Achieving the Water-Energy Conservation “Sweet Spot"

July 30, 2019

Drought in corn field

Achieving the Water-Energy Conservation “Sweet Spot”

While the focus on climate change has driven reduced energy consumption, attention to water conservation is increasing because fresh water supplies are diminishing. At the current consumption rate, this situation will only get worse. By 2025, two-thirds of the world’s population may face water shortages, according to the World Wildlife Fund[1] 

When it comes to building design, however, achieving both energy and water conservation can be challenging. In the average office building, approximately one-third of average water consumption can be traced back to the operation of heating, ventilation and air conditioning systems, according to Engineered Systems[2]. Electricity generation requires water

for cooling, and the movement and treatment of water requires energy. The ideal solution is one that takes the total environmental impact of consuming both water and energy into consideration.

Addressing both energy and water conservation in your cooling systems approach can help prepare your organization for reduced resources and the water conservation regulations that could well accompany them.

The Conundrum

Juggling fulfillment of simultaneous water and energy conservation ― without an understanding of available options ― can leave building engineers and building operators frustrated.

  • If you are most concerned about energy conservation - then water-cooled chiller systems seem ideal for your building

  • If water conservation is most critical, then air-chilled systems become the obvious choice

How do you optimize both water conservation and energy conservation at the same time?

The optimal solution typically incorporates a careful assessment of – and balance of ― utility rates and incentives, efficiency requirements, your local climate conditions, and in many cases, the power-generation mix.  

Thermal energy storage [3] can downsize the chiller capacity. Shifting creation of cooling to periods of lower ambient temperatures can offset energy and water consumption at the source. A proper evaluation takes into account improved thermos-cycle electricity generation efficiency at night, lower distribution losses and increased grid efficiency by using renewable energy resources.

Tips to Identifying the Best Solution

Here are key tips to help drive your decision-making process:

Check available incentives and utility rates - Buildings and codes and utility rates may incentivize one form of conservation over another. In California, for example, Title 24 building codes[4] promote more energy efficiency by restricting use of air-cooled chillers for projects over 300 tons. However, there are multiple exceptions to this limitation. For example, you can exceed 300 tons of air-cooled chiller if paired with thermal energy storage.  

  • In areas served by complex utility rates, pairing air-cooled chiller systems with thermal energy storage has similar annual costs as a premium efficiency water-cooled system but consumes no water /disposal/ chemicals on-site and can reduce 25 percent of total water consumption.  


Consider the building use - The average U.S. household uses 254 gallons per day. When it comes to data centers, however, a report from the US Department of Energy’s Lawrence Berkeley National Laboratory[5] found that the nation’s data centers collectively consumed 165 billion gallons of water in 2014 a number they expect to creep upwards to 174 billion gallons in 2020. As the cost of water continues to rise with demand, the issue becomes one of both economics and sustainability.

Gather and evaluate other metrics -The delicate energy-water nexus balance you’d like to achieve requires a wealth of data for informed decision-making:

  • Consider water consumption from both the building and power source standpoint

  • Evaluate total water consumption from potential air-cooled and water-cooled chillers you are considering. Compare evaporation rates at the site (consumption by cooing tower) and source (consumption at power plant) by doing the following

    • Compare compressor and heat of rejection energy of water-cooled chiller systems and an air-cooled chiller

    • Determine cooling tower water consumption. Ignore waste water which is not consumption, according to ASHRAE® Journal[6]

    • Calculate key conversion factor to relate average thermos-electric type power generation to changes in fresh water supply.  A process detailed in the January 2019 ASHRAE Journal [7]. This capability represents a critical tool for engineers.

  • Conduct a life-cycle analysis. In most cases the result will be that air-cooled chillers use more power and less water than water cooled. However, the total installed cost of air-cooled may be much less.

  • Implement controls strategies that can have more agility due to changes in water and energy use

  • Develop a scheduled maintenance plan which can in many cases extend the life of the equipment and help ensure the HVAC system is performing optimally as equipment, sensors, etc. ages, or there are changes in building occupancy, utility rates

  • Review electric rates, energy procurement opportunities and assess the opportunity to reduce demand or shift energy to off-periods

  • Review the needs of your geographic region if they should factor into the process

  • Consider the role of cleaner energy resources with thermal energy storage to optimize the impact of water and energy use


Evaluate Different Potential Solutions - Thermal storage plays well with others and can be combined with a variety alternative energy resources. Partner with a solution provider who has the design tools and experience to identify the best fit for your needs:

  • Since an air-cooled system can use 75 percent less water than a water-cooled system, potentially offering a quicker payback, consider thermal storage options in combination with an air-cooled system to keep energy costs down while optimizing water conservation.

  • Evaluate the use of solar power with thermal energy storage and an air-cooled chiller system. Solar energy generated on-site can power an air-cooled chiller to create cooling and charge the thermal energy storage system. When the sun isn’t shining, the storage kicks on.

  • Assess the addition of wind power to reduce water consumption. Wind outputs typically lull during the day, during time of peak energy demand and increase when the building is not operating. Charging thermal energy storage systems during periods of higher wind energy can help reduce overall energy use.

  • Don’t write off water-cooled chiller systems, however. They can be combined with thermal energy storage to improve energy efficiency and increase large cooling capacity creating a manageable footprint and cost-related benefits comparable to the use of air-cooled chillers.

While energy conservation is important and rightfully so, the landscape is changing today, literally, as water conservation moves to the forefront. While there are various approaches to water conservation, addressing cooling in buildings represents a critical component. Considering environmental and cost-related factors for both water and energy conservation can help you find the water-energy nexus “sweet spot.” Working with experts at an energy service company such as Trane®, can help you formulate the best solution for your organization.


[1] World Wildlife Fund, Water Scarcity Overview,, Overview

[2] Johnstone, Henry, P.E., Engineered Systems, May 2019, “The Energy Water Nexus and The Role of Air Conditioning,” p. 38

[3] Trane Commercial HVAC Website, Energy Storage System,

[4] California Energy Commission, 2019 Building Energy Efficiency Standards for Residential and Nonresidential Buildings,

[5], Arman Shehabi, Sarah Josephine Smith, Dale A Sartor, Richard E Brown, Magnus Herrlin, Jonathan G Koomey, Eric R Masanet, Nathaniel Horner, Inês Lima Azevedo, William Lintner United States Data Center Energy Usage Report, US Department of Energy’s Lawrence Berkeley National Laboratory 2016,

[6] Poole, James, P.E., and Lessans, Mark, CEM, ASHRAE Journal, January 2019, “A Balanced System Approach to the Water-Energy Nexus,”  p. 24,

[7] Poole, James, P.E., and Lessans, Mark, CEM, ASHRAE Journal, January 2019, “A Balanced System Approach to the Water-Energy Nexus,” p. 24,