Consider Water-Side Economizers

In a large data center, a mechanical chiller¹ often provides a continuous supply of chilled water for use in air handling units² to cool the data center. A water-cooled chiller uses a separate “condenser” water loop to take away the heat gained in the chiller’s refrigerant³ and release the heat in an evaporative cooling tower.⁴

Chiller-less Cooling

When it is cold and dry enough outside, the condenser water itself can be used to cool the chilled water directly through a separate heat exchanger (“water-side economizer”) and the chiller can be bypassed. Please see Figure 1 below with chilled water loop in dark blue and yellow and the condenser water loop in light blue and orange.  Water side economizers save enormous amounts of energy because they cool water without operating the mechanical chiller. That’s why data center operators may refer to the use of economizers as “free cooling.”

Risk Reduction

Water-side economizers offer cooling redundancy because they can provide chilled water in the event that a mechanical chiller goes offline. This can reduce the risk of data center down time.

Savings and Costs

During water-side economizer operation, the cost of producing chilled water is reduced by up to 70%.⁵  A number of studies have indicated favorable paybacks on water side economizer retrofits.  For example:

• A 2014 DOE study of three federal data centers and proposed efficiency measures concluded that investment in a water-side economizer retrofit pays back in 2.3 years.⁶ 
• Water-side economizer retrofit projects tend to offer better return on investment for larger data centers. A paper published by ASHRAE demonstrates this point.  It cites a one year payback for a water-side economizer retrofit project on a chiller plant serving a data center with a 2,000 ton cooling load.  A smaller data center with a 110 ton cooling load had a payback of 5 to 10 years.⁷
• Wells Fargo built an economizer into its Minneapolis data center facility at a cost of about $1 million.  The bank envisions saving up to $450,000 annually on its energy costs, since the water-side economizer takes advantage of the cold seasonal temperatures in Minnesota.⁸

Water side economizer retrofits can sometimes be too costly. Two ENERGY STAR certified data centers — operated by BNY Mellon (PDF, 600 KB) and Target (PDF, 609 KB) — examined water-side economizers and found unfavorable paybacks.

Tips and Considerations

• Water side economizers are best suited in climates where the wet bulb temperature is lower than 55°F for 3,000 hours or more.⁹ This describes the majority of the United States, barring some areas in the Southwest and portions of the Southeast. (See Figure 2 below.)

  

• Water-side economizers can be integrated with the chiller or non-integrated. Integrated water-side economizers are the better option because they can pre-cool water before it reaches the chiller. Non-integrated water-side economizers run in place of the chiller when conditions allow.¹⁰
• Data centers that use containment, along with a cold aisle or hot aisle configuration, can use higher supply air temperatures. These higher temperatures can be maintained using a higher chilled water set point of 55°F or warmer. This higher chilled water temperature allows water-side economizers to be used more often, saving energy.
• Adequate physical space is a major concern when considering water-side economization. Oracle attempted a retrofit and concluded that the additional parts–new "pipes, valves, and controls"–required physical space that existing facilities simply did not have available. In addition, they had concerns about cooling tower operation and performance at freezing ambient conditions, and about control issues with switching between condense mode and economizer chiller mode.  In the end, they settled on new variable speed chillers that were extremely efficient at low ambient outdoor temperatures.¹¹
• Water side economizers use a great deal of water in their evaporative cooling towers. Digital Realty recently deployed a new type of free-cooling system: one that uses pumped-refrigerant economization and consumes no water.¹²