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.

Water vs. electricity in air cooling

The price of electricity in South Africa is guaranteed to escalate with 8% in 2014 and is amongst the most expensive in the world. Protek has been providing air cooling using water for the last 30 years and suddenly, it becomes a more affordable option to cool buildings with air.

This is mainly because the capital and operational costs for evaporative cooling is lower than for conventional air-conditioning and it is getting cheaper.

Comfort

We design our air cooling to provide our customers with 24 °C air at around 60% relative humidity. What we really want to know is for how many hours per year would our evaporative coolers result in temperatures above this design condition.

Lower the temperature

To achieve the target temperatures, it is not enough to just evaporate water into air. Protek has a two stage cooling process that can lower the air by another 2 °C below wet bulb. This might not sound like much, but in a world where our ambient wet bulb temperatures vary between 17 and 21 °C (excluding the lowveld and KZN coastal areas) it results in a feasible design for acceptable room temperatures around 23 to 25 °C.

Energy availability

The amount of KVA you can get in South Africa is limited by the ability of the national provider to provide you and this is becoming more limited. Many large complexes today are limited by the KVA allocated to them by Eskom. Even if you have more money, you cannot get more electricity.

Challenges

Using water to cool air lowers cost and is more energy friendly, but nobody can control the relative humidity, which affects the amount of cooling you can get from air. The real challenge is therefore control.

To control the process, you need to look at historical figures so that customers can get an idea of how their air conditioning would have performed in years before.

So what is interesting is that in Brackenfell in 2012, two stage evaporative cooling would for 23 hours in that year, have had resulting temperatures of above 21.4 °C while the external temperature was above 22.3 °C.

To put that in perspective, it means that for 23 hours in the total year, the evaporative cooler would have produced temperatures that were too high for human comfort. That is less than 1 day per year.

Figure 1: Tdb vs temperatures provided by the evaporative cooler for Brackenfell (Western Cape) for 2013.

(Y-axis = T ambient dry bulb; X-axis is temp from 2 stage evap cooler;body is hours per annum)

Demonstrating control

Evaporative cooling has progressed from swamp coolers where satisfying customer comfort requirements was a hit and miss affair. As this article shows, it is now possible to use real world historical data to calculate the efficiency of two stage evaporative cooling.

In figure 1, it is shown that two stage evaporative cooling will exceed 26 °C room temperature for 23 hours over a 4 summers period (which is 5.75 hours per year). Additionally, for 70 hours per 4 years, the temperature will be above 25 °C (17.5 hours per year) and a total of 11 days above 24 °C (2.75 days per year).

Single stage evaporative cooling will exceed 24 °C for 32 days per year.



Figure 2 Two dimensional histogram of evaporative cooling unit temperatures vs. ambient dry bulb temperatures.

Door closed due to aircon

How often do you see that in high summer. Butchers, grocers and small shops, all desperately trying to keep their doors shut in order to keep cool air in, naturally resulting in customers being kept out.

Our evaporative coolers are full fresh air systems. What this means is that our cool air requires you to keep your doors and windows open in order evacuate the "used" air.

There is a much better natural fit between evaporative cooling and shops since:

• The operational cost of evaporative cooling is a fraction of conventional cooling because of the reduced electrical load. 
• The total amps drawn by the evaporative coolers is much smaller than conventional cooling. 
• Because evaporative cooling provide bigger volumes of cool air, rather than the small volumes of very cold air produced by conventional cooling, you obtain a fresh breeze in the shops. 
• But most significantly is that fact that evaporative cooling wants you to open doors and windows in order to evacuate the cold air and allow the injection of new fresh air.

South African Government departments turn to Evaporative Cooling

The Department of Environmental Affairs recently bought 24 Evaporative coolers from Protek to provide cooling for their flagship offices in Pretoria.

What makes the project unique is not only that the whole building has been done using two stage evaporative cooling, but that the building also received a 6 star energy efficient rating.

Not only was two stage evaporative cooling instrumental in acquiring the 6 star rating but also worked out much cheaper than a conventional cooling system in both running and capital costs.

Factors such as limited supply and the escalating cost of electricity is driving large office buildings to consider evaporative cooling.

And evaporative cooling has progressed to the point where consistent supply temperatures can be achieved in order to provide effective cooling to buildings using only water.

The DEA is the second government department after the Department of Trade and Industry (DTI) who makes use of Protek evaporative cooling.

Pretoria climate data with IWAC supply temperatures

Here is the Pretoria climate data posted before, but with the IWAC supply temperatures super imposed.

The lines indicate the number of days per year that the IWAC will provide supply air above the specific temperature datum.

Pretoria climate data

Included is a 2D histogram for Tdb/Twb for the Pretoria Forum, 14h00, 1956-1960 for the months Dec-Feb.

Here is the actual bin data for the graph above.

Unit images

A Compact Single Stage with centrifugal fan

A Two Stage cooler connected to vertical ducting

The Two Stage unit seen from the side. Note the dry cooling coil (loads of small copper pipes protruding from the side)

A number of small Single Stage units ready for loading

Two Stage double units where the secondary air is blow out vertically and the primary air horizontally

A Two Stage evaporative cooler mounted on a roof