By IAN DEMPSTER, Optimum Energy, Seattle
Conflicts between energy production and water availability are on the rise, even in areas not traditionally associated with water supply constraints. These two critical resources are linked: The collection, pumping, conveyance, treatment and discharge of water requires large amounts of energy; likewise, energy production uses large amounts of water, for mineral extraction and mining, fuel production, hydropower and power plant cooling. And demand for both resources is growing.
The World Resources Institute estimates that by 2050, global economic activity will increase five-fold, global population will increase over 50 percent, global energy consumption will increase nearly three-fold, and global manufacturing activity will increase at least three-fold. Balancing sustainability against conventional mass production creates conundrums for business. One of the biggest is how to spread the benefits of industrialization worldwide without creating unsustainable impacts on water and other natural resources.
Buildings are part of the solution
Building owners have to be part of the solution. They’re already feeling the pressure: rising energy costs and concerns about global warming are forcing some water managers to seek ways optimize the energy efficiency of their water systems and reduce overall water use. One potentially large source of water savings that building owners and operators often overlook is HVAC systems.
When considering building water use, people immediately think of bathrooms, kitchens and maybe irrigation, and most stop there. But studies by the California Urban Water Conservation Council and the U.S. Environmental Protection Agency show that an HVAC system may account for 28 to 48 percent of a building’s water consumption, with restrooms and kitchens using 31 to 37 percent, and landscaping accounting for only 18 to 22 percent. While water consumption varies by climate and building type, that’s a huge source of potential water savings.
Fortunately, cutting this waste is a happy byproduct of optimizing HVAC systems to reduce energy consumption and costs. Improving the efficiency of a building’s central plant (a large part of the HVAC system), including automating components for real-time optimal performance, can cut chiller water use by thousands of gallons.
An ocean of savings
Water savings from HVAC optimization depend on the size of the central plant, the amount of space being conditioned, geographical location and the flexibility of the building’s occupants. In large buildings with temperature flexibility, the savings can be huge:
- A 1,000,000-square-foot university campus with 3,800 tons of cooling power is saving 570,800 gallons of water a year;
- A 3,000,000-square-foot airport with 7,000 tons of cooling power is saving 918,000 gallons of water a year;
- A 1,000,000-square-foot biomedical facility with 9,000 tons of cooling power is saving 1,509,000 gallons of water a year;
- A 2,250,000-square-foot office building with 7,600 tons of cooling power is saving 1,200,000 gallons of water a 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 200,000-square-foot research laboratory building that operates 24/7 with 1,050 tons of cooling power is saving 364,921 gallons of water a year;
- A 340,000-square-foot manufacturing facility with 1,300 tons of cooling power is saving 1,000,650 gallons of water a year;
- A 470,000-square-foot medical center with 2,250 tons of cooling power is saving 200,000 gallons of water a year;
- A 400,000-square-foot hospital with 2,000 tons of cooling power is saving 742,000 gallons of water a year;
- A 500,000-square-foot data center with 3,000 tons of cooling power is saving 778,518 gallons of water a year.
How optimization saves water
Water savings from HVAC systems are linked with energy savings, which makes addressing the problem a two-for-one sustainability win. If a facility’s cooling tower is using more than 3 gallons of water per ton-hour of cooling, the HVAC system is running inefficiently. HVAC optimization can cut that usage to 2.5 to 2 gallons per ton-hour of cooling while reducing energy use and costs.
Water loss from cooling towers comes mainly from evaporation, and secondarily from blowdown, the intentional draining of water from the system. As water evaporates, the dissolved solids in the water become more concentrated—and more detrimental to the system. Part of the reason operators perform blowdown is to limit scaling and fouling and to help get rid of bacteria. Legionnaires’ disease, for example, emerges from mismanaged water systems, including HVAC systems. Managers have to add chemicals to stop bacterial growth and then perform blowdown to control dissolved solids and maintain required chemical concentrations. That water must be replaced with fresh water.
The size and efficiency of the chiller plant and how much the building gets cooled are the primary factors in a commercial building’s water usage. When a system is more efficient, less water circulates and less water 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 to 0.75 kilowatts per ton of cooling, facility operators can cut water usage by 10 percent or more. The end result is cost savings, reduced chemical usage and a decreased carbon footprint.
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 for a particular space at a particular time. For example, an optimized system might blow less air in the morning and increase the air conditioning as the building heats up during the day.
Reducing the level of cooling also helps, because it reduces evaporation. Buildings with some temperature flexibility—such as hotels, warehouses and offices — can save thousands of gallons if managers turn up the thermostat just a degree or two. For instance, nudging the temperature up from 67 degrees to 68 or 69 will reduce the load on air handling systems and cut back on the use of chilled water, and most building occupants won’t feel a difference.
Optimizing chilled water production, reducing ton-hours of cooling and automating air handling in an HVAC system together can result in water savings of more than 20 percent, based on Optimum Energy project data. And it can reduce energy consumption and costs by 20 to 50 percent.
Selecting the best technologies
A good HVAC optimization solution will calculate the most efficient operation of the whole system—whether that’s one building or an entire campus—in real time, automatically and continuously optimizing the performance of the chiller plant. It will also track savings.
In addition, new water treatment technologies can help building owners and operators better maintain their cooling towers, keeping the condenser system efficient and clean. These technologies include continuous cooling tower performance monitoring and data analytics tools that track the efficiency of both water and energy usage.
Non-chemical options for cooling tower water conditioning, such as ion exchange and electrostatic field generators, can also be helpful. An ion exchange column (periodically recharged with ordinary salt) can provide softened makeup water with the calcium and magnesium ions removed and replaced with sodium ions, which do not contribute to scale. Electrostatic field generators are a class of precipitation induction devices for water treatment systems that show some promise in reducing scaling potential and minimizing biological growth. These devices enable the precipitation of calcium without the formation of scale, allowing operation of the cooling towers with fewer blowdown cycles.
Owners should select a water treatment vendor based on their commitment to water conservation and the cost to treat makeup water and maintain a cooling tower to the highest recommended system water cycle of concentration. Make sure the vendor understands that water efficiency is a priority, and that the vendor has a solid reputation for results in this area. Not every vendor wants to serve a conservation-oriented client because that usually means selling fewer chemicals (savings on chemicals is an indirect but not insignificant additional benefit of water conservation).
LEED points recognize the value of optimization
Taking advantage of these techniques to help ensure that an evaporative heat rejection system uses the least amount of water required will also ensure that the system is saving energy, help maintain optimized system performance, minimize system maintenance and maximize system life.
On top of those benefits, if a building’s LEED rating is important, all of these measures provide additional LEED points. Cooling tower water management has been part of LEED for Existing Buildings: Operations & Maintenance (EB:O+M) for years, but LEED v4 also brings it into the building design and construction (BD+C) rating systems, such as LEED for New Construction (LEED-NC). Reducing water lost through blowdown is the focus of LEED v4’s cooling tower credit. Projects pursuing the credit need to increase the number of cycles through which water can recirculate before it is removed by blowdown. Depending on its chemistry, the blowdown water can be captured and reused in appropriate applications, such as irrigation — particularly if it is mixed and diluted with captured rainwater or other water sources. Taking this extra step earns the Cooling Tower Water Management credit, which adds up to two points for BD+C projects, and up to four for EB:O+M.
Optimizing HVAC systems to power buildings with the least possible energy and water use — while maintaining comfort and staying within required operating parameters — clearly has enormous financial and sustainability advantages. Realizing those advantages is becoming more and more urgent as water supplies throughout the world face growing demand. It is critical that all water-consuming systems — including hidden users like HVAC systems—optimize their use of this resource.
The author is senior director of product innovation at Optimum Energy and a certified energy manager (CEM). He directs multiple R&D projects, drawing on a 16-year engineering career that spans three continents.