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Photo: Broad USA
Broad USA recently installed its IX Multi Energy Chiller Heater for an electricity and air-conditioning cogeneration system at Raritan Valley Community College in New Jersey.

Efficiency and reliability are two of the most sought-after performance attributes in chilling equipment for distributed generation (DG) systems. Some recent technological advancements in this equipment, detailed below, demonstrate a marketplace focus on these attributes.

A Design Overview
Chillers, which generate cold water via the compression of a refrigerant to remove heat, are key pieces of equipment used for cooling buildings. This equipment is also used to provide cooling for specific operations such as file servers. A chiller-equipped cooling process has two major stages: low-temperature cooling and high-temperature heat exchange.

In the first stage, refrigerant passes through an evaporator that uses the refrigerant to pull heat from chilled water and evaporates. Next, the water vapor is compressed and converted into a liquid again by a condenser that cools the vapor. The condensed liquid refrigerant passes into an evaporator section through an expansion device that converts the condensed liquid refrigerant to cold refrigerant. The chilled water then passes through coils in an air handler to remove heat from the air used to cool a building. In a closed-loop cooling system, warm water produced via heat transfer from the building’s air-conditioning system returns to the evaporator and the cycle starts over.

Mechanical chillers vary by the type of compressor used. Some of the major types of compressors are reciprocating, rotary, and centrifugal.

Reciprocating chillers use a compressor equipped with a crankshaft turned by an electric motor; as this occurs, several pistons compress and heat the water vapor before discharging it to the condenser. The pistons are designed with intake and exhaust valves that can be opened to allow piston idling in proportion to reduced chilled-water demand. An alternative design for varying unit output is the use of a bypass for the heated vapor, and some reciprocating chillers have valve-equipped pistons in addition to a bypass that allows a further output reduction.

A rotary chiller has a compressor with two mated grooved rotors that compress gas as they rotate in unison. A sliding inlet valve or variable speed drive (VSD) on the motor is used to control the output. Rotary chillers can be either screw type or scroll type. Rotary scroll compressors have an orbiting scroll and a fixed scroll. The orbiting scroll rotates around the fixed scroll, creating pockets of compression between the two scrolls. These pockets of compression trap the water vapors and increase the amount of compression as the water vapors are sucked through a discharge port in the center of the compressor.

A centrifugal chiller is designed for high output and a small footprint, and has a compressor equipped with an impeller that compresses the refrigerant. These units can be equipped with both inlet vanes and VSDs for output variation. An alternatively designed compressor called a frictionless centrifugal is configured for high efficiency through the use of a rotor shaft and impeller configuration that is suspended in a magnetic field. The compressor is equipped with a variable-speed, direct-drive Direct Current (DC) motor.

An alternative to mechanical chillers is the absorption chiller, which uses a heat source such as natural gas or district steam, i.e., heat supplied via steam or hot water to a group of buildings to a central location, to produce a cooling cycle without mechanical compression.

VFD-Equipped Screw Chiller
In 2007, Evergreen 23XRV chillers were installed at the 17-story Crowne Plaza Hotel Philadelphia–Cherry Hill, the largest hotel in southern New Jersey. The hotel and conference center needed new chillers to cool guest rooms and conference facilities throughout the year. Other priorities included energy efficiency that would qualify the facility for the New Jersey SmartStart program, which is designed to provide incentives for purchasing equipment that offers significant long-term energy savings, and quiet operation.

The hotel runs under low-load conditions and the units were chosen due to their efficient operation in servicing functions such as cooling a crowded ballroom during a winter wedding. An additional consideration was the fact that the unit’s VSD is IEEE-519 compliant, eliminating the need for field-installed harmonic filters or a harmonic study of the building’s electrical system.

Two of the units were installed in a chiller room on the 17th floor, directly above guest rooms, making quiet operation and vibration control critical. The manufacturer claims that the efficient operation qualified Crowne Plaza for a $100-per-ton rebate from the SmartStart program. The chillers also reportedly saved the owner $90,000 in initial costs.

Carrier Corp. recently developed the Evergreen 23XRV chiller to allow system duplication across geographic and varied applications. The company calls it the first available integrated water-cooled variable-speed screw chiller. The unit is equipped with a variable-frequency drive (VFD) designed for high efficiency and its capacity is 300–550 nominal tons (1,055–1934 kW).

The unit utilizes VFDs, which prevent energy waste by precisely matching motor speed with cooling requirements. These drives use electronics to vary the frequency of input power to the motor to control motor speed. VFDs address a common operating scenario for Standard 550/590-2003 chillers, according to the Air Conditioning and Refrigeration Institute: running at part-load, or off-design conditions, 99% of the time.

Additional benefits of VFD use for this application, according to the manufacturer, include the lowest available starting current, a reduction in reduce thermal and mechanical stresses on motors and belts, easy installation, a high power factor that eliminates the need for external power factor correction capacitors, and a lower kilovolt-ampere (kVA) that helps alleviate voltage sags and power outages.

The manufacturer explains the differences in compressor designs in analyzing the ability of the Evergreen 23XRV to operate at high efficiency. Centrifugal compressors are dynamic compression devices that continuously exchange angular momentum between a rotating impeller and steadily flowing refrigerant to create compression. The way in which lift—the amount of work the compressor performs on the refrigerant—is reduced in a centrifugal chiller is to modulate guide vanes, a.k.a. pre-rotation vanes, at the impeller inlet while the mass flow of refrigerant moving through the compressor is reduced. The more the vanes are closed, the company argues, the more inefficiency is introduced into the system.

Screw compressors, which have been used in HVAC applications for nearly three decades, are classified as positive displacement compressors, meaning that a volume of gas is trapped with an enclosed space and reduced, the company points out. A drive rotor coupled to the motor turns and moves a driven rotor, similar to a gear set. Positive displacement compressors can develop the same amount of lift at any speed, so mechanical loaders can be replaced entirely by speed control. A screw compressor with a tri-rotor, such as the Evergreen 23XRV, is designed to allow for shorter rotors and about 5–10% greater efficiency than a twin-rotor design. The tri-rotor is also designed to balance thrust to reduce bearing loads.

Cogeneration-Powered Cooling
Raritan Valley Community College, with a 240-acre campus in North Branch, NJ, recently investigated the use of onsite power generation that included a cogeneration system to provide relief to the growing campus’ peak energy demands. The cogeneration system uses the Broad IX Multi Energy Chiller Heater, along with a 1.4-MW internal combustion (IC) engine powered primarily by natural gas and a Broad waste heat-fired unit. The electrical energy generated from the IC engine provides a portion of the electricity for the campus and the thermal energy from the IC engine provides supplementary air conditioning.

An IC Engine produces waste heat in two forms: hot water and exhaust gases. The exhaust gas that leaves the generating equipment is about 850˚F. Hot water from the engine jacket, lube oil cooler, and intercooler is about 200˚F and is rejected to the cooling water via a cooling tower. The primary circuit uses a propylene glycol mixture and the secondary circuit in the heat exchanger circulates cooling water through the cooling tower.

A typical cogeneration system incorporates a waste heat boiler to capture the heat from the exhaust and convert it to steam or hot water. At first, a typical single-stage steam- or hot water-fired unit was considered; using this equipment, but the equipment that would have been required to capture the waste heat was deemed too costly for the project. In addition, the chiller would be shut off in the winter months to provide heating for the HVAC system, reducing equipment utilization. Finally, combining heat from the engine hot water and heat-recovery hot-water heaters would have limited the system to the use of lower-efficiency single-stage equipment that would have reduced the potential useful form of energy by 25–30%, according to Broad.

The multi-energy unit ultimately chosen is designed so that exhaust gases are taken directly into the machine to convert the exhaust energy into chilled water or hot water, and the glycol mixture from the engine hot water is taken directly into the same unit to produce additional chilling capacity. Capacity control is achieved by regulating the exhaust flow and the glycol mixture flow into the machine. During winter, the exhaust heat, as well as the engine hot water, is converted into hot water to supply building HVAC requirements.

The chiller reportedly uses 6.8 kWh of electrical power to generate chilled water during peak summer months and less than 2 kW to produce hot water for heat. Additionally, the system is said to achieve total efficiency of more than 77% on this project.

Specialized IT Facility Cooling
A unit that provides cooling for a specific application is the APC InRow RD, which is designed for the cooling of information technology (IT) infrastructure facilities. As IT grows increasingly robust, IT facilities—many of which are powered by DG systems—are requiring greater cooling capacity. With consolidation, power over Ethernet and virtualization increasing the complexity of traditional server rooms, a cooling issue can contribute to power problems.

APC by Schneider Electric is addressing these cooling requirements by releasing to the North American market the unit, a 300-mm, row-based, direct-expansion cooling system for network closets, server rooms, and data centers.

The unit is designed for real-time capacity monitoring, simple operation, and variable-speed fans for energy reduction during off-peak hours. A modular design is configured for scalability as demand increases, up to 10 kW. Designed for installation within the row of racks, the unit captures hot exhaust air directly from the aisle and distributes cool air through the front of the unit in an attempt to ensure that temperatures are held at set point conditions.

The manufacturer points out that the unit was conceived to address a common issue faced by organizations operating complex server rooms: serious power problems that can accompany facility expansion. The idea behind development of the unit was to move the source of cooling closer to the heat load, preventing the air from mixing and providing a predictable cooling architecture and flexibility in installation.

Photo: Broad USA The system was deemed more efficient and a better year-round utilization of equipment than a typical cogeneration system that incorporates a waste heat boiler.

Expanded Absorption Chiller Offerings
Yazaki Energy Systems Inc. has launched two new, larger-capacity additions to its line of Underwriters Laboratories (UL)-listed natural gas-driven double-effect absorption chiller-heaters. These 150 and 200 refrigeration ton models have higher Coefficient of Performance (C.O.P.) ratings and larger cooling capacities than the company’s previous “K” series machines, according to the manufacturer. Their fuel efficiency has been increased to a full load C.O.P. of 1.20, and integrated Part-Load Values are 1.32 for both models, resulting in up to 30% higher fuel efficiencies than previous models, the company says. The models also have solid state controls and burner technologies for precise chilled-water control.

The company has also announced a new addition to its line of model WFC-S series hot water-driven absorption chillers, the model WFC-SC5. The new unit has a rated capacity of 60,000 Btu per hour and uses 190˚F hot water to drive the cycle, providing the company’s line of single-effect chillers and chiller-heaters with smaller dimensions and capacities. The new model is currently available only to a developing specially trained dealer group that will also employ trained installation and service personnel.