Effect of cooling strategies on overall performance of a hybrid personal cooling system incorporated with phase change materials (PCMs) and electric fans

Highlights

• Effect of cooling strategies on HPCS's performance was examined.

• Fan control could suppress skin temperature rise to 34 °C by > 15 min.

• Fan control could also cut down energy consumption by 3.5 W h.

• PCM control reduced 37.3% physical load and RPE ratings by 3.5–4.2 units.

• Cooling strategies chosen for this work could improve HPCS's overall performance.

Abstract

The effect of four cooling strategies on cooling performance of a hybrid personal cooling system (HPCS) incorporated with phase change materials (PCMs) and electric fans in a hot environment (i.e., T_air = 36 ± 0.5 °C, RH = 59 ± 5%) was investigated. Twelve healthy young male participants underwent four 90-min trials comprising 70 min walking and 20 min resting periods. Cooling strategies adopted in this work were CON (control), PCM-control (PCMs were removed at the end of exercise), Fan-control (fans were switched OFF during the initial 20 min) and PCM&Fan-control (fans were turned ON after 20 min exercising and PCMs were removed after the 70-min exercise). Results demonstrated that the control of electric fans could suppress the mean skin temperature rise to 34.0 °C by over 15 min and also cut down the energy consumption of the HPCS from 15.6 W h to 12.1 W h over the entire 90-min trials. Thus, it is recommended that fans should be turned off at the beginning of hot exposure and switched on once participants felt warm. Our findings also showed that the removal of fully melted PCM packs from the HPCS could enhance the evaporative cooling effect brought about by air circulation. The removal of melted PCMs significantly reduced the physical load by 37.3% and ratings of perceived exertion (RPE) were decreased by 3.5–4.2 RPE units. This could also help quickly restore the PCM energy for future usage. In summary, cooling strategies demonstrated in this work could improve HPCS's overall cooling performance on workers while working in the studied hot environment.

Introduction

Outdoor workers performing strenuous physical tasks in hot and humid environments are often suffering from great heat stress and severe discomfort, which could greatly limit the duration of continuous work and thereby reduce their work productivity (Dukes-Dobos, 1981; Li et al., 2016; Zander et al., 2015). Uncompensated heat stress induces excessive sweating, increased heart rate and cutaneous blood flow as well as causes a progressive rise in both core and skin temperatures (Kenny and Jay, 2013). In order to mitigate heat stress effects on workers and to extend effective working duration in hot environments, various types of personal cooling clothing based on a single cooling technique have been developed over the past five decades, e.g., air-cooled clothing (Ham, 1965; Hadid et al., 2008; Zhao et al., 2013), liquid-cooled clothing (Bartkowiak et al., 2017; Guo et al., 2015; Bartkowiak et al., 2017, 2017; Wang et al., 2019), evaporative cooling clothing (Konz et al., 1974; Heled et al., 2004; Weder et al., 2008), and clothing incorporated with phase change materials (PCMs) (Speckman et al., 1988; Shim et al., 2001; Gao et al., 2010; House et al., 2013; Hamdan et al., 2016). In general, air-cooled and liquid-cooled clothing can be classified as active cooling strategy, which requires external energy supplies to draw excessive body heat (Chan et al., 2015). In contrast, passive cooling clothing utilizes PCMs (instead of external energy supply) to draw body heat during phase change. Liquid-cooling clothing is effective in mitigating body heat strain of workers while working in hot conditions (Teunissen et al., 2014; Wang et al., 2019). Nevertheless, they are heavy, bulky and may limit the user motion while working outdoors. Dry ice incorporated clothing is also effective in mitigating heat strain, but they may cause local body cold discomfort due to the huge amount of heat absorbed during sublimation (Heled et al., 2004; Wang et al., 2019). Of all aforementioned cooling strategies, air-ventilation and PCM cooling are convenient for use by outdoor workers because they are portable and light-weight and, more importantly, they will not restrict the body movement.

In recent years, the development of hybrid cooling clothing systems (HPCSs) incorporated with more than one cooling technique became a hot topic (Song et al., 2016; Wang and Song, 2017). Lu et al. (2015) developed hybrid cooling clothing incorporated with PCMs and electric fans and found that the new HPCS demonstrated good potential for body cooling in both hot-humid and hot-dry environments. Song and Wang (2016) assessed the performance of a HPCS incorporated with both electric fans and PCMs and found that the HPCS was effective in mitigating body heat strain while exercising in a hot environment. Chan et al. (2017) also developed a HPCS incorporated with PCMs and electric fans and found that the HPCS could notably improve the workers’ perceptual heat strain in a limited resting duration. With the help of a numerical model, Wan et al. (2018) performed numerical simulations and found that PCMs in the HPCS could draw over 50% heat from the hot environment (i.e., 36 °C, 59% relative humidity [RH]) during a phase change cycle. Hence, majority of the PCM cooling energy was wasted during phase change. Itani et al. (2019) designed a hybrid PCM-fan vest and compared its cooling performance with a PCM vest under various ambient temperatures of 35, 40 and 45 °C. Results illustrated that the hybrid PCM-fan vest was recommended at high ambient temperatures due to its superior performance to the PCM vest. Kang et al. (2018) carried out numerical simulations and proposed to add additional insulation pads in between PCMs and the ambient environment in a standard HPCS to save PCM cooling energy and reduce heat absorption by PCMs from the surrounding hot environment during melting. Numerical results demonstrated that adding an insulation pad with a thermal insulation of 0.233 m² K/W could decrease the heat absorption by PCMs from the hot environment to <20%. More recently, in a human trial study, Udayraj et al. (2019) found that the usage of 5 mm thick expanded polyethylene insulation between PCM and clothing outer layer could significantly decrease the skin and torso temperature rise in a hot environment (36 °C, 59%RH) and thereby contribute to extended body cooling duration.

Despite many research studies conducted on the cooling performance of HPCS in recent years, there is lack of sufficient knowledge regarding proper management of PCMs and the ventilation cooling in HPCS during use in hot environments. Based on the findings of previous studies on the HPCS and existing understanding of heat and moisture transfer mechanisms in the HPCS, the following critical issues can be noted in relation to the HPCS:

• Electric fans incorporated in the HPCS circulate the ambient air around the human body through the clothing microclimate. This enhances convective heat transfer and evaporative heat transfer (i.e., sweat evaporation, if any) and thereby enhances the evaporative heat transfer from the body to the environments (positive effects). However, the circulation of ambient air from hot environments in the microclimate results in early melting of PCMs and higher sensible heat transfer from the air to the body if the ambient air temperature was higher than the body temperature (negative effects). During the initial phase of the operation when significant sweat production is not present, the circulation of warm/hot air in the clothing microclimate may not be good as it will not contribute much to the body heat removal. An appropriate strategy to utilize the fans in the HPCS system to achieve a better positive body cooling needs to be derived to address this issue.

• Our recent studies (Kang et al., 2018; Udayraj et al., 2019) revealed that PCMs were melted completely after 70 min heat exposure at T_air = 36 °C. Fully melted PCMs could not provide any body cooling but instead they increase clothing weight and impose a physical burden on the body. Moreover, PCM packs may hinder sweat evaporation and moisture transmission through clothing because they are moisture-impermeable.

In order to address the above issues and to effectively manage the cooling energy (from air ventilation and PCMs) of the HPCS, three energy cooling strategies were formulated in the present work and investigated by carrying out human trials consisted of 70 min exercising and 20 min resting in a hot environment (36 °C, 59%RH). The three cooling strategies chosen for this work are 1) keeping electric fans being turned off during the initial 20 min (denoted as Fan-control); 2) removing completely melted PCMs at the beginning of the recovery period (at the 70th min) while keeping the ventilation fan on throughout the entire 90 min (denoted as PCM-control), and 3) a combination of Fan-control and PCM-control strategies (i.e., fans were switched ON after 20 min of the trial and PCMs were removed from the HPCS after 70 min of the trial, denoted as PCM&Fan-control). The purposes of this study were to examine the effect of three cooling strategies on HPCS's cooling performance compared to its normal operation (i.e., the control case, CON) and to find out the optimum cooling strategy for the HPCS to reduce energy consumption. It was hypothesized that the cooling strategies chosen for this study could enhance the HPCS's cooling performance without compromising wearers' thermal comfort.