Off-Peak HVAC is Once Again Hot

When it comes to energy, facility managers and owners don't like surprises. Unfortunately, trends of the past few years have done little to promote any sense of stability. Utility deregulation is still a work in progress in most states, with California's recent supply woes offering an extreme example. Even as states like California scramble to put more generating facilities on line, last winter's volatile prices for natural gas and heating oil, combined with the threat of rolling blackouts, signal to many that the days of relatively inexpensive energy are over.

To alleviate some of this uncertainty, facility owners are looking for options that will help improve the energy efficiency of their building systems. In areas where incentives for off-peak usage are available, there is renewed interest in thermal energy storage technology. These approaches call for water or other chemical mixtures to be chilled using electricity generated when demand is low—usually between the hours of 6 p.m. and 7 a.m.—and stored for later use to help cool individual buildings or an entire campus. Thermal ice storage systems have been particularly attractive for schools and other educational facilities, where the bulk of the building's energy demand occurs during the day.

Ice storage and other thermal energy systems are by no means new. Their applications have been limited in recent years by the reluctance of some energy providers to offer significant incentives for off-peak usage such as reduced kilowatt-hour rates and, in some cases, partial funding to help offset the high initial investment in equipment. In those areas where deregulation is on track, however, many believe that these inducements will be revived to some extent as competing energy providers vie to serve those customers with multiple facilities. Under the right circumstances, off-peak thermal ice storage may well be the ideal solution to the challenge of providing reliable comfort at a reasonable price.

Thermal ice storage systems fall into two categories: full and partial storage systems.

• Full storage systems produce substantial amounts of ice overnight to handle virtually the entire cooling load for the following day. As such, the air conditioning chiller runs only a minimal amount of time—if at all—during peak hours.
• Partial ice storage systems combine a smaller ice system with a smaller-capacity water chiller that also handles the initial cooling demand during the morning hours. The ice storage tanks supplement the chiller as loads increase during the day.

Partial ice storage systems enjoy wider use for many reasons. To generate enough ice to handle a substantial cooling load, full storage systems require larger, more expensive equipment, including a full-size chiller. Unless the off-peak kilowatt-hour rates are very attractive, the power demands of generating large amounts of ice would likely negate any off-peak cost savings. Partial systems, on the other hand, require chillers that are smaller than those designed for conventional air-conditioning systems. The combination of downsized equipment and long-term energy savings can help offset high up-front costs and accelerate the payback period. In some cases, the savings generated by a thermal ice storage system have reduced the payback period of an ice storage system with a 50-year life expectancy to as little as 36 months.

In addition to promoting energy savings, partial ice storage systems themselves are highly efficient. Most usually require eight hours to refreeze fully thawed ice tanks. Under normal conditions, however, the refreezing time is substantially less, because most buildings do not require the entire supply of ice to handle the day's cooling needs. In most parts of the U.S., for example, the system may use up only about 35% of its stored ice during spring and autumn days with normal temperatures.

This feature is particularly attractive for school buildings, which have vastly reduced cooling demands during the extreme summer months. Even then, the use of sophisticated controls enables the system to direct cooling to those areas that remain operational, while providing limited cooling to unused sections.

Most thermal ice storage systems are also easy to maintain. Static ice-storage tanks have no moving parts; they simply freeze and thaw during each cooling cycle and require only periodic fluid-level inspections. Otherwise, the equipment life cycle compares favorably to conventional HVAC components.

Perhaps the most valuable characteristic of a thermal ice storage system is its flexibility. A system can be designed as part of a new construction project or incorporated into an existing air-conditioning system. In fact, the oversized chillers found in many large buildings designed in the early 1980s can easily accommodate the addition of partial ice storage equipment with few—if any—modifications.

While thermal ice storage systems offer cost-saving benefits for a variety of buildings and applications, they are not a panacea for energy-efficiency challenges. Engineers must carefully weigh a number of factors in determining whether such a system is economically and operationally feasible.

The primary consideration is the availability of off-peak power rates from the energy provider, and the difference between on- and off-peak costs. In addition, the utility's off-peak hours must provide sufficient time to generate ice for the following day, and a building's location and use patterns must be considered.

Regions, such as the South, that experience lengthy stretches of extreme temperatures, will probably require more constant cooling capacity than areas with more moderate weather patterns. A building with around-the-clock operations would not be a likely candidate for thermal ice storage, except in cases where large portions of the facility are not in use during off-peak hours. Other variables that can determine whether thermal ice storage is a feasible system include:

• Building size and expected cooling load.
• Other facility-wide electrical requirements.
• Chiller sizing.
• Availability of other cost-effective supplemental energy options such as geothermal sources.
• Availability of financial assistance for integrating such systems to offset equipment costs.

Thermal ice storage merits consideration for any project—commercial, institutional or industrial—where peak power pricing opportunities are available. The technology has proven itself time and again as a reliable way to save money and improve a cooling system's overall efficiency. And in a world where uncertain energy pricing and availability is the rule, these are good qualities for any HVAC system to have.