Project profile: University science building
Davis Street Building at the University of Findlay has 26 science laboratories and classroom spaces. This project includes HVAC system installation, indoor air control, and building control system.
Project name: Davis Street Building
Project type: New construction
Engineering Firm: Greensleeves LLC
Building type: Educational facilities
Location: Findlay, OH
Timeline: December 2010 - April 2011
The University of Findlay's Davis Building was completed in 2012 and provides an approximately 42,000 sq ft addition of science classrooms and related spaces. The project includes 26 laboratories and classroom spaces, plus additional offices, conference rooms and support spaces.
Challenges and solutions
Science buildings with multiple fume hoods and high ventilation rates are often the highest net energy-consuming buildings per sq ft on a campus. The project team was challenged by the University to create a building that had a minimal energy usage while providing a safe and comfortable educational environment.
The design process began immediately with extensive energy modeling as the architect worked through early concepts of massing, fenestration and wall construction types. The final envelope design consisted of high mass walls (concrete blocks with cores filled with sand) enveloped with exterior insulation covered by architectural metal.This insulated thermal mass was then leveraged in the design of the HVAC system, enabling the interior to absorb peak heating and cooling loads in a manner that "time shifted" the peak loads by several hours. This also allowed a reduction in the peak load seen by the central plant - the capacity of the central heat pump is nominally 60 tons or equivalent to 700 building sq ft/ ton - again extremely low for this type of building.
The design of the HVAC system commenced in parallel to the architectural design process. A central geothermal heat pump system (a 60 ton magnetic-bearing chiller that can produce up to 90 tons under certain conditions) providing chilled water and hot water was selected as it allowed for an innovative method of coupling sensible cooling devices directly to the geothermal earth heat exchanger (GHX). This would not have been possible with traditional distributed unitary water-to-air geothermal heat pumps. The central geothermal energy plant simultaneously makes hot (95 F) and chilled (45 F) water for heating and cooling, feeding the outside air system as well as the chilled beams, reheat coils and thermally massive radiant heating/cooling system.
A hybrid wet/dry closed-circuit cooling tower (nominal 30 ton capacity) was selected to provide both daily and seasonal pre-conditioning of the GHX. Three hydraulically separated geothermal earth heat exchangers using vertical HDPE loops were sized and are controlled to provide different fluid temperatures and to provide direct sensible cooling via radiant cooling and active chilled beams. When the building cooling load exceeds the heating load the control system determines whether to direct the surplus thermal energy into the GHX for later use/later rejection or to reject it to the closed-circuit cooling tower if that process consumes less energy or costs less. If the heating load exceeds the cooling load the heat deficit to the central heat pump is taken from the GHX.
An 18,000 cfm variable volume dedicated outside air system (DOAS) using dual energy recovery wheel technology (one total energy wheel, one sensible energy only wheel) supplies conditioned outside air to the active chilled beams and hot water reheat coils for each space. This system recovers energy and moisture, heats, cools and dehumidifies the ventilation air as required.
Thermally-massive radiant heating and cooling using embedded PEX tubing in the concrete floors and the active chilled beams can use fluid directly from the geothermal loops for sensible cooling without engaging chiller operation. A seven-zone geothermal variable refrigerant flow system was used for stairwell and vestibule conditioning.
An air quality monitoring system tracking volatile organic compounds (VOCs), CO2, particle counts, and wet bulb air temperature to ensure that the air quality within the spaces is being maintained. The air quality monitoring system takes air samples from each space on a rotating basis and conveys the samples to a central air quality testing station where the air is analyzed for CO2, VOCs, and wet bulb temperature. Should one of the monitored items exceed a setpoint, the ventilation rate in the space is automatically increased. In the event of a solvent spill in a lab area, the system automatically initiates a high air volume flush mode to rapidly remove the contaminants.
The zone-level wet bulb temperature sensing allows monitoring and control of the dewpoint temperature to ensure that the radiant cooling and active chilled beams to not enter into a mode where unwanted condensation might occur as well as providing the needed humidity control for Thermal Comfort. If the potential for condensation is detected, the sensible cooling device in that zone is disabled until the conditions change to allow it to return to normal operation. The above signals are used to reset the DOAS unit supply air humidity levels. The zone-level CO2 monitoring allows reduction of the ventilation rate when a space is lightly occupied and allows identification of potential under-ventilation if that might occur.
During the past year the air quality monitoring system took 164,000 air samples and 98.99% of the time all air samples tested at CO2 levels below 1,000 ppm indicating good compliance with the design process that used ASHRAE 62.1-2010. 90% of all samples were at 500 ppm or below, 97% at 700 ppm or below.
Indoor air control
This building houses the University's Industrial Hygiene Program and the faculty is anticipating accessing the indoor air quality data for educational use.
All of the fume hoods have a variable volume fume hood control system that senses an operator standing in the breathing zone and sash position to provide the maximum safety and contaminant capture while minimizing exhaust air volume and energy use. The lab areas with fume hoods have active space pressurization control systems to provide a negative lab space pressure relative to the corridors for contaminant control.
The control system uses anticipatory predictive algorithms for the geothermal heat exchanger (GHX) seasonal and daily pre-conditioning. This can minimize both heat rejection and compressor energy use as compared to traditional "real-time" control that would initiate closed-circuit cooling tower (CCCT) operation when the GHX temperature simply exceeds a setpoint. This means that the CCCT may operate during the night or during winter months to pre-condition the GHX for summer cooling and to minimize summer daytime CCCT operation. Significant reductions in CCCT energy use and water use can be achieved by operating in winter instead of summer due to lower ambient temperatures.
Building control system
The control system measures and "learns" the actual building thermal load imposed on the GHX and adjusts the preconditioning algorithms in relation to this intelligent model.
The cooling (chilled water) system energy efficiency ratio (EER or Btu transferred per watt of energy consumed) when using GHX water directly for sensible cooling can approach 150-200 EER of pumping energy versus a typical chiller EER of 15 to 20. The GHX predictive control system also allows for an annual reset of the GHX mean earth temperature to prevent temperature "creep" in this cooling dominant application.
Sub-metered electrical use by HVAC system, lighting and receptacle loads allows the University to track energy consumption and know very specifically where all the energy is going.
Design phase energy modeling indicated an approximately 50% reduction in energy cost from a baseline ASHRAE 90.1-2007 building modeled per Appendix G. Following system commissioning, the energy model was modified to reflect the actual fan and pump heads as well as current temperature setpoints and occupancy schedules. Since the building has come on line in August 2012, the energy consumption has tracked this model well, with the overall actual energy consumption being 7% less or a total of 57% than the adjusted energy model. Current Site Energy Utilization Index (EUI) is 64.0 kBtu/sq ft; quite low for a science building with many fume hoods.
The application of thermally massive radiant cooling and heating increases the thermal comfort by addressing the mean radiant temperature of the space directly through a reset of the radiant surface temperature. To date, no thermal comfort-related complaints have been relayed to the Engineer. The control system looks at conditions from the previous day in addition to the current conditions, then predicts when a peak cooling load might occur and then pre-conditions the radiant cooling slab in anticipation of the cooling event. This allows the floor slab to absorb some of the peak cooling load and thereby reduce the peak cooling load on the geothermal heat pump energy plant.
The geothermal heat pump energy plant consisting of the magnetic-bearing chiller, pumps, variable speed drives, power wiring and controls was factory-assembled at an ISO-9001 facility and shipped to the site in five (5) portions for site assembly. This reduced construction and commissioning time as well as risk related to varying on-site conditions and quality control.
Several conclusions can be drawn from this project; the primary one being that the application of anticipatory predictive controls on a geothermal HVAC system can provide additional energy saving benefits not possible with traditional real-time control approaches. Additionally, significant system efficiencies and energy cost reductions are possible in climates where the mean earth temperature allows some portion of the cooling load to be addressed directly by GHX loop water without the operation of a chiller. Finally, actual building operation is always different from a design energy model and controls that track, analysis and adapt to these differences will provide more optimum operation over time than a control system that is simply commissioned and left in a static operational condition.