Wednesday 15 October 2014

Cooling air with Ice


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

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 db-wb 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


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

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

Conclusion

The conclusion of this analysis is that by installing ice storage in this case, the operating energy requirements, specifically the chiller size can be reduced by between 8 and 15 times the traditional chiller size.