Do Evaporative Cooling Systems Use Too Much Water?
One of the issues that will ultimately confront designers when considering evaporative cooling solutions for data centers, or any other application for that matter, is the issue of water use. Considering that water is already an increasingly scarce resource in many areas, with projections that the situation will only get worse, it is important to consider this issue. In the data center design world the common metric of efficiency is known as the PUE but many people want to use a new metric, WUE, to reflect the water use efficiency of the data center.
Evaluating water use efficiency can be pretty tedious and is very dependent upon the location, the type of HVAC cooling system being used, and the type of electrical power plant providing the power to the building. Why do we consider the electrical power plant? Because we need to remember that electrical power plants consume vast amounts of water in the process of generating electricity and that about 70% of the power generated is lost in transmission and generation inefficiencies.
The US Geological Survey has published average water consumption rates for various types of power plants. Nuclear plants consume, on average, as much as 720 gallons per MW-h. Natural gas fired plants, commonly used for peaking plants, consume on average as much as 180 gallons per MW-h. And, coal fired plants, representing almost half of all US power plants, use on average 480 gallons per MW-h.
An additional impact of power generation is the increase in local water temperature near the cooling discharge of the typical “once-thru” plant design. (Recirculating designs are rare due to the huge increase in construction costs that they incur) This increased water temperature has ecological impacts on fish and plant life near the power plant. There is also an ecological impact near the cooling water intakes as the suction pressure of the cooling pumps is such that small fish and aquatic animals are pulled into the intake screens where they die.
So in order to accurately assess whether or not an evaporative cooling solution uses too much water it must be compared to the water use by the power plant that is providing the energy to the HVAC systems that are being compared. This becomes difficult due to the wide range of system types and locations but let’s look at a representative example using some common rules of thumb.
A typical indirect evaporative cooling unit, such as the Aztec ASC-50, will use about .25 kw/ton of energy to provide roughly 150 kw of cooling capacity. A packaged DX rooftop unit with and EER of 10.1 will use about 1.15 kw/ton to do the same amount of work. And a modern air cooled chiller will use about .88 kw/ton for the same cooling effect.
The table below shows the power required, in kw, to produce 50 tons of cooling effect from these three system types. The Aztec water consumption is a combination of the electrical power required plus the evaporation and bleed rates for the evaporative cooling process.
Using the figures from the US Geological Survey we calculated the water use for the two most common power plant types (coal is used to generate 50% of US power and natural gas is used to generate 20%). You can see that, for the most commonly used power source, the Aztec ASC indirect evaporative cooling system actually uses LESS water than the air cooled HVAC systems for the same amount of work. Of course this does not consider the environmental impact of the heated water discharge into the local reservoir from the electric power plant.
Some people would suggest that hydroelectric power is the way to counter water loss but, according to the US Geological Survey study, hydro power is actually a huge water user. The report indicates that, on average, a hydro plant will use 1,430 gallons per MW-h…roughly double that of the coal-fired plant.
Although natural gas fired power plants use less water than the Aztec evaporative cooling system there are some economic considerations. According to a 2009 study by the investment banking firm of Lazard, Ltd. a natural gas primary plant generates electricity at the rate of roughly 10.2 cents/kwh. A natural gas peaking plant generates electricity at the rate of roughly 34.2 cents/kwh. A new integrated gasification combined cycle (IGCC) coal plant generates electricity at the rate of roughly 11.0 cents/kwh. The blended electricity rate for an all natural gas utility would be significantly higher than an all coal utility. History also indicates that the cost of electricity from natural gas fired utilities fluctuates more than for coal fired utilities due to the volatility of natural gas prices.
So…do evaporative cooling systems use too much water? When you look at “source energy water use” of power plants versus the local use of an evaporative cooling unit the answer is usually “NO”. And, finally, consider that a local system offers load diversity that the power plant cannot offer, meaning that the local system can be completely shut off, using no water at all when there is no load.