ASHRAE Journal - August 2019 - 49

TECHNICAL FEATURE

Co o

ling

Coi

Humidity Ratio (lb Vapor/lb Air)

typical AC cooling coils,
FIGURE 4 Theoretical psychrometric paths of outdoor air in the control system (yellow), outdoor air in the upgraded system (blue),
as previously described
and exhaust air in the regeneration airstream (red).
on the psychrometric chart, involves a
change in enthalpy of
Standard Airstream
22 Btu/lb (51 J/g). On
Upgraded Airstream
the other hand, proRegeneration Airstream
Des
ir)
icca
nt R
Des
ry A
cessing the outside air
D
i
ege
c
b
l
c
ant
nera
u/
t
D
B
ﬁrst through the dehue
ti on
(
h
u
y
mid
p
l
a
i
ﬁ
h
t
c
ati o
midiﬁcation unit and
En
n
Heat Exchanger
Heating Coil
then through the heat
exchanger (assuming
Cooling Coil
Heat Exchanger
50% efﬁciency) removes
4.3 Btu/lb (10 J/g) load
from the cooling coil, or
roughly 20% of the load.
Dry-Bulb Temperature (°F)
In our case, where
RTUs have constant
fresh air intake of 2,400 cfm (1133 L/s), this translates to
TABLE 1 Auxiliary energy loads for desiccant system.
46,200 Btu/h (14 kW) of energy savings per AC unit. This
COMPONENT
ANNUAL HOURS
ENERGY USE (MBTU/YR)
represents the theoretical maximum energy load that is
Booster Fan
4,380
4
removed from the cooling system on a particular design
day. Instead, this load is handled by recovering sensible
Desiccant Belt Motor
1,222
0.3
energy through the heat exchanger, and by supplying
Heating Coil
1,222
35
thermal energy in the desiccant regeneration cycle to
remove latent energy.
Using TMY-3 data for southeast Michigan, the theoreti- 34 MBtu (36 MJ) in thermal energy annually. This highlights again the fact that savings in such a system are
cal annual energy savings can be estimated by compargenerated by the substitution of electric energy with
ing cooling coil loads in both the upgraded system and
more cost effective thermal sources such as solar, natuthe standard system. The standard system has a total
ral gas, waste heat, or other.
annual load of 408 MBtu (431 MJ), as compared to the
It is important to note that the application of this
upgraded system load of 373 MBtu (394 MJ), resulting in
dehumidiﬁcation system is for a single-zone RTU that
theoretical savings of 35 MBtu (37 MJ) annually.
discharges air directly after the cooling coil. The proThe change in enthalpy leads to electricity savings
posed design revolves around reducing the load on this
on the air-conditioning unit. However, to be properly
RTU by processing the air before the cooling coil. A more
evaluated, we must also consider the auxiliary loads
introduced with the retroﬁt unit. This includes the static common desiccant wheel processes air after the cooling coil. The post-cooling coil system is beneﬁcial to
pressure drop through the desiccant system and heat
reheat systems, which heats the air after the cooling coil,
exchanger, which increases loads on the fans, heat supplied to the regeneration heater and power to the motor- but not to single-zone systems. Dehumidiﬁcation after
the cooling coil removes the need for a sensible heat
ized conveyor belt. Estimates for each of these auxiliary
exchanger and has a very different psychrometric pathloads are provided in Table 1. Taking into consideration
way and associated energy savings.
each of these auxiliary loads reveals that the modiﬁed
system generates 31 MBtu/yr (33 MJ/yr). Taking into
Methods
consideration each of these auxiliary loads reveals that
Data collection was done using three types of
the modiﬁed system generated 31 MBtu/yr (33 MJ/yr) in
electric energy savings, but also consumes an additional hardware, and two types of software. The hardware
A U G U S T 2 0 19

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