By Ian Dempster, CEM
From the June 2017 Issue
Many facility managers do not know how much water is being wasted through their buildings’ HVAC systems. Fortunately, cutting this waste is a welcome byproduct of optimizing HVAC systems to reduce energy consumption and costs. Improving the efficiency of a central plant’s HVAC system, including automating components for real-time optimal performance, can cut chiller water use by thousands of gallons. The water energy nexus is a useful place to examine when conservation of both of these resources are a facility management goal.
Commercial buildings often draw more water for cooling and heating than for any other use. Water consumption varies by climate and building type, but studies by the California Urban Water Conservation Council and the U.S. Environmental Protection Agency show that HVAC systems may account for 28% to 48% of a building’s water consumption, with restrooms and kitchens using 31% to 37%, and landscaping accounting for 18% to 22%.
Energy and water savings go hand in hand. The main factors determining a commercial building’s water usage are the size and efficiency of the chiller plant and the maintenance that cooling operators perform. An HVAC optimization solution should reduce system energy usage and costs, decrease water usage in chiller plants, and track savings. It could also calculate the most efficient operation of the whole system—whether for one building or an entire campus—in real time, continuously optimizing the performance of the chiller plant.
Evaporation is the chief cause of water loss from cooling towers, followed by blowdown—the intentional draining of water from the system. Cooling towers typically run hot, but not hot enough to kill bacteria. As water evaporates, dissolved solids in the water become more concentrated—and more dangerous. Legionnaires’ disease, for example, emerges from mismanaged water systems, including those related to HVAC. Managers add chemicals to stop bacterial growth and then perform blowdown to get rid of the chemicals. That water, too, must be replaced with fresh water. In a more efficient system, less water circulates and less needs to be flushed out, reducing the need for chemicals.
When chillers and pump motors function more efficiently, they transfer less heat into the plant’s condenser system and thus reduce evaporation. By improving plant efficiency from 1.0 to 0.75 kilowatt per ton of cooling, facility operators can cut water usage by 10% or more. The end result is cost savings and reduced chemical usage.
Less cooling also helps, because it causes less evaporation. Buildings with some temperature flexibility—such as hotels, warehouses, and offices—can save thousands of gallons of water by turning up the thermostat just a degree or two. This practice will reduce the load on air handling systems and cut back on the use of chilled water, and most occupants won’t feel the difference
HVAC optimization also reduces the amount of water the air handling system uses. Advanced optimization software can calculate the right amount of air to condition a space at a particular time; typically, that means less in the morning and increasing as a building heats up.
Combined, optimizing chilled water production, reducing ton-hours of cooling, and automating air handling in an HVAC system can lead to a 21% improvement in water usage, according to results of projects undertaken by my firm.
Savings From The Water Energy Nexus
Water savings from HVAC optimization depend on the size of the central plant, the amount of space being cooled, geographical location, and building occupant needs. In large buildings with temperature flexibility, the savings can be significant, as mentioned above.
- A 1.5 million square foot airport with 12,000 tons of cooling power is saving 4,013,000 gallons of water per year.
- A 2.25 million square foot office building with 7,600 tons of cooling power is saving 1,200,000 gallons of water per year.
Smaller commercial buildings and those with more exacting temperature standards can see impressive results through HVAC optimization as well, by varying factors such as water flows, pump speeds, and fan speeds while maintaining set temperatures.
- A 400,000 square foot hospital with 2,000 tons of cooling power is saving 742,000 gallons of water per year.
- A 220,000 square foot office building that operates 24/7 with 2,200 tons of cooling power is saving 364,921 gallons of water per year.
- A 500,000 square foot data center with 3,000 tons of cooling power is saving 778,518 gallons of water per year.
The U.S. Geological Survey estimates that buildings consume 47 billion gallons of water daily in the U.S., and HVAC systems are typically responsible for 44% of this energy consumption, according to the U.S. Energy Information Administration. Optimizing HVAC systems to power buildings with the least possible energy and water use—while staying within required operating parameters—can deliver enormous financial and sustainability benefits for facility management.
Dempster is senior director of product innovation at Optimum Energy and a certified energy manager. He directs multiple simultaneous R&D projects for the Seattle, WA-based company, drawing on a 16 year engineering career that spans three continents.
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