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R. Kumar et al.
and related adverse effects. Various technologies such as hot water bottles, electric
pads, electric blankets, and insulating clothing are being used to provide the thermal comfort for inhabitants living in extremely cold conditions (Watson and Watson
1976). The insulating clothing maintains body temperature by reducing the rate of
heat transfer between the human body and surrounding ambient. This method may
be effective for a shorter time duration as there is no heating source. The relief time
using hot water bottles is less as it utilizes the specific heat of water (4 kJ kg
−1 ) and
also provides heating in a large temperature window, not soothing to the human body.
The electric power based heating products utilizes a temperature controller such as
a thermostat to regulate the temperature of these devices. In the case of any failure
of the temperature controller, the temperature of the heating device may shoot and
cause serious skin burn causality. Further, these devices are not cost effective and not
suitable for outdoor applications. The coal-based heating devices such as Kangaries,
Bukharies release CO 2 , CO gases and are dangerous for the health. These heating
systems are also prone to the skin cancer incidences (Wani 2010). The phase change
material (PCM) based technologies such as PCM heat packs are very promising for
thermal comfort under such conditions and therapeutic applications at the time of
requirement. It utilizes a phase change material as thermal energy storage media. The
thermal energy storage capacity of PCM is very high as compared to the sensible
storage materials such as water because of the latent heat of fusion of PCMs while
changing the phase of material from solid to liquid. For example, the latent heat values (and melting temperature) of some of salt hydrates CaCl 2 .6H 2 O, LiNO 3 .3H 2 O,
Na 2 CO 3 .10H 2 O, Na 2 SO 4 .10H 2 O are 174–191 kJ kg
−1 (29–30 °C), 296 kJ kg
−1
(30 °C), 267 kJ kg
−1 (32 °C), and 241 kJ kg
−1 (32.4 °C), respectively (Barrett et al.
1984; Barrett and Best 1985). Further, it releases heat in the very limited temperature
range (near phase change temperature of PCM), therefore it may be more effective
for body heating applications. The hydrated salt PCMs are being used in the PCM
heat packs. The hydrated salt PCMs are known for their large supercooling and may
remain in a metastable supercooled liquid state far below PCM’s melting temperature before solidification. This property of hydrated salts makes them very useful for
storing heat at the time of availability and releasing it later at the time of requirement
(Barrett et al. 1984; Barrett and Best 1985). These PCM heat packs are reusable,
reliable, and cost-effective, providing heat at body soothing temperature (Ulman and
Valentin 1983). Among numerous PCM based heat packs, the sodium acetate trihydrate (SAT) based heat packs has attracted more attention due to its high latent heat
(~270 kJ kg
−1 ), suitable melting temperature (~58 °C), large supercooling (up to −
10 °C), and stability against large numbers of charging cycles, making it reusable,
and very cost effective (Kimura and Kai 1985). These PCM based heat packs use
a metallic disk containing multiple grooves to start nucleation in metastable supercooled liquid SAT for releasing heat at the time of requirement (Kapralis et al. 1990;
Sandnes and Rekstad 2006). These heat packs can be recharged by heating packs in
hot water for 25–30 min. Rohitash Kumar et al. developed SAT, ethylene glycol and
water-based PCM suitable for body warming in extremely cold climates and medical
applications such as treatment of muscle cramp, frostbite etc. (Kumar et al. 2017;
R. Kumar et al.
and related adverse effects. Various technologies such as hot water bottles, electric
pads, electric blankets, and insulating clothing are being used to provide the thermal comfort for inhabitants living in extremely cold conditions (Watson and Watson
1976). The insulating clothing maintains body temperature by reducing the rate of
heat transfer between the human body and surrounding ambient. This method may
be effective for a shorter time duration as there is no heating source. The relief time
using hot water bottles is less as it utilizes the specific heat of water (4 kJ kg
−1 ) and
also provides heating in a large temperature window, not soothing to the human body.
The electric power based heating products utilizes a temperature controller such as
a thermostat to regulate the temperature of these devices. In the case of any failure
of the temperature controller, the temperature of the heating device may shoot and
cause serious skin burn causality. Further, these devices are not cost effective and not
suitable for outdoor applications. The coal-based heating devices such as Kangaries,
Bukharies release CO 2 , CO gases and are dangerous for the health. These heating
systems are also prone to the skin cancer incidences (Wani 2010). The phase change
material (PCM) based technologies such as PCM heat packs are very promising for
thermal comfort under such conditions and therapeutic applications at the time of
requirement. It utilizes a phase change material as thermal energy storage media. The
thermal energy storage capacity of PCM is very high as compared to the sensible
storage materials such as water because of the latent heat of fusion of PCMs while
changing the phase of material from solid to liquid. For example, the latent heat values (and melting temperature) of some of salt hydrates CaCl 2 .6H 2 O, LiNO 3 .3H 2 O,
Na 2 CO 3 .10H 2 O, Na 2 SO 4 .10H 2 O are 174–191 kJ kg
−1 (29–30 °C), 296 kJ kg
−1
(30 °C), 267 kJ kg
−1 (32 °C), and 241 kJ kg
−1 (32.4 °C), respectively (Barrett et al.
1984; Barrett and Best 1985). Further, it releases heat in the very limited temperature
range (near phase change temperature of PCM), therefore it may be more effective
for body heating applications. The hydrated salt PCMs are being used in the PCM
heat packs. The hydrated salt PCMs are known for their large supercooling and may
remain in a metastable supercooled liquid state far below PCM’s melting temperature before solidification. This property of hydrated salts makes them very useful for
storing heat at the time of availability and releasing it later at the time of requirement
(Barrett et al. 1984; Barrett and Best 1985). These PCM heat packs are reusable,
reliable, and cost-effective, providing heat at body soothing temperature (Ulman and
Valentin 1983). Among numerous PCM based heat packs, the sodium acetate trihydrate (SAT) based heat packs has attracted more attention due to its high latent heat
(~270 kJ kg
−1 ), suitable melting temperature (~58 °C), large supercooling (up to −
10 °C), and stability against large numbers of charging cycles, making it reusable,
and very cost effective (Kimura and Kai 1985). These PCM based heat packs use
a metallic disk containing multiple grooves to start nucleation in metastable supercooled liquid SAT for releasing heat at the time of requirement (Kapralis et al. 1990;
Sandnes and Rekstad 2006). These heat packs can be recharged by heating packs in
hot water for 25–30 min. Rohitash Kumar et al. developed SAT, ethylene glycol and
water-based PCM suitable for body warming in extremely cold climates and medical
applications such as treatment of muscle cramp, frostbite etc. (Kumar et al. 2017;