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International Journal of Air-Conditioning and Refrigeration
World Scientific Publishing Company
1
On climatic control of Wrap-Around Heat Pipe (WAHP) enhanced
dehumidifier in outdoor air systems
Mridul Sarkar
Integrated Environmental Solutions India Pvt. Ltd.
Pune-411021, India.
mridul.sarkar@iesve.com; mridul.rns@gmail.com
Although wrap-around heat pipes (WAHP) are widely used for enhanced dehumidification systems in
tropical and hot-humid climates, very few literature resources have actually reported any control
methodology applicable for WAHP dehumidifier systems for an entire year operation. In the present
work, a methodology is proposed for an outdoor air unit equipped with a WAHP based dehumidifier
along with other auxiliary components. For this, the ambient conditions over an entire year are
categorized into six constituent regions based on temperature and humidity levels. The proposed method
involves defining specific control sequences corresponding to ambient conditions under each
psychrometric region for tandem operation of system components. Fundamentally, this establishes a sort
of segregated climatic control protocol for maintaining acceptable levels of humidity and temperature
inside the conditioned spaces during whole year operations. An energy simulation study is performed
for two DOE prototype office buildings with six representative weather locations ranging from
extremely hot and humid to dry and hot as per ASHRAE’s climate classification. Results show that a
100% outdoor unit comprising of WAHP based dehumidifier system equipped with proposed climatic
controls save cooling plant energy in a range of 1.5-19%, when compared with a similar WAHP based
outdoor unit without any climatic controls and auxiliary components. The proposed controls also enable
the outdoor air unit in maintaining comparatively better indoor environmental conditions (temperature
and relative humidity), resulting in lesser number of building occupied hours drifting away from
prescribed indoor temperature and relative humidity limits than its basic counterpart without any climatic
controls. This substantiates the intent and applicability of proposed control methodology for WAHP
based dehumidifier systems.
Keywords: Wrap-around heat pipe (WAHP); dehumidifier; climatic control; humidity; energy
1. Introduction
With an exponential increase in global energy
demand, it is evidently clear that energy resources
should be used in a discreet and responsible manner in
order to conserve them for future generations. In
process industry and building sector, a significant
amount of energy is wasted through the discharge
streams. Mechanical devices like heat exchangers
enable the extraction or recovery of energy from the
flue stream to cool or heat the process stream, thereby
saving energy and increasing the overall efficiency of
the system. However, like any other mechanical
equipment, heat exchanger too needs timely
maintenance and can increase the overall size or weight
of the system. A heat pipe system can be an answer to
all these limitations.
Heat pipes are special types of recuperative heat
exchangers that are gaining a lot of popularity and
recognition in industrial and scientific communities. Its
ability to transport heat over large distances1-3 without
considerable losses makes it a suitable choice as an
energy-conserving agent for many applications. The
compact design and high thermal conductance allows a
heat pipe to transfer energy even with small temperature
differences4. Literature sources have even reported that
for equal lengths, the overall thermal conductivity of
heat pipes can be many time higher than copper5. The
2 Mridul Sarkar
absence of any electrical input for operation and ease of
design and manufacturing3 are some of the other
advantages of heat pipes over conventional heat
exchangers.
Heat pipes find various applications in diverse
fields like air conditioning4, thermal management of
data centers6, thermal energy storage systems (TES)7,
solar collectors8, electronic cooling9, water heating
systems10 etc. Despite the application, the total heat
transfer capability of a heat pipe is dependent on the
type of refrigerant used and restricted by various heat
transfer limits at specific working temperatures1,11,12.
Investigation of the usage of heat pipes for heating
ventilation and air conditioning (HVAC) systems is an
important field of study. El-Baky et al.3 explored the
applications of heat pipes for heat recovery in air
conditioning and studied the effect of mass flow rate
ratios and evaporator inlet temperatures on system
performance. Noie-Baghban et al.11 presented
theoretical and experimental investigations of a
methanol-based heat pipe for heat recovery systems
serving hospital surgery rooms. Experimental
investigations by Jouhara et al.12 are aimed towards the
performance study of air handling units utilizing heat
pipes. Yau13 showed the impact of heat pipes on the
energy efficiency of dehumidification systems through
the simulation model of an HVAC system utilizing dual
heat pipe heat exchanger for an operating theater in
tropical climate of Malaysia. Ahmadzadehtalatapeh14
investigated the performance of an air conditioning
system with a heat pipe based heat exchanger and
verified that it meets the comfort criteria recommended
by ASHRAE through TRNSYS simulation.
Many researchers have also evaluated the economic
potential of heat pipes for building air conditioning.
Jouhara15 conducted a detailed study on the energy and
cost saving potential of heat pipes and concluded
significant cooling and heating energy savings for
ventilation. Zhang et al.16 presented simulation results
showing the energy conservation potential of heat pipes
for dedicated outdoor air handling units serving office
buildings in Hong Kong.
It is quite evident from the literature review that the
heat pipe is an active topic of research and
development. Its application in building HVAC has
proved significant improvement over conventional
systems. However, the studies so far have been
restricted to the performance of heat pipes systems for
particular weather conditions and no generalized
methodology has been defined to control the system
based on climatic conditions over an entire year. This
paper aims towards the theoretical development of such
a method for a chilled water dehumidifier system
equipped with WAHP. The overall objectives of this
article are outlined as follows:
Proposing a methodology for climatic control of a
WAHP based outdoor air system for year round
operation.
Presenting a simulation study by applying the
proposed control methodology for a 100% outdoor
air system and comparing it with a similar system
without implementing the climatic controls.
2. Basic Wrap Around Heat Pipe (WAHP)
System
A heat pipe system transfers thermal energy from
one spot to another without utilizing any moving
component for refrigerant flow. The movement of
refrigerant inside a heat pipe takes place due to the
differences in density and pressure between the two
phases of refrigerant that creates a pulsating effect. The
transport of liquid refrigerant to the evaporator can be
gravity assisted or through capillary effect. In a WAHP
dehumidifier system, a dehumidifier coil is encased by
heat pipe in such a way that evaporator section of the
heat pipe is placed just ahead of the cooling coil and
precools the incoming air before it is further cooled and
dehumidified by the coil. Subsequently, the cold and
dehumidified air is reheated as it passes through the
condenser section and supplied to the space. The
precooling of air at evaporator converts the liquid
refrigerant into vapor, which is transported to the
condenser through connecting tubes. As the
dehumidified air passes through the condenser, heat is
transferred from hot refrigerant to the cold air. Due to
this, refrigerant vapor condenses into liquid and
transported back to the evaporator via gravity and the
whole cycle is repeated. Fig.1 (a) shows the schematics
of a basic WAHP dehumidifier system. WAHP tubes
can also be fitted with electrically operated solenoid
valves to control its overall effectiveness and reheat
temperature by controlling the refrigerant flow through
heat pipe system17. The number of solenoid valves
depend on the design and size of heat pipe and affected
by the total number of heat pipe rows and outdoor
temperature range for WAHP dehumidifier operation18.
Conventional dehumidifier systems employ a
cooling coil (chilled water or DX-based) to cool and
dehumidify air and a reheat coil to elevate its supply
Climatic control of Wrap-Around Heat Pipe (WAHP)
3
temperature. Due to subsequent cooling and heating
operations, both the coils work continuously, which
leads to supplemental heating energy consumption for
attaining the required supply temperature and
maintaining comfortable room condition. For
dehumidification, the coil must sensibly cool the air to
its saturation limit before cooling it down further to the
required coil outlet temperature. Compared to a
conventional dehumidifier, a WAHP dehumidifier
system not only saves the reheat energy, but also
reduces the sensible heat ratio (SHR) of the coil by
precooling air before admitting it into the dehumidifier
coil. This allows dehumidification of air by the coil to
the required temperature at a comparatively higher coil
apparatus dew point (ADP) temperature and reduces
overall coil load and size. Fig.1 (b) shows
psychrometric plots of a conventional dehumidifier
system and a WAHP dehumidifier system, respectively.
In both cases, the dehumidifier coils are assumed to
cool and dehumidify air to the same temperature.
3. HVAC system description
3.1. Proposed WAHP system
Air conditioning systems are designed primarily to
supply air for maintaining acceptable comfort level
inside a space. The supply conditions depend on various
factors like room cooling loads, room thermostat
settings, permissible room humidity levels, outdoor
weather, operational schedule, etc. Out of all these
factors, variations in external weather conditions play a
critical role in ascertaining the type of air conditioning
system. Hence, air conditioning system should be
flexible enough to adapt with any climatic condition to
control the supply conditions appropriately.
A WAHP dehumidifier works well for hot and
humid weathers, where high SHR of conventional
dehumidifier substantiates the application of heat pipe
Fig.1 (a) Schematic of Wrap-Around Heat Pipe (WAHP) dehumidifier system. (b) Psychrometric plots of processes through conventional
dehumidifier and a WAHP dehumidifier.
4 Mridul Sarkar
device. However, variations in outdoor conditions
around the year do not assure efficient performance of
an air conditioning system that is designed specifically
for a particular climatic condition. Hence, a number of
auxiliary components must be incorporated into the
system to cater the off-design conditions too. Fig.2
shows schematic representation of the proposed WAHP
dehumidifier system that includes various other
auxiliary components, enabling the system to function
at varying ambient conditions round the year. In fact,
the position and placement of these components as
shown in the figure are also important from system
design and operation point of view. This system
configuration fulfills two important objectives of the
proposed climatic control methodology:
Firstly, it restricts the temperature range for the
operation of WAHP module, which not only allows
reduction in the sizes of dehumidifier and WAHP
coils, but also limits the number of control valves
required to control WAHP.
Secondly, the tandem operation of these auxiliary
components allow a complete all year round system
operation, including the periods when dehumidifier
coil is not required to work.
The detailed function of each of the auxiliary
components along with the underlying principle will be
explained in section 4.
3.2. Principle
The supply dry bulb and dew point temperatures of
outdoor air are controlled by modulating condenser and
dehumidifier coil outlet temperatures, respectively.
Referring to Fig.2, the supply temperature from WAHP
module is given by the condenser outlet temperature
(point 4):
A WAHP precools incoming air (point 1 to 2)
through its evaporator, thereby reducing its dew point
depression (DPD) before passing it through the
dehumidifier coil. Customization in design and
operation of WAHP dehumidifier may result in air to
exit the coil at almost saturated condition. This is
possible if the system (including auxiliary components)
operates in such a way that the maximum limit on the
entering temperature to WAHP is restricted (e.g. by
precooling coil or by heat exchanger component as in
Fig.2). As a result, precooling by WAHP evaporator
will be sufficient to allow the dehumidifier coil to cool
and dehumidify the process air stream to its required
supply dew point temperature at a comparatively higher
apparatus dew point temperature (ADP). Hence,
without loss of generality, the coil exit temperature
(point 3) can be approximately equal to the supply dew
point temperature:
Therefore, by applying Eq. (2), the temperature
effectiveness of a WAHP is written as:
With a rise in inlet air temperatures, a WAHP with
fixed temperature effectiveness supplies air at a higher
temperature due to increased reheat through the
condenser. This also leads to higher dehumidifier coil
loads. Hence, these dehumidifier systems that operate
at high inlet air temperature require larger sizes of heat
Fig.2 Schematic representation of proposed WAHP system for climatic control.
(1)
(2)
(3)
Climatic control of Wrap-Around Heat Pipe (WAHP)
5
pipe and dehumidifier coils. Also, to control the overall
effectiveness of WAHP, multiple solenoid valves will
be required to modulate the flow of refrigerant through
heat pipe tubes. The number of solenoid valves and
their opening and closing ascertain the temperature
effectiveness range for controllability of the
dehumidifier system. Broader the WAHP inlet
temperature band, higher will be the number of valves
required. Decreasing this temperature bandwidth results
in reduction of dehumidifier and WAHP coil sizes, as
well as number of control valves. As discussed in
section 3.1, one of the main reasons for including
various auxiliary components in the proposed WAHP
dehumidifier system is to reduce the temperature band
of WAHP inlet temperature. For precise climatic
control, the WAHP either operates within this reduced
temperature band or completely staged off depending
on the dehumidification requirements. In this work, the
lower limit of the WAHP inlet temperature is set equal
to the common cooling set point. When incoming air
temperature is lesser than or equal to this temperature
limit, all the control valves will be opened to allow the
maximum refrigerant flow through WAHP and to
maintain the required discharge temperature after
condenser reheat. Hence, the lower limit of the WAHP
inlet temperature is given by:
Considering the temperature band to be tband, the upper
limit of WAHP inlet temperature is given by:
The choice of tband is entirely subjective and depends
upon the weather conditions and system size. For the
sake of presenting a general concept of system control
from a theoretical aspect, tband can be set equal to the
required condenser reheat temperature after cooling and
dehumidification of air down to the required supply
dew point temperature by dehumidifier coil. Thus
Therefore, the controlled band for WAHP inlet
temperature is given by applying Eq. (4), (5) and (6) as:
From Eq. (3) and (7), the maximum and minimum
limits on the WAHP temperature effectiveness in the
proposed system configuration are given by:
When the outdoor air temperature is within this
temperature band defined by Eq. (7), only the WAHP
and dehumidifier coil operates with temperature
Fig.3 (a) Psychrometric plot of the process for psychrometric region-1. (b) Psychrometric plot of the process for psychrometric region-2.
1 set
lower
tt
1 set band
upper
t t t
band cond
tt
(4)
(5)
(6)
(8)
(7)
(9)
6 Mridul Sarkar
effectiveness varying between the maximum and
minimum limits. At other conditions, the auxiliary
components first operate in tandem to bring the WAHP
entering temperature within the prescribed temperature
band followed by WAHP and dehumidifier coil
operations. This allows a complete and segregated
seasonal control over the supply conditions from the
outdoor air unit.
4. Climatic Control (CC) methodology
Depending on the design supply temperatures and
humidity levels for the effective control of WAHP
system, closely bounded psychrometric regions can be
defined. Weather conditions corresponding to a
particular region dictates the operation of all the system
components (Fig.2) for global control over supply
conditions. Similar approach for precise control of
WAHP can be found in an engineering white paper (see
ref. 19) published by manufacturer. However, the
present work formalizes the whole methodology based
on a holistic approach that is applicable for any climate.
This section defines the control sequences for each
psychrometric region.
4.1. Region-1
Fig.3 (a) shows the psychrometric plot of the whole
operation of the proposed system for this region. In this
region, the external air DBT and humidity ratio are
lower than the prescribed supply temperature and
supply humidity ratio, respectively. This implies that
heating and humidification are required in order to
bring the temperature and humidity ratio of outdoor air
as close as possible to the supply condition. To achieve
this objective, a heat wheel will first recover heat from
return air stream to preheat the supply air (process a to
b) before the steam humidifier adds moisture to increase
Fig.4 (a) Psychrometric plot of the process for psychrometric region-3. (b) Psychrometric plot of the process for psychrometric region-4.
Fig.5 (a) Psychrometric plot of the process for psychrometric region-5. (b) Psychrometric plot of the process for psychrometric region-6.
Climatic control of Wrap-Around Heat Pipe (WAHP)
7
its humidity ratio (process b to e). If required, the
auxiliary pre-heat coil operates to raise the air
temperature up to the supply level (process e to f). The
WAHP and dehumidifying coil will be staged off
during the whole process.
4.2. Region-2
The DBT of outdoor air is higher than the
prescribed supply temperature in this psychrometric
region. However, the humidity ratio of air in this region
is lower than the prescribed supply level, which calls
for humidifier operation. Fig.3 (b) shows the
psychrometric plot of the process corresponding to this
region. If outdoor DBT is higher than the return air
temperature, operating the heat recovery wheel and
evaporative cooler will allow precooling of the supply
air (process a to b). However, if outdoor DBT is lower
than the room temperature, both the heat wheel and
evaporative cooler are turned off. Subsequently, the
humidifier adds moisture to the outdoor air stream
(process b to e) to bring its humidity ratio as close as
possible to the supply level. The auxiliary pre-cool coil
cools the humid air until the temperature reaches the
prescribed supply DBT (process e to 1). Similar to
psychrometric region-1, both the dehumidifying coil
and WAHP will not be operational in this case too.
4.3. Region-3
Outdoor air corresponding to this region can be
admitted directly into the WAHP module without
operating any other supporting components in the
system. Fig.4 (a) shows the operation of WAHP for
psychrometric region 3. Modulating the temperature
effectiveness by operating the solenoid valves in the
WAHP module will be sufficient to bring the outdoor
condition to the prescribed supply humidity ratio and
temperature (process a to 4). If the dew point depression
of the outdoor air is lower than the design condenser
reheat (i.e. at high humidity ratio near the apex of
region-3), moisture condensation will take place during
precooling phase. Since conditions corresponding to
psychrometric region-3 will allow seamless operation
of WAHP as per theoretical design considerations, this
climatic region will be referred here as “controlled
psychrometric region”.
4.4. Region-4
Fig.4 (b) shows the process for psychrometric
region-4. In this psychrometric region, outdoor DBT is
lower than the return air temperature but humidity ratio
is higher than the supply humidity level. The air is first
preheated by passing through the heat wheel (process a
to 1), where energy is recovered against the return air
stream. Now air is passed through the WAHP module,
which is operated at the maximum temperature
effectiveness to bring the outlet temperature closer to
the supply DBT (process 1 to 4) and the auxiliary pre-
heat coil provides any further temperature raise required
(process 4 to f). Even though, both the dehumidifier and
pre-heat coils work sequentially in this mode, cooling
and heating energies expended are still lower than a
conventional dehumidifier system comprised of
dehumidifier and reheat coils.
Table 1. Control summary of proposed WAHP dehumidifier system.
Psychrometric
Region
Heat
Wheel
Evaporative
Cooler
Humidifier
Pre-cool
Coil
WAHP
Dehumidifier
Preheat
coil
1
On
Off
On
Off
Staged off
Off
On
2
On/Off
On/Off
On
On
Staged off
Off
Off
3
Off
Off
Off
Off
Staged on
On
Off
4
On
Off
Off
Off
Staged on
On
On
5
On
On
Off
On
Staged on
On
Off
6
On
On
Off
On
Staged on
On
Off
8 Mridul Sarkar
4.5. Region-5
Fig.5 (a) shows the complete psychrometric process
of WAHP operation for psychrometric region-5. For
outdoor conditions belonging to this psychrometric
region, a reduction in dry bulb temperature is sufficient
for transitioning into the controlled psychrometric
region. This is done by operating the heat wheel first to
precool the outdoor air (process a to b). To increase the
air-to-air heat recovery, the return air temperature can
be reduced further by operating the evaporative cooler
(process r to c). Pre-cool coil can provide any
supplementary cooling required for bringing down the
air temperature from region-5 to region-3 (process b to
1). The remaining steps of the process are the same as
for region-3, which involve WAHP and dehumidifying
coil operations (process 1 to 4).
4.6. Region-6
The expanse of outdoor DBT in this zone coincides
with that of region-5. However, the humidity ratio in
this zone is higher than the controlled psychrometric
zone. The only difference between the processes for
psychrometric regions 5 and 6 is that the pre-cool coil
must cool and dehumidify the incoming air stream to
reduce its humidity and temperature, enabling it to
reach psychrometric region-3 (process b to 1). Fig.5 (b)
shows the psychrometric process for this climate zone.
Table1 summarizes and tabulates the controlling
sequence of various auxiliary components of the
proposed system applicable to each psychrometric
region.
5. Simulation study
5.1. Model description
Two prototype building models representing a
small and large office are chosen in this simulation
study. These buildings are part of the building model
stock referenced and documented by US DOE for
whole building simulation analyses20. Hence, their
basic geometry and relevant model input data
(thermostat set points, relative humidity, plug loads,
occupancy, lighting, constructions, ventilation,
infiltration and operation profiles) are as per US DOE
recommendations. Fig.6 shows the schematics of these
office-building models. The small and large office
buildings have 1 and 12 floors (excluding basement),
respectively. As per Fig.6, each floor is divided into
core and perimeter areas. For simplicity, spaces with
identical thermal characteristics and exterior exposure
are combined together to form separate thermal zones
that are conditioned by their respective air conditioning
systems. There are 5 and 16 such thermal zones created
in the small and large office buildings, respectively. The
methodology for determining appropriate supply
conditions based on thermal zoning and its influence on
indoor environmental conditions are explored in
literature source (Ref. 21). Only those energy end
usages are considered in the models that explicitly
affect the building air conditioning loads. That is why
energy consumption by elevators, service hot water and
exterior lights are not reported in the results. In the large
office model, data center and IT closets are not included
Fig.6 Schematics of DOE prototype models: (a) Small office model; (b) Large office model
Climatic control of Wrap-Around Heat Pipe (WAHP)
9
for HVAC system simulations, since internal load
variations are almost absent in these spaces and they are
not occupied regularly.
5.2. Weather locations
For this simulation study, six locations are selected
that consumes substantial energy for comfort cooling
during an entire year. ASHRAE standard 169 (Ref.22)
classifies various locations across the globe into
specific climate zones depending upon their annual
cooling degree-days (CDD), annual mean temperatures
and annual precipitation levels. This standard
segregates the climates of various locations into humid
and dry types based on annual precipitation levels.
Table 2 summarizes the ASHRAE climatic
classifications applicable to these selected locations.
To ascertain the pertinence of dehumidification
requirement for a particular climate zone, an entity
similar to CDD termed as dehumidification degree-days
(DDD) is defined as:
The base dew point temperature (td-base) applied for
defining DDD is set equal to 12.8oC, which is the
maximum limit on room dew point temperature
prescribed for hot and humid climates by ASHRAE
design guide (see Ref. 24). Table 2 also shows the DDD
obtained by applying Eq. (10) for the selected climatic
locations. Except for climate zone 0A, the other climate
zones show significant concentration of DDD during
the summer months that calls for continuous cooling
and dehumidification processes from dehumidifier
systems. Moreover, despite being classified as dry
climate type (climates zones: 0B, 1B and 2B), the high
design dew point temperatures prescribed by ASHRAE
for designing and selecting appropriate
dehumidification systems23 justifies the application of
WAHP in these selected locations. The seasonal swings
of temperature and humidity levels in these locations
stimulates HVAC systems to perform diverse
operations from cooling and dehumidification to
heating and humidification for maintaining comfortable
conditions inside the spaces. The simulation weather
files of these selected locations required for running
energy simulations are readily available in .epw format
from Energy Plus website25. IES VE (VE-2017) is the
software tool applied for running energy simulations
and post processing of results26.
5.3. HVAC system specification
In this simulation study, a comparative analysis of
two different configurations of 100% outdoor unit
comprising of WAHP based dehumidifier system is
presented. The two configurations of outdoor units
considered are as follows:
no CC: “no CC” is the system label used for the
baseline configuration considered in this study. This
is the most basic WAHP dehumidifier system
without any other auxiliary components for
segregated climatic control. The WAHP is
considered controllable and conditioned air from
the outdoor unit is supplied to each of the spaces via
VAV boxes with terminal reheat coils.
w CC: “w CC” is the system label applied for the
proposed or improved configuration. The schematic
of this system configuration is depicted as per Fig.2
and all the features of the climatic control
Table 2. Weather locations for energy simulation study
Location
ASHRAE weather Classification22
Annual Cooling
Degree Days
(CDD)23
Dehumidification
Degree Days
(DDD)
Design
DPT
(oC)23
Climate
Zone
Climate description
Singapore
0A
Extremely hot and humid
6614
4192
27.1
Abu-Dhabi, UAE
0B
Extremely hot and dry
6619
1942
29.2
Hong Kong
1A
Very hot and humid
5179
2501
26.9
New Delhi, India
1B
Very hot and dry
5591
1611
27.5
Tel-Aviv, Israel
2A
Hot and humid
3830
873
26.1
Cairo, Egypt
2B
Hot and dry
4536
691
23.2
(10)
d j d base
jd j d base
tt
DDD ; t t
24
10 Mridul Sarkar
methodology presented previously in section 4 are
applicable for the operation of this system. Similar
to the baseline configuration, the WAHP is
considered controllable and conditioned air from
the outdoor unit is supplied to each of the spaces via
VAV boxes with terminal reheat coils.
The common salient features of HVAC system for
simulation of the considered system configurations are
as follows:
A separate 100% outdoor air unit serves each and
every thermal zone in both of the building models.
The load calculations are performed in IES VE by
applying ASHRAE heat balance methodology27.
Actual temperature effectiveness data from WAHP
manufacturer’s design software is considered for
the simulation of WAHP dehumidifier system28.
A chilled water plant supplying to the dehumidifier
and pre-cooling coils in both office models.
Electric preheat and terminal reheat coils for heating
requirement.
Constant supply temperatures through dehumidifier
coils and flow modulation via terminal VAV
(variable air volume) boxes to regulate space
Fig.7 Cooling plant energy summary of the two building models.
Fig.8 Annual cooling plant energy vs. annual chiller load.
Climatic control of Wrap-Around Heat Pipe (WAHP)
11
temperatures and humidity. This control strategy for
maintaining acceptable humidity levels at part load
conditions is as per Ref.29.
The chilled water plant is sized and modeled as per
ASHRAE 90.1-Appendix G methodology30.
Identical design airflow rates, flow turndown ratio,
supply temperatures and equipment capacities
corresponding to a particular location and building
type are considered for both HVAC system
configurations.
The fans in HVAC system with climatic controls are
considered to operate at higher design static
pressures (~25% higher than the conventional
WAHP dehumidifier configuration) due to
additional pressure drops across auxiliary
components. WAHP systems both with and without
climatic controls are designed for same supply flow
rates. Hence, external static pressure drops are same
in both the system configuration types. The 25%
extra fan static pressure in the proposed case is
accounted due to additional internal static pressure
drop across various auxiliary components (precool,
preheat coils, heat exchanger, humidifier,
evaporative cooler), which are as per fan pressure
drop correction data from literature sources30,31.
This additional pressure drop leads to higher fan
heat addition that causes a cascading effect on
chiller load increment.
5.4. Results and discussion
5.4.1. Cooling energy and load summary
Fig.7 summarizes the annual energy consumption
by cooling plant in the two office models. The cooling
plant energy includes energy consumed for running the
chiller compressors, heat rejection through condenser
pumps and cooling towers and chilled water distribution
pumps to supply chilled water to dehumidifier and
precooling coils. In each of the building models,
lighting and plug loads are considered as process loads
and thus, energy consumption by these end uses
remains the same in every simulation scenario. To
compare the cooling performance of the two system
configurations in each climate, the overall percentage
of plant energy saving is defined by:
Comparing the plant energy from Fig.7, the WAHP
dehumidifier systems equipped with climatic controls
performs comparatively better than conventional
WAHP dehumidifier systems without any auxiliary
components and saves plant energy between 1.5-19%.
It should be noted that WAHP dehumidifier systems
with auxiliary components and climatic controls show
these levels of net plant energy savings despite some
penalty imposed by higher heat addition from
distribution fans on net chiller loads. It is known from
literature study that cooling plant energy has a strong
linear correlation with cooling degree-days (CDD) for
a location32. Fig.8 shows a similar trend between annual
chiller loads and annual cooling plant energies, which
corroborates this inference on global climatic levels.
Hence, weather locations belonging to zone type 0 show
the highest cooling plant energies followed by zone
types 1 and 2. Similar number of CDD of each climatic
type (dry and wet) for a particular climate zone (0, 1 or
2) and observed annual chiller loads and cooling plant
energies imply that substantial energy consumption by
cooling plant for cooling and dehumidification of
supply air is not only limited to wet climates but are
significant in dry climates too.
Looking at the results specific to climate zones
considered, it is observed that substantial energy
savings are seen for dry climate zones (like Abu Dhabi:
0B, New Delhi: 1B and Cairo: 2B) as compared to wet
climate zones (Singapore: 0A, Hong Kong: 1A and Tel
Aviv: 2A) in both the models. The main assertion for
this observation is that the climatic controls work better
with wide temperature and humidity fluctuations (more
number of hours in psychrometric regions 5 and 6)
allowing operation of other components, specifically
precooling coil and heat recovery device, in tandem
with WAHP dehumidifier to bring down the net
dehumidification load. This is commonly seen in case
of dry climate zones considered. However, the spreads
of humidity and temperatures are quite confined to
specific psychrometric regions in case of wet climate
zones that limits the operation of other auxiliary
components. In summary, the savings in energy
consumption by the proposed WAHP dehumidifier
configuration can be attributed to the number of
operating hours corresponding to particular
psychrometric regions defined in section 4 that affects
(11)
12 Mridul Sarkar
the extent of utilization of its auxiliary components.
This aspect will be explored in detail in section 5.4.3.
5.4.2. Indoor air quality
Space temperature and relative humidity (RH) are
the two thermal parameters based on which the
qualitative performance of the proposed dehumidifier
system for indoor air quality is accessed. To do so,
number of occupied hours are determined from
simulation results, when indoor air temperatures and
relative humidity are not within their predefined
acceptable ranges. Such occupied hours are termed as
unmet hours. From the weekly schedules of occupancy
considered for the building models (as per Ref. 20),
such unmet hours are screened out from the occupied
periods between 8:00 am to 10:00 pm and 9:00 am to
8:00 pm for large and small offices, respectively. The
acceptable ranges for space temperatures and relative
humidity are as follows:
Temperature range: Between occupied minimum
set point minus 1oC and occupied maximum set
point plus 1oC. Indoor temperatures falling outside
this range imply conditions of overcooling and
overheating, respectively.
RH range: Space RH range with minimum limit of
40% to a maximum limit of 60% during occupied
periods. This RH range is as per recommendation
from Ref. 33 for healthy and comfortable living
environments.
As far as acceptable number of unmet hours are
concerned, ASHRAE 90.1 standard appendix G30 states
that the acceptable number of temperature unmet hours
in a validated energy model should not exceed 300.
However, acceptable unmet hours for RH is not defined
in any literature source. Hence, by generalization, a
maximum threshold of 300 hours is considered for RH
unmet hours too.
Fig.9 shows histogram plots of temperature and RH
unmet hours corresponding to each of WAHP
dehumidifier configurations considered for both of the
office models. For example in simulation scenario “1B:
no CC” for small office model, the temperature unmet
hours and RH unmet hours are reported as 46 and 692,
respectively. This means that out of all the occupied
(and hence conditioned) hours in this building, there are
46 hours when space temperatures are outside the
required indoor temperature range (either lower than
minimum set point minus 1oC or higher than maximum
set point plus 1oC). Similarly, the air conditioning
Fig.9 Temperature and relative humidity (RH) unmet hours of the two building models.
Climatic control of Wrap-Around Heat Pipe (WAHP)
13
systems are unable to maintain the building RH for 692
occupied hours during which, space RH are either lower
than 40% RH or higher than 60% RH. The results depict
that in all the considered cases, temperature unmet
hours remain below the standard threshold of 300. This
shows that the basis of cooling loads and equipment
sizing in the building simulation models complies with
ASHRAE 90.1 energy modeling criteria and hence
theoretically validated. Despite this, for some climate
zones, system with climatic controls result in higher
temperature unmet hours than conventional WAHP
system. This is seen for climate zone 0A in case of small
office model and climate zones 0A, 1A and 2B in case
of large office model from Fig.9. The unmet hours in
these cases are actually due to overheating instances
because of higher heat addition from supply fans that
are sized at higher design pressures to account for
additional internal pressure drops at various auxiliary
components added for enhanced climatic control. In
general, even though these components enable the
WAHP system to maintain the required supply DPT,
greater fluctuations in supply DBT with airflow rates
are observed due to comparatively higher fan heat
addition from supply fans. As a result, WAHP systems
with climatic controls tend to overheat the spaces to a
marginally higher extent in the aforementioned cases.
The moisture level in the air affects the thermal
comfort perceived by occupants. Due to this,
maintaining appropriate RH levels inside a conditioned
space is equally important for air conditioning systems.
This performance of the proposed dehumidifier system
is assessed by the number of RH unmet hours obtained
from the simulation results. Fig.9 summarizes the RH
unmet hours for both of the WAHP dehumidifier
configuration in each climate zone. The plots show the
consistency of the proposed WAHP dehumidifier
equipped with different auxiliary components and
climatic controls in maintaining acceptable range of
space RH for both the models. Especially in dry climate
zones (0B, 1B and 2B), this observation is quite
expected, since the proposed climatic control
methodology allows imperative actions that includes
both dehumidification and humidification depending on
weather conditions. In climate zone 0A, both the system
configurations are able to maintain the RH levels under
acceptable range during entire occupied periods. This is
a clear indication that in climate zone 0A, continuous
dehumidification through WAHP dehumidifier in both
the system configurations is sufficient to maintain the
acceptable thermal conditions inside the spaces and any
other intervention for maintaining humidity is not
required. In climate zones 1A and 2A, the proposed
WAHP dehumidifier system performs comparatively
better in maintaining required RH levels inside the
space than its baseline counterpart. Despite the
dehumidifier being predominantly active in outdoor
units serving both the office buildings in these climate
zones, operation of auxiliary components in proposed
system maintains the required space RH levels, which
is not possible for basic WAHP dehumidifier system
without climatic controls. Considering the total number
of occupied hours in both the building models, the
proposed configuration successfully achieves in
maintaining the space RH between the minimum (40%
RH) and maximum (60% RH) limits for more than 95
and 92% of the time in small and large offices,
respectively.
5.4.3. Climatic control utilization
As per preceding sections, a clear relationship
between the proposed climatic controls and weather
conditions is established. In this section, a simple
methodology to assess the extent of utilization of
proposed climatic controls based on climatic data is
introduced that aids in projecting the energy savings
from the application of these control protocols.
Segregating weather conditions of a location into 6
constituent psychometric regions as per section 4 aids
in streamlining the operation of proposed WAHP
dehumidifier system for total control over supply
conditions. On the other hand, a basic WAHP
dehumidifier system without any supporting
components is not capable of providing complete
control throughout varying weather conditions. In order
to justify the savings achievable by applying the
proposed climatic control methodology, it is important
to understand how the basic WAHP dehumidifier
configuration is expected to operate in these
psychrometric regions. The expected behavior of the
basic WAHP dehumidifier system in varying weather
conditions is tabulated in Table 3. The savings from the
proposed climatic controls are triggered in those
14 Mridul Sarkar
psychrometric regions, where the cooling loads on
dehumidifier coil in basic configuration is much higher
than in the proposed configuration. From Table 3, one
can see that psychrometric regions 2 (savings in
sensible coil load, no dehumidification load), 5 and 6
are such regions, where maximum scope of cooling
plant savings is concentrated. Despite this, it is evident
from the preceding sub section that the proposed
WAHP dehumidifier configuration provide superior
control over indoor conditions than the basic
counterpart. This is also clear from Table 3.
To quantify the utilization factor for operation of
climatic controls, the cooling degree hours are defined
for cooling-only regions. Since, weather conditions
belonging to psychrometric regions 2, 3, 4, 5 and 6
prompt WAHP dehumidifier systems to perform
cooling operations, the annual cooling degree hours
(CDH) is given by:
The base temperature considered here for computation
is the average supply temperature from all the thermal
zones in each climate zone. Similarly, CDH for all the
climatic conditions falling in psychrometric region-3
and 4 are given by:
Average supply DPT from all the thermal zones is
applied here for identifying region-3 boundary for each
building model in every climatic zone. Since, savings
from cooling plant energy in regions 3 and 4 are
minimal; the climatic control utilization ratio should not
include the CDH of cooling system operating in region-
3 and region-4. This factor is hence given by:
Fig.10 Cooling plant energy saving percentages vs. climatic control utilization ratios.
(12)
(15)
(13)
j
cool all j sup avg
t reg2,3,4,5,6
j sup avg
CDH t t
tt
j
cool reg3 j sup avg
t reg3
j sup avg j
CDH t t
t t & t region3
j
cool reg4 j sup avg
t reg4
j sup avg j
CDH t t
t t & t region4
cool reg3
cool all cool reg4
cool all
CDH
CDH CDH
CC utilization ratio= CDH
(14)
Climatic control of Wrap-Around Heat Pipe (WAHP)
15
Table 4 summarizes each region specific CDH and the
corresponding climatic control utilization ratio by
applying Eq. (12)-(15). The data clearly shows higher
CDH for regions 5 and 6 in dry climate zones results in
higher utilization ratios than the wet climate zones. In
addition, a large number of climatic conditions are
clustered in psychrometric regions 3 and 4 in case of
wet climate zones that leads to lower utilization ratios
in these zones. Fig.10 shows the scatter plots of
percentages of cooling plant energy savings against
Table 4. Annual CDH in proposed psychrometric regions
Climate
Zone
Office Building
model
Region-2
Region-3
Region-4
Regions-5 &
6
CC utilization
ratio
0A
Small
0
60832
1807
20258
0.24
Large
0
58976
1807
22114
0.27
0B
Small
10508
17587
5854
48929
0.72
Large
10912
16656
5792
49519
0.73
1A
Small
350
34858
6953
7665
0.16
Large
396
33119
6906
9404
0.20
1B
Small
14737
18453
2684
31440
0.69
Large
15277
16807
2621
32609
0.71
2A
Small
4967
13347
6329
4390
0.32
Large
5269
12174
6125
5466
0.37
2B
Small
13966
10321
5714
12994
0.63
Large
14145
9899
5687
13263
0.64
Table 3. Behavior of basic WAHP dehumidifier system in proposed psychrometric regions.
Psychrometric
Region
Expected behavior by basic configuration
1
The WAHP and dehumidifier coil are inactive.
The terminal reheat coils are active.
Supply air will is not able to maintain the minimum required RH in space.
Since no cooling is involved, no saving by cooling plant is expected, when the proposed configuration is
compared with basic configuration.
2
WAHP dehumidifier becomes active when outside temperature is greater than the required supply
temperature.
The dehumidifier coil sensibly cools the air down to the required supply dew point followed by reheat
through the WAHP condenser.
Supply air is not able to maintain the minimum required RH in space since there is no component to add
humidity in the air.
By applying climatic controls, savings in cooling plant energy is expected because of higher sensible
cooling by dehumidifier coil in the basic configuration.
3
The operation of the basic WAHP dehumidifier configuration is exactly same as the proposed WAHP
dehumidifier configuration.
No savings in cooling plant energy by incorporating climatic controls into the basic configuration.
4
The operations of basic and proposed configurations are almost similar in this region.
Instead of the central preheat coil (as in proposed case), the terminal reheat coils provide the required
heating energy to the supply air.
Savings in cooling plant energy is insignificant by incorporating climatic controls.
5 & 6
WAHP dehumidifier is active.
The sizes of dehumidifier and WAHP coils will be greater since the dehumidifier coil loads are higher in
this region.
Significant savings in cooling plant energy by incorporating climatic controls into the basic configuration.
16 Mridul Sarkar
climatic control utilization ratios determined for both
building models. The plot shows a strong correlation
between the two parameters. This implies that higher
cooling energy savings is expected at higher climatic
control utilization ratios under the purview of defined
control protocols for WAHP based dehumidifier
systems to treat and supply outdoor air. Additionally,
this ratio also serves as an indicator to ascertain whether
adjoining auxiliary components for supporting all year
round operations would be beneficial from cooling
energy savings point of view. Based on this, an engineer
can decide whether climatic controls are required for
system design and selection right from early design
stages.
5.4.4. Uncertainty analysis
As per literature study, uncertainty analysis is
applied in building simulation studies as an aiding
instrument for checking robust design information,
quality assurance, and reliability and influence of
design parameters handled by simulation tools34. In this
section, the influence of controlled inlet temperature
band (tband) on the design and performance of proposed
WAHP dehumidifier system is studied through a simple
uncertainty analysis. For this study, this temperature
band is varied between 1 to 10oC with an increment of
1oC applied to each WAHP dehumidifier system in both
the office models that are simulated for every climate
zone selected in this work. Hence, 60 simulation runs
are done each for small and large office models. The
annual cooling energies obtained from these simulation
runs for each of the climate zone are compared with the
corresponding cooling energies of the proposed design
test cases showcased in subsection 5.4.1. The
percentage deviations in these energies help in
understanding and quantifying how different inputs for
tband offsets the simulation results as compared to
considering design condenser reheat for tband in climatic
controls.
Fig.11 depicts the variation of annual cooling loads
with tband. The results show that for majority of cases,
the increment in annual cooling loads are lower than
10% for both of the office models, when compared to
that of proposed configuration reported in subsection
5.4.1. This observation is due to the combined effects
of increase in dehumidifier loads and decrease in
precooling coil loads (for transitioning into controlled
zone) with an expansion of WAHP inlet temperature
band. However, the deviations of annual cooling
energies in both the models primarily vary in a range of
Fig.11 Variation of annual cooling loads with controlled WAHP inlet temperature band.
Climatic control of Wrap-Around Heat Pipe (WAHP)
17
± 8% as shown in Fig.12. The left side in Fig.12 depicts
the number of simulation test cases or frequency of test
scenarios with results belonging to specific percentage
deviation bins. The normality plots on the right hand
side in Fig.12 confirms normal distribution of
percentage variations of annual chiller energy across 60
simulation scenarios considered for both of the office
models. On careful observation, it can be concluded
from Fig. 12 that deviations in annual cooling energy
vary between -7 to +7.4% and -6 to +7.8%, in small and
large office models, respectively, at a confidence
interval of 95% with the considered tband range.
6. Conclusions
A segregated climatic control methodology for
WAHP enhanced dehumidifier systems is proposed in
this work. For this, the annual climatic conditions of a
location are divided into six constituent psychrometric
regions and specific system control sequences are
defined for each region. This allowed synchronized
operation of auxiliary system components along with
the main WAHP module based on outdoor temperature
and humidity levels for maintaining acceptable indoor
conditions throughout an entire year. Such climatic
control protocols allowed the proposed WAHP
dehumidifier system configuration to perform
comparatively better than the basic WAHP
dehumidifier configuration at part load conditions.
From an energy simulation study of two DOE prototype
office-building models, the acquired annual cooling
energy savings from the proposed system configuration
are in 1.5-19% range over the six climate zones
considered. These percentage savings of cooling plant
energy are shown to be dependent on the control
utilization ratios that in-turn are explicitly influenced by
psychrometric conditions corresponding to the system
supply and ambient states. Additionally, the proposed
WAHP dehumidifier system configuration maintains
the temperature and RH unmet hours below 300 for
more than 95% and 92% of occupied hours in small and
Fig.12 Frequency distribution and normality plots of percentage deviations of annual cooling energies across
simulation scenarios.
18 Mridul Sarkar
large office models, respectively, thereby proving its
superior capability in controlling indoor air quality. In
conclusion, the proposed system configuration and
control methodology proved to be an energy efficient
alternative for outdoor air supply units in climates with
continuous as well as seasonal dehumidification
requirements.
Nomenclature
Symbols
CDD Cooling Degree Days
CDH Cooling Degree Hours
DDD Dehumidification Degree Days
PE Cooling plant energy
t Temperature (oC)
Δt Temperature differential (oC)
Greek symbols
ε Sensible effectiveness
Subscripts
1 Evaporator inlet
2 Evaporator outlet / dehumidifier coil inlet
3 Dehumidifier coil outlet / condenser inlet
4; cond Condenser outlet
band Temperature bandwidth
cool-all All conditions that require cooling
cool-reg3 Conditions belonging to psychrometric region-3
cool-reg4 Conditions belonging to psychrometric region-4
d-base Base room dew point temperature
d-j Ambient dew point at jth hour (oC)
d-sup Supply dew point (oC)
j jth hour (8760 hrs a year)
max. Maximum
min. Minimum
no-CC Without climatic controls
s Supply condition
set Space set point
sup-avg Average supply condition
w-CC With climatic controls
Acknowledgments
The author acknowledges no conflict of interest. This
research did not receive any specific grant from funding
agencies in the public, commercial, or not-for-profit
sectors. The author would like to thank the anonymous
reviewers for their valuable comments.
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