Wednesday, 8 January 2014

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.


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.


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.