©2014 This excerpt taken from the article of the same name which appeared in ASHRAE Journal, vol. 56, no. 5, May 2014.
By Stephen Hamstra, P.E., Member ASHRAE
About the Author
Stephen Hamstra, P.E., is chief technology officer at Greensleeves in Zeeland, Mich. He is an ASHRAE-certified High Performance Building Design Professional and a member of the Western Michigan ASHRAE Chapter.
Science buildings with multiple fume hoods and high ventilation rates are often the highest net energy-consuming buildings per square foot on a campus. However, the Davis Building at the University of Findlay in Findlay, Ohio, which is made up of lab and classroom spaces, offices, conference rooms and support spaces, has a site EUI of only 64 kBtu/ft2 (202 kWh/m2).
The design process began 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 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 strategy also allowed a reduction in the peak load seen by the central plant—the capacity of the central heat pump is nominally 60 tons (211 kW) or equivalent to 700 ft2/ton (18.5 m2/kW)—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 [211 kW] magnetic-bearing chiller that can produce up to 90 tons [317 kW] 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 [35°C]) and chilled (45°F [7°C]) water for heating and cooling, feeding the outdoor 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 [105 kW] capacity) was selected to provide both daily and seasonal preconditioning 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 (8495 L/s) variable volume dedicated outdoor 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 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, volatile organic compounds (VOC), and wet-bulb temperature.
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