indirect evaporative cooling: interaction between thermal
performance and room moisture balance
M. Steeman1, A. Janssens1, M. De Paepe2
1
Department of Architecture and Urban Planning, Ghent University, Ghent, Belgium
2
Department of Flow, Heat and Combustion Mechanics, Ghent University, Ghent, Belgium
[email protected]
Abstract
In an indirect evaporative cooling (IEC) installation the extracted air is cooled by means of
adiabatic humidification. By passing over an air/air heat exchanger this air cools down the
supply air. A clear interaction can be observed between the relative humidity of the extracted
air and the thermal comfort realized in the building. To be able to predict the performances of
this technique well, a good knowledge of the indoor relative humidity is thus important.
This paper presents the results of measurements carried out in the summer of 2006 in a nonresidential building at the Belgian coast which makes use of indirect evaporative cooling. An
evaluation of the indoor summer comfort is made and the interaction between the thermal
performance and the indoor humidity is investigated. Furthermore, using the multizone
building simulation program TRNSYS, simulations were performed to evaluate the
parameters influencing the room moisture balance.
1
Introduction
Different moisture sources exist in buildings e.g. people, plants, cooking and showering …
Apart from them, the relative humidity in a room is influenced by ventilation with outdoor air,
infiltration and the exchange of moisture with walls and furniture. Hygroscopic materials such
as wood and textile are able to dampen out relative humidity variations and provide therefore
a healthier indoor climate [1]. Svennberg et al. [2] have shown that furniture plays an
important role in the buffer capacity of a room.
Nevertheless it is difficult to assess the buffer capacity of the indoor climate because of the
uncertainty of many parameters: the material moisture properties vary with the relative
humidity and for many materials there is a lack of available data [3] [4]. Furthermore, it is
difficult to predict the exact surface of buffering material in the indoor climate. Due to this
most building simulation programs, e.g. Trnsys, predict the relative humidity in a simplified
way [5] [6].
One of the HVAC applications of which the thermal performances depend on the indoor
humidity is indirect evaporative cooling (IEC), an interesting passive cooling method in
which a clear interaction can be observed between the relative humidity of the return air and
the thermal comfort realized in the building.
This paper focuses on the interaction between the thermal performance of this cooling
technique and the indoor relative humidity. The first part explains the principles of IEC, the
second part discusses the measured performance of an existing installation in a non-residential
building in Oostende. The third part presents some results from a parameter study performed
with a simple simulation model.
2
Operation of indirect evaporative cooling
In indirect evaporative cooling (Figure 1) the return air passes over the wet side of an air/air
heat exchanger. By adiabatic humidification the air stream is cooled. At the same time fresh
air flows over the other side of the heat exchanger and cools down. This method differs from
direct evaporative cooling in which the supply air stream is directly humidified. By
humidifying the return air the dry bulb temperature of the supply air can be lowered without
increasing its humidity ratio. In this way a more comfortable indoor climate can be obtained.
In figure 2 the states of both airstreams during the process are schematically presented making
use of a Mollier chart. During humidification, the return air follows lines of equal wet bulb
temperature until saturation, while the horizontal lines indicate the state of the supply air.
moisture
relative
humidity
100%
SUPPLY AIR
indirect
cooling
2
1
3
RETURN AIR
adiabatic
humidification
temperature
Figure 1: Indirect Evaporative Cooling [7]
Figure 2: State of the airstreams in IEC
Because of his low cooling loads and his moderate humidity the Belgian climate is suitable
for the use of this technique. Applications in new or retrofit buildings with low internal gains
and without specifications of close temperature or humidity control are favourable. In winter
the heat exchanger can be used for heat recovery thus preheating the outdoor air [8].
3
Experimental determination of thermal performance
3.1 Indirect evaporative cooling installation in Oostende
Measurements were carried out in the cafeteria of a holiday centre located at the Belgian
Coast (Oostende) in the summer of 2006. Temperature and relative humidity were measured
in the installation at all stages of the process as well as in the cafeteria. Also the position of
the valves, pumps and fans were registered. The cross flow heat exchanger which is used has
a total width of 900mm, a length of 1950mm and is made out of polypropylene. Both the
supply and return fan can work in two stages with a maximum air flow rate of 7100m³/h.
The installation can operate at different stages. When the outdoor air is fresh enough in
summer, the air flow rate is increased and the air is used for free cooling. As soon as the
outdoor air temperature is too high, the fresh air is adiabatically cooled by moistening the heat
exchanger. If the desired indoor temperature is not yet reached, active cooling may contribute
to lower the temperature of the supply air.
3.2 Discussion of the measured performance
3.2.1 Thermal summer comfort
Figure 3 presents the measured temperatures from 18 until 22 July. Measurements were done
every five minutes. The periods in which the IEC is working are indicated. Figure 1 shows
where the temperature of the airstreams is measured.
For the first two days the temperature of the supply air downstream from the heat exchanger
section did not differ much from that of the outdoor air. The fresh air was clearly bypassed
allowing free cooling. The temperature measured behind the evaporator was always lower
than before, showing active cooling has been working the whole measured period.
temperature (°C)
40
max 13.9°C with IEC
35
30
1
free cooling
25
2
20
3
15
10
18/jul/06
19/jul/06
20/jul/06
fresh outdoor air
supply air behind evaporator
IEC
21/jul/06
22/jul/06
supply air behind heatexch
cafetaria
Figure 3: Measurements of IEC in Oostende (18/07/2006 - 22/07/2006)
During the measured days the outdoor temperature varied from 20°C to 35°C. The average
temperature measured in the cafeteria was 26°C. Making use of evaporative cooling, a
maximum temperature drop of 13.9°C was noticed on 19/07 when the outdoor temperature
reached 34.8°C. On average a cooling of 3.3°C could be achieved with this method within the
measuring period. At night free cooling was used for some hours to cool down the cafeteria.
The indoor summer comfort was evaluated using the Adaptive Temperature Limits Indicator
(ATG) developed in The Netherlands which is based on the adaptive comfort theory of de
Dear and Brager [9] - [11]. They state that the degree in which people accept the indoor
climate depends on their expectations and thus is partly influenced by the outdoor climate.
In this method, two types of buildings are distinguished, depending on the availability of
operable windows and the possibility to adjust the indoor climate and clothing. The studied
building was determined to be a so-called beta-building as it is top cooled by a mechanical
cooling installation and every two occupants do not have an operable window available.
Thermal comfort is divided into three levels. The standard level B corresponds to 80%
thermal acceptability and thus means a ‘good’ indoor thermal comfort. Level A (90%
acceptability) is applied in buildings with high performance requirements to thermal comfort
and level C (65% acceptability) should only be applied for temporary situations in existing
buildings.
In figure 4 the evaluation of the indoor climate is given for July, August and September 2006.
On the horizontal axis the running mean outdoor temperature is presented, which is derived
from the external temperature from the current day and that of the three preceding days. The
vertical axis shows the indoor operative temperature. In the graphs only the occupancy hours
are taken into account (08-22h). From the measurements of the outdoor temperature could be
derived that July 2006 was the warmest month [12]. During 79.3% of the occupancy hours in
July, a good indoor climate was obtained. In August and September 2006 this was
(°C)
Tmax 65% acceptability
Tmax 80% acceptability
Tmax 90% acceptability
Tmin 90% acceptability
Tmin 80% acceptability
Tmin 65% acceptability
i,o
33
32
31
30
29
28
27
26
25
24
23
22
21
20
19
18
17
6
T
T
i,o
(°C)
respectively 98% and 88%. Thus, although active cooling was working most of the time, the
cafeteria did not always meet the criterion for good indoor thermal comfort.
8
10 12 14 16 18 20 22 24 26 28
T
i,o
T
Tmax 65% acceptability
Tmax 80% acceptability
Tmax 90% acceptability
Tmin 90% acceptability
Tmin 80% acceptability
Tmin 65% acceptability
8
10 12 14 16 18 20 22 24 26 28
(°C)
(°C)
e,ref
33
32
31
30
29
28
27
26
25
24
23
22
21
20
19
18
17
6
33
32
31
30
29
28
27
26
25
24
23
22
21
20
19
18
17
6
T
e,ref
(°C)
Tmax 65% acceptability
Tmax 80% acceptability
Tmax 90% acceptability
Tmin 90% acceptability
Tmin 80% acceptability
Tmin 65% acceptability
8
10 12 14 16 18 20 22 24 26 28
T
e,ref
(°C)
Figure 4: Indoor summer comfort during respectively July, August and September 2006 in Oostende.
Table1: Percentage of occupancy hours the indoor thermal comfort is situated in class A, B and C
July ‘06
Aug ‘06
Sept ‘06
Class A
51.6%
83.8%
57.2%
Class B
79.3%
98.0%
88.0%
Class C
93.1%
99.4%
96.7%
Regarding humidity comfort criteria the indoor air humidity should be, according to
recommendations in EN 13779 between 30 and 70% [13]. Measurements have shown that this
requirement was satisfied in the cafeteria during the whole measurement period (JulySeptember).
3.2.2 Hygrothermal interaction
The relative humidity of the return air plays an important role in the performance of IEC
because it is moistened in the installation. As moisture buffering in hygroscopic materials is
able to reduce humidity peaks in the indoor climate a better performance of this technique is
expected. In that case a larger amount of moisture can be evaporated in the return air before
saturation is reached.
Figure 5 shows that there is a linear relation between the amount of cooling of the supply air
and the difference between the dry bulb temperature of the supply air entering and the wet
bulb temperature of the return air leaving the heat exchanger. The smaller this latter
temperature difference, the less the fresh air can be cooled because saturation is reached faster
when humidifying the return air. This can as also be seen on figure 3. In figure 5 only the
period during the measurements (from 18/07 to 22/07) in which the evaporative cooling was
working is taken into account.
The effectiveness ε of an indirect evaporative cooling system describes its thermal
performance and is given by:
ε=
θ s ,1 − θ s , 2
θ s ,1 − θ 'r ,1
(1)
In Eq. (1) θs,1 and θs,2 are respectively the temperature of the supply air entering and leaving
the heat exchanger. θ'r,1 is the wet bulb temperature of the return air entering the heat
exchanger. The effectiveness thus compares the actual temperature difference realized by IEC
with the maximum possible difference. Calculated from the measurements (the slope in figure
5) the effectiveness was averagely 82.5%.
temperature difference of supply air realized with IEC (°C)
16
14
y = 0.825x - 0.5338
2
R = 0.996
12
10
8
effectiveness ε
6
4
2
0
0
2
4
6
8
10 12 14 16 18 20
temp difference dry bulb supply & wet bulb return air (°C)
Figure 5: Interaction between humidity level and thermal performance in Oostende. (18/07/2006 - 22/07/2006)
4
Simulations
4.1 Parameter study
The definition of effectiveness ε in Eq. (1) can be used as a first step in evaluating the
performance of an IEC system having a constant effectiveness. As the wet bulb temperature
of the return air depends on the moisture balance indoor, the influence of moisture production,
moisture buffering and ventilation rate can be easily studied. Therefore in Trnsys [6] a simple
model was built for calculating the temperature of the supply air in order to evaluate the
indoor temperature in every time step. For the indoor environment, the BESTEST building
was used [14].
The indoor climate consisted of a volume of 129.6m³ of which all walls are built from 15cm
aerated concrete. Assuming that all boundaries are adiabatic, it is possible to study only the
effect of the IEC installation on the indoor conditions. The outdoor climate is stationary 30°C
and 50% relative humidity. There is no infiltration, no radiation and no cooling or heating
device.
Every day from 09-17h a moisture production of 0.35kg/h and a heat production of 600W due
to five persons occupying the room (seated, light work) existed in the building [6][15]. The
air is assumed to be well mixed and there was no other ventilation present then the IEC. To
model the moisture buffering the Simple Capacitance Ratio model from Trnsys, assuming that
the capacity of the air is fictively increased, was used. Runs were carried out with hourly
values until stationary conditions were reached.
It is easy to understand that a better effectiveness of the system will result in a lower supply
temperature and therefore in a more comfortable indoor climate. This model assumed a
constant effectiveness of 85%.
4.1.1 Moisture production
Several simulations were run in which the indoor moisture production was varied. First, from
the basic case A, only the moisture production was increased (due to e.g. extra activities:
cases B and C), later also the heat production was enlarged (due to e.g. larger occupancy:
cases D and E).
As the indoor moisture production increases, the IEC system cools the supply air less,
resulting in a less comfortable indoor climate. Furthermore with higher moisture productions
the variations in indoor temperature enlarge due to larger variations in the supply temperature.
From figure 7 the interaction between heat and moisture production is demonstrated: a larger
occupancy of the room causes a higher cooling demand due to internal gains, but at the same
time the supply temperature increases due to the larger moisture production.
Table 2: Moisture production
Occupation
& activities
A
B
C
D
E
5persons
5pers+extra
5pers+extra
10pers
20pers
relative humidity (%)
temperature (°C)
28
90
80
26
70
40
RHin A
24
30
RHin B
20
20
1080
10
1104
1128
1152
1176
0
1200
RHin C
indoor air
indoor air
supply air
supply air
increasing moisture
& heat production
27
25
50
21
28
26
24
22
Tin A
E
29
Tin C
60
23
prod
Tin B
25
Heat
production
(W)
5*120
5*120
5*120
10*120
20*120
average temperature (°C)
30
100
27
Extra
activities
(kg/h)
/
1.00
2.00
/
/
Moisture
production
(kg/h)
5*0.07
5*0.07
5*0.07
10*0.07
20*0.07
D
22
D
A
21
C
C
B
A
23
E
B
increasing moisture
production
20
0
0.5
1
1.5
moisture production (kg/hr)
2
2.5
Figure 6 and 7: Influence of indoor moisture production on thermal performance of IEC (ε 85% / n 2h-1 / Ratio5)
4.1.2 Moisture buffering and ventilation rate
Simulations were performed with and without taking into account moisture buffering in the
indoor climate. A capacitance ratio of five equals with three walls (+/- 52.5m²) finished with
gypsum board, while a ratio of ten equals with all walls and ceiling (+/- 123.6m²) finished
with gypsum.
From figure 8 can be seen that the average indoor temperature does not change much for
different buffer capacities. Obviously this can be explained by the fact that moisture buffering
does not have a large effect on the average relative humidity, but rather dampens out the
variations. This was also shown by [2] by experiments. In other words with moisture
buffering the amount of cooling is enlarged during periods where moisture is buffered while
when moisture is released the supply air will be less cooled. On average the thermal
performance achieved with the IEC system will be similar.
As a last parameter the ventilation air change rate was varied from 1 to 5h-1. High air flow
rates cause more moisture to be removed from the room providing a lower supply temperature
and therefore a better indoor climate. On the other hand the variations in relative humidity and
thus in supply air temperature are dampened out (figure 9).
relative humidity (%)
100
Tin no
buffer
90
Tin
80
buff5
70
Tin
buff10
60
prod
50
temperature (°C)
30
29
28
27
26
40
25
30
20
24
10
23
1080
1104
1128
1152
1176
0
1200
RHin no
buffer
RHin
buff5
RHin
buff10
Figure 8: Influence of moisture buffering on thermal
performance of IEC
(ε 85% / n 1h-1 / prod. 0.35kg/hr moisture & 600W)
5
temperature (°C)
32
relative humidity (%)
100
Tin n1
90
Tin n2
80
30
28
70
26
60
50
24
40
22
30
20
20
10
18
1080
1104
1128
1152
1176
0
1200
Tin n5
prod
RHin
n1
RHin
n2
RHin
n5
Figure 9: Influence of air flow rates on thermal
performance of IEC
(ε 85% / Ratio5 / prod. 0.35kg/hr & 600W)
Future work
The measurements have only been the first step in studying indirect evaporative cooling. A
simulation model for a cross flow wet surface heat exchanger is recently being developed and
will be validated against the measurements. The aim is to use the model in the Trnsys –
environment to be able to simulate buildings where IEC is applied.
Further research will also be done on calculating the system’s effectiveness. In such a way the
thermal performance of an installation could be easily assessed without calculating the whole
model thus permitting a much smaller calculation time, e.g. in the design faze of an IEC
installation.
6
Conclusions
Indirect evaporative cooling is an interesting passive cooling technique in which the thermal
performance depends mainly on the indoor humidity. Measurements carried out in July 2006
show that by using this technique the supply temperature can be cooled up to 14°C. Despite
the active cooling present to further cool the supply air, an evaluation by the adaptive
temperature limits indicator showed that the indoor thermal comfort was not satisfied during
the whole measured period.
Some first simulations with a simple model based on the system’s effectiveness showed that
the knowledge of the inner moisture balance is essential to predict the performance of IEC
well. The indoor moisture production plays the most important role in lowering the indoor
temperature, while the amount of moisture buffering material rather influences the stability of
the climate. Higher ventilation rates also help to lower the indoor temperature.
Further research will be carried out using a wet surface heat exchanger model allowing a
better understanding of the parameters influencing this evaporative cooling technique.
7
Acknowledgements
This PhD research is established with financial support of the Flemish Institute for the
Promotion of Innovation by Science and Technology in Flanders (IWT).
8
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