Evaluation of a hybrid personal cooling system using a manikin operated in constant temperature mode and thermoregulatory model control mode in warm conditions

Abstract

In this study, the cooling effect of a portable hybrid personal cooling system (PCS) was investigated on a sweating manikin operated in the constant temperature (CT) mode and the thermoregulatory model control (TMC) mode. Both dry (i.e., no sweating) and wet manikin tests (i.e., sweating) were performed in the CT mode in a warm condition (30℃, 47% relative humidity (RH), air velocity v_a = 0.4 m/s). For the TMC mode, two case studies were simulated: light work condition (30℃, 47% RH, air velocity v_a = 0.15 m/s, duration: 60 min, metabolic rate: 1.5 METs) and construction work condition (30℃, 47% RH, v_a = 1.0 m/s, 40 min exercise [5.5 METs] and 20 min rest [1.2 METs]). Four test scenarios were selected: fans off with no phase change materials (PCMs) (i.e., Fan-off, the Control), fans on with no PCMs (i.e., Fan-on), fans off with fully solidified PCMs (i.e., PCM+Fan-off) and fans on with fully solidified PCMs (i.e., PCM+Fan-on). Under the dry condition, the cooling rate in PCM+Fan-off during the initial stage (e.g., 55 and 50 W for the first 15 min and 20 min, respectively) was higher than that in Fan-on (i.e., 45 ± 1 W); under the wet condition, the cooling rate in PCM+Fan-off (e.g., 45 W for 10 min) was much lower than that in Fan-on (i.e., 282 ± 1 W). The hybrid PCS (i.e., PCM+Fan-on) provided a continuous strong cooling effect. Simulation results indicated that ventilation fans or PCMs alone could provide sufficient cooling while doing light work. For the intensive work condition, the PCS in all three scenarios (i.e., PCM+Fan-off, Fan-on and PCM+Fan-on) exhibited beneficial cooling, and the hybrid PCS showed an optimized performance in alleviating heat strain during both exercise and recovery periods. It was thus concluded that the PCS could effectively remove body heat in warm conditions for moderate intensive activities.

Keywords

personal cooling system phase change materials ventilation fans cooling effect

Heat stress has been considered as one of the main threats to the health and safety of occupational workers.¹ The magnitude of heat stress depends upon environmental factors, clothing types and human activity levels. In particular, for people who are wearing protective clothing and/or performing physically demanding work in hot environments, uncompensable heat stress can rapidly increase the body heat storage and thereby they can develop heat-related illnesses and it can eventually cause deaths.^1,2 The best way to tackle the heat stress incidents is prevention. Developing a personal cooling system (PCS) has been regarded as one of the feasible methods to fight against heat stress incidents.³ Over the past few decades, many types of PCSs have been developed to alleviate body heat stress while working in warm and hot environments, for example cooling vests with ice packs or phase change material (PCM) packs or frozen gel strips,^4–9 garments with liquid cooling systems^10,11 and garments with forced air ventilation.^12–16 Liquid cooling tubed garments (LCGs) provided the best effect in dissipating extra body heat.¹⁷ However, the bulky and heavy LCGs require external pumps, heat exchangers or power sources and, hence, they may severely restrict the user’s mobility and compromise their cooling effectiveness.¹⁸

Different cooling systems cool the human body through different heat transfer mechanisms. For example, PCMs, frozen gel strips, cold air cooling tubes and liquid cooling tubes cool the body mainly through conduction. In contrast, ventilation fans cool the human body through both convection (if the ambient temperature is lower than the skin temperature) and sweat evaporation. The PCMs may not be effective in all environmental conditions due to the limited conductive heat transfer and the weight concerns. Similarly, ventilation fans might not provide effective convective and evaporative heat dissipation in hot humid conditions, especially when the human skin surface is dry at the initial working period.¹⁹ The combination of two or more cooling strategies can provide extra cooling pathways and thus may dramatically reduce thermal strain.

A few hybrid PCSs combined with both air and liquid cooling techniques were developed.^11,20–22 Cadarette et al.²⁰ and DeCristofano et al.²¹ compared the cooling performance of an air–liquid hybrid system with air- and liquid-cooled systems while doing high-intensity exercise in hot environments. It was found that the hybrid air–liquid vest in the liquid cooled mode showed significantly greater thermal strain than the air-cooled system. However, no investigation was made on the hybrid air–liquid system in both air and liquid cooled mode. Kim et al.¹¹ found that the hybrid liquid and air ventilation cooling significantly reduced physiological strain and improved performance time in a hot environment (i.e., 35℃, 50% relative humidity (RH)). A hybrid cooling uniform for construction workers was developed based on the PCMs and air ventilation fans.²³ However, its cooling efficiency has not been extensively investigated yet. More recently, a novel hybrid PCS incorporated with PCMs and air ventilation fans was developed and evaluated by a ‘Newton’ sweating manikin in two hot conditions.¹⁹ It was found that the hybrid PCS could provide effective cooling power in hot environments. Nevertheless, the efficiency of the hybrid PCS during light and strenuous work in warm conditions was unclear.

In this study, the cooling effect of a hybrid PCS was evaluated on a ‘Newton’ thermal manikin operated in the constant temperature (CT) mode and the TMC mode in warm conditions. It was hypothesized that the hybrid PCS could effectively alleviate heat strain in different working environments.

Methodology

Hybrid personal cooling system

A hybrid PCS, including a jacket and long pants incorporated with PCMs and ventilation fans, was engineered (see Figure 1). The details of the hybrid PCS can be found in Lu et al.¹⁹ The PCS was equipped with four small ventilation fans and 24 PCM packs. Two ventilation fans were located at the lateral lower back regions, whereas the remaining two were fixed at the lateral pelvis regions. The bottom hems of the jacket and the waist of the pants were sewn with elastic straps. Twenty-four PCM packs were placed at the chest, back, upper arms and front thigh regions. The PCS was designed to fit the body tightly at the body areas where the PCM packages were located, and special ventilation pathways were designed to optimize the airflow circulation within the clothing microclimate. The main ingredients of the PCMs are sodium sulfate decahydrate (Na₂SO₄·10H₂O) and additives. The melting temperature (T_m) of the PCMs is 21℃ and the latent heat of fusion is 144 J/g. The total mass of the PCM packs is 2.06 kg. The total latent heat of the 24 PCM packs is 297.2 kJ. All PCM packs were kept in a refrigerator at about 6.0℃ for at least 12 h for solidification. A polyester T-shirt and a single set of briefs were worn under the PCS.

Figure 1.

The newly developed hybrid personal cooling system.

Thermal manikin

A 34-segment Newton thermal manikin (Thermetrics LLC, Seattle, WA, USA) was used. Its segmental surface temperature can be individually controlled, and the segmental heat loss can be recorded by ThermDAC® software (version 8.2.26.0). In this study, both the CT mode and the TMC mode were used to evaluate the cooling effect of the PCS in warm conditions. The total surface area of the manikin is 1.697 m². For the CT mode, the surface temperatures of all segments were set to 34.0℃. For the TMC mode, the manikin was controlled using the ManikinPC² program developed by ThermoAnalytics Inc. (Calumet, MI). The manikin was first preheated to a thermal neutral condition. The defined manikin segmental temperatures under the thermoneutral condition are shown in Table 1. Once the defined thermal neutral condition was reached, the TMC mode was immediately activated. The manikin surface temperature and the sweating rate were controlled in analog to the prediction of the thermoregulatory model based on the Fiala model.^24,25 The real-time heat flux observed from the manikin was used as the feedback to the thermoregulatory model to determine physiological prediction outputs for the next time interval.²⁵ All the predicted subjective perceptions were calculated based on the Berkeley thermal comfort model,²⁶ using a nine-point thermal sensation scale (ranges from –4 to +4): ‘–4’ corresponds to a rating of very cold, ‘–3’ cold, ‘–2’ cool, ‘–1’ slightly cool, ‘0’ neutral, ‘1’ slightly warm, ‘2’ warm, ‘3’ hot and ‘4’ very hot. The predicted mean skin temperature, T_sk, the hypothalamic temperature, T_hy, and overall thermal sensations were recorded.

Table 1.

Manikin segmental temperatures under the defined thermoneutral condition.

Segments	Temperature (^oC)	Segments	Temperature (^oC)
Face	35.50	Waist	34.49
Head	35.24	Low back	34.70
Right forearm front	33.48	Right pelvis front	34.25
Right forearm back	33.48	Right pelvis back	34.81
Left forearm front	33.32	Left pelvis front	33.91
Left forearm back	33.32	Left pelvis back	35.00
Right upper arm front	33.27	Right thigh front	33.86
Right upper arm back	33.27	Right thigh back	33.95
Left upper arm front	33.33	Left thigh front	33.79
Left upper arm back	33.33	Left thigh back	34.05
Right hand	34.69	Right shin	33.78
Left hand	34.58	Right calf	33.66
Upper chest	34.79	Left shin	34.10
Shoulders	34.85	Left calf	33.49
Stomach	34.48	Right foot	30.73
Mid back	34.70	Left foot	30.77

Test protocol and test conditions

All manikin tests were performed in a climatic chamber, where the air temperature and RH were 30 ± 0.5℃ and 47 ± 5% RH (i.e., the partial water vapor pressure: 2.0 kPa), respectively. For all manikin tests operated in the CT mode, the v_a was set to 0.4 ± 0.1 m/s. The CT tests were performed under both dry condition (i.e., no sweating) and wet condition (continuous sweating). For wet tests, the sweating rate was set at 1200 ml/h·m² to simulate the human body in a heavy sweating condition, and the water flow was heated up to 34.0℃. Segmental heat losses under the four test scenarios (i.e., Fan-off, Fan-on, PCM+Fan-off, PCM+Fan-on) were recorded at 1 min intervals. The test duration of Fan-off and Fan-on was 3 h, whereas the test duration of PCM+Fan-off and PCM+Fan-off was set to 7 h so as to ensure the total area-weighted heat loss of the hybrid PCS was reached at equilibrium at the end of the test, For the TMC mode, two case studies were simulated, that is, Case 1: light work condition (air velocity v_a = 0.15 m/s, metabolic rate = 1.5 METs, simulation duration: 60 min) and Case 2: construction work condition (v_a = 1.0 m/s, metabolic rate = 5.5 METs (40 min) and 1.2 METs (20 min), total duration: 60 min). Prior to performing the simulation, the thermal manikin was heated up to reach the defined thermal neutral status, and the simulation mode was activated immediately once the thermal neutral condition was reached. The mean skin surface temperature, T_sk, the hypothalamic temperature, T_hy, and the overall thermal sensations under the four test scenarios were reported at 1 min intervals.

Calculations

The area-weighted total heat loss was reported. To evaluate the cooling effect, the cooling power of the PCS (i.e., the total power supply to the manikin while wearing the PCS minus the manikin power when no cooling was presented) in Fan-on and PCM+Fan-on was calculated by deducting the mean steady-state heat loss in Fan-off (i.e., the Control) from the total heat loss observed in Fan-on and PCM+Fan-on. Similarly, the cooling power of the PCS in PCM+Fan-off was determined by deducting the steady-state heat loss in PCM+Fan-off from the total heat loss in PCM+Fan-off. The cooling effect of PCMs in the PCM+Fan-off and PCM+Fan-on scenarios was calculated by deducting the mean steady-state heat loss in PCM+Fan-off and PCM+Fan-on from the total heat loss in PCM+Fan-off and PCM+Fan-on, respectively.

Statistical analysis

The heat loss and the cooling power of the PCS in four difference test scenarios at different time points were analyzed by Paired Samples t-tests using SPSS v.20.0 software (IBM Corp., Armonk, NY, USA). The significance level was set at p < 0.05.

Results

Dry heat loss (the CT mode)

In the dry condition, the observed heat losses under the four different test scenarios are as displayed in Figure 2. Generally, the heat loss of Fan-on was significantly higher than that of Fan-off (i.e., heat loss of Fan-off versus Fan-on was 25 ± 1 and 52 ± 1 W/m², respectively) (p = 0.00). The addition of PCMs caused a higher body heat loss at the initial testing stage and it gradually decreased to a steady state. There was no significant difference in the observed total heat loss between PCM+Fan-on and Fan-on after 170 min. Similarly, no significant difference in the total heat loss was found between PCM+Fan-off and Fan-off after about 283 min.

Figure 2.

Total observed heat losses under four test scenarios in the dry condition.

Figure 3 shows the cooling power of the PCS during the time course in the three test scenarios (i.e., Fan-on, PCM+Fan-off, PCM+Fan-on). It was clear that the PCM+Fan-on and Fan-on exhibited a much higher cooling power than that of PCM+Fan-off. For PCM+Fan-on, the total cooling power ranged from 42 to 122 W, and the mean cooling power in Fan-on was 45 ± 2 W. The PCS in PCM+Fan-off lost its cooling power after about 283 min. It indicated that ventilation fans were more effective in body cooling than PCMs in the studied dry condition in terms of cooling duration and magnitude. It was also obvious that the cooling duration of PCMs was shorter when the fans were turned on (i.e., the cooling durations of PCM+Fan-off and PCM+Fan-on were 283 and 170 min, respectively).

Figure 3.

The cooling power of the personal cooling system in phase change material (PCM)+Fan-off, Fan-on and PCM+Fan-on in the dry condition.

Heat loss in the wet condition (the CT mode)

Figure 4 shows the total observed heat losses of the four different test scenarios in the wet condition. The observed total heat loss in Fan-off and Fan-on was 167 ± 1 and 333 ± 3 W/m², respectively. The PCMs provided a 110 and a 10 min cooling duration in PCM+Fan-off and PCM+Fan-on, respectively. There was no significant difference in the total heat loss between Fan-off and PCM+Fan-off when the PCMs were completely melted (p > 0.05). Similarly, there was no significant difference in the observed heat loss between Fan-on and PCM+Fan-on when PCMs were fully melted (p > 0.05).

Figure 4.

Total observed body heat losses under the four testing scenarios in the wet condition.

Figure 5 presents the cooling power of the three test scenarios (i.e., Fan-on, PCM+Fan-on and PCM+ Fan-off) in the wet condition. The PCM+Fan-on and Fan-on continuously cooled the body, for example the observed total cooling power ranged from 264 to 315 W in PCM+Fan-on, and the mean cooling power in Fan-on was 282 ± 5 W. With regard to PCM+Fan-off, it could only cool the whole body at the initial 110 min. The ventilation fans produced a much higher cooling power than the PCMs in the wet condition.

Figure 5.

The cooling power of the personal cooling system in phase change material (PCM)+Fan-off, Fan-on and PCM+Fan-on in the wet condition.

Thermophysiological and psychological responses (the TMC mode)

Figure 6 presents the mean skin temperature, T_sk, the hypothalamic temperature, T_hy, and the overall thermal sensation (TS) of Case 1 (i.e., light work condition). The mean skin temperature T_sk curves of the four scenarios showed similar change trends. The T_sk dropped at initial the 3–13 min, and then continuously increased until the end of the test. The three scenarios (i.e., Fan-on, PCM+Fan-off and PCM+Fan-on) exhibited lower T_sk than the Fan-off (i.e., control). The final mean skin temperatures T_sk in Fan-off, Fan-on, PCM+Fan-off and PCM+Fan-on were 35.0℃, 34.0℃, 33.7℃, 32.3℃, respectively. The mean skin temperature difference between Fan-on and PCM+Fan-off at the end of the simulation was 0.3℃. The T_hy curves in the four test scenarios developed a similar trend, exhibited a decrease at the initial 3–5 min, and then gradually increased until the end of the test. All TS values decreased at the initial stage, then gradually increased, and finally reached a plateau. The TS in PCM+Fan-on dropped from 0 (thermal neutral) to –3.4 (between ‘Cold’ and ‘Very cold’), and the final TS was rated at –1.7 (cool). The TS in Fan-off dropped from 0 (thermal neutral) to –0.5 (between ‘Thermal neutral’ and ‘Slightly cool’), and finally increased to 1.6 (a rating between ‘Slightly warm’ and ‘Warm’).

Figure 6.

Evolution of the predicted mean skin temperature (a), the hypothalamic temperature (b) and the overall thermal sensation (c) during the time course under four test scenarios (Case 1: 60 min light work).

Figure 7 presents the mean skin surface temperature, T_sk, the hypothalamic temperature, T_hy, and the overall thermal sensation (TS) of Case 2 (i.e., construction work). The mean skin temperature T_sk curves of the four scenarios showed a similar development trend. The T_sk dropped at the initial 2–7 min, then continuously increased until the 40th min; finally it dropped steeply until the end of the simulation. The Fan-off (i.e., control) exhibited the highest T_sk, and the PCM+Fan-on showed the lowest T_sk (the final T_sk values in Fan-off and PCM+Fan-on were 34.3℃ and 32.3℃, respectively).

Figure 7.

Further, the T_hy curves in the four test scenarios showed a similar trend, that is, the T_hy decreased at the initial 2 min, and then it continuously increased to 38.1–38.5℃ until the 40th min. It dropped to 37.4–37.8℃ during the last 20 min recovery period. Generally, the PCM+Fan-on showed the lowest T_hy, whereas the Fan-off showed the highest T_hy. All TS curves decreased at the initial stage, then gradually increased to 3.4–4.0 (i.e., a rating between ‘Hot’ and ‘Very hot’) after the 40 min exercise. The post-simulation TS values in PCM+Fan-on and Fan-off were –1.6 (i.e., a rating between ‘Slightly cool’ and ‘Cool’) and 2.1 (i.e., ‘Warm’), respectively.

Discussion

The cooling performance of the newly developed hybrid PCS in different test scenarios has been investigated by using a thermal manikin operated in the CT mode and the TMC mode under moderate warm environments. The hybrid PCS in PCM+Fan-on showed a continuous cooling, whereas the PCS cooling effect difference between PCM+Fan-off and Fan-on was determined by the test condition. The cooling duration of PCMs varied in different scenarios and test conditions. The TMC tests indicated that single ventilation fans or PCMs could provide effective cooling while doing light work; the PCS in all three test scenarios (i.e., PCM+Fan-off, Fan-on and PCM+Fan-on) exhibited effective cooling while doing intensive work. Also, the hybrid PCS showed the optimized performance in alleviating heat stress during both exercise and recovery periods. The hybrid PCS could effectively dissipate body heat in warm conditions while doing both light work and strenuous work.

Comparison of the cooling power

In warm conditions where the air temperature is lower than the mean skin temperature, both sensible (i.e., conduction, convection and radiation) and insensible heat loss are two major pathways to dissipate the metabolic body heat. In the dry condition (i.e., no sweating), the four ventilation fans (i.e., Fan-on) could circulate air flow over the body, exhibited a significantly higher dry heat loss than that of Fan-off (i.e., 25 ± 1 and 52 ± 1 W/m² in Fan-off and Fan-on, respectively) and, hence, resulting in a cooling power of 45 ± 2 W. In the wet condition (i.e., continuous sweating), the Fan-on scenario also had a higher total heat loss (the sum of the dry heat loss and the evaporative heat loss) than the Fan-off scenario (i.e., 167 ± 1 and 333 ± 3 W/m² in Fan-off and Fan-on, respectively), resulting in a constant cooling power of 282 ± 5 W. This indicated that the cooling power due to sweat evaporation in Fan-on was about 237 W, which was 5–6 times the cooling power in the dry condition. The reported results were in good agreement with previous findings that ventilation clothing greatly increased the evaporative heat loss, and thereby reduced the body heat strain.^12–16,19

If the fans were turned off, the PCM packs in the developed PCS provided some cooling effect during the initial test period. The cooling duration provided by the PCMs in PCM+Fan-off under the dry condition was much longer than that in the wet condition. The cooling energy of PCMs in the dry condition and the wet condition was 235.2 and 79.8 kJ, respectively. Our results demonstrated that the evaporative cooling in the wet condition significantly decreased the cooling effect of PCMs. The results were in line with our previous study demonstrating that the cooling effect of PCMs in hot wet conditions under the hot dry environment (i.e., 34℃, 28% RH) was lower than that under the hot humid environment (i.e., 34℃, 75% RH).¹⁹

The cooling efficiency of PCMs and ventilation fans varied in different test conditions. Under the wet condition, the cooling rate in PCM+Fan-off (e.g., 45 W for 10 min) was much lower than that in Fan-on (i.e., 282 ± 1 W), which further confirmed Barwood et al.’s¹⁰ findings that the whole body fanning was the most effective and practical cooling strategy. Under dry conditions, the cooling rate of the PCS in PCM+Fan-off during the initial testing stage (e.g., 55 and 50 W for the first 15 and 20 min, respectively) was higher than that in Fan-on (i.e., 45 ± 1 W). It was thus evident that PCMs could provide more effective cooling than ventilation fans when the human skin surface was relatively dry at the initial working stage. In addition, the observed short cooling duration provided by the PCMs in the wet condition was consistent with previous studies by Smolander et al.²⁷ and Lu et al.¹⁹.

Further, few studies have been reported on the performance evaluation of hybrid PCSs.^11,19 Kim et al.¹¹ found that the combination of liquid and ventilation air cooling significantly improved the work duration and reduced physiological strain in a hot environment (i.e., 35℃, 50% RH). Lu et al.¹⁹ assessed the cooling power of the hybrid PCS using a sweating manikin operated in the CT mode under two hot environments (34℃). It was found that the hybrid PCS (i.e., PCM+Fan-on) provided the highest cooling effect during the constant sweating. In this study, the cooling power of PCM+Fan-on in both dry and wet conditions showed the greatest cooling power (refer to Figures 3 and 5), which was consistent with our previous study¹⁹ and further reconfirmed the effectiveness of this newly developed hybrid PCS under moderate warm conditions.

In actual work conditions, industrial workers are often required to perform strenuous work in warm and/or hot environments; the hybrid PCS (PCM+Fan-on) used in this study could provide effective conductive cooling at the initial working stage (e.g., a cooling rate of 94 W for the first 15 min and 89 W for the first 20 min in the dry condition), which was higher than that in Fan-on (i.e., 45 W). With the gradual development of sweat secretion, the human skin surface gradually got wet; ventilation fans then started to provide strong evaporative cooling (e.g., a cooling power of 282 ± 5 W in Fan-on). Compared with existing portable air ventilation systems, the developed hybrid PCS showed much better cooling performance at the initial working stages in the warm condition. There was a high potential for application of similar hybrid PCSs to alleviate the body heat strain in such settings as light and construction work in warm conditions.

Thermophysiological and psychological responses

The application of the hybrid PCS did not affect the development of the hypothalamic temperature, whereas it evidently affected the mean skin temperature. For light work (e.g., a metabolic rate of 1.5 METs), the hypothalamic temperature (i.e., the hypothalamic temperature) in the four studied scenarios was relatively stable throughout the 60 min simulation. The mean skin temperature varied in the four scenarios (i.e., the final T_sk values in Fan-off, Fan-on, PCM+Fan-off and PCM+Fan-on were 35.0℃, 34.0℃, 33.7℃ and 32.3℃, respectively). Gao et al.⁷ investigated the effect of the PCM cooling vest on combating heat waves in a hot environment (34℃, 60% RH) and found that the PCM cooling vest could alleviate the heat strain and improve wear thermal comfort, but it had no effect on the development of hypothalamic temperature. The PCM cooling vest could decrease the torso temperature by about 2.5℃ at the simulated light work. This further confirmed the results in our study, although the decrease of T_sk caused by PCMs (i.e., 1.3℃) was much lower. This was mainly because only the torso temperature was compared in Gao et al.’s study.⁷ The T_sk in PCM+Fan-off was lower than that in Fan-on, which might be caused by the higher cooling rate in PCM+Fan-off during the initial simulation stage (see Figure 3). Nishihara et al.⁵ investigated the performance of a cooling vest while doing light activity (i.e., 1.4 METs) in a warm condition (i.e., 30.2℃, 37% RH) and found that the PCM cooling vest decreased the skin temperature and the observed mean skin temperatures lay within the thermal neutrality zone. This was in line with the findings in this study. The thermal sensation in Fan-off gradually increased from 0 to 1.6 (i.e., a rating between ‘Slightly warm’ and ‘Warm’). In Fan-on, the rated post-simulation thermal sensation was 0.1 ± 0.2, showing a thermal neutral state. The application of PCMs in the PCS caused a sharp drop in the thermal sensation at the initial simulation period, resulting in a cold sensation (TS was rated at around –3). There was an obvious rise in TS at about the 45th minute in PCM+Fan-off, and then it stayed at around 1.0 (i.e., ‘Slightly warm’). This again demonstrated that the PCMs could provide a limited effective cooling duration, which was in good agreement with our manikin results obtained from the CT mode. The sudden increase of TS might indicate that the PCMs have been fully melted. The rated TS in PCM+Fan-on was maintained at –1.7 (i.e., between ‘Slightly cool’ and ‘Cool’) throughout the 1 h activities. The results indicated both Fan-on and PCM+Fan-off might provide appropriate thermal comfort, whereas the strong cooling effect provided by PCM+Fan-on forced the wearers to perceive a ‘Cool’ thermal sensation in the simulated light work condition. Thus, the hybrid PCS (i.e., PCM+Fan-on) could be applied to alleviate body heat strain while doing more intensive work.

For more intensive construction work, the hypothalamic temperatures in the four scenarios coincided during the initial 15 min, then they showed different increments and rose to between 38.1℃ (i.e., PCM+Fan-on) and 38.5℃ (i.e., Fan-off). At the initial simulation period, there was almost no or very little sweat production, thus the dry heat loss (<100 W/m²) was much lower than the metabolic rate (i.e., 320 W/m²), resulting in a sharp increase of the hypothalamic temperature. As the skin surface got gradually wet, the evaporative heat loss (i.e., 282 W/m²) provided by the four ventilation fans greatly decreased the increment of hypothalamic temperature. Hence, PCM+Fan-on developed the lowest hypothalamic temperature after the 40 min simulation.

During the resting period, there was no significant difference among the decrease rates of hypothalamic temperature in the four scenarios. In contrast, the skin temperatures varied largely among the four scenarios (e.g., the skin temperature after the 40 min simulation in PCM+Fan-on and Fan-off was 33.8℃ and 36.5℃, respectively), showing a 2.7℃ difference, which was in line with the results reported in the light work condition. Thermal sensations after the 40 min exercise in the four scenarios increased to 3.4–4 (i.e., between ‘Hot’ and ‘Very hot’). During the 20 min resting period, the rated TS values in the four scenarios, PCM+Fan-on, PCM+Fan-off, Fan-on and Fan-off, were –1.6 (i.e., between ‘Slightly cool’ and ‘Cool’), 0.4 (i.e., approaching ‘Thermal neutral’), 0.9 (i.e., ‘Slightly warm’) and 2.1 (i.e., ‘Warm’), respectively. Thus, subjective perceptions presented in this study demonstrated that the PCS could effectively alleviate body heat strain while performing such an intensive work in a warm condition.

In this study, the PCMs added to the hybrid PCS weighed about 2.06 kg, and it was estimated that the human metabolic rate could increase by about 6.5 W/m².¹⁹ Unfortunately, predicted physiological and psychological responses by the thermoregulatory model controlled sweating manikin failed to evaluate the effect of added PCMs weight on the human metabolic rate. In a recent human trial study,²⁸ the same hybrid PCS was found to be effective in alleviating body heat strain while exercising in a hot humid environment. Nevertheless, the impact of PCMs on the total cooling effectiveness of the PCS in warm conditions should be further assessed by human trial tests in the future.

Conclusions

In this study, the cooling efficiency of a hybrid PCS incorporated with PCMs and ventilation fans was investigated on a thermal manikin operated in the CT mode and the TMC mode in warm conditions. Under the dry condition, the cooling rate in PCM+Fan-off during the initial stage (e.g., 55 and 50 W during the first 15 and 20 min, respectively) was higher than that in Fan-on (i.e., 45 ± 1 W). In contrast, the cooling rate in PCM+Fan-off under the wet condition (e.g., 45 W for 10 min) was much lower than that in Fan-on (i.e., 282 ± 1 W). The hybrid PCS (i.e., PCM+Fan-on) provided a continuous strong cooling effect. The thermoregulatory model simulation results demonstrated that the hybrid PCS could effectively remove the body heat in warm conditions at different activity intensities. For the light work, ventilation fans or PCMs alone were able to provide the wearers with a thermal comfort condition, whereas the wearer might feel slightly cool using the hybrid PCS in PCM+Fan-on. For the intensive work, the hybrid PCS in all three scenarios (i.e., PCM+Fan-off, Fan-on, PCM+Fan-on) showed a beneficial effect in cooling the manikin body, and the hybrid PCS in PCM+Fan-on exhibited the best performance in alleviating body heat strain in both exercise and recovery periods. Finally, it should be emphasized that all test results presented in this study were obtained from a thermal manikin under laboratory conditions, and cautions should be taken when applying those results to actual working environments.

Footnotes

Declaration of conflicting interests

The authors declared no potential conflicts of interest with respect to the research, authorship and/or publication of this article.

Funding

The authors received no financial support for the research, authorship and/or publication of this article.

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