One of the biggest challenges for evaporative cooling is to demonstrate control in design. We have done that now by running simulations that can show you your historical room temperatures for a specific air conditioner.
As can be seen in the historical temperatures for Windhoek in Namibia, a Two Stage evaporative cooler would have produced air hotter than 15.7 °C for only 20 hours during the summer months of 2013/2014.
Figure 1: The Two Stage evaporative cooler temperatures off the unit during the summer 2013/2014
The question now is whether there is a cost effective way to manage the 20 hours in applications that are concerned about this issue.
We simulated a large hall (3000 m2) with 3000 people in it.
Outside temperature was assume to be 30/20 °C Tdb/Twb which would be typical for Pretoria/Johannesburg
The space design condition was 22 °C with 60% RH.
Firstly, we simulated three hours in the morning each reflecting Tdb incrementally raising from 20, 25 to 30 °C (9:00, 10:00 and 11:00 hours)
Using a compressor
To size a DX system, we assume that each person must receive 5l of fresh air per second. It can be seen in Table 1 that the DX chiller size required to provide cooling for people in the room, would be 670 kW.
h outside air
kJ/kg
66
Room Air WB
°C
18
h room air
kJ/kg
57
Air from the outside
5 l per person per second
kg/s
15
Outside air total heat
OATH
kW
133
DX Chiller size
RTH+OATH
kW
670
Table 1: DX Chiller size calculation
h outside air

kJ/kg

66
 
Room Air WB

°C

18
 
h room air

kJ/kg

57
 
Air from the outside

5 l per person per second

kg/s

15

Outside air total heat

OATH

kW

133

DX Chiller size

RTH+OATH

kW

670

Using ice storage to lower the DX chiller size
The problem here is that the cooling load is very focused on a few hours and if one could store the energy to cool the room, then a smaller compressor can be used to store the energy over time.
The next part of the simulation first calculated what the load in the room would be for the 3 hours. The RSH increases from 93, 101 to 109 w/m^2 for the hours in question.
Hours of the day
09:00
10:00
11:00
RSH
Total
93
101
109
Facade
w/m2
8
16
24
Lights
w/m2
15
15
15
People sensible
w/m2
70
70
70
People latent
w/m2
70
70
70
RTH
w/m2
179
RSHF
RSH/RTOTAL
0.57
0.59
0.61
The evaporative cooler will still do the bulk of the work, with the ice coil just filling in when required.
Hours of the day
09:00
10:00
11:00
Tdb
°C
20
25
30
Twb
Constant dewpoint
°C
17.0
18.5
20.0
T sa db
118% total cooling of dbwb depression
°C
16.5
17.3
18.2
Tsa wb
altitude correction for wb (80% dry and 90% wet cooling)
°C
16.3
16.9
17.6
The specific size of the Two Stage evaporative cooler is given below.
Q=RSH
kW
279.0
303.0
327.0
Cp
kW/kg.C
1
1
1
dT
Troom  Tsupply air
°C
9
9
9
m (Q=mCPdT)
Q=mCpdT
kg/s
31
34
36
And now comes the coil performance requirements to augment the two stage evaporative cooler.
Energy removed by ice coil
h_2S_off
kJ/kg
52.6
54.9
57.1
h_coil_off
kJ/kg
39.2
39.2
39.2
dh
kJ/kg
13.4
15.6
17.9
Energy removed
Airflow * dh
kW.h
416
527
652
Hours chiller operation
hrs
20
Total energy removed in 3 hours
Subtotal of energy removed over 3 hours
kW.h
1594
Energy per hour required for 20 hours
Compressor size
kW
80
Hours of the day

09:00

10:00

11:00
 
RSH

Total

93

101

109
 
Facade

w/m2

8

16

24
 
Lights

w/m2

15

15

15
 
People sensible

w/m2

70

70

70
 
People latent

w/m2

70

70

70
 
RTH

w/m2

179
 
RSHF

RSH/RTOTAL

0.57

0.59

0.61

Hours of the day

09:00

10:00

11:00
 
Tdb

°C

20

25

30
 
Twb

Constant dewpoint

°C

17.0

18.5

20.0

T sa db

118% total cooling of dbwb depression

°C

16.5

17.3

18.2

Tsa wb

altitude correction for wb (80% dry and 90% wet cooling)

°C

16.3

16.9

17.6

Q=RSH

kW

279.0

303.0

327.0
 
Cp

kW/kg.C

1

1

1
 
dT

Troom  Tsupply air

°C

9

9

9

m (Q=mCPdT)

Q=mCpdT

kg/s

31

34

36

Energy removed by ice coil
 
h_2S_off

kJ/kg

52.6

54.9

57.1
 
h_coil_off

kJ/kg

39.2

39.2

39.2
 
dh

kJ/kg

13.4

15.6

17.9
 
Energy removed

Airflow * dh

kW.h

416

527

652

Hours chiller operation

hrs

20
 
Total energy removed in 3 hours

Subtotal of energy removed over 3 hours

kW.h

1594
 
Energy per hour required for 20 hours

Compressor size

kW

80

Comparing the DX system with the ice storage alternative
It can be seen that the DX Chiller must be 8 times larger in capacity to supply the 670 kW vs the 80 kW required by the ice coil.
DX Chiller size
RTH+OATH
kW
670
Glycol Chiller Size
operating the no off hours in 37
kW
80
Chiller size increase
X
8
The amount of ice required to store the energy.
Size of ice tanks
latent heat of melting
kJ/kg
334
Kg ice required to supply 1355 kJ
kg
14601
The electrical connection size for the two installations.
Electrical connection GLYCOL
Air flow@600pa+Chiller@COP=3
kW
58
Electrical connection DX
kW
254
DX Chiller size

RTH+OATH

kW

670

Glycol Chiller Size

operating the no off hours in 37

kW

80

Chiller size increase

X

8

Size of ice tanks
 
latent heat of melting

kJ/kg

334
 
Kg ice required to supply 1355 kJ

kg

14601

Electrical connection GLYCOL

Air flow@600pa+Chiller@COP=3

kW

58

Electrical connection DX

kW

254

Sensitivity analysis
The initial study was done for ambient temperatures of 30/20. WHat will happen in drier climates where the ambient is 30/19 or even 30/18?
At 30/19, the chiller needs to be even smaller than before and a 10 times size reduction can be achieved.
DX Chiller size
RTH+OATH
kW
613
Glycol Chiller Size
kW
59
Chiller size increase
X
10
At 30/18, the solution is even more favourable with a 15 times size reduction in the ice storage chiller size.
DX Chiller size
RTH+OATH
kW
558
Glycol Chiller Size
kW
38
Chiller size increase
X
15
DX Chiller size

RTH+OATH

kW

613

Glycol Chiller Size

kW

59
 
Chiller size increase

X

10

At 30/18, the solution is even more favourable with a 15 times size reduction in the ice storage chiller size.
DX Chiller size

RTH+OATH

kW

558

Glycol Chiller Size

kW

38
 
Chiller size increase

X

15
