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Holistic HVAC Design

©2013 This excerpt taken from the article of the same name which appeared in ASHRAE Journal, vol. 55, no. 4, April 2013.

By Abdel K. Darwich, P.E., C.Eng., Member ASHRAE

About the Author
Abdel K. Darwich, P.E., C.Eng., is an associate principal with Guttmann and Blaevoet Consulting Engineers in Sacramento, Calif. He is the co-editor of the 4th edition of the ASHRAE Greenguide and member of SSPC 62.1 and TC 2.8. Darwich is an ASHRAE certified Healthcare Facility Design Professional.

The Segundo Services Center (SSC) is the final addition to the Segundo residential area master plan of the University of California, Davis (UCD). It provides services for about 1,800 students and connects a variety of buildings and spaces to form a vibrant student community in a sustainable way. These services include an academic advising center, computer center, recreation room, laundry room, service desk, mail center and convenience store. The SSC is also the home for student housing administrative services including residential education, facilities services, dining services, and a maintenance shop.

The SSC is a new three story 34,500 ft2 (3206 m2) building that was completed and occupied in August 2011. The project is seeking LEED-NC 2.2 Gold and has participated in the California statewide Savings by Design program.

 

Energy Efficiency Measures

The project used a design-build delivery method with the contract awarded through a design competition. This allowed the team to have a holistic approach, with all disciplines collaborating to deliver a cost-effective sustainable design. The energy efficiency measures used in the project included:

  • Hybrid chilled beam variable air volume (VAV) system. One 100% outdoor air (OA) AHU was used to serve the internal spaces through a conventional VAV system and provide primary air to the chilled beams for the perimeter spaces. The all-water chilled beam system allowed the total building cfm to be in the range of 0.5 cfm/ft2 (0.24 L/[s·m2]), which resulted in substantial fan energy savings. The added pumping energy was still low enough that it didn’t offset the fan energy savings.
  • Heat recovery and indirect evaporative cooling systems on the main air-handling unit. An evaporative cooling system was added on the exhaust airstream, allowing the return air to be cooled to slightly above its wet-bulb temperature (66°F [19°C]) before being passed on to the energy recovery air-to-air heat exchanger. The combination of the evaporative cooling and the heat recovery allows outside air in summer to be cooled from 106°F to about 81°F (41°C to about 27°C). In the winter, the heat recovery system allows air to be heated from 24°F to about 51°F (–4°C to about 11°C).
  • Demand control ventilation strategy implemented by installing a VAV box upstream of the chilled beams. Most high-occupancy spaces served by the chilled beams have their primary air requirements governed by ventilation requirements rather than by cooling demand. Carbon dioxide sensors were added to these spaces that added a demand control ventilation loop to the VAV control and allowed reduction of primary airflow to unoccupied spaces.
  • Daylighting and occupancy sensor controls. Daylighting modeling was performed for strategic locations, and solar tubes were provided as deemed necessary to minimize lighting energy use. The tubes have additional control dampers that can be manually adjusted by occupants to control brightness. The current installed lighting design with control credit has an installed lighting power density (LPD) of 0.676 W/ft2 (7.28 W/m2). Without taking into account lighting control credits, the installed LPD is 0.74 W/ft2 (7.97 W/m2), which is approximately 25% less than the allowed LPD as defined by the California Energy Code.
  • High performance glazing systems with integrated shading systems (on the west façade) have excellent daylighting potential without heat load penalty. The structure’s windows are low-e double-paned windows. The façade has an approximate 31% window-to-wall ratio.
  • Optimized high performing building opaque envelope. The typical opaque wall construction is metal framed with R-15 batt insulation between the studs, with an additional R-5 rigid insulation board on the exterior of the framing system to reduce bridging through the metal studs. The resulting U-factor of this wall system is 0.103. The metal spandrel wall has an overall U-factor of 0.111. The roof is a metal deck roof with continuous R-30 insulation above it. The aged reflectance of the roof is 0.53 and the thermal emittance is 0.87.
  • On-demand pumping control sequence allowing the building to shut down its chilled water pumps and use the available pressure from the campus chilled water network when possible. This was accomplished by incorporating a three-stage pumping control sequence. In Stage 1, both chilled water pumps are turned off, and the building chilled water demand is fed directly from the campus site loop. Once any of the AHUs’ valves reach 95% of the open position, Stage 2 is enabled turning on one chilled water pump. Once this pump reaches 99% of its full speed, Stage 3 is enabled turning on the second chilled water pumps. This sequence has allowed pumping energy savings, especially in the shoulder seasons.

As modeled, the project was able to achieve 38% energy savings with respect to a Standard 90.1-2004 baseline on an energy cost basis for both electricity and steam. The metered electricity and steam use for 2012 are shown in Figure 1 and Figure 2 along with the modeled values and the baseline values. So far, the building is exceeding the expectations for both electricity and steam use, except for the first few months in the year for steam.

On an energy time-dependent valuation (TDV) basis, the project achieved 34.3% energy savings based on California Title 24-2005. The project participated in the California statewide Savings by Design program, which contributed $126,000 of owner incentives and $28,000 of design team incentives.

 

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