By Paul Valenta, LEED AP
The HVAC system is rarely thoroughly studied or analyzed until there is a problem. In the daily activities of running a business or managing a building, HVAC is out of sight, out of mind until things get too hot or cold. Then, productivity drops and costs increase as chillers limp along with Band-Aid fixes that result in expensive after-hour service calls. Replacing the chiller with a newer version (i.e. the ‘like-for-like’ method) is all too often the chosen solution, simply because it is the quickest option. However, this can represent a missed opportunity.
Air-cooled chillers typically last 15 to 20 years, and water-cooled systems perform even longer—they are lengthy commitments for building owners. This means performing a change-out presents a great chance to optimize the existing cooling method and match it with a new system selected for optimal lifecycle value.
Optimizing the distribution piping
Twenty and 30 years ago, chilled water systems were designed with a constant volume pumping requirement of 0.15 L/s (2.4 gpm) per ton. The change in flow is proportional to the change in speed, and the change in energy use is proportional to the cube of the change in speed. Essentially, this means pumping less fluid allows it to stay in the coil a little longer, resulting in energy and cost savings.
This physical property drives chilled water cooling system designs to variable volume pumping and designs that require lower flows. A like-for-like chiller change almost always overlooks significant energy and cost savings opportunities (as much as payback within two years) in the chilled water evaporator pumping system.
In most areas of the country, electricity for commercial customers is half price at night. The rates are complicated and come in many different formats, but at the end of the day, rates are greatly reduced at night. A missed opportunity that occurs in a like-for-like chiller change-out is capturing the nighttime electricity savings; a simple replacement will not allow a building to capture savings that come from disconnecting energy consumption or cooling production from cooling demand.
Capturing off-peak electricity savings can be accomplished during chiller replacement projects by installing a hybrid cooling system that features a chiller and thermal energy storage (TES). TES systems also work well with low-flow systems to capture even more savings. There are several kinds of thermal storage systems in the marketplace, but most use water or ice to store energy.
Energy storage employs a standard packaged chiller to produce ice during the night using less expensive off-peak energy and store it in thermal energy storage tanks. The stored ice provides cooling the following day to meet the building’s air-conditioning requirements during higher priced times. Depending on local electricity rates, cooling costs can be reduced from 20 to 40 percent by employing energy storage cooling systems.
This can be illustrated by a hypothetical situation where a building’s two 250-ton air-cooled chillers need replacement. The like-for-like change would require a purchase of a new pair of 250-ton chillers. A hybrid cooling system with ice storage would require two 150-ton chillers and 1500-ton-hours of storage, saving more than $30,000 annually based on eight months of operation with a summer demand peaking building and a utility rate of six cents per kilowatt-hour and a demand charge of $12 per kW. The installed cost of this option is a 25 percent premium over the like-for-like change, but the return on investment (ROI) is just over 20 percent.
Demand and response
With a little more engineering analysis, revenue can also be generated with TES cooling systems. In a growing number of areas in the country, demand response savings will be available to buildings with energy storage by making them virtual generators.
In 2012, the Federal Energy Regulatory Commission (FERC) approved Rule 719, which establishes the rate wholesale market participants must pay retail customers for reducing purchases of electric energy during peak-demand periods. In particular, FERC now requires market participants to pay the full locational marginal price (LMP) for electricity that is not consumed; it treats non-consumption of energy as the equivalent of producing energy. Simply put, the customer can resell its stored energy back to the market, typically through a third party that aggregates customers and determines when the best time is to buy or sell electricity to the grid.
Whether related to piping optimization, hybrid cooling, or virtual generation, missed HVAC opportunities often result from inaction or lack of planning when chillers need replacement. Building owners can be more aware by taking advantage of opportunities to build relationships with local building association and trade groups, including CSI. This enables them to be part of the discussion—through chapter meetings, trade shows, and presentations—and stay abreast of new technology and emerging perspectives.
Through consultation with design professionals and manufacturers, it is also important for building owners to know the expected life of their chiller, planning accordingly by gathering information about how the system operates. Trend logs about how the chillers work during different times of the year, and throughout the day, can be invaluable to optimizing its replacement. Gathering other information about pressure drops, pump operation, and demand logs can offer data that will help define the right replacement system choice.
Gathering data about energy is important. Getting to know the utility representative to understand a building’s consumption and the tariff and rebate choices available is invaluable.
Paul Valenta, LEED AP, is the North American sales manager for Calmac Manufacturing Corp. He is a member of the American Society of Heating, Refrigerating, and Air-conditioning Engineers (ASHRAE), and has been in the HVAC industry for more than 25 years. Valenta can be contacted via e-mail at email@example.com.