the absorption core are negligible, the cooling capacity of a VDC pad can be estimated as the rate of latent heat using the following equation,
Q ¼ JADHev
(2)
Where J is the average evaporation flux, A the area of evaporation (i.e., the effective membrane area) of the pad, and DHev the latent heat of water evaporation, which is taken as a constant of 2400 J/g in the experimental temperature range. 2.3. Prototype VDC garment The VDC garment prototype was composed of 4 Type I and 8 Type II VDC pads attached to a XL T-shirt (Ergodyne co., St. Paul, MN, USA). The sleeves of the T-shirt were cut off from the shoulders. As shown in Fig. 3, there were two Type I pads at the upper position (the chest or the upper back) and four Type II cooling pads at the lower part (the abdomen and the lower back) on each side of the T-shirt. Each cooling pad was attached to the T-shirt using four Velcro sets. The total weight of a VDC garment is 3.4 kg. The total area of VDC pads was 0.4 m2 with a total effective membrane area of 0.24 m2. All human experimental trials were carried out with a 22 year old male (170 cm) weighing87 kg. Pictures of the subject dressed in the cooling garment are shown in Fig. 4. During the experimental trials, the participant was required to perform 60-min of treadmill walking at 3 mph and 2% incline in an environmental chamber regulated at 40 C and 50% RH (relative humidity) while wearing : 1) a cotton T-shirt and shorts (Control); 2) the VDC garment on top of a cotton T-shirt and shorts (VDC); 3) a Nuclear Biological Chemical (NBC) protective garment on top of a cotton T-shirt and shorts; 4) an NBC on top of a Climatech CM2000 ice vest (Clima Tech Safety, White Stone, VA, US) plus a cotton T-shirt and shorts (ice-pad/NBC); and 5) NBC on top of VDC garment plus cotton Tshirt and shorts (VDC/NBC). For all the trials, core temperature was estimated by measuring rectal temperature using a paediatric thermocouple probe (Mon-atherm General Purpose Temperature Probe, Mallinckrodt Medical,
Fig. 4. Participant wearing a prototype VDC cooling garment
St-Louis, MO, USA) inserted to a minimum of 12 cm past the sphincter. 3. Results 3.1. Performance of VDC pad Fig. 5 presents the surface temperature profiles for the cooling core and absorption core in a typical cooling pad experiment.
Fig. 3. Arrangement of the six cooling packs on one piece of the cooling garment. The entire garment consisted of two pieces, the front piece and the back piece, which had the same pad arrangement.
Fig. 5. Temperature profiles of the surface of the cooling core (Tc) and that of the absorption core (Tad). Initially, the cooling pad contained 100 g water in the cooling core and 30 g LiCl powder in the absorption core. Experiments were carried out at the maximum vacuum achievable by the vacuum pump (28.8 In Hg or 3.725 kPa) at a 37 C environment inside an incubator. The vacuum remained constant after the 5-min initialization period.
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Table 2 Summary of VDC pad performance testing results Initial cooling core surface temperature ( C) Final cooling core surface temperature ( C) Temperature difference for cooling core ( C) Evaporated water (g) Effective membrane area (m2) Average evaporation flux (J, g/h m2) (Eq. 1) Estimated cooling capacity (Q, W/m2) (Eq. 2)
22.7 20.7 2.0 16.8 0.03 560 373.3
0.05 0.05 0.05 0.05 0.0001 50 38
Mean values of two trials. Experiments were carried out using Type I VDC pads.
During the initialization stage, the cooling core surface temperature decreased from the starting temperature of 22.7e16.9 C at 5 min, and reached the lowest temperature of 16.0 C within 10 min. Water temperature started to increase afterward and reached 20.7 C at the end of the 60-min test. This temperature was still sufficient to maintain efficient heat absorption from the 37 C environment given that the temperature difference between the water in the cooling core surface and the environment was still 16.3 C. On the absorption core surface, the initial temperature was 33.5 C, which reached 41.5 C at the end of the 5-min initialization stage and rose to 47.3 C at the end of the test. It is worth mentioning that the initial temperature of the cooling core surface was always lower and that of the absorption core surface always higher than the room temperature because water evaporation (endothermic) and vapour absorption (exothermic) took place in the pad fabrication process, although at very low rates. As shown in Table 2, the total mass of water evaporation during the 60-min test was 16.8 0.05 g through an evaporation surface of 0.03 m2, corresponding to an average evaporation flux of 560 50 g/h m2. It should be noted that the evaporation fluxes at different time points varied with time because important parameters such as water temperature and extent of desiccant saturation changed with time in the cooling process. The average flux is however a necessary approximation as we did not have the capacity to measure real time water evaporation without damaging the cooling pad. The cooling capacity of the VDC pad was calculated to be 373.3 38 W/m2 using Eq. 2. This value is calculated from the average evaporation flux and therefore represents the mean cooling capacity of the pad during the experimental period. 3.2. Performance of VDC garment during exercise in the heat
As shown in Fig. 6, the core temperature of the control increased from a baseline resting value of 37.1e37.8 C (or 0.7 C above resting levels) at mid-point of exercise (i.e., 30 min) and continued to increase, albiet at a lower rate up, for the duration of exercise. Endexercise core temperature achieved an elevated state of hyperthermia of 38.1 C, an equivalent increase of 1.1 C above the baseline. In sharp contrast, for the VDC test, the core temperature increased by only 0.2 C above the baseline resting values (37.3 C) after 30 min of exercise. The core temperature increased 0.5 C to reach 37.8 C by the end of the 60 min of exercise when the subject wore VDC garment. The much lower core temperature increase in the VDC test (0.2 C at 30 min and 0.8 C at 60 min) than that of the control (0.7 at 30 min and 1.1 C at 60 min) indicates a much lower change in body heat content and therefore body heat storage during work performed in the heat. 3.3. Effectiveness of the VDC garment vs. ice-cooling vest on reducing the level of thermal strain during work performed with the NBC protective garment The effectiveness of the VDC garment in reducing the level of thermal strain while wearing heavily insulated protective garments (NBC suit) was assessed by comparing core temperature responses during exercise in the heat with a commercially available icecooling vest (Climatech CM2000, Clima Tech Safety, White Stone, VA, US). The weight of the ice vest was 4.1 kg. As shown in Fig. 7, the NBC trial had to be terminated at w32 min as rectal temperature of the subject was approaching 38.5 C. Whereas the NBC trial was terminated for health and safety precautions at 32.0 min into exercise, core body temperature during the Ice-Pad/NBC trial for the same time point was only 38.0 C, reaching a similar elevation as that of the NBC trial only after the full 60 min of exercise. In other words, the icecooling vest increased exposure time by 87.5% when the subject wore NBC suit wherein core temperature increased at a reduced rate. Furthermore, the VDC garment was more effective than the commercial ice/pad garment at attenuating the increase in core body temperature, and therefore body heat storage, as evidenced by the core body temperature profiles. The core temperature increased from a baseline resting value of 37.1e38.1 C at the end of the 60-min trial, corresponding to a core temperature increase of 1.0 C, 25% lower than the 1.25 C core temperature elevation experienced in the Ice-Pad/NBC trial (from 37.15e38.4 C).
As noted above, the human experimental trials were undertaken to evaluate the performance of the VDC garments. The data are presented in Fig. 6.
Fig. 6. Core temperature profiles during the 60-min exercise performed in the heat when the participant wore 1) shorts only (Control) and 2) shorts and VDC (VCD).
Fig. 7. Core temperature response during exercise in the heat for the Control, NBC, IcePad/NBC, and VDC/NBC conditions.
Y. Yang et al. / Applied Thermal Engineering 47 (2012) 18e24
4. Discussion 4.1. Unpowered vacuum desiccant cooling (VDC) The VDC is in essence a desiccant cooling device enhanced by the vacuum gap that separates the absorption and cooling cores. Desiccant cooling was proposed previously in developing a manportable microclimate cooling device [23] and a subminiature cooling pad [22]. In these designs, however, the vacuum was not involved, leading to rather mild cooling capacities. The function of the vacuum gap in VDC is twofold: 1) enhancing water evaporation by facilitating vapour diffusion from the water surface to the desiccant surface; and 2) enhancing thermal insulation between the absorption core and the cooling core to prevent the heat released from vapour absorption in the absorption core from being transferred to the cooling water in the cooling core. The VDC can also be viewed as a type of vacuum cooling, which is a rapid cooling technology that has been widely employed in industrial applications such as food processing and storage [24,25]. Conventional vacuum cooling requires a high performance vacuum pump and continuous power supply to maintain the driving force for evaporation during the entire cooling process, which greatly reduces the portability of the system and therefore is not suitable for personal cooling. In the VDC, however, the vacuum pump was required only for a short initialization period (w5 min for a cooling pad) during which vacuum was generated. Maintenance of vacuum in the process was achieved by vapour absorption by desiccant in the absorption core. As a result, the VDC could be effectively regarded as an unpowered cooling technology that allows cooling, once initiated, to continue for a prolonged period of time without requiring power supply. LiCl was selected as the desiccant for the VDC packs and garments because of its large vapour absorption capacity. In the experiments, LiCl powders were used as the starting material and it was converted to aqueous solution when sufficient amount of vapour was absorbed. The resulted concentrate aqueous LiCl solution could also absorb vapour. The powder and aqueous LiCl solution were held in place by the cotton towels in the absorption core and prevented from leaking into the vacuum gap by the semipermeable membrane. Regeneration of the desiccant could be achieved by drying the LiCl soaked towels in heat. A key feature of the VDC pad was the use of the hydrophobic membrane. The membrane had a liquid entrance pressure of water (LEPw) of 3.0 bar, which was much larger than atmospheric pressure (1.0135 bar), the highest trans-membrane pressure could occur in a VDC pad. LEPw is by definition the trans-membrane pressure at which water would enter the pores of a membrane (i.e., wetting). In other words, the Teflon membrane was capable of retaining liquid water but allowing the selective passage of vapour due to its large LEPw. As a result, it provided a semi-permeable barrier that prevented the liquid water in the cooling core from entering the vacuum gap or the absorption core while allowing water evaporation to occur continuously. Hydrophobic membrane has long been employed in different evaporative cooling systems to provide the barrier that separates the liquid water and the vapour, or in other words, to provide the surface for water evaporation [26,27]. These membranes evaporative cooling processes, however, typically involve sweep gas evaporation rather than vacuum evaporation and are designed for air conditioning or air humidification [27,28]. The VDC technology integrates vacuum cooling, desiccant cooling, and membrane technology to create a novel device that is independent of ambient conditions, could provide sufficient cooling capacity, and is sufficiently lightweight for specialty applications such as personal cooling. It provides major advantages
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including: 1) employing the large latent heat of water evaporation (approximately 2400 kJ/kg, more than 7 times of the latent heat of ice fusion) for cooling; 2) large evaporation flux which is independent of ambient humidity due to the existence of a vacuum gap; 3) no power supply or heavy equipment required except in the short initialization period; 4) water as coolant is non-toxic, cheap, and easily available for recharging. 4.2. VDC garment for personal cooling The results in Fig. 6 demonstrate that the VDC garment could substantially mitigate the metabolic heat accumulation in the body, reduce the rate of core body temperature increase and therefore greatly extend the safe working period for workers in heat. The results shown in Fig. 7 suggest that the VDC cooling garment was more efficient than the commercial ice-pad vest in attenuating the increase core body temperature thereby ensuring a longer safe working period under the specified conditions. Furthermore, these results demonstrate that VDC could effective alleviate the heat stress for workers in heat wearing heavily insulated NBC suit. It is interesting to notice that the body temperature profile of the VDC/NBC trial, as shown in Fig. 7, increased almost linearly after 10 min in exercise. In contrast, while the core temperature profile of the Control was comparable with that of the VDC/NBC trial in the first 10 min into the exercise, it had a larger slope in the 10e23 min period and a smaller slope in the period after the 23-min point, indicating a higher rate of temperature increase in the first period and a lower one in the second. This phenomenon can be explained by the fact that the participant was completely covered in the VDC/ NBC trial. Consequently, the body depended solely on sensible heat transfer for heat dissipation. On the contrary, in the Control trial, both sensible heat transfer and sweat evaporation were available for heat dissipation and sweating became more dominant in the later stage of exercise when core body temperature was at an elevated level. The energy expenditure is typically 290 kcal/h (alias 337.3 W) for a 70 kg man in active exercise. Since the oxidation efficiency of aerobic metabolism is approximately 30% [29], it is reasonable to assume 10% overall metabolic efficiency, i.e., 90% of the energy expenditure is converted to metabolic heat, which leads to a metabolic heat production rate of approximately 304 W. The average body surface of a 1.65 m adult weighing 73 kg is 1.8 m2 [30]. Assuming that the total effective cooling area of a human body is 1 m2, the desired cooling capacity is therefore 304 W/m2 or above. A cooling capacity of 373.1 W/m2 was achieved with the VDC pad, indicating that it could provide sufficient capacity for personal cooling during heat stress conditions. This is partially confirmed by the experimental results with the prototype VDC garment. Despite that the total cooling surface of the VDC garment was only 0.4 m2, the garment was able to significantly alleviate metabolic heat accumulation in the body of the participant. 5. Conclusions A man-portable cooling garment based on VDC technology was developed. The VDC cooling pads were estimated to have a cooling capacity of 373.1 W/m2 in an environment of 37 C and 50% RH. Experiments with prototype VDC garments weighing approximately 3.4 kg and covering 0.4 m2 body surface demonstrated that these garments could deliver effective heat stress mitigation to humans working in the heat. Further improvement in the design of cooling pad and cooling garment are expected to greatly improve the reliability, cooling capacity, work duration, and costeffectiveness and therefore pave the way to its commercialization.
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Acknowledgements Financial support from the Natural Science and Engineering Research Council of Canada (NSERC) and Canadian Institute of Health Research (CIHR) are gratefully acknowledged. References [1] M.J. Howes, C.A. Nixon, Development of procedures for safe working in hot conditions, in: R.V. Ramani (Ed.), Proceedings of the 6th International Mine Ventilation Congress, SME, Pittsburgh, 1997, pp. 191e197. [2] P.A. Hancock, I. Vasmatzidis, Effects of heat stress on cognitive performance: the current state of knowledge, Int. J. Hyperthermia 19 (2003) 355e372. [3] T. McMorris, J. Swain, M. Smith, J. Corbett, S. Delves, C. Sale, R.C. Harris, J. Potter, Heat stress, plasma concentrations of adrenaline, noradrenaline, 5hydroxytryptamine and cortisol, mood state and cognitive performance, Int. J. Psychophysiol. 61 (2006) 204e215. [4] L. Nybo, Exercise and heat stress: cerebral challenges and consequences, Prog. Brain Res. 162 (2007) 29e43. [5] S.S. Cheung, Neuromuscular response to exercise heat stress, Med. Sport. Sci. 53 (2008) 39e60. [6] A.M. Hunter, A. St Clair Gibson, Z. Mbambo, M.I. Lambert, T.D. Noakes, The effects of heat stress on neuromuscular activity during endurance exercise, Pflugers Arch. 444 (2002) 738e743. [7] S. Razmjou, Mental workload in heat: toward a framework for analyses of stress states, Aviat. Space Environ. Med. 67 (1996) 530e538. [8] Misaqi F.L, Inderberg J.G, Blumenstein P.D, Naiman T. Heat stress in hot U.S. Mines and Criteria for Standards for Mining in Hot Environments, edited by Mining Enforcement and Safety Administration USDotI: MESA, 1976. pp. 1e47. [9] N.E. Flomenbaum, L.R. Goldfrank, R.S. Hoffman, M.A. Howland, N.A. Lewin, L.S. Nelson, Goldfrank’s Toxicologic Emergencies, McGraw-Hill, 2006, 255e265. [10] R.C. Luginbuhl, L.L. Jackson, Heat-related deaths among crop workers e United States, Morb. Mortal Wkly. Rep. (MMWR) (1992e2006) 649e653. [11] M.A. Tuck, Personal cooling in hot workings, J. Mine. Vent. Soc. South Africa 53 (2000) 80e84. [12] S. Rissanen, I. Jousela, J.-R. Jeong, H. Rintamaki, Heat stress and bulkiness of chemical protective clothing impair performance of medical personnel in basic lifesaving tasks, Ergonomics 51 (2008) 1011e1022. [13] T. Bernard, C. Ashley, J. Trentacosta, V. Kapur, S. Tew, Critical heat stress evaluation of clothing ensembles with different levels of porosity, Ergonomics 53 (2010) 1048e1058.
[14] G. Havenith, E. den Hartog, S. Martini, Heat stress in chemical protective clothing: porosity and vapour resistance, Ergonomics 54 (2011) 497e507. [15] P.A. Bishop, S.A. Nunneley, S.H. Constable, Comparisons of air and liquid personal cooling for intermittent heavy work in moderate temperatures, Am. Ind. Hyg. Assoc. J. 52 (1991) 393e397. [16] A.D. Flouris, S.S. Cheung, Design and control optimization of microclimate liquid cooling systems underneath protective clothing, Ann. Biomed. Eng. 34 (2006) 359e372. [17] B. Zalba, J.M. Marin, L.F. Cabeza, H. Mehling, Review on thermal energy storage with phase change: materials, heat transfer analysis and applications, Appl. Therm. Eng. 23 (2003) 251e283. [18] I. Hunter, J.T. Hopkins, D.J. Casa, Warming up with an ice vest: core body temperature before and after cross-country racing, J. Athl. Training 41 (2006) 371e374. [19] W. Bendkowska, A. Bogdan, K. Kopias, M. Klonowska, Thermal manikin evaluation of microclimate cooling vests containing PCMs, in: ITC and DC: Book of Proceedings of the 4th International Textile, Clothing and Design Conference e Magic World of Textiles (2008). [20] S.E. Perez, B.J. Tooker, R. Nunez, Theoretical study of a personal cooling garment that utilizes natural evaporation, ASHRAE Trans. (1994). [21] M. Rothmaier, Design and performance of personal cooling garments based on three-layer laminates, Med. Biol. Eng. Compt. 46 (2008) 825e832. [22] D.Y. Lee, Y.S. Hwang, Subminiature cool pad applying sorption cooling effect, HVAC & R Res. 12 (3B) (2006) 797e806. [23] L.R. Grzyll, W.C. Balderson, Development of a man-portable microclimate absorption cooling device, in: Proceedings of the 32nd Intersociety Energy Conversion Engineering Conference (1997), pp. 1646e1651. [24] D.W. Sun, L. Zheng, Vacuum cooling for the food industry: a review of recent research advances, Trends Food Sci. Tech. 15 (2004) 555e568. [25] H.M. Ozturk, H.K. Ozturk, Effect of pressure on the vacuum cooling of iceberg lettuce, Int. J. Refrig. 32 (2009) 395e403. [26] N.T. Charles, D.W. Johnson, The occurrence and characterization of fouling during membrane evaporative cooling, J. Membr. Sci. 319 (2008) 44e53. [27] D.W. Johnson, C. Yavuzturk, J. Pruis, Analysis of heat and mass transfer phenomena in hollow fiber membranes used for evaporative cooling, J. Membr. Sci. 227 (2003) 159e171. [28] A. Chiari, Air humidification with membrane contactors: experimental and theoretical results, Int. J. Ambient Energy 21 (2000) 187e195. [29] A.C. Guyton, J.E. Hall, Textbook of Medical Physiology, W.B. Elsevier Saunder, Philadelphia, 2000. [30] C.R. Underwood, E.J. Ward, The solar radiation area of man, Ergonomics 9 (1966) 155e168.
Contents lists available at SciVerse ScienceDirect
Applied Thermal Engineering journal homepage: www.elsevier.com/locate/apthermeng
Man-portable personal cooling garment based on vacuum desiccant cooling Yifan Yang a, Jill Stapleton b, Barbara Thiané Diagne a, Glen P. Kenny b, Christopher Q. Lan a, * a b
Department of Chemical and Biological Engineering, University of Ottawa, 161 Louis Pasteur Private, Ottawa, Ontario, Canada K1N 6N5 Human and Environmental Physiology Research Unit, School of Human Kinetics, University of Ottawa, Ottawa, Ontario, Canada
a r t i c l e i n f o
a b s t r a c t
Article history: Received 30 January 2012 Accepted 5 April 2012 Available online 13 April 2012
A man-portable personal cooling garment based on the concept of vacuum desiccant cooling (VDC) was developed. It was demonstrated with cooling pads that a cooling capacity of 373.1 W/m2 could be achieved in an ambient environment of 37 C. Tests with human subjects wearing prototype cooling garments consisting of 12 VDC pads with an overall weight of 3.4 kg covering 0.4 m2 body surface indicate that the garment could maintain a core temperature substantially lower than the control when the workload was walking on a treadmill of 2% inclination at 3 mph. The exercise was carried out in an environment of 40 C and 50% relative humidity (RH) for 60 min. Tests also showed that the VDC garment could effectively reduce the metabolic heat accumulation in body with subject wearing heavily insulated nuclear, biological and chemical (NBC) suit working in the heat and allow the participant to work safely for 60 min, almost doubling the safe working time of the same participant when he wore NBC suit only. Ó 2012 Elsevier Ltd. All rights reserved.
Keywords: Evaporative cooling Vacuum cooling Absorption Membrane Personal cooling garment
1. Introduction Numerous occupations expose workers to hot environmental settings, where they are regularly subjected to excessive heat stress. In addition to reduced worker performance and productivity [1], heat stress may be an underlying cause for many workplace accidents and injuries, as it impairs mental alertness and concentration [2e4], motor dexterity and coordination [5], muscle fatigue [6,7]. The increased physical discomfort associated with elevated body temperature promote irritability, anger and other emotional states, which often cause workers to overlook safety procedures or to divert attention during hazardous tasks [8]. The tragic death of a United Kingdom soldier who died on duty when trying to dismantle an improvised explosive device in Afghanistan without wearing a heavily armoured bomb suit due to the extreme discomfort caused by heat stress in the high temperature highlighted the need of efficient personal cooling systems. Furthermore, exposure to excessive heat could cause severe symptoms that may threaten of the life of workers. In 2002, 1816 active American soldiers were reported injured from heat related causes [9]. From 1992 to 2006, 68 U.S. farm crop workers were reported to have passed away due to excessive heat stress [10]. Healthy humans regulate core temperature to maintain a near constant level (w37 C) regardless of environmental conditions. To
* Corresponding author. Tel.: þ1 613 562 5800x2050. E-mail address: [email protected] (C.Q. Lan). 1359-4311/$ e see front matter Ó 2012 Elsevier Ltd. All rights reserved. doi:10.1016/j.applthermaleng.2012.04.012
do this, a balance must be maintained between the heat produced within the body and the heat lost to the environment via a combination of dry heat exchange and evaporative heat loss. Ultimately, the ability to offset increases in core temperature during heat exposure depends on the ability of the body to dissipate heat through skin to the immediate microenvironment surrounding it. Many cooling technologies have been employed to manage heat stress in daily activities, which can be classified into space cooling and personal cooling according to the immediate subject of cooling. Space cooling is the cooling of the interior space of a building or a room and is implemented at a relatively large scale. In occasions space cooling is very costly, impractical, or even impossible. Such cases include the hot outdoor environments and large work spaces such as steel mills, foundries, mines [11] and metallurgy plants. In these occasions, personal cooling (PC), which is designed to cool the immediate surroundings of the wearer, is more practical and cost-effective. PC acts directly on the microenvironment of the individual and is also referred to as microclimate cooling (MC). In scenarios where impermeable protective clothing is required, personal cooling garment also serves as the most effectively means of heat stress management [12e14]. Three different types of PC garments have been developed in the last several decades: i) fluid cooled garments (FCGs), also known as refrigeration aided PC systems, ii) phase change material (PCM) cooling vests, and iii) evaporative PC devices, also known as evaporative cooled garments (ECGs). FCGs employ air, water or non-toxic aqueous solutions as the coolant and are generally recognized as the most efficient PC technologies at present [15].
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They can achieve continuous cooling at capacities ranging from 600 W to1000 W [16]. Nevertheless, they are limited by lack of portability, due to the requirement of refrigeration unit and continuous power supply. PCM systems employ phase change materials, which are substances having the ability to absorb substantial amount of latent heat when changing from solid to liquid state in a temperature range that is suitable for personal cooling [17]. PCM garments are portable but offer only very limited cooling capacities. Some commercial vests containing up to 5 lbs of PCMs have been reported to remain cool for about 2 h at a cooling capacity of up to 95 W [18,19], which is much lower than what is achieved with FCGs. ECGs harness the large latent heat of water evaporation for cooling [20e23]. The latent heat of water evaporation, i.e., around 2400 kJ/kg in the temperature range of 0e30 C, promises the potential of creating effective ECGs using a small amount of water. The main drawback of conventional ECGs is that they do not work well in highly humid environments due to small evaporation flux. The objective of this study is to evaluate the feasibility of a novel cooling technology, the vacuum desiccant cooling (VDC), for microclimate cooling. In contrast to conventional vacuum cooling that require continuous vacuuming throughout the cooling process, VDC requires vacuuming only for a short period of initialization and can operate detached from a vacuum pump for a prolonged period of time afterward. It was demonstrated in the study that a portable VDC garment could provide sufficient capacity for personal cooling. 2. Experimental methods 2.1. VDC pad The cross-sectional structure of a VDC pad is shown in Fig. 1A. In this study, cooling pads of two different sizes were fabricated. The larger ones (Type I) had the dimensions of 180 250 mm with an
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effective membrane area of 0.030 m2 (150 200 mm); and the smaller ones (Type II) had dimensions 140 190 mm with an active membrane area of 0.015 m2 (100 150 mm). The honeycomb spacers were 16 mm thick. A completed cooling pad was 25e30 mm thick, which included the additional thickness of a cooling core and an absorption core. As depicted in Fig. 1A, the VDC pad was composed of four major components, a cooling core, a spacer, an absorption core and an outer bag. The cooling core (Figs. 1B and 2A) was a hybrid bag containing an amount of water as specified in Table 1. The hybrid bag was made of a soft impermeable plastic water bag (Seal-aMealÒ, Sunbeam Product, Inc. Rye, NY, USA) with one side replaced with a semi-permeable Teflon (PTFE) membrane (Sterlitech, Kent, WA, US). A piece of soft cotton cloth (Towel III) was used to cover the membrane for protection from possible damage caused by the edges of honeycomb matrixes. A piece of perforated aluminum foil was used to cover the Towel III to minimize the radioactive heat transfer from the hot absorption core to the cold cooling core. The membrane was a microporous hydrophobic membrane (Teflon membrane) that had a pore size of 0.2 m, a thickness in the range of 0.061e0.081 mm, and a liquid entrance pressure of water (LEPw) of 4.1 bars. The perforated aluminum foil had approximately 20 holes per square centimetre (diameter of approximately 1 mm), was attached to the cloth. The spacer (Fig. 2B) was made of honeycomb, which served to separate the cooling core from the absorption core. The absorption core (Figs. 1B and 2C) was composed of two layers of cotton towel glued together with lithium chloride (LiCl) powder being sandwiched in the middle of them. And the outer bag (Figs. 1B and 2D) was made of a plastic bag connected to a piece of ¼" polyvinyl chloride (PVC) tubing of 150 mm in length. To assemble a functional VDC pad, the prepared cooling core and the spacer were put together with the spacer sitting on top of the cooling core. The pre-prepared absorption core, which was sealed
Fig. 1. A. Schematic diagram of the cross-sectional view of a VDC cooling pad. B. Schematic diagram of the cross-sectional view of absorption core and cooling core (drawing not in proportion).
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Fig. 2. Parts of a VDC pad. A) The cooling core (water bag); B) The spacer (two layers of honeycomb); C) The absorption core (desiccant holder); D) The outer bag with vacuum tubing; and E) The cooling core side of an AVMEC pad (food grade blue dye was added for detection of water leakage).
in a storage bag, was taken out of the bag and placed on top of the spacer. After the cooling core, spacer and absorption core were aligned with each other, an outer bag was used to envelop the three-layer structure and the opening of the outer bag was sealed air-tight quickly using a Seal-A-MealÒ VS107 sealer (Sunbeam Product, Inc. Rye, NY, USA). The cooling pad could then be initialized using a high performance vacuum pump to start the cooling process. The cooling core side of an initialized VDC pad is shown in Fig. 2E. Blue dye was added into water for the purpose of detecting water leakage. The dotted pattern was generated because the soft cooling core was pushed against the hard spacer by vacuum and therefore revealed the pattern of the honeycomb matrix.
Cole-Parmer, Montreal, QC, Canada), which could deliver a maximum vacuum degree of 29.0 In Hg (corresponding to an absolute pressure of 3.048 kPa) and a free air capacity of 32.5 L/min at room temperature. The vacuum pump was turned on for 5 min to achieve the highest vacuum degree, after which the vacuum tubing of the pad was sealed and disconnected from the pump. The disconnected pad was then put in an incubator (MaxQ 5000, GENEQ Inc, Montreal, QC, Canada) maintained at a constant temperature of 37 C. Temperature of the surface of the absorption core and that of the surface of the cooling core were monitored by taping the probe of an Oakton Temp-300 dual-input type K thermocouple (Cole-Parmer, Montreal, QC, Canada) onto each surface. The mean evaporation flux was estimated by the following equation:
2.2. Cooling (VDC) pad experimental setup and procedures
J ¼
Type I cooling pad (Table 1) was used in cooling pad experiments to evaluate their performance. The cooling pad was first connected to a high performance vacuum pump (WZ-07061-11,
where Mi is the initial water mass (g), Mf the final water mass at the end of test (g); A the area for evaporation, i.e., the effective membrane area of the cooling core (m2) and t the testing time (h).
Mi Mf
. At
(1)
Table 1 Summary of some key features of structural components for VDC pad Type I and Type II. Pad
Component
Description
Dimensions (mm)
Weight (g)
Type I
Cooling Core Absorption Core
Hybrid bag containing 100 g water Two layers of cotton towels sealed into a bag with 30 g LiCl powder spreading evenly inside Polypropylene honeycomb (two-layer) Multi-layered PA/PE (Polyamide/Polyethylene) bag
180 250 Same as above
121 1.0 95 1.0
150 200 16 240 300
54 32 332 62.5
Type II
Spacer Outer Bag Subtotal Cooling Core Absorption Core
Spacer Outer bag Subtotal
Hybrid bag containing 50 g distilled water Two layers of cotton towels sealed into a bag with 15 g LiCl powder spreading evenly inside Polypropylene honeycomb (two-layer) Multi-layered PA/PE (Polyamide/Polyethylene) bag
140 190 Same as above
100 150 16 185 p 540
1.0 1.0 1.0 g 1.0
55 1.0
30 1.0 42 1.0 210 1.0 g
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Assume that the sensible heat change of water and the internal heat exchange between the cooling core and the absorption core are negligible, the cooling capacity of a VDC pad can be estimated as the rate of latent heat using the following equation,
Q ¼ JADHev
(2)
Where J is the average evaporation flux, A the area of evaporation (i.e., the effective membrane area) of the pad, and DHev the latent heat of water evaporation, which is taken as a constant of 2400 J/g in the experimental temperature range. 2.3. Prototype VDC garment The VDC garment prototype was composed of 4 Type I and 8 Type II VDC pads attached to a XL T-shirt (Ergodyne co., St. Paul, MN, USA). The sleeves of the T-shirt were cut off from the shoulders. As shown in Fig. 3, there were two Type I pads at the upper position (the chest or the upper back) and four Type II cooling pads at the lower part (the abdomen and the lower back) on each side of the T-shirt. Each cooling pad was attached to the T-shirt using four Velcro sets. The total weight of a VDC garment is 3.4 kg. The total area of VDC pads was 0.4 m2 with a total effective membrane area of 0.24 m2. All human experimental trials were carried out with a 22 year old male (170 cm) weighing87 kg. Pictures of the subject dressed in the cooling garment are shown in Fig. 4. During the experimental trials, the participant was required to perform 60-min of treadmill walking at 3 mph and 2% incline in an environmental chamber regulated at 40 C and 50% RH (relative humidity) while wearing : 1) a cotton T-shirt and shorts (Control); 2) the VDC garment on top of a cotton T-shirt and shorts (VDC); 3) a Nuclear Biological Chemical (NBC) protective garment on top of a cotton T-shirt and shorts; 4) an NBC on top of a Climatech CM2000 ice vest (Clima Tech Safety, White Stone, VA, US) plus a cotton T-shirt and shorts (ice-pad/NBC); and 5) NBC on top of VDC garment plus cotton Tshirt and shorts (VDC/NBC). For all the trials, core temperature was estimated by measuring rectal temperature using a paediatric thermocouple probe (Mon-atherm General Purpose Temperature Probe, Mallinckrodt Medical,
Fig. 4. Participant wearing a prototype VDC cooling garment
St-Louis, MO, USA) inserted to a minimum of 12 cm past the sphincter. 3. Results 3.1. Performance of VDC pad Fig. 5 presents the surface temperature profiles for the cooling core and absorption core in a typical cooling pad experiment.
Fig. 3. Arrangement of the six cooling packs on one piece of the cooling garment. The entire garment consisted of two pieces, the front piece and the back piece, which had the same pad arrangement.
Fig. 5. Temperature profiles of the surface of the cooling core (Tc) and that of the absorption core (Tad). Initially, the cooling pad contained 100 g water in the cooling core and 30 g LiCl powder in the absorption core. Experiments were carried out at the maximum vacuum achievable by the vacuum pump (28.8 In Hg or 3.725 kPa) at a 37 C environment inside an incubator. The vacuum remained constant after the 5-min initialization period.
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Table 2 Summary of VDC pad performance testing results Initial cooling core surface temperature ( C) Final cooling core surface temperature ( C) Temperature difference for cooling core ( C) Evaporated water (g) Effective membrane area (m2) Average evaporation flux (J, g/h m2) (Eq. 1) Estimated cooling capacity (Q, W/m2) (Eq. 2)
22.7 20.7 2.0 16.8 0.03 560 373.3
0.05 0.05 0.05 0.05 0.0001 50 38
Mean values of two trials. Experiments were carried out using Type I VDC pads.
During the initialization stage, the cooling core surface temperature decreased from the starting temperature of 22.7e16.9 C at 5 min, and reached the lowest temperature of 16.0 C within 10 min. Water temperature started to increase afterward and reached 20.7 C at the end of the 60-min test. This temperature was still sufficient to maintain efficient heat absorption from the 37 C environment given that the temperature difference between the water in the cooling core surface and the environment was still 16.3 C. On the absorption core surface, the initial temperature was 33.5 C, which reached 41.5 C at the end of the 5-min initialization stage and rose to 47.3 C at the end of the test. It is worth mentioning that the initial temperature of the cooling core surface was always lower and that of the absorption core surface always higher than the room temperature because water evaporation (endothermic) and vapour absorption (exothermic) took place in the pad fabrication process, although at very low rates. As shown in Table 2, the total mass of water evaporation during the 60-min test was 16.8 0.05 g through an evaporation surface of 0.03 m2, corresponding to an average evaporation flux of 560 50 g/h m2. It should be noted that the evaporation fluxes at different time points varied with time because important parameters such as water temperature and extent of desiccant saturation changed with time in the cooling process. The average flux is however a necessary approximation as we did not have the capacity to measure real time water evaporation without damaging the cooling pad. The cooling capacity of the VDC pad was calculated to be 373.3 38 W/m2 using Eq. 2. This value is calculated from the average evaporation flux and therefore represents the mean cooling capacity of the pad during the experimental period. 3.2. Performance of VDC garment during exercise in the heat
As shown in Fig. 6, the core temperature of the control increased from a baseline resting value of 37.1e37.8 C (or 0.7 C above resting levels) at mid-point of exercise (i.e., 30 min) and continued to increase, albiet at a lower rate up, for the duration of exercise. Endexercise core temperature achieved an elevated state of hyperthermia of 38.1 C, an equivalent increase of 1.1 C above the baseline. In sharp contrast, for the VDC test, the core temperature increased by only 0.2 C above the baseline resting values (37.3 C) after 30 min of exercise. The core temperature increased 0.5 C to reach 37.8 C by the end of the 60 min of exercise when the subject wore VDC garment. The much lower core temperature increase in the VDC test (0.2 C at 30 min and 0.8 C at 60 min) than that of the control (0.7 at 30 min and 1.1 C at 60 min) indicates a much lower change in body heat content and therefore body heat storage during work performed in the heat. 3.3. Effectiveness of the VDC garment vs. ice-cooling vest on reducing the level of thermal strain during work performed with the NBC protective garment The effectiveness of the VDC garment in reducing the level of thermal strain while wearing heavily insulated protective garments (NBC suit) was assessed by comparing core temperature responses during exercise in the heat with a commercially available icecooling vest (Climatech CM2000, Clima Tech Safety, White Stone, VA, US). The weight of the ice vest was 4.1 kg. As shown in Fig. 7, the NBC trial had to be terminated at w32 min as rectal temperature of the subject was approaching 38.5 C. Whereas the NBC trial was terminated for health and safety precautions at 32.0 min into exercise, core body temperature during the Ice-Pad/NBC trial for the same time point was only 38.0 C, reaching a similar elevation as that of the NBC trial only after the full 60 min of exercise. In other words, the icecooling vest increased exposure time by 87.5% when the subject wore NBC suit wherein core temperature increased at a reduced rate. Furthermore, the VDC garment was more effective than the commercial ice/pad garment at attenuating the increase in core body temperature, and therefore body heat storage, as evidenced by the core body temperature profiles. The core temperature increased from a baseline resting value of 37.1e38.1 C at the end of the 60-min trial, corresponding to a core temperature increase of 1.0 C, 25% lower than the 1.25 C core temperature elevation experienced in the Ice-Pad/NBC trial (from 37.15e38.4 C).
As noted above, the human experimental trials were undertaken to evaluate the performance of the VDC garments. The data are presented in Fig. 6.
Fig. 6. Core temperature profiles during the 60-min exercise performed in the heat when the participant wore 1) shorts only (Control) and 2) shorts and VDC (VCD).
Fig. 7. Core temperature response during exercise in the heat for the Control, NBC, IcePad/NBC, and VDC/NBC conditions.
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4. Discussion 4.1. Unpowered vacuum desiccant cooling (VDC) The VDC is in essence a desiccant cooling device enhanced by the vacuum gap that separates the absorption and cooling cores. Desiccant cooling was proposed previously in developing a manportable microclimate cooling device [23] and a subminiature cooling pad [22]. In these designs, however, the vacuum was not involved, leading to rather mild cooling capacities. The function of the vacuum gap in VDC is twofold: 1) enhancing water evaporation by facilitating vapour diffusion from the water surface to the desiccant surface; and 2) enhancing thermal insulation between the absorption core and the cooling core to prevent the heat released from vapour absorption in the absorption core from being transferred to the cooling water in the cooling core. The VDC can also be viewed as a type of vacuum cooling, which is a rapid cooling technology that has been widely employed in industrial applications such as food processing and storage [24,25]. Conventional vacuum cooling requires a high performance vacuum pump and continuous power supply to maintain the driving force for evaporation during the entire cooling process, which greatly reduces the portability of the system and therefore is not suitable for personal cooling. In the VDC, however, the vacuum pump was required only for a short initialization period (w5 min for a cooling pad) during which vacuum was generated. Maintenance of vacuum in the process was achieved by vapour absorption by desiccant in the absorption core. As a result, the VDC could be effectively regarded as an unpowered cooling technology that allows cooling, once initiated, to continue for a prolonged period of time without requiring power supply. LiCl was selected as the desiccant for the VDC packs and garments because of its large vapour absorption capacity. In the experiments, LiCl powders were used as the starting material and it was converted to aqueous solution when sufficient amount of vapour was absorbed. The resulted concentrate aqueous LiCl solution could also absorb vapour. The powder and aqueous LiCl solution were held in place by the cotton towels in the absorption core and prevented from leaking into the vacuum gap by the semipermeable membrane. Regeneration of the desiccant could be achieved by drying the LiCl soaked towels in heat. A key feature of the VDC pad was the use of the hydrophobic membrane. The membrane had a liquid entrance pressure of water (LEPw) of 3.0 bar, which was much larger than atmospheric pressure (1.0135 bar), the highest trans-membrane pressure could occur in a VDC pad. LEPw is by definition the trans-membrane pressure at which water would enter the pores of a membrane (i.e., wetting). In other words, the Teflon membrane was capable of retaining liquid water but allowing the selective passage of vapour due to its large LEPw. As a result, it provided a semi-permeable barrier that prevented the liquid water in the cooling core from entering the vacuum gap or the absorption core while allowing water evaporation to occur continuously. Hydrophobic membrane has long been employed in different evaporative cooling systems to provide the barrier that separates the liquid water and the vapour, or in other words, to provide the surface for water evaporation [26,27]. These membranes evaporative cooling processes, however, typically involve sweep gas evaporation rather than vacuum evaporation and are designed for air conditioning or air humidification [27,28]. The VDC technology integrates vacuum cooling, desiccant cooling, and membrane technology to create a novel device that is independent of ambient conditions, could provide sufficient cooling capacity, and is sufficiently lightweight for specialty applications such as personal cooling. It provides major advantages
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including: 1) employing the large latent heat of water evaporation (approximately 2400 kJ/kg, more than 7 times of the latent heat of ice fusion) for cooling; 2) large evaporation flux which is independent of ambient humidity due to the existence of a vacuum gap; 3) no power supply or heavy equipment required except in the short initialization period; 4) water as coolant is non-toxic, cheap, and easily available for recharging. 4.2. VDC garment for personal cooling The results in Fig. 6 demonstrate that the VDC garment could substantially mitigate the metabolic heat accumulation in the body, reduce the rate of core body temperature increase and therefore greatly extend the safe working period for workers in heat. The results shown in Fig. 7 suggest that the VDC cooling garment was more efficient than the commercial ice-pad vest in attenuating the increase core body temperature thereby ensuring a longer safe working period under the specified conditions. Furthermore, these results demonstrate that VDC could effective alleviate the heat stress for workers in heat wearing heavily insulated NBC suit. It is interesting to notice that the body temperature profile of the VDC/NBC trial, as shown in Fig. 7, increased almost linearly after 10 min in exercise. In contrast, while the core temperature profile of the Control was comparable with that of the VDC/NBC trial in the first 10 min into the exercise, it had a larger slope in the 10e23 min period and a smaller slope in the period after the 23-min point, indicating a higher rate of temperature increase in the first period and a lower one in the second. This phenomenon can be explained by the fact that the participant was completely covered in the VDC/ NBC trial. Consequently, the body depended solely on sensible heat transfer for heat dissipation. On the contrary, in the Control trial, both sensible heat transfer and sweat evaporation were available for heat dissipation and sweating became more dominant in the later stage of exercise when core body temperature was at an elevated level. The energy expenditure is typically 290 kcal/h (alias 337.3 W) for a 70 kg man in active exercise. Since the oxidation efficiency of aerobic metabolism is approximately 30% [29], it is reasonable to assume 10% overall metabolic efficiency, i.e., 90% of the energy expenditure is converted to metabolic heat, which leads to a metabolic heat production rate of approximately 304 W. The average body surface of a 1.65 m adult weighing 73 kg is 1.8 m2 [30]. Assuming that the total effective cooling area of a human body is 1 m2, the desired cooling capacity is therefore 304 W/m2 or above. A cooling capacity of 373.1 W/m2 was achieved with the VDC pad, indicating that it could provide sufficient capacity for personal cooling during heat stress conditions. This is partially confirmed by the experimental results with the prototype VDC garment. Despite that the total cooling surface of the VDC garment was only 0.4 m2, the garment was able to significantly alleviate metabolic heat accumulation in the body of the participant. 5. Conclusions A man-portable cooling garment based on VDC technology was developed. The VDC cooling pads were estimated to have a cooling capacity of 373.1 W/m2 in an environment of 37 C and 50% RH. Experiments with prototype VDC garments weighing approximately 3.4 kg and covering 0.4 m2 body surface demonstrated that these garments could deliver effective heat stress mitigation to humans working in the heat. Further improvement in the design of cooling pad and cooling garment are expected to greatly improve the reliability, cooling capacity, work duration, and costeffectiveness and therefore pave the way to its commercialization.
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Acknowledgements Financial support from the Natural Science and Engineering Research Council of Canada (NSERC) and Canadian Institute of Health Research (CIHR) are gratefully acknowledged. References [1] M.J. Howes, C.A. Nixon, Development of procedures for safe working in hot conditions, in: R.V. Ramani (Ed.), Proceedings of the 6th International Mine Ventilation Congress, SME, Pittsburgh, 1997, pp. 191e197. [2] P.A. Hancock, I. Vasmatzidis, Effects of heat stress on cognitive performance: the current state of knowledge, Int. J. Hyperthermia 19 (2003) 355e372. [3] T. McMorris, J. Swain, M. Smith, J. Corbett, S. Delves, C. Sale, R.C. Harris, J. Potter, Heat stress, plasma concentrations of adrenaline, noradrenaline, 5hydroxytryptamine and cortisol, mood state and cognitive performance, Int. J. Psychophysiol. 61 (2006) 204e215. [4] L. Nybo, Exercise and heat stress: cerebral challenges and consequences, Prog. Brain Res. 162 (2007) 29e43. [5] S.S. Cheung, Neuromuscular response to exercise heat stress, Med. Sport. Sci. 53 (2008) 39e60. [6] A.M. Hunter, A. St Clair Gibson, Z. Mbambo, M.I. Lambert, T.D. Noakes, The effects of heat stress on neuromuscular activity during endurance exercise, Pflugers Arch. 444 (2002) 738e743. [7] S. Razmjou, Mental workload in heat: toward a framework for analyses of stress states, Aviat. Space Environ. Med. 67 (1996) 530e538. [8] Misaqi F.L, Inderberg J.G, Blumenstein P.D, Naiman T. Heat stress in hot U.S. Mines and Criteria for Standards for Mining in Hot Environments, edited by Mining Enforcement and Safety Administration USDotI: MESA, 1976. pp. 1e47. [9] N.E. Flomenbaum, L.R. Goldfrank, R.S. Hoffman, M.A. Howland, N.A. Lewin, L.S. Nelson, Goldfrank’s Toxicologic Emergencies, McGraw-Hill, 2006, 255e265. [10] R.C. Luginbuhl, L.L. Jackson, Heat-related deaths among crop workers e United States, Morb. Mortal Wkly. Rep. (MMWR) (1992e2006) 649e653. [11] M.A. Tuck, Personal cooling in hot workings, J. Mine. Vent. Soc. South Africa 53 (2000) 80e84. [12] S. Rissanen, I. Jousela, J.-R. Jeong, H. Rintamaki, Heat stress and bulkiness of chemical protective clothing impair performance of medical personnel in basic lifesaving tasks, Ergonomics 51 (2008) 1011e1022. [13] T. Bernard, C. Ashley, J. Trentacosta, V. Kapur, S. Tew, Critical heat stress evaluation of clothing ensembles with different levels of porosity, Ergonomics 53 (2010) 1048e1058.
[14] G. Havenith, E. den Hartog, S. Martini, Heat stress in chemical protective clothing: porosity and vapour resistance, Ergonomics 54 (2011) 497e507. [15] P.A. Bishop, S.A. Nunneley, S.H. Constable, Comparisons of air and liquid personal cooling for intermittent heavy work in moderate temperatures, Am. Ind. Hyg. Assoc. J. 52 (1991) 393e397. [16] A.D. Flouris, S.S. Cheung, Design and control optimization of microclimate liquid cooling systems underneath protective clothing, Ann. Biomed. Eng. 34 (2006) 359e372. [17] B. Zalba, J.M. Marin, L.F. Cabeza, H. Mehling, Review on thermal energy storage with phase change: materials, heat transfer analysis and applications, Appl. Therm. Eng. 23 (2003) 251e283. [18] I. Hunter, J.T. Hopkins, D.J. Casa, Warming up with an ice vest: core body temperature before and after cross-country racing, J. Athl. Training 41 (2006) 371e374. [19] W. Bendkowska, A. Bogdan, K. Kopias, M. Klonowska, Thermal manikin evaluation of microclimate cooling vests containing PCMs, in: ITC and DC: Book of Proceedings of the 4th International Textile, Clothing and Design Conference e Magic World of Textiles (2008). [20] S.E. Perez, B.J. Tooker, R. Nunez, Theoretical study of a personal cooling garment that utilizes natural evaporation, ASHRAE Trans. (1994). [21] M. Rothmaier, Design and performance of personal cooling garments based on three-layer laminates, Med. Biol. Eng. Compt. 46 (2008) 825e832. [22] D.Y. Lee, Y.S. Hwang, Subminiature cool pad applying sorption cooling effect, HVAC & R Res. 12 (3B) (2006) 797e806. [23] L.R. Grzyll, W.C. Balderson, Development of a man-portable microclimate absorption cooling device, in: Proceedings of the 32nd Intersociety Energy Conversion Engineering Conference (1997), pp. 1646e1651. [24] D.W. Sun, L. Zheng, Vacuum cooling for the food industry: a review of recent research advances, Trends Food Sci. Tech. 15 (2004) 555e568. [25] H.M. Ozturk, H.K. Ozturk, Effect of pressure on the vacuum cooling of iceberg lettuce, Int. J. Refrig. 32 (2009) 395e403. [26] N.T. Charles, D.W. Johnson, The occurrence and characterization of fouling during membrane evaporative cooling, J. Membr. Sci. 319 (2008) 44e53. [27] D.W. Johnson, C. Yavuzturk, J. Pruis, Analysis of heat and mass transfer phenomena in hollow fiber membranes used for evaporative cooling, J. Membr. Sci. 227 (2003) 159e171. [28] A. Chiari, Air humidification with membrane contactors: experimental and theoretical results, Int. J. Ambient Energy 21 (2000) 187e195. [29] A.C. Guyton, J.E. Hall, Textbook of Medical Physiology, W.B. Elsevier Saunder, Philadelphia, 2000. [30] C.R. Underwood, E.J. Ward, The solar radiation area of man, Ergonomics 9 (1966) 155e168.