Normal air-conditioning is primarily driven by a building design, solar radiation and then lastly by the local climate (cloud cover etc) at a specific location.
Evaporative cooling is additionally also dependent on the humidity at a location.
Most consultancies approach this problem by selecting a design day, something that broadly corresponds to a day when it is very hot and pretty humid. You cannot select the worse day of the year to size your air conditioner because this will result in a very expensive oversized solution. It is normal practice to accept that you will be 1 or 2 °C over specification for up to 40 hours a year (5 days a year).
The design day approach is problematic in that it can easily result in an over design. Imagine a location that is pretty cold but have a very hot summer month (high altitudes)? Ok, so you over designed the solution by 20%. No biggie.
But when you try and work out the operating cost of this solution, knowing the exact daily/hourly temperature distribution is very important.
ProtekCooling has developed a simulation package that will simulate the performance of your evaporative cooler for historical temperature and humidity data.
So now, for the first time, you can get an idea of how much it would have cost to operate a DX only system vs. an evaporative cooler.
Water as a scarce resource
A major complaint against evaporative cooling is that what you save in electricity, you lose in paying for water.
This argument proves to be invalid for two reasons:
- It turns out that the total cost of water consumption for an evaporative cooler per year, even at elevated rates, is only a 1/5 of the cost of electricity for the year. When you look at your electricity savings, typically 50% of the comparable cost of DX electricity, then the water cost less than 1/5 of the savings you make in electricity.
- In many of the locations for industrial cooling, the annual rainfall exceeds the water consumption requirements. So by collecting rain water, which is Ph neutral and relatively easy to use in evaporative cooling, you can remove your dependency on external water supplies.
Here are the results for a real world application. It is a new mall in Transvaal, with approx 20000 m^2 floor space (15 000m^2 under evaporative cooling). The client required 220 m^3/s of Two Stage air to condition the mall to 24 °C.
Cost of electricity
To do the whole mall with DX, would have resulted in operating costs of approx R1,100,000 per year.
Moving to Two Stage Evaporative cooling, reduced that cost to about R500,000, less than 50% of DX.
Cost of water
If water was purchased from the municipality at R25/kL, the annual costs would have been about R110,000
Availability of water
Rainfall in this region is in the order of 600mm per year. Collected from the roofs of 20 000m^2, this would provide 12 kL of water per year at zero cost (you have to pay for the storage tanks).
The units in this simulation consumed 4 kL per annum.
Total cost of operation
The total cost of operation for Two Stage cooling is about R610,000 per year compared to R1,100,000 for DX. This is about 55% of the DX installation.
If the Evaporative Coolers are assumed to have cost R8,000,000 capital layout, assuming the cost of electricity stays constant, the whole plant would have been paid off after 16 years. Note that this is not the payback time (which is the time it requires to pay for the additional cost of evaporative cooling vs DX) because the capital costs for both plants are about the same. This means that after 16 years, you could have paid for the complete evaporative cooling plant from the operational savings compared to a DX plant.
The unit state through the year
The number of hours that the unit would be above specification (24 deg C) and how much it would be over specification.
The unit supply air temperature vs ambient conditions. The red and purple lines are the ones that show the supply air temperature for single and two stage units.
The energy consumption of the different types of units. It is clearly visible that nearly at all temperatures, the evaporative cooler requires significantly less energy that the DX system.