Central Energy Plants

Since the first water-tube boilers made over one hundred thirty years ago, central energy plants have provided a consolidated energy source for multiple building complexes. Central utility plants now include boilers, chillers, thermal storage, cogeneration equipment, electrical gear and monitoring, and energy management systems. The primary benefits of central utility plants, compared to smaller local systems, are reduced operating costs, better maintainability, less downtime, and easier servicing. The major disadvantage is first cost.

Thermal Storage

Concerns related to overtaxing the nation’s electrical infrastructure have increased the interest in thermal storage. By operating chillers at night when electrical plants have reserve capacity, thermal storage systems generate chilled water or ice for use the next day. Owners benefit because electrical consumption costs are less during these off-peak periods, and operating chillers at night reduces the facility’s electrical billing demand. Some utilities offer financial incentives to owners installing thermal storage systems. When installed concurrently with a central energy plant, thermal storage can reduce the installed chiller capacity or increase the capacity of an existing plant without increasing electrical demands on the utility.

The larger the storage capacity, the greater the economic benefit. However, large exposed tanks can challenge even an engineer’s aesthetic sensibilities. Options are available for exterior tank finishes that blend architecturally with common materials such as brick or stone. Owners may highlight the tank as a statement of their energy-conscious design or may conceal the tank by burying it.

Cogeneration

Facilities having the right proportion of year-round heating demand and base electrical load can benefit from the simultaneous production of heat and power from a cogeneration system. Combustion gas turbines can drive power generators while concurrently generating steam or hot water via waste heat. The combined effect of producing both power and heat from the combustion of natural gas or fuel oil makes combustion turbines a viable choice for owners considering on-site power generation.

Similar to thermal storage for building cooling, combustion turbines have the ability to reduce the installed capacity of central boiler plants.

Variable Speed Technologies

By using variable speed drives, owners can improve energy use by reducing equipment speed during periods of low demand. In the case of cooling towers, variable frequency drives also reduce stress on bearings by reducing fan cycling. Variable flow chilled water systems result in increased water temperature differentials, which not only allow the use of smaller piping but also enable the chillers to operate more efficiently.

Originally applied to air handling unit fans and then large pumps, variable speed technology is now used on chillers, cooling towers, domestic water booster systems, and other motors where a diversity of demand or a need to continuously regulate flow or pressure exists. Input line reactors are used in facilities having a low tolerance for electrical interference from harmonics generated by variable frequency drives.

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Equipment Selections

Traditionally, vapor compression style chillers have used electrically-driven motors as the prime mover, and absorption chillers have used steam or hot water. Today, options are available that use natural gas as the energy source either by fueling an engine-generator set that serves the electric motor or by coupling a natural gas-fired engine with a centrifugal compressor. In absorption machines, natural gas may also be used to drive the cooling cycle.

Some owners may benefit from using medium voltage electrode boilers for large heating plants in lieu of traditional natural gas-fired boilers.

Commissioning

Once completed, a central energy plant requires thorough commissioning of operational sequences, operating modes, setpoints, and energy management parameters. Thorough documentation of equipment submittals, spare parts availability, and troubleshooting procedures help maintenance staffs provide continuous system operation. Training of owner personnel and resolution of warranty period issues complete the commissioning process.

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