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.

Evaporative cooling simulation

It is always challenging to understand how well evaporative cooling will fit a specific requirement.

For that reason, we have started interfacing with Wunderground to obtain historical temperature data for specific locations. We then simulate the performance of an evaporative cooler so that the client can get an idea of the performance of the cooler.

Here is an example of two locations: The first is Monte Vista on the Cape flats. A pretty humid location where this client had a specific need to condition a large storage location.

The second was Oliver Tambo Airport in Johannesburg, elevation nearly 1400 m and much dryer.

Figure 1: Predicted summer temperatures for Monte Vista

In Figure 1 it can be seen that the evaporative cooler would produce air at a temperature higher than 17.5 ° C for a significant number of hours during the summer. The reason for this is that the environment is relatively humid.

Figure 2: Predicted summer temperatures for Oliver Tambo Airport

In contrast, Figure 2 shows that evaporative cooling will only spend a small number of hours outside of specification up in Johannesburg where it is much dryer.

This performance of the units are quantified in Figure 3 and 4.

Figure 3: Monte Vista hours over specification 

Figure 4: ORT hours over specification

It is clear that at ORT, that the evaporative cooler will produce air that is 1 ° C over specification for slightly more than 2 % of the summer hours. Air at 2 ° C over-specification will be less than 1 % of the total summer hours.

Evap cooling vs. air conditioning

Comparing evaporative cooling with traditional air conditioning

Why do we evaporate water?

The fact that the energy derived from the evaporation of water is the driving force between the cooling potential of evaporative cooling establishes beyond doubt that evap cooling uses much less electrical energy to accomplish an energy reduction in the air, required to remove a heat load from any occupied space.

The efficiency of evaporation is however directly proportional to the moisture content in the air per se which then confirms that the more arid the region, the more cooling can be required.

With the development of two stage evaporative cooling (or dry/wet cooling), the geographical areas suitable to evaporative cooling can be vastly expanded, as two stage evaporative cooling cools their air through 20-25 °C in arid areas and 10-15 °C for the wetter areas. This then also establishes the limitations of the system from a cost point of view.

Generally in South Africa the two stage evaporative cooling developed by Protek cools the air to 19 °C for the worst climate of Polokwane and Rustenburg and down to 14 °C for the arid areas of Kimberley and Bloemfontein.

In general the overall temperature drop is 120% of wet bulb depression (Ambient dry bulb - Ambient wet bulb temperature).

Due to the upper comfort limit of 25 °C and 26-27 °C for industrial applications, the economic feasibility then limits two stage evaporative cooling to a supply air of below 20 °C. Fortunately this is possible for the complete South Africa excluding Mpumalanga, Limpopo lowveld regions and the KZN coastal area.

Single stage evaporative systems can only achieve between 80-90% of the wet bulb depression which generates air at 4-6 °C higher than two stage evaporative cooling and is thus only feasible as an alternative for human comfort in the more arid areas. Provided that the increased humidity is acceptable. The humidity of single stage is 10% higher over two stage evaporative cooling due to the process which sprays water into hot air as opposed to two stage evaporative cooling that sprays water into cooler precooled air.

What is the total cost of owning and operating evaporative coolers?

How feasible is two stage evaporative cooling compare to traditional compressor based air conditioning?

The running cost of cooling for two stage evaporative cooling is between 25-30% lower than packaged air conditioners. Capital expenditure comparison depends on the following params:

• Geography: The more arid, the more cost effective two stage evaporative cooling
• Application: The higher the floor temp the more cost effective two stage evaporative cooling.
• Building construction: The less airtight the building, the more cost effective two stage evaporative cooling.
• Fresh air requirements: The higher the requirement, the more beneficial two stage evaporative cooling.
• Other factors:
• Limited availability of electrical power may compel the use of evaporative cooling.
• Unavailability of water might favour traditional air-conditioning.
• Limitation on RH, the higher the limit, the more AC units are favoured.
• Space on ducting: Less ducting favours aircon
• Heating: Two stage evaporative cooling can be equipped with heating like any aircon. With gas, electricity, water. With high air volume floor distribution is better with two stage evaporative cooling provided the fresh air is properly limited.

Cost study to establish the capex and running cost

This study focused on a moderate climate such as Pretoria.

• Tambient - 25 °C
• Floor area - 1000 m2
• Climate - Pta, Schoemans street WB40.
• Tinside - 24 °C /60 % RH
• Fresh air package: 1.5 l/m2

Control modes:

Evap cooling - 3 steps control

• Ventilation
• Single Stage
• Two stage

Package unit - 3 steps control

• Ventilation
• 50% cooling
• 100% cooling

COP overall including air movement air conditioner 2.5.

Study done for constant volume and variable volume as alternatives:

System	Two stage evaporative cooling	Packaged AC
RH	62	62
GTH - EC (kW)	-	95
Fresh air	100%	1.5 l/s.m2
Tambient	35/20 °C	35/20 °C
Troom	24 °C/60% RH	23 °C/50% RH
Tsupply air (Db/Wb)	18/17.2 °C	13/12 °C
Airflow m3/s	10.3	6.9
kWH/annum Constant Volume	27118 (57%)	47430
kWH/annum Variable Volume	14604(36%)	39792
Capex Constant Volume
Unit	R220 000	R270 000
Ducting	R220 000	R170 000
Total cost	R440 000	R440 000

Conclusion

The saving of approx 20 000 kWH/annum would favour two stage evaporative cooling considerably.

Sensitivity analysis in respect to capex shows the following results in R/m2 for equipment and ducting only.

Tambient (°C)	City/Town	Two stage evaporative cooling	AC
33/19.4	Pretoria East	R318/m2	R314/m2
35/18.3	Bloemfontein	R292/m2	R304/m2
32/19	Johannesburg East	R331/m2	R312/m2
30/18	Oliver Thambo Airport	R297/m2	R302/m2
33.5/17	Windhoek	R264/m2	R294/m2
39/19/1	Kimberley	R335/m2	R352/m2

Capacity has been increased to the warmer climates to allow for the increased facade loads.

In general CAPEX for two stage evaporative cooling is equivalent or lower by up to 10% compared to AC.

Who else makes use of Evap cooling?

Industrial areas

They tend to favour two stage evaporative cooling more due to the nature of the building structure which are normally not airtight and operations requiring roller shutter doors and good entering and leaving the building.

Floor temp also tend to be on the higher side and 27 °C/50RH is not uncommon to achieve for two stage evaporative cooling.

For Rosselyn with a 30% fresh air infiltration load and RSH 110W/m2 the capex as follows:

• two stage evaporative cooling industrial plant R410/m2
• Packaged AC R450/m2

Running costs will again be 40-50% lower for two stage evaporative cooling.

Office blocks

We would refer you by Toon Herman for extensive analysis that has been done on office blocks.

Agricultural

Agricultural products normally needs a higher RH to preserve product life and prevent dehydration and two stage evaporative cooling has proven in terms of stage evaporative cooling lf to be ideally suitable for export grape cooling.