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.