Study on the Performance of Personal Heating in Extremely Cold Environments Using a Thermal Manikin

Abstract

Cold protection for outdoor workers is crucial for their health and thermal safety in winter. Personal heating is considered an effective measure to solve the problem, which can significantly improve thermal comfort. However, according to the present studies, a uniform assessment of different personal heating measures is hard to obtain. This study explored four typical types of personal heating measures (electrically heated garment, electrically heated garment with an aerogel layer, electrically heated seat, and chemically heated insole) in different cold environments. Clothing insulation, effective heating power (P_eff), and heating efficiency ($\eta$) were measured by a thermal manikin with a constant temperature in nine environmental conditions. Three levels of two critical environmental factors (air temperature (T_a): −5 °C, −10 °C, and −15 °C; air velocity (V_a): <0.1 m/s, 0.5 m/s, and 1.0 m/s) were crossed orthogonally to form the nine environmental conditions. The results indicated that T_a had no significant effect on clothing insulation, while elevated V_a significantly decreased clothing insulation. When V_a increased from 0 m/s to 1 m/s, the air layer inside the garment was squeezed, causing a 0.6–0.9 clo decrease in total clothing insulation. Decreased T_a and elevated V_a reduced the P_eff and $\eta$ of electrical heating measures while they improved the P_eff and $\eta$ of chemical heating insoles. The P_eff and $\eta$ of the garment dropped to 8.2 W and 21%, respectively, at −15 °C and 1.0 m/s. In addition, the aerogel layer could effectively improve the P_eff and $\eta$ of the garment. The improvement was weakened by decreased T_a and elevated V_a. The corrective power values of personal heating measures in different environments were calculated to guide the design and application of personal heating.

1. Introduction

1.1. Background

Extreme cold weather has frequently occurred worldwide in the past few years, increasing the risk of cold exposure [1,2]. Many outdoor occupational activities, such as emergency rescue and electrical maintenance, are performed outdoors. Those workers would experience cold exposure in winter, decreasing work performance [3,4,5], and increasing health risks [6,7,8]. However, it is unrealistic to achieve cold protection by increasing the ambient temperature when it is outdoors. Effective measures are needed to protect outdoor workers from cold exposure.

1.2. Personal Heating Measures for Cold Protection in Cold Environments

To avoid the risk of disease and physical injury, enhance thermal comfort, and reduce the restriction on body movement caused by multi-layer garments, various personal heating measures, which provide heat to garments through integrated heating units, have been developed, such as heated gloves, heated insoles, heated socks, and heated garments. Personal heating measures for the hands and feet can improve finger dexterity and work efficiency during prolonged cold exposure [9,10,11,12]. Ma et al. [9] tested the heating performance (HP) of electrically heated gloves at V_a of 0.17 m/s and 0.5 m/s and T_a of 2.5 °C. The results indicated that electrically heated gloves enhanced the hand’s thermal sensation and skin temperature (T_skin) at both V_a. In different studies, the heating power required for the electrically heated gloves varied due to the thermal insulation of the gloves and the environmental parameters. Brajkovic [11] reported that a heating power of 14 W could maintain the T_skin of hand at 34.1 °C by wearing the arctic gloves at −25 °C. Streets [12] suggested that conductive polymer gloves with a heating power of 15 W were able to maintain T_skin of hand at 35 °C, and knitted gloves with a heating power of 19 W were able to maintain T_skin of hand at 27 °C in a cold environment of −32 °C. In addition, subjects wearing heated insoles with a power of 6 W experienced a drop in toes’ T_skin to 20 °C after three hours of cold exposure. Without the heated insoles, the subjects withdrew from the experiment early at the 75th min because of the intolerable cold.

Moreover, various forms of heated garments, such as air-heated and electrically heated garments (EHG), have been developed for different scenarios. In a study by Rapaport et al. [13], subjects were exposed to a −34 °C environment wearing a full-body air-heated garment. T_skin of the exposed hand could be maintained above 21 °C for one hour. Brajkovic et al. [11] used an electrically heated vest (EHV) to heat the subject’s trunk. T_skin of fingers and toes were maintained in a comfortable range (22–25 °C.) during three-hour cold exposure at −15 °C. Ducharme et al. [14,15] similarly demonstrated the effectiveness of an EHV in maintaining T_skin and finger dexterity of human extremities in a −25 °C environment. The power of the EHVs was 108–111 W, and the heating temperature was maintained at 42 °C in the above studies.

Some heating systems have been applied in military and civilian, based on long-term experimental studies. However, previous studies can only provide limited references due to the specific research subjects and application scenarios. In addition, it is not easy to make a uniform assessment of the effectiveness of personal heating measures from various studies due to the differences in environmental conditions and clothing. In a similar condition, the corrective power (CP) proposed by Zhang et al. [16] could reflect the performance of heating measures on the improvement of the thermal environment around the human body and has been widely used to assess the performance of various types of personal thermal comfort devices.

1.3. Influence of Environmental Parameters on the Cold Protective Performance of Garments

Clothing is essential to protect the human body from cold exposure. Multi-layer garments squeeze the internal air layer and reduce the thickness, significantly decreasing the clothing insulation of the garments [17]. In addition, environmental parameters have significant effects on the cold protective performance (CPP) of the garment. G. Havenith et al. [18] found that V_a (or walking speed of the thermal manikin) significantly affected clothing insulation. The clothing insulation decreased by about 20% at V_a within 0.15–0.7 m/s, compared to windless conditions. Oliveira et al. [19] studied the effect of the temperature difference between T_skin of the thermal manikin and T_a on clothing insulation. They found that the value of clothing insulation may decrease when the temperature difference is too small. However, as the temperature difference exceeded a specific value, it no longer affected the clothing insulation value.

Environmental parameters also influence the HP of actively heated garments. For heating measures, $\eta$ and P_eff are generally used to evaluate HP. Wang and Lee [20] tested the HP of an EHV at 0 °C and −10 °C using a thermal manikin, indicating that decreased T_a significantly reduced the $\eta$ of the vest. Wang et al. [21] studied the effect of V_a on the $\eta$ of EHV and reported that elevated V_a significantly reduced the $\eta$. The closer the EHV was to the body in the same garment system, the higher the $\eta$ was. However, in the study by Song et al. [22], elevating V_a from 0.4 m/s to 1.0 m/s significantly decreased the total clothing insulation of the EHV, while elevated V_a had a minor effect on the P_eff and $\eta$.

The conclusions of the studies are different due to the differences in environmental condition settings, garment combinations, and electrical heating unit performance in different studies. Therefore, strict restrictions on environmental parameters and applicable conditions are required when testing garments’ CPP and HP. The weakening effect of the most adverse environmental condition on performance should be considered when designing the garments to achieve better performance in the actual scenario. In addition, the effect should be quantified to leave the corresponding design parameters margin.

1.4. Objectives

This study studied the following four typical types of personal heating measures: EHG, EHG with an aerogel layer, electrically heated seat (EHS), and chemically heated insole (CHI). Total clothing insulation related to CPP and the following two critical parameters related to HP: P_eff and $\eta$ were measured by a thermal manikin operated in the constant temperature mode in nine environmental conditions. Three levels of two critical environmental factors (T_a: −5 °C, −10 °C, and −15 °C; V_a: <0.1 m/s, 0.5 m/s, and 1.0 m/s) were crossed orthogonally to form the nine environmental conditions. The objectives of this study are as follows:

• To investigate and quantify the effects of T_a and V_a on CPP and HP;
• To analyze the effect of the new material aerogel on the CPP and HP of EHG;
• To calculate and explore the CP of different personal heating measures.

2. Methods

2.1. Thermal Manikin

Human subject experiments or thermal manikin tests are usually used to evaluate the thermal performance of personal heating measures because they can reproduce the way the personal heating measures are used correctly. Considering the repeatability of experimental procedures and avoiding individual differences, the thermal manikin is the most common tool used by most researchers to evaluate different personal heating measures and measure clothing insulation [20,21,22,23,24]. American and international standards have addressed the measurement specifications of clothing insulation by thermal manikin [25,26]. Therefore, the measurement method of the thermal manikin was used in this study.

A 22-segment thermal manikin (PT TEKNIK, Esbjerg, Denmark) was used in this experiment. As shown in Figure 1a, the physical characteristics of the thermal manikin are similar to those of a medium-sized male, with a height of 171 cm and a surface area of 1.67 m². The flexible joints enable it to simulate various body postures, such as sitting and standing. The shell of the thermal manikin is made of glass fiber-reinforced resin, the joints are connected by aluminum, and the pure nickel heating wires distributed under the shell are uniformly wound on the body surface. According to the physiological structure of the human body, the thermal manikin is divided into 22 segments (i) with defined surface areas A_j (m²) (Figure 1b). The surface area of each body segment is listed in

Table 1. Body segments and corresponding areas of the thermal manikin.

Body Segment	Area (m2)	Body Segment	Area (m2)
Left/Right (L/R). Foot	0.053	Scull	0.054
L/R. Foreleg	0.115	L/R. Hand	0.0457
L/R. front Thigh	0.0957	L/R. Forearm	0.042
L/R. back Thigh	0.0957	L/R. Upper arm	0.083
Pelvis	0.046	L/R. Side Chest	0.0894
Back side	0.076	L/R. Side Back	0.0894
Head	0.08	Total	1.6738

. The heating flux and skin surface temperature of 22 body segments can be controlled and detected independently. The accuracy of T_skin control is ±0.1 °C in the range of 25–45 °C, and the accuracy of measuring heat flux is ±1% in the range of 0–300 W/m². The body segment and the precision of the T_skin and heat flux of the thermal manikin meet the requirements of ASTM F1291 [26] for measuring clothing insulation.

The thermal manikin has the following three operation modes: proportional-integral (PI) control mode, locked power mode, and comfort mode. In this study, the thermal manikin operated in the PI control mode, in which the skin temperature of each segment (T_{skin, i}) was controlled, and the heating power (P_i) was recorded. Considering the cold exposure effects on T_skin of the human body [27,28] and the testing requirement, the T_{skin, i} of 22 segments was set to a constant value of 32 °C according to ISO 9920 [25]. In addition, the PI control was applied to P_i as Equation (1). Considering the heating rate and stability of the heating system, the proportional constant K was set to 100, and the integral constant $\tau$ was set to 50.

$$P_{\mathrm{i}}=\mathrm{K}\left(\Delta T_{\mathrm{i}}+\frac{1}{\tau }\int \Delta T_{\mathrm{i}}\mathrm{dt}\right){, P}_{\mathrm{i}} \ge 0, \mathrm{i}=1–22$$

(1)

where the $\Delta T_{\mathrm{i}}$ was the difference between the setting value and T_{skin, i} of the skin surface temperature at segment i.

2.2. Experimental Site and Test Conditions

The experiment was conducted in a 3.62 m × 2.67 m × 2.29 m cryogenic chamber (Figure 2). Thermal manikin remained sedentary during the experiment. In addition, a wind deflector was set between the fan and thermal manikin to ensure the uniform transition of cold air. The three T_a conditions involved in the experiment included −5 °C, −10 °C, and −15 °C, and the three V_a conditions included 0 m/s, 0.5 m/s, and 1.0 m/s. The two variables, T_a and V_a, were cross-combined into nine environmental conditions.

2.3. Development of Personal Heating Devices

(a)

The EHG (Figure 3a) was developed for this experiment, which included a jacket with aerogel insulation and graphene heating. The jacket was made up of four layers of materials, namely, windproof and waterproof materials, an aerogel insulation layer, a graphene flexible heating layer, and a fleece fabric inner layer. It well combines passive insulation and active heating in the material arrangement. The eight flexible graphene sheets were distributed in the position of the chest, abdomen, back, and upper arms for local heating. The size of each piece of graphene sheet was 10 × 15 cm for the chest and abdomen and 10 × 20 cm for the back and upper arms. The heating power of graphene sheets could be adjusted in three gears through the control button in the pocket to meet the dynamic individual heating demands. In low, medium, and high gear, the surface temperatures of graphene sheets were 35–40 °C, 40–45 °C, and 45–50 °C, respectively (measured at T_a of 20 °C), while the heating powers were 90 W/m², 135 W/m², 180 W/m² correspondingly. In this experiment, the high gear was used to heat the thermal manikin locally, and the heating power was 25.2 W. The aerogel insulation layer has excellent thermal insulation properties;

(b)

The EHS (Figure 3b) provided heating to the buttocks of sedentary people. The heating seat was characterized by heating rapidly and uniformly. The maximum heating power was 50 W, and the maximum surface temperature could reach about 70 °C (measured at an ambient temperature of 20 °C). The other surface of the heating seat was well insulated to ensure one-way heat transmission and reduce heat loss to the environment;

(c)

The chemically heated insole (CHI) (Figure 3c) was similarly designed to offset fast cooling of the feet, a major both locally and whole-body in cold environments. The insole used the heat generated by the oxidation reaction between the iron powder filled inside and the air to warm the foot. The surface temperature of the heated insole can reach 42 °C, and adequate operating time was fully ensured for a single experiment.

2.4. Experiment Conditions

Under the nine environmental conditions, the CPP and HP of six clothing combinations or improvement measures in

Table 2. Experimental conditions.

Condition Number	Clothing Combination/Improvement Measure
1	Without aerogel layer, without local heating (CG)
2	With aerogel layer, without local heating (WANH)
3	Without aerogel layer, with local heating (NAWH)
4	With aerogel layer, with local heating (WAWH)
5	With heated seat (WHS)
6	With heated insoles (WHI)

were discussed, respectively. Among them, Condition 1 was regarded as the control group.

The basic clothing of thermal manikin was the typical winter outdoor suit, including a long-sleeved sweater and jacket on the upper body, warm pants and windproof trousers on the lower body, socks and cotton-padded shoes on the feet, and some warm-keeping accessories such as hat, scarf, mask, and gloves. The specific clothing information is shown in

Table 3. Clothing used in the experiments.

Garment Description	Materials	Size	Weight (g)
Hat	Acrylic fiber	–	98
Scarf	Acrylic fiber	–	156
Mask	Cotton	–	12
Long-sleeved sweater	Fabric: cotton Lining: polyester fiber	XL	440
Jacket	Fabric: polyester fiber Lining: fleece Filler: aerogel	XL	1800
Gloves	Fabric: poly urethane leather Lining: long fuzz	–	202
Warm pants	Fabric: cotton Lining: short fuzz	XL	418
Windproof trousers	Fabric: polyester fiber Lining: fleece	XL	364
Socks	Cotton	–	32
Cotton-padded shoes	Fabric: suede Lining: short fuzz	42 (Europe)	682

. All the clothes selected for the experiment were well-fitting for the thermal manikin.

2.5. Environmental Parameters Measurement

T_a, V_a, relative humidity (RH), and globe temperature (T_g) were measured using calibrated quick-response instruments during the experiment. The instruments are shown in

Table 4. Information on the instruments.

Measured Parameter	Instrument	Specification	Manufacturer
Ta and RH	WSZY-1 thermometer	Accuracy: ±0.3 °C and ±3% Range: −40–100 °C and 0–100%	TianJianhuayi Co., Ltd., Beijing, China
Tg	HQZY-1 globe thermometer	Accuracy: ±0.3 °C Range: −20–80 °C	TianJianhuayi Co., Ltd., Beijing, China
v	Swema-03 Omnidirectional anemometer	Accuracy: ±0.04 m/s Range: 0.05–3.00 m/s	Swema AB Co., Ltd., Farsta, Sweden

and Figure 4. T_a and RH were measured at 0.1 m, 0.6 m, and 1.1 m from the ground using the WSZY-1 thermometer (Figure 4a). V_a was measured at 0.6 m from the ground by the Swema-03 universal micro-wind speed probe (Figure 4b) during the experiment. T_g was measured 0.6 m above the ground using an HQZY-1 globe thermometer (Figure 4c). T_a, RH, and T_g were recorded at intervals of 10 s, and V_a was recorded at intervals of 1 s to meet the measurement requirement and investigate their variance.

2.6. Evaluation Indicators for Cold Protection and Heating Performance

The clothing insulation was used to evaluate the cold protection of clothing. Clothing insulation can be divided into total insulation (I_t), effective insulation (I_clu), and intrinsic insulation (I_cl). The total insulation is defined as the thermal resistance from the skin surface to the environment, which includes the I_clu and the thermal insulation of the air layer around the skin surface when nude (I_a). I_cl is defined as the thermal resistance of the clothing over a nude thermal manikin when the outer air layer is removed, taking into account the variation in external surface area. The local I_t of each body part is calculated by Equation (2), using T_{sk, i}, heat flux of the body part (H_i), and T_a. The unit of clothing insulation is clo (1 clo = 0.155 (m² °C)/W). The parallel method was used to calculate the whole-body I_t via Equation (3). Then, I_cl is calculated using Equations (4) and (5). [25] I_a is corrected by the area factor (f_cl) related to the I_cl, according to ISO-9920.

$$I_{\mathrm{t},\mathrm{i}}=\frac{T_{\mathrm{s}\mathrm{k},\mathrm{i}}-T_{\mathrm{a}}}{0.155\times H_{\mathrm{i}}}$$

(2)

$$I_{\mathrm{t}}=\frac{\left({{\displaystyle \sum }}_{i=1}^{22}{f}_{\mathrm{i}}·T_{\mathrm{s}\mathrm{k},\mathrm{i}}\right)-T_{\mathrm{a}}}{0.155\times {{\displaystyle \sum }}_{i=1}^{22}{f}_{\mathrm{i}}·H_{\mathrm{i}}}$$

(3)

$$I_{\mathrm{t}}=I_{\mathrm{cl}}+\frac{I_{\mathrm{a}}}{f_{\mathrm{cl}}}$$

(4)

$$f_{\mathrm{cl}}=1+0.25I_{\mathrm{cl}}$$

(5)

where f_i is the proportion of the surface of body segment i to the total surface area.

The actual performance of the heating devices is assessed using P_eff and $\eta$ [22]. P_eff is the reduction in power output of the thermal manikin to maintain a constant skin temperature after using personal heating devices. P_eff is calculated by Equation (6), using the difference between H_i with and without heating ($\mathsf{\Delta }{H}_{\mathrm{i}}$) weighted by corresponding A_i. $\eta$ is defined as the ratio of P_eff to the actual output power of the personal heating devices, which is calculated via Equation (7). In addition, the heating effect provided by personal heating devices can be characterized by equivalent homogenous temperature (EHT), which quantifies local human body heat loss in any non-uniform condition by comparing it against exposure to an equivalent homogenous still-air ambient environment [16]. The overall EHT was used to calculate the corrective power (CP) to compare different personal heating devices’ effects on the whole body. The CP values quantify the ability of personal heating devices to improve the thermal environment around the human body. The overall EHT and CP are calculated by Equations (8) and (9), respectively [23].

$$P_{\mathrm{eff}}={{\displaystyle \sum }}_{i=1}^{22}\Delta H_{\mathrm{i}}{A}_{\mathrm{i}}$$

(6)

$$\eta =\frac{P_{\mathrm{eff}}}{\lambda P_{\mathrm{in}}}$$

(7)

$$EHT=T_{\mathrm{sk}}-H\times I_{\mathrm{cl}}\times 0.155$$

(8)

$$CP=EH{T}^{*}-EH{T}_{\mathrm{ref}}$$

(9)

where $\lambda$ is the heat conversion efficiency equal to 0.95; P_in is the input power of the personal heating devices; EHT* is the EHT with personal heating devices; EHT_ref is the EHT without personal heating devices.

2.7. Test Procedure

Each experimental condition was tested under the nine environmental conditions. When the T_{sk, i} (I = 1–22) fluctuation was lower than 0.1 °C, the thermal manikin was considered stable. According to the recommended procedure in ASTM F1291 [26], the test flow is shown in Figure 5. In the test, each experimental condition was tested at least three times, and each test lasted 90 min to ensure that the manikin reached a stable state. The test results are reliable when the difference between the tested clothing insulation results and the mean is less than 10%.

2.8. Data Analysis

Data were presented as mean ± standard deviation (SD). A two-way analysis of variance for repeated measures (RM ANOVA) (with T_a and V_a as the independent variables) was performed to determine if there was any significant difference in the clothing insulation, P_eff, and $\eta$ between different clothing scenarios. The statistical significance of the RM ANOVA was considered at p < 0.05 (indicated by “*”) and highly significant at p < 0.01 (indicated by “**”). When a T_a × V_a interaction demonstrated statistical significance, the simple effects of each factor were analyzed individually. If the interaction had no statistical significance, the effects of the two factors were considered independently, and the main effect of each factor was analyzed.

3. Results

3.1. Environmental Parameters

As shown in

Table 5. Measured environmental parameters during the experiment (Mean ± SD).

Planned		Measured
Ta (°C)	Va (m/s)	Ta (°C)	Va (m/s)	RH (%)	Tmr (°C)
−5	0.1	−4.9 ± 0.2	<0.10	70 ± 3	−4.3 ± 0.2
−10	0.5	−10.1 ± 0.1	0.51 ± 0.05		−9.4 ± 0.3
−15	1.0	−14.6 ± 0.2	1.10 ± 0.10		−14.3 ± 0.2

, the measured T_a and V_a were consistent with the expected settings. RH was maintained within 70 ± 3%. In addition, the environmental parameters control accuracy met the test requirement.

3.2. The Cold Protection and Heating Performance of Garments at Different T_a

Figure 6 shows the I_cl of CG, WANH, NAWH, and WAWH conditions at three T_a (all v < 0.1 m/s). The mean and SD of the I_cl of the same garments are marked in red. The difference in the I_cl of the same garments at different T_a is marked as a percentage. Compared with the CG, the I_cl of WANH increased to 2.13 clo, indicating that the aerogel layer provided good thermal insulation. The I_cl of NAWH was 2.10 clo, slightly lower than that of WANH. In addition, the I_cl of WAWH significantly increased to 2.46 clo. The I_cl increase due to local heating increased from 0.14 clo (the difference between CG and NAWH) to 0.33 clo (the difference between WANH and WAWH), indicating that good insulation can significantly improve the effectiveness of local heating. Moreover, the difference in I_cl of the same garment at different T_a did not exceed 10%, suggesting that T_a did not cause significant differences in the I_cl of the same garment.

Figure 7 depicts the variations in P_eff of NAWH and WAWH conditions at three T_a. The P_eff of the heated garments significantly decreased as T_a dropped. Additionally, P_eff of WAWH was always higher than NAWH, but the difference between the two conditions narrowed as the T_a dropped.

Table 6. $\eta$ of EHGs at different T_a.

Ta (°C)	NAWH	WAWH
−5	42%	95%
−10	35%	52%
−15	22%	33%

presents $\eta$ of NAWH and WAWH conditions at three T_a. $\eta$ of the EHGs significantly reduced with T_a decreased. A comparison of the $\eta$ of NAWH and WAWH indicated that the aerogel insulation significantly improved $\eta$. However, the improvement was weakened by decreased T_a, with the most significant improvement occurring at −5 °C, where the $\eta$ increased from 42% to 95%.

3.3. The Cold Protection and Heating Performance of Garments at Different V_a

Figure 8 illustrates I_t of CG, WANH, NAWH, and WAWH conditions at three V_a. Since the ambient temperature changes in the experiments had little effect on the thermal resistance of the garments, each bar in Figure 7 represents the mean value of the I_t under the three T_a. As shown in Figure 7, the I_t of all four garment combinations significantly decreased (p < 0.01) with the V_a increase. When V_a increased from 0 m/s to 1 m/s, the air layer inside the garment was squeezed, which caused a 0.6–0.9 clo decrease in I_t. Furthermore, the I_t of the garment did not show a linear variation with V_a. When V_a increased from 0 m/s to 0.5 m/s, the decrease in I_t was more significant than that when V_a increased from 0.5 m/s to 1.0 m/s.

Figure 9 and

Table 7. $\eta$ of EHGs at different V_a.

Va (m/s)	−5 °C		−10 °C		−15 °C
	NAWH	WAWH	NAWH	WAWH	NAWH	WAWH
0	42%	95%	35%	52%	22%	33%
0.5	33%	68%	32%	50%	21%	33%
1	30%	59%	30%	49%	21%	33%

show P_eff and $\eta$ of the heated garments with and without aerogel insulation under nine environmental conditions, respectively. When the T_a was −5 °C, the P_eff and $\eta$ of the garment significantly decreased as V_a increased. The P_eff and $\eta$ of the garment no longer changed significantly with increasing V_a at −10 °C and −15 °C, indicating that the effect of V_a on the P_eff and $\eta$ of the garment gradually diminished as the T_a decreased. The P_eff and $\eta$ of the heated garments with aerogel insulation were higher than those without aerogel insulation, indicating that aerogel insulation can effectively enhance the HP of the heated garments. However, the enhancement effect of aerogel insulation decreased with decreasing T_a and increasing V_a.

3.4. Evaluation of the Heating Performance of the Heated Seat under Different Environmental Conditions

Overall, I_t was challenging to reflect the variation in the performance of the heated seat under different environmental conditions owing to a small surface area directly heated. Therefore, the local I_t of the back thigh (I_{t, back thigh}) was chosen as the indicator for comparison. Figure 10 shows the effect of the heated seat on I_{t, back thigh,} under different environmental conditions. The heated seat significantly increased I_{t, back thigh} by 1.2 clo in windless conditions and by 0.7 clo at a V_a of 1 m/s. The improvement at the same V_a was almost independent of the T_a change.

The phenomenon that the thermal insulation value decreased with elevated V_a may be directly attributed to the following two reasons: one was the reduction of intrinsic clothing insulation caused by V_a with the heating off; the other was the weakened improvement in the thermal insulation of the local heating with elevated V_a. Therefore, the influence of elevated V_a on I_{t, back thigh} should be excluded when calculating P_eff and $\eta$ of the heated seat. Figure 11 and

Table 8. $\eta$ of EHS in different experimental conditions.

Ta (°C)	Va (m/s)
	0	1
−5	23%	25%
−10	24%	24%
−15	25%	26%

show the calculated P_eff and $\eta$ of the heated seat under different environmental parameters. As shown in Figure 11, the P_eff of the heated seat had no significant difference at various T_a and V_a. However, the $\eta$ can only reach about 25% because of the significant heat loss caused by direct exposure to low-temperature environments of a partial area of the heated seat.

3.5. Evaluation of Insole Heating Performance under Different Environmental Conditions

Figure 12 shows the effect of heated insoles on the local I_t of the foot (I_{t, foot)} under different environmental conditions. Because of the small ratio of the foot area directly heated, it is difficult to reflect variations in the HP with I_t under different environments. The I_{t, foot} is chosen for comparison. As shown in Figure 11, the heated insoles increased I_{t, foot} from 0.6 to 0.7 clo, and offset the effect of decreasing I_{t, foot} produced by increasing V_a (when V_a increased from 0 m/s to 1 m/s, I_{t, foot} had no significant difference).

Variations in P_eff of the heated insoles under different environmental conditions are shown in Figure 13. P_eff of the heated insoles increased with decreasing T_a and elevated V_a. This was attributed to a sufficient oxidation reaction of the internal iron powder owing to the higher oxygen permeability of insoles caused by elevated V_a. It is consistent with the findings of Song et al. [22] in the study of a chemically heated garment, which indicated that the P_eff of the chemically heated garment had a slight improvement as V_a increased from 0.4 m/s to 1.0 m/s.

4. Discussion

4.1. Influence of Environmental Parameters on Local Heating Performance

T_a had no significant effects on the garments’ intrinsic insulation and the additional thermal insulation of the seat, which is consistent with the findings of Wang and Lee [20]. The effects of T_a on the HP of different types of personal heating measures differed. For the EHG, there was no significant change in the I_cl of EHGs (NAWH and WAWH) as T_a decreased, indicating that EHG was able to maintain its enhancement of I_cl as T_a decreased. However, $\eta$ of EHGs decreased significantly with T_a decreased, indicating that the HP of reducing heat loss from the human body was weakened. This could be explained by the study of Wang and Lee [20] on the temperature distribution inside EHG at 0 °C and −10 °C. They reported that the decrease in T_a would change the temperature distribution in the microenvironment of the garments. The decreased temperature of the heating layer increased the heat transfer temperature difference from the human body to the garment, resulting in increased heat loss. In addition, $\eta$ decreased significantly with T_a. The decrease in T_a further increased the temperature difference between T_a and the heating layer, which was significantly greater than that between the heating layer and T_skin. Therefore, the heat dissipation from the heating layer to the external environment increased. This is consistent with the results of Wang and Lee [20]. They evaluated the $\eta$ of the electrically heated vest (EHV) in cold environments at different T_a. Adding an insulation layer (aerogel) between the heating layer and the external environment reduced the heat dissipation, significantly improving the P_eff and $\eta$ of the EHG. The P_eff and $\eta$ of WAWH were more than twice that of NAWH in a −5 °C windless environment. $\eta$ of WAWH was as high as 95%. For the EHS, the drop in T_a has no significant effect on the P_eff and $\eta$. This may be attributed to the following two factors: part of the surface of the EHS was directly exposed to the cold environment, resulting in large heat loss; the thermal resistance between the skin and EHS was large, resulting in its poor HP. As for the CHIs, the P_eff and $\eta$ rose with decreased temperature, which may be related to the variation in chemical reaction rate.

Elevated V_a significantly reduced the garments’ intrinsic insulation and the additional thermal insulation of the seat. The effect of V_a on the HP of different types of personal heating measures varied. Due to the position of EHS, its HP was worse than that of EHG and CHI integrated into the winter clothing system. Moreover, the pressure from body weight made EHS and trouser surfaces close together. Therefore, the P_eff and $\eta$ of EHS did not change significantly with elevated V_a. I_t decreased exponentially with elevated V_a. In addition, elevated V_a weakened the improvement of heating on the I_t of EHG. The effects of V_a on P_eff and $\eta$ were closely related to T_a. Song et al. [22] evaluated the CPP of EHG in a cold environment at 2 °C, indicating that V_a had a minor effect on P_eff and $\eta$. However, elevated V_a reduced the P_eff and $\eta$ of EHG in a cold environment at −5 °C in our study, consistent with the findings of Wang et al. [21]. at 4.5 °C. The P_eff and $\eta$ of EHG had no significant variation with elevated V_a in the cold environment at −10 °C and −15 °C. According to the above results, the effect of V_a and T_a combination on the P_eff and $\eta$ is difficult to clarify, and further studies on this need to be conducted in the future.

4.2. Corrective Power of Personal Heating Measures

A comparison of the corrective power (CP) for the four personal heating measures under different environmental conditions is shown in Figure 14. the CP values for the personal heating measures did not change significantly with T_a. For the three forms of electrical heating with constant power in the experiments (the heated garments without and with an insulation layer and the heated seat), elevated V_a led to significant decreases in CP values. The CP of chemical heating insoles appeared to increase slightly with elevated V_a. the CP of heated garments with an insulation layer can reach up to 8 K or more. The following four heating measures were ranked from highest to lowest according to CP values: EHG (WAWH), EHG (NAWH), EHS, and CHI. The CP values of EHGs were higher owing to the larger surface area covering the human body compared to the other two heating measures. In addition, the aerogel insulation layer effectively improved the CP value of EHG. According to the study of Deng et al. [29,30], CHI can significantly improve local and overall thermal comfort in practical applications. Therefore, the energy efficiency ratio of thermal comfort of CHI (the improvement of thermal comfort/energy consumption) is substantial, although the CP value of CHI was the lowest.

4.3. Limitations and Future Work

Thermal manikin maintained a stable T_sk during the test to quantify the HP of personal heating measures. However, due to human physiological regulation [7], local thermal sensitivity differences [31], and blood flow distribution characteristics [32,33], the actual local skin temperatures are not uniformly stable [27,34], affecting the actual heating performance of personal heating measures. In addition, subjective thermal perception is equally critical for evaluating the operational effectiveness of heating measures. Therefore, the actual application effect of personal heating measures requires further study by human subject experiments.

Moreover, latent heat loss is neglected since sensible heat loss from the skin will be the main component of heat loss from the body surface due to the large temperature difference between the skin and the ambient in extremely cold weather. In addition, people would not sweat at low activity intensities, so the latent heat dissipation from the skin would be the diffuse heat dissipation of the skin, and its value would be negligible. However, the moisture permeability of clothing is essential for outdoor workers with higher activity intensities. The latent heat loss from the skin is a concern and is influenced by intrinsic clothing insulation and materials. Further study should be conducted on this point.

5. Conclusions

This study quantified and evaluated the “weakening effect” of cold and windy environments on the CPP of clothing and the enhancement brought by personal heating measures through thermal manikin tests in a cryogenic chamber. The effects of environmental parameters on the HP of personal heating measures were discussed. The main conclusions are as follows:

(1)

The effects of environmental parameters on basic CPP were analyzed. T_a had no significant effect on clothing insulation. However, elevated V_a significantly decreased the clothing insulation. When V_a was elevated from 0 m/s to 1 m/s, the clothing insulation of typical winter garments worn by the thermal manikin decreased from 0.6 to 0.9 clo;

(2)

The effects of environmental parameters on the HP of different personal heating measures were analyzed. Decreased T_a and elevated V_a reduced the P_eff and $\eta$ of electrical heating measures. $\eta$ was no longer affected by variations in T_a and V_a when $\eta$ dropped to a certain level (EHS: about 25%). In addition, the effect of V_a is related to T_a. However, the effect of V_a and T_a combination on the P_eff and $\eta$ is challenging to clarify, and further studies on this need to be conducted in the future. For the chemical heating measure, HP instead increased with the decreased T_a and elevated V_a;

(3)

The improvement of the new material aerogel on the CPP and HP of EHG was obtained. I_cl of WANH increased by 0.2 clo, equal to the improvement of NAWH, indicating the aerogel layer provided good thermal insulation. In addition, the aerogel layer significantly improved the HP of EHG. The P_eff and $\eta$ of WAWH were more than twice that of NAWH in a −5 °C windless environment. $\eta$ of WAWH was as high as 95%. However, the enhancement effect of aerogel insulation decreased with decreased T_a and elevated V_a;

(4)

The CP values of different personal heating measures were calculated and investigated. In the nine environmental conditions in this study, the CP of EHG (WAWH), EHG (NAWH), EHS, and CHI could reach 5.8–8.4 K, 3.2–4.3 K, 1.9–3.9 K, and 0.3–1.1 K, respectively. However, the energy efficiency ratio of thermal comfort of CHI (the improvement of thermal comfort/energy consumption) is substantial, although the CP value of CHI was the lowest. Therefore, the actual application effect of personal heating measures should be further studied by human subject experiments.