According to a Spanish proverb, it takes much silver to find gold. This saying holds true if what you seek is a Leadership in Energy and Environmental Design (LEED) Gold rating—except, in this case, the silver you invest is engineering expertise.

The May issue of HPAC Engineering contained an article on the U.S. Environmental Protection Agency’s (EPA’s) Region VII Science and Technology Center in Kansas City, Kan.,1 which was awarded LEED Gold certification in 2003. Gold certification is awarded to buildings that earn 39 to 51 points under the U.S. Green Building Council’s 69-point LEED rating system (www.usgbc.org/LEED).

The EPA’s only dioxin analysis laboratory, the Science and Technology Center features a laboratory fume-hood system utilizing variable-speed fans, heat-recovery coils, and high-quality fume hoods and control valves; chilled- and hot-water systems employing variable-speed pumping throughout; a chilled water system with a heat-recovery system that runs hot condenser water through a heat exchanger to provide hot water for dehumidification when required; and, for the administrative and office areas, a high-quality variable-air-volume (VAV) system with carbon-dioxide- (CO2-) controlled ventilation. All of this is controlled with an off-the-shelf control system running standard control algorithms.

The facility is controlled with a Web-based, native-BAC net building-automation system (BAS). High-speed, 32-bit controllers are used on the air-handling units (AHUs) and mechanical-room equipment, while 16-bit controllers are used on the VAV boxes. The building’s 10/100-mbps Ethernet forms the backbone of the control network, with 156- kbps Arcnet segments running to the field controllers. The fume-hood controllers run on a Modbus network and connect to the rest of the system through a Modbus-to-BACnet/Ethernet gateway. Hard-wired connections provide monitoring and alarming.

This article will describe energy-saving features involving administrative area VAV control and laboratory-ventilation control and discuss the integration of different systems and protocols into a single-user interface.

ADMINISTRATIVE-AREA VAV CONTROL
The control algorithms were carefully designed to minimize energy use. For example, the administrative-area VAV box controllers use a learning adaptive optimal-start routine with outsideair- temperature compensation to ensure the system is not turned on until the last possible moment in the morning. Rather than schedule the AHU that supplies these zones to start at a fixed time every morning, it waits until the optimal-start routine in one of its zones calls for cooling. On a cool, pleasant spring morning, the AHU may not start until workers arrive. On a hot, muggy August morning, however, the AHU may start a couple of hours earlier to make certain the building is cooled before workers arrive.

The temperature of the supply air coming from the AHU is similarly controlled by the cooling requirements of the zones. A set-point-optimization routine adjusts the AHU supply air temperature based on the need for cooling in the zones. Whenever the proportional integral-derivative (PID) loop in a zone calls for more than 80 percent of the maximum available cooling, the zone controller sends a cooling request to the AHU. The AHU, in turn, adjusts its supply-air set point between 55 and 62 F, based on the number of zones requesting cooling. By carefully matching the start-up time and AHUsupply-air temperature to the actualneeds of the zones, the building providesa comfortable work environment whilestill meeting the LEED criteria. See thethermal-comfort floor plan in Figure 1. Any zone that is not red is within the comfort envelope.

FIGURE 1. Thermal-comfort floor plan

In the administrative areas, considerable savings are possible through careful control of the outdoor-air dampers. An enthalpy economizer compares the enthalpy of the outdoor air with that of the return air to determine if free cooling is possible. When the enthalpy of the outdoor air is less, the mixed-air dampers modulate to provide mixed air at or slightly below the cooling set point to minimize energy used by the cooling coil. When the enthalpy of the outdoor air is greater, the dampers close to provide the minimum required ventilation. A request-based control scheme tells the AHU if the zones are occupied. If the zones are unoccupied, no ventilation is required. If they are occupied, the dampers open to provide a minimum 5-percent outdoor air. That this percentage is so low is because a CO2 control scheme will open the dampers farther if necessary.

CO2 in the return- and outdoor- air streams is monitored continuously. As long as the return-air CO2 is within 200 ppm of the outdoor-air level, the damper is allowed to remain at the 5-percent position. If the difference rises above 200 ppm, the damper opens farther, until it reaches 100-percent outdoor air at a difference of 1,000 ppm. In practice, the damper rarely needs to open beyond the 5-percent minimum, yielding significant energy savings.