ASHRAE Journal - October 2020 - 30

TECHNICAL FEATURE

Dollars (Millions)

(0.79 gr/lb). For this purpose, a mixing section is
TABLE 1 Energy consumption cost comparison (kWh/yr).
used to pass on the required quantity of process
OPTION 1
OPTION 2
OPTION 3
OPTION 4
air to the hygroscopic wheel considering moisSupply Fan
225,137
225,115
225,137
225,137
ture content of about 7.3 g/kg (1 gr/lb) off the
Extract Fan
40,231
40,231
40,231
40,231
hygroscopic wheel.
Energy Wheel Motor
2,628
2,628
2,628
2,628
The downside of this system is the need for
CHW Coil
408,433
141,914
360,928
278,648
high grade heated regenerative air (typically
DX
Coil
627,236
about 30% to 35% of process air at 100°C [180°F]
Electric Heater
769,022
18,887
above ambient) to regenerate the desiccant
Reactivation Fan
8,833
3,475
media (silica gel) for the adsorption process,
Hygroscopic Wheel Motor
2,628
2,628
which typically involves using an electric
resistance heater. Also, the processed air out
Reactivation Heater
1,770,958
696,674
of the hygroscopic wheel is dry but heated to a
Total Energy Consumption per Year (kWh)
1,445,452 1,056,012
2,411,343
1,249,422
higher temperature because of heat exchange
Total Operational Cost ($/Year)
$82,391
$60,193
$137,447
$71,217
with regenerative air. Regeneration is the most All device consumption measured in kWh/year. Rate = $0.057/kWh
energy-intensive process in this option on top of
a chilled water cooling coil (post-cooling coil)
FIGURE 2 Energy consumption cost comparison.
used to sensibly cool the heated process air.
2.5
1,770,958
Option 4 is devised to minimize the energy
consumption from the regeneration process of
2
the desiccant system by using a hybrid chilled
water cooling coil and hygroscopic wheel for
1.5
dehumidification. The strategy is to further
769,022
dry the air from 7.3 g/kg (1 gr/lb) in Option 3 to
1
696,674
about 3.3 g/kg (0.5 gr/lb) by using a precooling
627,236
chilled water coil between the enthalpy wheel
360,928
408,433
0.5
278,648
141,914
and the hygroscopic wheel. The precooling
40,231
40,231
40,231
40,231
coil after the enthalpy wheel cools the air to
225,137
225,137
225,137
225,115
0
Option 1
Option 2
Option 3
Option 4
the dew-point temperature, condensing the
Note: All device consumption measured in kWh/year. Some energy consumers may not be visible due to
moisture content in the air and giving drier air
insignificant consumption or may not be applicable to a certain option.
at 8.8 g/kg (1.3 gr/lb) to the hygroscopic wheel,
Reactivation Heater
Hygroscopic Wheel Motor
Reactivation Fan
Electric Heater
which further reduces the moisture content
DX Coil
CHW Coil
Energy Wheel Motor
Extract Fan
Supply Fan
to 3.3 g/kg (0.5 gr/lb). Note that since the air
became drier, this configuration needs less
volume of air to be treated to meet the latent load. This
than necessary. This essentially reduces waste energy
reduces the overall size of the hygroscopic wheel sysin the system. This will be further evaluated in the next
tem, which in turn reduces the energy required for the
section to analyze the reduction in energy consumption
regeneration process.
compared to the baseline.
The post-cooling coil will also require less energy
Energy Consumption Comparison
compared to Option 3 since the quantity of hot and dry
To evaluate these options from an energy consumpprocess air mixing with return air has also been reduced
tion perspective, the bin method was used by computing
significantly. Since the outdoor fresh air intake for
bin hours based on ASHRAE International Weather for
ventilation is defined by Standard 62.1-2016, the same
Energy Consumption. For the purpose of this evaluaquantity of air is treated at the precooling coil (after
tion, a spreadsheet tool was developed by the author to
energy recovery from the enthalpy wheel) and then
compute energy consumption at each step of all system
bypassed from the hygroscopic wheel to avoid wasting
configurations. A basic energy model based on bin hours
energy by treating more air volume for the latent load
30

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ASHRAE Journal - October 2020

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