Hybrid HVAC Slashes School's Energy Costs
February 4, 2015 - HVAC
When Clark County in Kentucky replaced its circa 1960s high school with a 30 percent larger facility, monthly utility costs actually dropped by up to 35 percent, in part because of the new school’s HVAC design.
The old 199,492-square-foot building had a four-pipe boiler/chiller system with fan coil units, and was averaging annual utility costs of approximately $194,040. The new 300,000-square-foot George Rogers Clark High School has a hybrid HVAC design featuring a geothermal well field that supplies active chilled beams, and dual wheel outdoor air dehumidification systems.
Originally specified as a conventional 200-foot, 80-well geothermal field with just ground source heat pumps, the design was replaced with 350 heat pumps with 542 active chilled beams and six SEMCO Pinnacle Series dedicated outdoor air (DOAS) heat recovery systems. The HVAC system’s estimated annual cost is $45,175, based on an electric rate of $.07/Kwh. In the first four months of operation, the new building’s preliminary utility bill figures were $41,800 less than the old building’s during the same period.
Some of the greatest efficiency is attributed to a 50 percent air flow reduction that chilled beam technology offers versus conventional forced-air distribution methods. The system’s 43 F dew point temperature allows for reduced air volume due to the humidity control from dual-wheel dehumidifiers that control both latent and sensible humidity. Lower humidity allows the school’s set point to average at 75 F.
The chilled beams, which are supplied by a water-to-water ground source heat pump (GSHP), never develop condensation. Five rooftop DOAS units (3,700 to 14,500-cfm) supply them with semi-neutral, super-dry air. Some areas with high ceilings that aren’t ideal for chilled beams, such as the cafeteria and gym, use their own DOAS to distribute cooling and heating via the GSHP loop.
The DOAS units feature variable-frequency drives, integrated GSHPs, and dew point discharge control that are all interfaced with the school’s building automation system.
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