Office buildings are typically comprised of perimeter zones extending 12 to 18 feet in from the outer wall that usually include large glass areas, and interior zones (which typically require cooling only). The perimeter spaces require both cooling and heating. Perimeter zones have widely varying loads due to changing sun position and weather. One area or side of the building may require cooling while adjacent spaces may simultaneously require heating.
Office buildings may also house a wide variety of tenants with divergent energy needs. The office building can include stores, restaurants, fitness centers, radio or television studios, photo processors, printers, and computer centers, each with different operating hours and occupancy levels. Design occupancy levels vary from 20 square feet per person for conference space and waiting rooms, to 75 square feet per person in clerical areas, or a low of 200 square feet per person in some private offices.
Obviously, weather, occupancy, lighting, and floor loads (computers, printers, copiers, and other office machinery) are the big energy users. However, building shape, design, and orientation can also have a major effect on energy use. Even the type and placement of trees surrounding the building have an impact.
Office buildings usually include both peripheral and interior zone spaces. The peripheral zones frequently have large window areas and may be considerably subdivided. These zones have variable loads because of changing weather and sun position and typically require heating in winter. During spring and fall seasons, one side of the building may require cooling, while another side requires heating.
The interior zone spaces usually require a fairly uniform cooling rate throughout the year because their thermal loads are derived almost entirely from people, lights, computers and other office equipment. As the cooling loads can vary from full to low or no-load conditions, interior systems may utilize variable air volume (VAV) control.
Office building occupancy schedules can vary widely. Many are occupied from approximately 8:00 a.m. to 6:00 p.m. Others are occupied by some personnel from as early as 5:30 a.m. to as late as 7:00 p.m. Some tenants may require night work schedules, usually not to extend beyond 10:00 p.m.
Some office buildings may include spaces which could operate 24 h per day, such as television or radio studios, printing operations, communications and computing centers, . Therefore, for economical air-conditioning design, the intended uses of an office building must be very well established before the type of system is selected and design is begun.
Occupancy varies considerably. In clerical work areas, the density can be as high as one person per 75 sq.ft. of floor area. Private office density may be as low 200 sq. ft. person. The occasional waiting rooms, conference rooms, or board rooms can have occupancies from none to as high as 20 sq. ft. per person.
A major part of the total cooling load is the lighting. In an office building the lighting and normal "floor" (equipment) electrical loads typically average from 2 to 5 watts per sq. ft. Architectural or other considerations may make them considerably higher. Buildings with computer systems and other electronic equipment can have electrical loads as high as 5 to 10 watts per sq. ft.
As these loads are critical to owner satisfaction, a careful estimate should be made of the amount, size, and type of computer equipment anticipated for the life of the building. This will help to size the air-handling equipment properly and provide for future changes in the air-conditioning equipment. In high electrical loading areas consider using exhaust air or water tubing to pick up this heat at the source.
About 30% of the total lighting heat output from recessed fixtures can be picked up by exhaust or return air and, therefore, should not enter into the cooling supply air requirements. The suspended ceiling is often used as a return air plenum with the room air drawn through the light fixtures into the space above the suspended ceiling.
The balance of the cooling load includes miscellaneous allowances (for fan heat, duct heat pickup, duct leakage, and safety factors), which should not exceed 12% of the total.
The variety of functions and range of design criteria applicable to office buildings have allowed the use of almost every available air-conditioning system. Low-rise and smaller buildings typically use unitary equipment, such as packaged combination roof-top conditioners (gas heat/electric cooling or heat pumps), often one unit per zone. Others use multi-zone units. In climatic areas with low heating requirements, perimeter radiation, often electric, combined with conventional cooling units may be more economical.
Other buildings may opt for individual units, one per zone, ranging from the low-cost packaged terminal units (PTAC) or heat pump (PTHP) units to water-loop water-to-air heat pump systems. Recent innovations and attention to reducing energy costs have led to the increased popularity of geothermal heat pump systems.
High-rise office buildings typically use central chilled/hot water systems with perimeter fan-coil units, or water-loop heat pump systems. Where these systems have been installed for the perimeter zones, separate all-air systems (typically variable air volume systems) have been generally used for the interior spaces and to supply perimeter zone ventilation.
A perimeter heating system separate from the cooling system may be preferable, since the air distribution devices can then be selected for a specific duty, rather than a compromise between heating and cooling performance. If the higher cost of additional air-handling or fan-coil units and ductwork is a factor, the designer may consider less expensive options, such as fan-powered terminal units with heating coils serving perimeter zones in lieu of a separate heating system. In climatic areas with low outdoor winter design temperatures, under-the-window perimeter radiation (hydronic or electric) may add to the occupant comfort by offsetting downdrafts.
System flexibility is essential as office procedures are constantly being revised, and basic building services should be able to meet changing tenant needs. For one owner or lessee buildings, the system may be designed without the degree of flexibility needed for a building where the use is not as well defined. Owner-occupied buildings may require considerable design flexibility, because the owner is more interested in the life-cycle costs, usually wants quality and will pay for it, as well as future alterations. The speculative builder wants low first cost, is typically not interested in life-cycle cost, and can generally charge alterations to tenants.
Tenants having hours of occupancy or design criteria that differ substantially from those of the main office building (such as data processing centers, stores, banks, restaurants, and entertainment facilities) may need to have their own systems.
While office buildings are designed to minimize outdoor air usage, recent attention to indoor air quality may dictate higher levels of ventilation air. The minimum volume of outdoor air should be maintained in variable volume air-handling systems.
Dry-bulb or enthalpy controlled economizer cycles should be considered for reducing energy costs. When an economizer cycle is used, systems should be zoned so that energy waste will not occur by heating outside air. This is often accomplished by a separate air distribution system for the interior and each major exterior zone.
Other energy services opportunities include obtaining consideration of thermal storage to reduce demand costs and equipment size, and geothermal heat pumps system as an energy saving and "green" alternative.
Energy services opportunities include:
- Upgrading the chillers to non-CFC refrigerants and/or higher efficiency,
- System revisions for better indoor air quality,
- Improving system controls, adding energy management system, recommending retraining of equipment operators (possibly providing the retraining),
- Adding economizer systems and/or improving controls on existing systems,
- Adding thermal storage to reduce demand costs.
Water heating in office buildings is not a major energy user as use is typically confined to lavatory use (washing hands, etc.) by the occupants and night-time use by the cleaning crews. Any needs for specialized users (food service, photo processors, etc.) must be treated separately.
Service hot water in office buildings is typically heated using electric- or gas-fired storage water heaters. If a boiler is used it should be separate from the main heating boiler.
Recommendations/Energy Services Opportunities
Convert gas heaters to off-peak electric with adequate storage. If tower water is available year-round, consider a water-to-water heat pump.
Small offices typically don't have cooking facilities, large offices are usually equipped with cafeteria style cooking.
Office building lighting typically uses fluorescent fixtures set in suspended ceilings. Other fixtures consist largely of exit signs and possibly some emergency lighting in otherwise dark hallways.
Recommendations/Energy Services Opportunities
Lighting efficiency improvement is the simplest energy saving strategy for commercial and industrial facilities. Lights typically consume from 15 to 40 percent of the annual energy use for most buildings and are usually less expensive to change than other energy systems.
Therefore, always consider lighting first in any review of potential energy savings projects. Consider more than energy savings when reviewing lighting retrofit economic opportunities. Additional considerations that should be included are:
- Lower cooling costs: improved lighting efficiency reduces most peak electric demand and may permit operation of fewer or smaller cooling systems,
- Increased productivity: better lighting may permit faster work patterns with fewer errors, hence increased productivity,
- Reduced absenteeism: improper lighting can cause glare which results in fatigue, headaches and absenteeism,
- Increased safety and security: proper light levels reduce the possibility of burglary as well as improve the safety of both employees and vehicular traffic in the area,
- Lower maintenance costs: some light sources have longer lamp life which could result in lower lamp replacement and labor costs.
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