By Michael Valenti, Senior Editor

The cost of heating and cooling buildings can represent up to 40 percent of a commercial building's energy bills. Many building owners use heat pump or ice storage systems to reduce the cost of heating and air conditioning, but these systems are not as effective as they might be because of their own control limitations. Similarly, the energy efficiency of defrosting systems used in supermarket display cases is hampered by their control performance.

Supermarkets lengthened the interval between defrosting their refrigerated display cases three times by using an EPRI algorithm designed to improve defrosting control.

The Electric Power Research Institute in Palo Alto, Calif., has developed controllers to make heat pump, ice storage, and supermarket defrosting systems more efficient, increasing their reliability and reducing wasted costs. EPRI is the largest energy and energy services research and development organization in the United States.

"Our intent was to take advantage of the latest developments in microprocessors to create intelligent controllers that could do a better job than the electromechanical controls now in use," said Mukesh Khattar, a mechanical engineer and team leader of heating, ventilating, and air conditioning technologies at EPRI. Khattar led the development of the controllers.

EPRI worked with GC Controls of Greene, N.Y., to develop the Smart Loop 2000 Controller, a device that maximizes the efficiency and performance of the total water loop heat pump, or WLHP, system. The WLHP system is particularly suited for buildings that must heat and cool different workspaces simultaneously. Electric motor-driven pumps circulate water from a cooling tower through piping, to remove heat from a portion of the building requiring cooling and carry the heated water to warm another zone. This heat exchange technique reduces energy bills for air conditioning and heating.

A drawback of the concept is that conventional cooling tower controls simply provide circulating water to operate at fixed set points, preventing the system from meeting demand shifts caused by sudden changes in occupancy, or in the weather, thus limiting the energy efficiency of the water loop. The fixed set points of electromechanical controls are inflexible and underuse the cooling tower at part load conditions, while the heat pumps must operate at a higher-than-optimal loop temperature.

EPRI researchers developed new control algorithms to optimize the operation of the WLHP system, based on four years of recording WLHP data at a commercial building in Connecticut. These algorithms are used to record data, determine whether heating or cooling mode is more energy efficient, and enable the controller to learn from building use to optimize heat pump efficiency. GC Controls integrated these algorithms into a microprocessor-based controller.

EPRI installed SmartLoop 2000 controllers in two Wisconsin commercial buildings to show their effectiveness in 1997 and 1998. One test building in Middleton was owned by Western Center Properties; the other, in Sun Prairie, was owned by Wisconsin Public Power Inc. EPRI found the SmartLoop algorithm saved heat pump energy in spring and fall by lowering cooling loop temperatures.

EPRI teamed with Johnson Controls Inc. of Milwaukee to develop a controller that would improve the performance of ice storage systems, providing building owners with an incentive to install this technology, or to upgrade existing systems. Ice storage systems use chillers to reduce the temperature of a water-glycol mixture to about 22°F at night, when electric power rates are at their lowest. The cooled fluid is circulated through coils in a storage tank, freezing the water contained in the tank.

At daybreak, the system computer activates pumps that transport the glycol out of the tanks and to a heat exchanger. Here, the glycol cools water sent to fan coil cooling units in each room when electric power rates are at their highest. In this way, ice storage systems reduce air conditioning costs.

EPRI reasoned that, as in the case of the WLHP systems, ice storage systems would benefit from improved controls. The EPRI/Johnson Controls NOC, standing for "near optimum controller," requires one-time inputs when installed that include an occupancy schedule, the utility rates, and basic information about the mechanical cooling system used. Once the NOC is commissioned, the controller monitors the building's electric load, cooling requirements, and ice inventory. A forecasting algorithm predicts the daily cooling requirements.

Field tests included the First Union Center in Philadelphia, home to the National Basketball Association 76ers and the National Hockey League Flyers. The 21,000-seat arena saved about $71,000 during spring 1998, representing 9.8 percent of utility costs.

The EPRI NOC improved the reliability of the ice storage system serving a school administration building in Kenosha, Wis., by reducing the frequency of out-of-ice conditions and guaranteeing that the cooling plant would meet load demand.

EPRI also aims to lower the cost of defrosting supermarket display cases, one of the most energy-intensive processes in supermarket refrigeration. The two major defrost systems use either electric heaters or reverse-cycle hot gas to accomplish their task. Employing electric heaters can consume one kilowatt-hour per day per linear foot of display case defrosted, or as much as 25 percent of all display case energy. The additional heat load from a defrost cycle increases the refrigeration compressor's running time and energy use.

The supermarket industry decided to improve its electric heater defrosters by replacing mechanical timers with optical, infrared, and temperature sensors that would activate the defrost cycle when they indicated that excessive ice had accumulated or that temperature had dropped below a preset point. However, the frequent sanitary hosing of the display cases with hot water takes a toll on the more sensitive of these instruments.

Rather than measuring ice buildup or temperature drop with sensors, EPRI designed the defrost controller to use the history of defrost cycling in each refrigerator case to make intelligent predictions of defrosting times. The system does not have direct sensing information on conditions in the case, but it avoids exposing delicate equipment to harsh, life-shortening conditions. By analyzing past defrost patterns, a proprietary EPRI algorithm predicts when the next defrost cycle is needed.

The new controller was field-tested over the course of the last three years in supermarkets, including Lunds in Minneapolis, Grand Union in Livingston, N.J., Miller's in Crescent City, Fla., and Shaw's near Boston. In one instance, the controller extended the intervals between defrost cycles by a factor of four during the winter, and reduced total defrost time as much as 66 percent compared to conventional time-initiated, temperature- terminated defrost control. Defrost time was reduced 34 percent annually.

EPRI designed its new control algorithm as a software addition to the Johnson Controls/Encore 2100 direct digital control system already used to command many supermarket refrigeration systems. A stand-alone version of the EPRI controller is scheduled to be introduced next year and will directly replace the standard mechanical timer controls used in most supermarkets today.


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