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A review of intensified conditioning of personal micro-environments: Moving closer to the human body

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Various systems and technologies have been developed in recent years to fulfil the growing needs of high-performance HVAC systems with better performance of energy efficiency, thermal comfort, and occupancy health. Intensified conditioning of human occupied areas and less intensified conditioning of surrounding areas are able to effectively improve the overall satisfaction by individual control of personalized micro-environments and also, achieve maximum energy efficiency. Four main concepts have been identified chronologically through the development of personal environmental conditioning, changing the intensified conditioning area closer to the human body and enhancing conditioning efforts, namely the task ambient conditioning (TAC) system, personal environmental control system (PECS), personal comfort system (PCS), and the personal thermal management system (PTMS). This review follows a clue of the concept progress and system evaluation, summarizes important findings and feasible applications, current gaps as well as future research needs.
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Energy and Built Environment 2 (2021) 260–270
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Energy and Built Environment
journal homepage: http://www.keaipublishing.com/en/journals/energy-and-built-environment/
A review of intensied conditioning of personal micro-environments:
Moving closer to the human body
Bin Yang
a
, Xin Ding
b
, Faming Wang
c
,
, Angui Li
a
a
School of Building Services Science and Engineering, Xi’an University of Architecture and Technology, Xi’an, China
b
Department of Civil Engineering, Purdue University, West Lafayette, USA
c
School of Architecture and Art, Central South University, Changsha, China
Keywords:
Task ambient conditioning
Personal environmental control system
Personal thermal management system
Various systems and technologies have been developed in recent years to full the growing needs of high-
performance HVAC systems with better performance of energy eciency, thermal comfort, and occupancy health.
Intensied conditioning of human occupied areas and less intensied conditioning of surrounding areas are able
to eectively improve the overall satisfaction by individual control of personalized micro-environments and also,
achieve maximum energy eciency. Four main concepts have been identied chronologically through the devel-
opment of personal environmental conditioning, changing the intensied conditioning area closer to the human
body and enhancing conditioning eorts, namely the task ambient conditioning (TAC) system, personal envi-
ronmental control system (PECS), personal comfort system (PCS), and the personal thermal management (PTM)
system. This review follows a clue of the concept progress and system evaluation, summarizes important ndings
and feasible applications, current gaps as well as future research needs.
1. Introduction
Building related energy consumption increases strikingly with the
ever-higher demand on building conditioning due to frequent heatwaves
and cold spells in recent years [1] . People spend 80-90% of their daily
time in indoor environments [2] . Sick building syndrome (SBS), one of
the most commonly seen phenomena caused by poor indoor air qual-
ity, has become a serious concern for modern oces. SBS could cause
headaches, dizziness, sore eyes and throat, or reduction of productiv-
ity [3] . A high concentration of pollutants may further induce chronic
respiratory diseases and even a reduction of the life expectancy [4] .
Under spatially uniform conditioning, the occupants cannot be eec-
tively prevented from indoor air pollutants. Meanwhile, energy waste
is unavoidable because of the intensied conditioning for non-occupied
spaces.
Abbreviations: AC, Air cooling; AH, Air heating; ASHRAE, American Society of Heating Refrigerating and Air-Conditioning Engineers; CH, Chemical heating;
CHC, Chemical heating clothing; COP, Coecient of performance; EC, Evaporative cooling; EH, Electric heating; EHC, Electrical heating clothing; FH, Fluid heating;
HVAC, Heating, ventilation, and air conditioning; HYC, Hybrid cooling; ITVOF, Infrared transparent and visible opaque fabrics; LC, Liquid cooling; PCC, Personal
cooling clothing; PCD, Personal comfort device; PCMC, Phase change material cooling; PCMH, Phase change material heating; PCMs, Phase change materials; PCS,
Personal comfort system; PE, Polyethylene; PECS, Personalized environmental control system; PEM, Personal environment module; PES, Personalized exhaust system;
PI, Passive insulation; PMV, Predicted mean vote; PP, Polypropylene; PPD, Predicted percentage dissatised; PTMS, Personal thermal management systems; PV,
Personalized ventilation; RH, Relative humidity (%); SBS, Sick building syndrome; TAC, Task ambient conditioning; TCV, Thermal comfort vote; TEC, Thermoelectric
cooling; TECU, Thermoelectric energy conversion unit; TSV, Thermal sensation vote.
Corresponding author.
E-mail address: dr.famingwang@gmail.com (F. Wang).
Buildings consume around 20% to 40% of the total energy consump-
tion. If it keeps the status quo, the rate of building energy consump-
tion growth would have an annual increase of around 2%. The colossal
amount of energy consumption also led to serious environmental prob-
lems. Energy eciency becomes an urgent call for governments and
societies in most countries. Reduction in energy consumption could not
only result in economic savings but also environmental improvements
[1] .
The Predicted Mean Vote/Predicted Percentage Dissatised
(PMV/PPD) model and adaptive model were commonly used for eval-
uating occupants’ thermal comfort. A uniformed indoor environment,
which is considered acceptable for over 80% percent of occupants,
is provided for all occupants, leaving around 20% percent of people
unsatised. To achieve a higher satisfaction ratio among groups of
people with large individual dierences, the heating, ventilation, and
air conditioning (HVAC) system is required to be operated at a higher
https://doi.org/10.1016/j.enbenv.2020.06.007
Available online 6 July 2020
2666-1233/Copyright © 2020 Southwest Jiatong University. Publishing services by Elsevier B.V. on behalf of KeAi Communication Co. Ltd. This is an open access
article under the CC BY-NC-ND license ( http://creativecommons.org/licenses/by-nc-nd/4.0/ )
B. Yang, X. Ding, F. Wang et al. Energy and Built Environment 2 (2021) 260–270
performance level, which ends up with a larger amount of energy
consumption but a relatively slight satisfaction ratio increment. New
systems, considering individual dierence, were developed as a solution
for solving the dilemma. As a non-uniformed conditioning system, the
new system provides a thermally comfortable micro-environment to
each user, which allows personalized control of the micro-environment.
Combining with the conventional HVAC system as a background
system, the new system has been proved having the capability of
ensuring over 90% percent of occupant’s thermal satisfaction, elevating
inhaled air quality (convective manner by fresh air) while reducing a
signicant amount of energy consumption [5–7] .
This paper describes the concept development of personal environ-
mental conditioning systems in a chronological order. Beginning from
early 1990s, the concept of “Task Ambient Conditioning (TAC) System
was proposed by combing several relatively conventional strategies such
as zoning and occupant-dened comfort to provide individual occupants
better thermal comfort. The controlled area included the occupant and
a part of his/her working area [8] . In the 2000s, the “Personalized En-
vironmental Control System (PECS) ”emerged as a further development
of TAC. A PECS focuses not only on occupant thermal comfort improve-
ment but also takes the air quality in the occupied area into considera-
tion. The controlled area shrank to the torso or head of the human body
[9] . Two sub-concepts, “Personal Comfort System (PCS) ”and “Person-
alized Ventilation (PV) System ”, aiming at enhancing thermal comfort
and air quality respectively, were proposed. The two sub-concepts were
applied to dierent conditions. After PECS, the concept was developed
again with “Personal Thermal Management System (PTMS) ”added in.
PTMS are primarily focused on the enhancement of occupant thermal
comfort. The controlled area is the microclimate close to the user’s skin.
The usage of PTMS enabled the conditioned area moving closer to users
rather than being limited in a x position [10] .
Presently, a very limited number of reviews have been published on
the conditioning of personal micro-climate [ 11 13 ]. The review paper
by Veselý and Zeiler [11] did not distinguish the TAC system and PECS,
rather than discussing the thermal comfort and energy performance of
the concept as a whole. Godithi et al. [12] overviewed technologies of
personal environmental control systems (PECS) including task/ambient
control (TAC) and personal ventilation (PV) systems. The most recent
development on the personal thermal management systems (PTMS) has
not reviewed. More recently, Rawal et al. [13] comprehensively sum-
marized various personal comfort system, but very limited information
was made on the personal thermal management systems. Another re-
view work comprehensively described numerous forms of ventilation
and air distribution systems, which included the PV system as a part of
the non-uniformed ventilation systems [14] . Studies of PTMS, as a con-
tinuation of the TAC and PECS, have not been systematically reviewed
and comprehensively evaluated yet. It should be mentioned that, in this
review, it is the authors’ intention to give brief introductions to TAC and
PECS, and an extensive overview was focused on PTMS.
During the development of aforementioned systems, the conditioned
area was gradually reduced and closed to the human body ( Fig. 1 ). A
comprehensive review of the TAC, PECS and PTMS is given, in terms of
their energy performance, thermal comfort management, improvement
of air quality, and relative applications.
2. Methodology
To conduct the searching process, “task ambient conditioning ”,
“personalized ventilation/cooling/heating ”, “personal comfort manage-
ment ”, “personal comfort system ”, “personal comfort device ”, “personal
environment control ”, “wearable thermal devices ”, “personal cooling
system ”, “personal heating clothing ”and related items were used as
keywords to search for inuenced publications through the database
of ScienceDirect, Web of Science, Wiley Online Library, China National
Knowledge Infrastructure.
Academic publications that developed the concepts of aforemen-
tioned systems, implemented new methods and technologies to test the
system performance, improved forms and components of the system for
better overall performance, designed and tested new applications, tested
system performance in varying conditions, are considered strongly re-
lated with the focuses of this paper.
Experimental data that is commonly reported and consistent with
theoretical are extracted. Distinguishing values and extreme values that
are considered representative of the performance of the systems, and
the conditions they were found are extracted and discussed.
The reviewed publications are sorted by their publication dates, list-
ing with an order as TAC system, PECS, and PTMS in the references,
which gives a clear clue of how this paper develops and how each sec-
tion is formulated.
3. TAC (Task Ambient conditioning)
3.1. Working principle and available devices
TAC is a strategy that distributes thermally conditioned air to spec-
ied thermal zones surrounding the occupants. TAC allows occupants
regulate supply air (e.g. air velocity, supply air volume, supply air tem-
perature, supply air direction) to achieve a better eect of personal
thermal satisfaction [ 5 , 6 , 8 , 15–19 ]. TAC contributes to less intensifying
requirements of ambient environments, and thereby, to reduce energy
consumption as well as provide comfortable and healthy work environ-
ments [7] . Attentions were mainly paid to provide local thermal comfort
without supplying 100% fresh air. TAC systems were typically designed
for commercial oce buildings. Forms of air terminal units of a TAC
system are commonly located closely to occupants. Dierent forms of
TAC systems may include raised-oor distribution unit, desk-mounted
unit, desk-edge-mounted type, ceiling-mounted grill, desk fan, desk-top
unit, partition unit, personal environment module (PEM), and combi-
nations of some of those systems, etc. In Fig. 2 , commercially available
TAC systems, including PEM and ClimaDesk, are demonstrated.
Multiple experiments have been performed in controlled environ-
ment chambers by using thermal manikins and human subjects. Field
studies were also performed to analyze practical applications under non-
uniform thermal environments created by TAC systems. Most cases were
based on temperate and tropical climates. Few modules have the heat-
ing function. Questionnaire surveys were frequently used to measure
the impact of TAC system. Sitting and standing positions were all tested
while walking activity was seldom tested.
3.2. Cons and pros
TAC systems signicantly improve thermal comfort. Supply air tem-
perature varies from 19°C to 25°C [20] . A temperature dierence up to
2.5°C between the ambient environment and controlled area was ob-
served [ 6 , 15 ]. Higher air velocity was found preferred by occupants
without cool air supply. It is found that the occupants will generally
increase the supply air velocity when they gained control. The maxi-
mum air velocity was reported being raised to around 3 m/s in some
rare cases [15] . The supply air volume has a strong impact on stratica-
tion [6] . With increasing supply air velocity, the risk of draft becomes
a serious concern. However, the initiative of control allowed a higher
tolerance of supply air velocity [ 6 , 18 , 21 ]. Although cool and fresh air
are preferred, occupants could show acceptability for surrounding air
temperature from up to 30.5°C with the usage of recirculating air [22] .
Air velocity higher than the ASHRAE standard limitation was found ac-
ceptable and even preferred by occupants. Occupants commonly report
a higher level of satisfaction due to better temperature and ventilation
conditions compared with the conventional systems [ 15 , 23 , 24 ]. Under
increased activity levels, the desk-top conditioning system could main-
tain temperature in workstation 1-2°C lower than ambient.
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B. Yang, X. Ding, F. Wang et al. Energy and Built Environment 2 (2021) 260–270
Fig. 1. The intensied conditioning area moving closer to the human body.
Fig. 2. Dierent prototypes of TAC systems [5–8] .
Air quality, even not be intended, is believed to be another most af-
fected category by the TAC system, especially being found promoted in
the breathing level. The air change eciency of the TAC system was re-
ported (1.4 to 2.7) higher than that of conventional displacement ven-
tilation [ 20 , 25 ]. To test the best performance of the oor-based task
ventilation system on improving indoor air quality, a task supply us-
ing 100% fresh air was conducted, and the age of air in occupancy
breathing level was found 20%~40% lower than that of the mixing
ventilation when supplying straight towards occupants. Concentration
of smoke particles when using the oor-based task ventilation system
could be 50% lower than the average workspace concentration. Stud-
ies found that the workspace controlled by TAC system could not be
completely protected from a pollution source located in adjacent work-
ing spaces under the operation of a task ventilation system. However,
the height where transmit pattern occurred is higher than the breathing
zone, which could leave seated occupants unaected. A novel param-
eter, calculated by the Archimedes number of the supply jets and the
ratio of total supply ow rate divided by the magnitude of internal heat
loads, was proposed and found to have a linear relationship with the
change in the age of air with height, which could serve as a new strat-
egy of evaluation [26] . Up to 25% decrease of the probability of allergy
and up to 50% decrease that of sick building syndrome (SBS) could also
be expected by TAC based on currently existing technologies and pro-
cedures [ 27 , 28 ].
Applications of TAC system could contribute to economic benets
by energy saving and cost reduction. By maintaining suitable thermal
comfort around the occupied area, ambient space conditions could be
controlled and maintained under a lower standard, which may result in
lower energy consumption of the background HVAC system. Up to 15%
of electricity usage could be saved by a TAC system comparing with us-
ing the conventional HVAC system (mixing ventilation) [7] . However,
under some circumstances, energy consumed by the air distribution sys-
tem could rise due to the increased amount of personalized air and in-
creased amount of fan in every air terminal device [24] . The increment
of occupant’s productivity, which will lead to a considerable amount of
economic interests increment, was believed to have a signicant posi-
tive relation with high overall-satisfaction of the working environment
[ 7 , 9 , 17 , 19 , 27 , 29 ]. A reduction of illnesses, allergy, asthma symptoms,
and other building related diseases, by using TAC, was also announced
that could lead to an increase in productivity [ 27 , 28 ]. However, re-
ported annual productivity improvement only varied 0.08% to 2.8% due
to increased thermal comfort and air quality by the application of TAC
system alone [7] .
There are still some research gaps remaining in this period of study
of TAC system. Although being claimed capable of operating the TAC
system to achieve thermal comfort in varying conditions, occupants typ-
ically operate the supply air velocity, direction, volume and other pa-
rameters infrequently (from daily to monthly) and insuciently [30] .
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B. Yang, X. Ding, F. Wang et al. Energy and Built Environment 2 (2021) 260–270
Fig. 3. Dierent prototypes of PECS (PV) [ 31 , 34 , 37–39 ].
Usually, the optimal energy saving potential and thermal comfort im-
provement cannot be fully achieved.
4. PECS (Personalized Environmental Control System)
4.1. Working principle and available devices
The concept of PECS, emerging in early 2000s, is based on the TAC
system. Aiming to not only thermal comfort but also air quality, clean,
cool and dry air are supplied to occupants. The controlled area of some
PECS is narrowed down to occupants’ breathing zone [ 9 , 28 ].
Forms of PECS include desk mounted systems [ 31 , 32 ], desk-edge
mounted systems [ 20 , 25 ], ceiling mounted systems [33–36] , chair-
based systems ([37], bed-based systems [ 38 , 39 ], and varying types of
combinations ( Fig. 3 ).
During the development of PECS, a dichotomous branch of the stud-
ies was documented. What should PECS mainly focus on, thermal com-
fort or air quality? In this section, personalized ventilation (PV) system
is considered as one branch of PECS followed by the personal comfort
system (PCS) as another branch.
The PV system, mainly focuses on the capability of improving in-
haled air quality to avoid cross infection, sick building syndrome (SBS)
and other building related illness [ 9 , 28 ]. By supplying 100% fresh air,
PV system can signicantly improve inhaled air quality in breathing
zone to achieve a triumph of strongly protecting users from cross infec-
tion and contaminant inhalation in high polluted area such as hospitals
and industrial circumstances, and places with a high occupant density
[40–44] .
Supply momentum and temperature of personalized air are consid-
ered having a high impact on the performance of the PV system [ 40 , 45 ].
The supply airow rate that has been put into test varies from 4 L/s up
to 23 L/s. In some cases, supply air velocity was reported could be ele-
vated up to 0.9 m/s [ 23 , 31 , 34 ]. Supply air velocity over 0.2 m/s could
break the thermal plume around the human body to reduce the mixture
of personalized air and ambient air. As a result, better inhaled air qual-
ity can be achieved [41] . The distance between air terminal devices
and occupants’ facial area also plays a critical rule in the inhaled air
quality. Therefore, the importance of proper air terminal devices design
which would determine characteristics of supplying airow was exten-
sively studied by researchers. Several nozzles were designed and tested
to compare ventilation eects and energy saving potentials [ 34 , 46–48 ].
Several methods and indices were developed to evaluate the eciency
of PV system. Compared with mixing ventilation, ventilation eective-
ness of the PV system is up to 50% higher.
Background ventilation system, working together with PV system,
has an impact on the pollutant removal eciency [49–51] . Dierent
types of background ventilation system would cause dierent mixture
between personalized supply air and surrounding polluted air. The less
the mixture process, the better the inhaled air quality. With dierent
combinations of background ventilation systems, PV systems might have
dierent performances, not only for air quality but also on thermal com-
fort.
The performance of PV system for occupants with lying positions
was tested within ward and bedroom environments [38] . PV system
can provide a thermally neutral environment to users [39] . Sleeping
quality is also reported higher than conditions without the PV system
[52] . Cross infection between patients can be avoided in hospital wards
[53] . To provide more personalized air towards occupants’ breathing
area, a “Personalized Exhaust System ”(PES) placing small exhaust ducts
around occupants has been introduced. PV system combined with the
PE system could extract exhaled air in a very short period of time and
increase the proportion of fresh air in inhaled air than the PV system
alone [54] .
For personal comfort system (PCS), more attention is paid to the as-
pects of personal thermal comfort. In winter condition, radiant heat-
ing devices (e.g. footwarmer, legwarmer, kneehole radiant panels,
hand/palm warmer, heated chair, etc.) are mainly used to stimulate lo-
cal body segments ( Fig. 4 ) [ 55 60 ]. Ambient room temperature can be
reduced to some extent. In summer condition, elevated air movement
created by dierent types of electric fans (e.g. ceiling fan, stand fan, ta-
ble fan, small USB fan, box fan, fan aided ventilated chair, etc.) or local
isothermal jets (e.g. nozzle, slot diuser in desk or partition, etc.) can
improve thermal comfort by enhanced isothermal convective cooling
[ 56 , 61–67 ]. Ambient room temperature can be elevated to some extent.
Compared with negative draft for neutral-slightly cool thermal environ-
ments, elevated air movement can be regarded as a positive factor for
neutral-slightly warm thermal environments. By using aforementioned
PCS, the dead band can be expanded to achieve energy eciency by less
intensied ambient heating/cooling during winters/summers [68] .
4.2. Cons and pros
Without supplying fresh air into personal environments, PCS can
only improve personal thermal comfort but not inhaled air quality.
That’s why PV system, as one competitive alternative, is necessary es-
pecially for heavily polluted and infectious premises, although the limit
of extended air duct exists. As further development of PCS, PTMS im-
prove personal thermal comfort by moving closer to the human body
and further narrowing down personal environment by mainly consider-
ing attirement.
5. PTMS (Personal Thermal Management System)
In indoor environments, there are four adaptive ways to help occu-
pants achieve individual thermal comfort. In non-air-conditioned build-
ings, the most often used method to adjust indoor thermal environments
is the opening of windows and/or doors [69] . Opening windows is con-
sidered as the most favored measures to improve air quality as well as
the indoor thermal comfort. In air-conditioned buildings, HVAC is the
most popular measure to provide occupants thermal comfort conditions.
It has been well known that the use of HVAC systems results in enormous
energy consumption [70–73] . Besides, HVAC conditioned thermal envi-
ronments could only ensure up to 80% of occupant satisfaction [74] .
Third, small personal comfort devices (PCDs) such as electric fans and
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B. Yang, X. Ding, F. Wang et al. Energy and Built Environment 2 (2021) 260–270
Fig. 4. Dierent prototypes of PECS (PCS) [55–68] .
radiators could also be used to improve individual thermal comfort [75–
80] . Compared to HVAC systems, PCDs consume much less heating or
cooling energy and the power consumption of personal comfort devices
is often less than 1500 Wh (watt-hour). The fourth method to main-
tain indoor thermal comfort is the adjustment of clothing [81] . Tradi-
tional clothing has a very limited capability to help occupants maintain
thermal comfort because traditional clothing serves as an unchangeable
thermal insulator between the human body and its surrounding envi-
ronment [82] .
In recent years, the concept of personal thermal management
emerges as a promising approach to further enhance individual thermal
comfort as well as to help pushing building energy consumption to the
minimum limit [ 83 , 84 ]. The main goal of personal thermal management
is to help enhance heat exchange between an individual body and wear-
able systems such as clothing incorporated with wearable cooling and
heating devices. In hot indoor environments, PTMS enhance body heat
dissipation so that the body temperature could be maintained in ther-
mally neutral range. Conversely, in cool or cold indoor environments,
PTMS help occupants preserve body heat or even receive heat energy
from either the PTMS or the surrounding environment. Hence, PTMS
have a much high energy eciency compared to other means such as
the usages of HVAC systems and PCDs. In addition, PTMS are portable,
exible, light-weight and have almost no restrictions on body move-
ment. Besides, PTMS are environmentally and ergonomic friendly and
consume minimal energy.
Generally, personal thermal management systems may be catego-
rized into two main groups: PTMS incorporated with cooling and/or
heating modules and PTMS made of specially designed materials or
with unique structures. Presently, heating and cooling modules/units
may be incorporated with clothing include air ventilation fans, vortex
cooling unit, thermoelectric module, phase change materials (PCMs),
warmers, resistance wires, Janus brous membrane, conductive fabric
pads, solar heating systems, thermoelectric heater and coolers, portable
refrigerator, pre-cooled air or liquid circulating tubes, dry ice and
frozen gels [ 82 , 85–94 ]. PTMS made of specially designed materials
or with unique structures may include fabrics made with yarns en-
capsulated with PCMs, infrared transparent and visible opaque fabrics
(ITVOF), metallic nanowire-coated textiles, ultrathin graphene paper,
graphene and carbon-based materials, and moisture management tex-
tiles [ 10 , 83 , 84 , 95–100 ].
The concept of PTMS incorporated with heating and cooling devices
such as ventilation fans, uid circulation tubing, PCMs, and resistance
wire is not new [ 101 , 102 ]. The energy draw of PTMS incorporated with
wearable heating/cooling devices is normally less than 30 W. Various
types of personal cooling clothing (PCC) have been developed during
past eight decades. In general, personal cooling mechanisms may be di-
vided into six categories: phase change material cooling (PCMC), evapo-
rative cooling (EC), liquid cooling (LC), air cooling (AC), thermoelectric
cooling (TEC) and hybrid cooling (HYC) combining more than one cool-
ing technique [89] .
PCM cooling replies on PCMs such as frozen gels, ice, and parans
to provide body cooling during melting. PCMs should be placed closely
to the human body to draw body heat. Cooling eect of PCMs is deter-
mined mainly by PCM’s melting temperature, specic heat, latent heat of
fusion, total mass of PCMs, body coverage area, location of PCMs inside
clothing, ambient temperature, and insulation between PCMs and the
ambient environment [103–106] . For example, the larger the amount
of PCMs being incorporated into clothing, the better the cooling per-
formance. Unfortunately, the entire PCM cooling system will be heavy.
Existing PCM cooling vests weighs about 1.5–2.5 kg per piece. The ef-
fect of a cooling vest incorporated with PCM packs (total weight of 2.2
kg) on occupant thermal comfort at 34°C with RH = 60% was examined
[87] . Results showed that the torso skin temperature of occupants was
decreased by 2-3°C and overall torso temperature remained at 33.3°C
during the 90 min experimental duration. Besides, overall thermal sen-
sation, torso thermal sensation and skin wittedness sensation have been
greatly improved by the PCM cooling vest.
EC may be achieved by wearing clothing incorporated with dry ice
or clothing with the capability to store moisture. Dry ice incorporated
clothing was found to be eective in mitigating heat strain in hot and
humid conditions, but they may cause local body cold discomfort due
to the huge amount of heat absorbed during sublimation [ 107 , 108 ].
Cooling performance of EC clothing with stored moisture is mainly de-
termined by ambient humidity and such EC clothing normally performs
good in dry environments because water evaporation is dependent on
the water vapour pressure gradient between the wetted surface and the
ambient condition. EC clothing is light, cheap, environmentally friendly
and convenient to be used as compared to other wearable cooling sys-
tems.
LC utilizes pre-cooled liquid, usually water, circulating in tubes over
the skin surface, to conduct the body heat away [102] . Performance of
LC clothing is aected by the inlet liquid temperature, tubing length,
the diameter of the tube, ow rate, body coverage area, location, tub-
ing density, clothing t and cooling control modes [107] . LC is powerful
because water has a very high specic heat. Nevertheless, LC clothing is
normally heavy, bulky and cold water may cause condensation on the
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B. Yang, X. Ding, F. Wang et al. Energy and Built Environment 2 (2021) 260–270
Fig. 5. A cooling collar incorporated with (a)
PCMs and (b) the portable liquid cooling vest.
tubing so that the wear comfort could be greatly aected ( Fig. 5 ). LC
technique could also be used for fabricating cooling scarfs so that vi-
tal body parts such as the neck (where the carotid arteries are located)
can be cooled down in warm indoor environments. A LC collar was de-
veloped and the eect of LC collar was explored on occupant thermal
comfort at 33°C environment [86] . The liquid temperature was main-
tained at 16°C. Results demonstrated that the occupant thermal comfort
has been greatly improved by using the LC collar in the studied warm
environment.
AC utilizes circulated natural air or pre-cooled air to enhance convec-
tive heat loss at the body surface. Existing AC systems can be engineered
by incorporating arrayed ne hoses with pores into high elastic fabrics
to direct air ow to the skin surface. The air ow is powered by an air
compressor and sometimes a Ranque–Hilsch vortex tube could be ap-
plied to pre-cool the supply air. An AC system could also be fabricated
by mounting small air ventilation fans to clothing at discrete clothing
locations such as the lateral back [ 109 , 110 ]. Such AC clothing circu-
lates natural air to the clothing microclimate so that excessive body heat
could be dissipated to the ambient environments via convection and/or
evaporation. AC clothing powered by small fan units could greatly im-
prove indoor occupant thermal comfort in warm indoor environments
at 32°C [90] . Air ventilation clothing consumed only about 5 Wh energy
whereas PCDs such as a traditional desk fan has a power consumption of
40 Wh. Hence, the use of air ventilation cooling clothing could provide
7-8% more energy saving as compared to desk fans. The AC shirt could
also signicantly reduce the occupant local temperatures at the scapula
and the chest in a hot environment (38°C, 45%RH) [111] . Occupants
reported cooler TSVs during the initial 10 min after the AC shirt was
worn. However, the cooling eectiveness of the AC shirt was not high
on occupants while performing light oce work under the studied high
temperature.
Over the past decade, the use of thermoelectric cooling (TEC) to fab-
ricate PTMS has garnered considerable research attention. Thermoelec-
tric devices are developed based on the so-called Peltier–Seebeck ef-
fect. Thermoelectric cooler serves as a micro heat pump that creates a
heat ux at the junction of two types of materials, which can be used
to generate either cooling or heating energy. Solid-state thermoelectric
modules are compact, light-weight, reliable, environmentally friendly
for refrigeration and more important, the cooling power is readily ad-
justable. A portable thermoelectric energy conversion unit (TECU) was
developed and incorporated into clothing [93] . A micro blower contin-
uously supplies TECU cooled ambient air to the clothing microclimate
through a tree-like rubber tube network, which was knitted into the
TEC clothing. An axial fan was located at the air rejection side. Ther-
mal manikin tests showed that the TEC clothing could provide a cool-
ing power of 24.6 W and expand the building temperature setpoints
by around 2.2°C. Thereby, the use of TEC clothing could result in 15%
building energy saving. Further, thermoelectric modules could also be
designed as wearable devices such as a bracelet or a head cooler ( Fig. 6 ).
The Embr Labs Inc. (Boston, MA) developed a commercial wearable
bracelet called Embr Wave wrist band which was worn on the inside
of the wrist and delivered cooling rhythms tuned to human temperature
perceptions. This wearable device has a cooling area of 6.25 cm
2 and
the amplitude of wave rhythms can be adjusted. The eect of wearing an
Embr wave bracelet on occupant thermal comfort was evaluated [94] .
Results demonstrated that the Embr bracelet could improve overall ther-
mal comfort, thermal sensation by 0.5–1.0 scale unit, which is roughly
equivalent to 2–3°C temperature dierence during the 3 to 45 min time
period of use. A high coecient of performance (COP = 1.5) wearable
TEC device was fabricated [112] . This TEC device was worn on the arm
and infrared thermo-images showed that the local temperature of skin
covered by the TEC device was 2.5°C lower than the rest of the skin. The
TEC device could expand the indoor ambient temperature zone to 36°C
and thereby save 20% of the energy for a typical building. Similarly,
a wearable TEC cooler was developed and applied to the wrist area for
personal cooling [113] . This newly developed TEC device was composed
of several p- and n-type TEC thermocouples that were connected elec-
trically in series and thermally in parallel. This TEC cooler could cool
down the human skin up to 8.2°C below the ambient temperature. Be-
sides, cost-benet analysis revealed that the cooling over material value
of the new TEC cooler was 5 times greater than commercial modules.
Despite the evident cooling eect on occupant thermal comfort shown
by TEC devices, the limitations of TEC are obvious. TEC devices con-
sume a considerable amount of electricity to generate cooling energy
and thereby they have a high requirement on battery capacity. If a bat-
tery with a reasonable weight is desired, the TEC has to be designed
in small size (e.g. a bracelet). Small sized TEC devices have a very lim-
ited cooling capacity and hence, cooling eect on the human body is
265
B. Yang, X. Ding, F. Wang et al. Energy and Built Environment 2 (2021) 260–270
Fig. 6. The Embr TEC bracelet (Embr Labs, Boston, MA, USA) and a commercial TEC forehead cooler.
relatively limited. On the other hand, TEC devices should cover a much
greater body surface area to provide signicant body cooling. High per-
formance TEC devices require large capacity batteries for extended us-
age duration, which makes the overall weight of the TEC cooling system
relatively heavy. In addition, presently the thermoelectric conversion ef-
ciency is still much lower compared to a refrigeration system [114] .
The majority of existing personal cooling systems (PCS) use one cool-
ing technique to provide cooling energy to occupants. One of the known
limitations of such PCS is that they function well in a certain range of
environmental conditions but their cooling performance under the rest
of thermal conditions tends to be pretty low. For instance, PCS incorpo-
rated with PCMs showed good cooling performance in warm and humid
environments under which both the dry heat and latent heat transfer are
restricted. In order to further extend the application of PCS to a wider
range of ambient environments, hybrid cooling (HYC) combines multi-
ple cooling techniques in a clothing system has been proposed [115] .
A portable HYC clothing was developed and its impact on the improve-
ment of occupant thermal comfort in a hot indoor environment (34°C,
65%RH) was investigated [89] . It was found that the HYC clothing could
remarkably improve overall thermal sensation votes (TSVs), skin wet-
ness sensation votes and thermal comfort votes (TCVs). Besides, thermo-
physiological parameters such as the mean skin temperature and total
sweat production were also greatly reduced while using HYC clothing
as compared to no cooling (i.e. control).
Despite the extensive application of PTMS incorporated with cooling
devices/systems to improve occupant thermal comfort in indoor envi-
ronments, aforementioned PTMS still require external energy input (i.e.,
supplied power). Presently the development of state-of-art PTMS with
zero or near-zero energy input becomes a hot topic. PTMS with zero or
near-zero energy input are developed by using novel materials or mate-
rials with specially designed structures [ 83 , 84 , 98 ]. The main principle
of such novel PTMS is that the material or the special structure enables
the heat exchange between the human body and its surrounding envi-
ronment and thereby increases wearer thermal comfort. One the promi-
nent examples is the infrared transparent visible opaque fabrics (ITVOF)
[83] . Presently potential materials for ITVOF include polyethylene (PE)
and polypropylene (PP). ITVOF are transparent in the infrared spectrum,
which allow passive radiative cooling. It has been well established that
the human body emits most the infrared radiation in the wavelength
between 9.3 and 9.7 um. Hence, ITVOF enable radiative heat dissipa-
tion on the human body. Traditional textiles made of natural and syn-
thetic bers do not allow body heat dissipation via infrared radiation
[83] . Thus, wearing ITVOF increases an occupant’s cooling rate. Obvi-
ously, the increment in a person’s cooling rate allows a higher ambient
temperature to achieve the same level of thermal comfort. For exam-
ple, a 23W increase in the body cooling rate allows a thermal comfort
ambient temperature increase from 23.9°C to 26.1°C (i.e., increase of
2.2°C) and thereby, signicant built energy savings could be expected
[83] . Nanoporous polyethylene (nanoPE) textiles were fabricated and
results from hotplate tests showed that the simulated skin temperature
was reduced by 2.7°C when being covered with nanoPE cloth as com-
pared to cotton textiles [84] . The actual performance of clothing made
with ITVOF on the improvement of thermal comfort in various indoor
conditions were examined [97] . The ITVOF clothing enables thermal
comfort at an ambient temperature of up to 27°C, which is 1.5°C higher
than the indoor thermal comfort upper limit recommended ASHRAE.
Therefore, the use of ITVOF clothing could save around 9-15% cooling
energy in a typical built environment. In additional to the aforemen-
tioned example on improvement of radiative heat transfer on the human
body, other approaches to enhance conductive, convective and evapo-
rative heat transfer on the human body could also be proposed. Detailed
methods to enhance other means of heat transfer mechanisms by using
near-zero energy input materials can be found in recently published lit-
erature reviews [ 99 , 116 ]. It should be noted that translating ndings
from such material-level studies into actual indoor applications should
be made with caution because the majority of evaluation tests reported
in material-level studies were performed on 2-dimensional equipment.
The test condition, test protocol, design specications of PTMS and in-
dividual dierence could largely aect the actual performance of PTMS
in indoor settings.
Over the past few decades, personal heating systems have also been
developed to improve occupant thermal comfort in various cool or cold
indoor environments. The concept of incorporated auxiliary heating into
clothing could be dated back to the early 1940s [117] . Properly designed
personal heating clothing is capable of providing the human body with
sucient heating energy in cool and cold indoor environments with no
access to central HVAC heating systems. Existing personal heating tech-
nology may be divided into four types: electrical heating (EH); chemi-
cal heating (CH), phase change material heating (PCMH), air/uid ow
heating (AH/FH) and passive insulation from special structured materi-
als (PI).
EH is the most practical and widely used heating technology for
personal heating. Electrical heating clothing (EHC) is slim, lightweight,
washable, exible and does not restrict body movement. An EHC sys-
tem is normally comprised of heating elements, a power source, and a
user interface [118] . The most often used heating elements may include
electrical resistance wires, electrically conductive rubbers, metalized
textile fabrics, intrinsic conductive polymers, carbon polymer heating
elements, carbon fabrics, graphene heaters, Janus brous membranes
and TEC heaters ( Fig. 7 ). EHC can greatly improve occupant thermal
comfort while working in unheated indoor environments during cold
weather. The eect of EH clothing on thermal comfort of university stu-
dents while studying in a cold classroom condition was investigated,
where the air temperature was 8.0°C and RH = 80% [88] . Results showed
that EHC could remarkably increase skin temperatures. Overall and lo-
cal TSVs and TCVs were improved in EHC compared to non-heating.
The heating performance of EHC and two PCD combinations including
a radiant heating panel & a heated table pad (i.e., HB1) and a heated
chair & a heated mattress (denoted as HB2) at two cool indoor tempera-
tures (15 and 18°C) was examined [91] . It was found that EHC received
266
B. Yang, X. Ding, F. Wang et al. Energy and Built Environment 2 (2021) 260–270
Fig. 7. An electrical heating fabric made from (a) the carbon ber, (b) the graphene fabric heater, and (c) a heated jacket incorporated with thermoelectric heating
modules.
better thermal acceptability over HB1 and HB2 at both two cool con-
ditions. EHC signicantly improved overall TSVs as compared to HB1.
EHC consumed less than 15 W power and this only accounted for 4.4%
and 14.8% of the total power draw of HB1 and HB2. Therefore, EHC
is much more eective than PCDs in enhancing occupant thermal com-
fort in unheated cool indoor environments. More important, EHC is su-
per energy saving as compared to traditional PCDs. EH technique can
be conveniently incorporated into various clothing, sleepingbags and
accessories including, but not limited to, hats, hoods, scarfs, jackets,
trousers, gloves/mittens, socks and insoles [ 92 , 118 ]. EHC accessories
are found eective for maintaining local skin temperature as well as im-
proving local body thermal comfort, which have a similar function with
PCDs such as foot warmers, palm warmers, heated keyboards, desk radi-
ators, heated chairs, heated tables and warm-barrels [ 57 , 59 , 119 ]. Nev-
ertheless, EHC accessories consume even less heating energy and their
power consumption is always less than 5 Wh.
Compared to the EH technique, other heating technologies received
less attention during the past few decades. Chemical heating clothing
(CHC) was designed and its eect on occupant thermal comfort in a
cold indoor environment at 8.0°C was examined [88] . Fourteen chemi-
cal body warmers (chemical ingredients: iron powder, activated carbon,
water, vermiculite and salt) were incorporated in an insulative vest and
kneecaps and the CHC had an eective heating power of 6.9 W. Results
showed that CHC could greatly improve skin surface temperature, local
skin temperature at the ngers as well as the nger blood ow. TSVs at
the hands and feet were also improved in CHC compared to non-heating.
Thermoelectric heating clothing (warm air was pre-heated by the ther-
moelectric unit and then being supplied to clothing microclimate using
a blower) was designed and its performance was assessed using a ther-
mal manikin [93] . The thermoelectric heating clothing could provide a
heating power of 18.5 W.
Similar to the development of personal cooling systems with near
zero energy input, a recent hot research direction on personal heating
is to develop personal heating systems with near zero energy input.
By properly controlling the conductive, convective and radiative heat
transfer avenues, novel PTMS with zero energy input could be achieved.
Nanophotonic structure textiles were fabricated using nanoporous met-
allized PE [96] . An infrared reective layer was constructed on an in-
frared transparent layer with embedded nanopores. The nanoporous
metallized PE textile exhibited a high infrared reectivity of 98.5% on
the outer surface of the textile, which could eectively suppress radia-
tive heat loss. Hotplate tests showed that the newly fabricated PE textile
could maintain a simulated skin surface temperature of 33°C at 15°C am-
bient temperature, which is <