Taking HVAC performance beyond the baseline: Combining geothermal, DOAS, and chilled beams

by Katie Daniel | December 9, 2015 2:30 pm

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by Thomas Rice
As consulting engineers continually specify more sustainable HVAC technologies, they must ensure their designs comply with a variety of standards. Of course, when possible, it is even better to go far beyond those baseline requirements. In this respect, the University of Findlay’s $11 million Davis Building is a role model for future commercial mechanical technology, as it is an example of a facility design exceeding minimum requirements of several American Society of Heating, Refrigerating, and Air-conditioning Engineers (ASHRAE) standards.

The 3900-m² (42,000-sf) building, south of Toledo, Ohio, is based on a geothermal concept, but it far surpasses conventional system designs that employ heat pumps or unitary air-handling units (AHUs). Instead, the building employs chilled beams and dedicated outdoor air systems (DOAS) for energy recovery and outdoor air.

The science facility, which consists of 19 labs, four classrooms, a 112-seat lecture hall, computer lab, and 15 faculty offices, recently won an ASHRAE Technology Award in the Educational Facilities−New Construction category. More importantly, the HVAC system is saving 57 percent more energy, which translates annually into a $59,000 energy savings and $7500 in reduced maintenance. Its energy use intensity (EUI) is only 64 kBtu/sf.¹

The HVAC system was designed by Stephen Hamstra, PE, LEED AP, an ASHRAE-certified High Performance Building Design Professional (HBDP) of consulting engineering firm, Greensleeves. RCM Architects was the building architect.

Geothermal and chilled beams
Typical geothermal systems deliver water pre-conditioned by inherent ground temperatures to forced-air systems such as heat pumps or air-handling units (AHUs). Fan energy is used to distribute air across these systems’ coils.

Instead of conventional design using heat pumps and air-handlers, the University of Findlay project used ceiling-mounted chilled beams, which are fin-and-tube heat exchangers distributed strategically throughout a room. Therefore, the geothermal water is pumped to chilled beams, which condition the space via convection. Warmer air rises up to the chilled beams and then falls downward as it cools. This results in a less drafty, more evenly tempered space, but most importantly, chilled beams boast fan energy reductions of up to 60 percent versus heat pumps and air-handlers.

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A central outdoor air system (i.e. DOAS) typically distributed small amounts of dehumidified outdoor air to the chilled beams to eliminate condensation on chilled beams, maintain comfortable room relative humidity levels and comply with ASHRAE 62, Outdoor Air Ventilation Requirements−Institutional Facilities. In comparison with a typical heat pump or AHU forced air system, considerably less air is distributed to the chilled beams at significant lower fan energy costs.

Geothermal HVAC systems are an ideal application for chilled beams. Utilization of higher-temperature chilled water (i.e. 14 C [57 to 58 F]) allows for a greater capacity of cooling with less mechanical energy. Further, the geothermal borefield is not warmed as quickly because the amount of rejected energy is less per gallon of water cooled.

Chilled beams improve the indoor environmental air quality through increased ventilation effectiveness, room quietness and good room air distribution. The geothermal side helps the exterior environment by reducing the amount of evaporative water consumed and carbon dioxide (CO₂) emissions. The combination of the two building technologies makes it possible to achieve a low kBtu/square foot energy consumption.

Project specifics
For the Findlay project, the geothermal system supplies 14- to 17-C (58- to 62-F) closed-circuit ground-source water to ceiling-mounted, active chilled beams for handling 70 to
80 percent of the sensible load. Chilled beams reduce fan energy by up to 50 percent versus conventional air-handling technologies, so they comply and typically surpass the requirements of ASHRAE 90.1, Energy Standard for Buildings, Except Low-Rise Residential Buildings.

Each chilled beam requires only 2.8 to 3.5 m³/minute (100 to 125 cfm) from the project’s 538-m³/minute (19,000-cfm) DOAS to prevent condensation. Additionally, chilled beams helped ASHRAE 90.1 compliance, because they reduced the required air-conditioning tonnage by approximately 25 percent versus a geothermal/heat pump design.

ASHRAE 62.1, Ventilation for Acceptable Indoor Air Quality, defines requirements for ventilation, air-cleaning, design, installation, commissioning, operations, and maintenance. It specifies minimum ventilation rates and other measurements intended to provide indoor air quality (IAQ) acceptable to human occupancy and that minimize adverse health effects. DOAS can be integral to ASHRAE 62.1 in that they typically have high MERV filtration (i.e. minimum efficiency reporting value) and specially sloped drain pans to minimize condensation accumulation that could potentially introduce mold to the supply airstream.

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The DOAS energy recovery is even more critical to surpassing ASHRAE 90.1 at the Davis Building. The 19 lab hoods exhaust considerably more air than similarly sized conventional buildings, thus greater amounts of conditioned outdoor air are required to maintain building pressurization.

The building’s embedded 12-mm (½-in.) diameter cross-linked polyethylene (PEX) floor tubing leverages the thermal mass of the three-story building’s poured concrete. In this radiant system, piping under or inside the floor distributes the geothermal assembly’s water. The temperatures cool or heat the slab, which in turn radiates to the spaces above and below.

All combined, the chilled beams, radiant flooring, and DOAS provide even cooling and heating temperatures that are well within the six-degree Fahrenheit differential guidelines of ASHRAE 55, Thermal Environmental Conditions for Human Occupancy. The three systems draw from the 107-m (350-ft) deep, 40-well borefield’s three hydraulically separated geothermal earth heat exchangers using vertical loops of high-density polyethylene pipe (HDPE).

“We see a tremendous opportunity with chilled beams and geothermal, especially north of the Mason-Dixon line where inherent ground temperatures of mid-50s (i.e. 11 to 13 C) can be used in chilled beams and radiant systems,” said Hamstra, a geothermal design engineer with more than 464,500 m² 
(5 million sf) of building space on his resumé. “Instead of the 20 EER (i.e. energy efficiency ratio) of a conventional chiller system operating a majority of the time, the University of Findlay system achieves an estimated 150 to 200 EER.”

The chiller, a 60-ton magnetic bearing model, runs about
20 percent less and is approximately four times smaller than
a conventional chiller in a similar sized building, which helped reduce the project’s equipment capital costs. The resulting 65 m² (700 sf)/ton is unprecedented for a science-oriented building with 15 draft hoods that were 1.2 to 1.8 m (4 to 6 ft) in length, according to Hamstra, who has specified three chilled beam projects.

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The combination of a smaller chiller and no heat pumps, which are many times the more traditional geothermal choice over chilled beams, also complies and surpasses maximum pounds of refrigeration required by ASHRAE 15, Safety Code for Refrigeration Systems, and ASHRAE 34, Designation and Safety Classification of Refrigerants.

While these two standards are not new, the ever-changing scope of refrigerant types and quantities make the Davis Building environmentally cutting-edge for a facility its size. Additionally, both ASHRAE 15 and 34 are satisfied because Hamstra sized the building accurately and employed energy recovery (i.e. DOAS) and chilled beam technology to reduce the facility’s connected refrigeration load.

Indoor air comfort
Hamstra specified 49 active chilled beams in 1.2-, 1.8-, and 2.4-m (4-, 6-, and 8-ft) lengths. As the ceiling-mounted chilled beams quietly cool air around their coils, the cooler air sinks while warmer air rises in a perpetual room air mixing scenario superior in air comfort and energy efficiency compared to fan coils, variable air volume (VAV), and other forced air methods.

Installed by local mechanical contractor, Jack’s Heating A/C & Plumbing, the chilled beams have adjustable slots (nozzles) to address specific areas in its airflow range. The balance of air and water temperatures is all controlled by the building automation system (BAS). Hamstra also supplied each chilled beam room with a localized hot water reheat coil.

“We reset the DOAS’ discharge temperatures based on what most zones are calling for,” he explained. “The localized reheat coils can provide a more precise temperature required in that particular zone.”

The outdoor air supplied through the active chilled beams also prevents potential floor condensation from the radiant system. Hot water for the chilled beams and radiant system is supplied via chiller condenser heat recovery. An adjacent campus building’s conventional boiler serves as a backup, if needed.

Perhaps the most differentiating factor from other geothermal designs is the project’s intuitive Greensleeves’ in-house software that optimizes efficiencies beyond traditional BAS parameters. 
By analyzing considerations such as set-point temperatures and historical data, and then interpreting space needs, the software provides several levels of predictive control based on the present and future temperature and humidity conditions of the building load, the loop and outdoor environment, while also managing the geothermal borefield. For example, the software might signal the chiller to increase the temperature of the radiant system’s thermal mass on a morning where trends predict additional cooling will be needed in the afternoon.

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The software also intuitively manages the borefield via the project’s hybrid wet/dry closed circuit cooling tower. The cooling tower can be used to pull heat out during cooler nights, without the expense of running the chiller, so the field can be more efficient the following day.

“The software perpetually runs building load analytics and then reprograms itself every five days for better efficiency in a self-adapting approach,” said Hamstra.

Other alternatives to conventional HVAC systems that are controlled either with the Greensleeves’ software or the BAS are:

• 
fume hood systems’ air-quality sampling;
• 
a packaged, factory-assembled energy chassis that includes the chiller, seven variable frequency drive (VFD) pumps ranging from 1.5 to 15-hp, valves, piping, air separators, heat exchanges, and sensors; and
• 
a seven-zone geothermal variable refrigerant flow (VRF) system used for stairwell and vestibule conditioning.

Economics up-front and over time
Hamstra said he sees chilled beams as continuing to take market share from heat pumps when it comes to geothermal designs.

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“However, designers must consider condensation and humidity control along with DOAS. Another important consideration is getting the greatest capacity and efficiency from a geothermal system by using the warmest water temperatures possible for cooling and the coolest for heating,” he explained.

Compared with conventional systems using a chilled water loop, chiller, cooling tower, and VAV boxes with hot water coils, the University of Findlay system is only $1/sf higher in equipment costs, which amounts to approximately $75,000. The additional equipment expense will be paid back immediately if the school receives a proposed energy rebate from local utility AEP Ohio’s energy-efficient product installation program.

The Davis Building is the most efficient of the university’s 232,260 m² (2.5 million sf) of academic/athletic and residence buildings, explained the school’s physical plant director, Myreon Cobb.

“The building’s performance monitoring is still ongoing, however we’re presently attaining the energy savings that was proposed,” he said.

CHOOSING DESICCANT WHEEL ENERGY RECOVERY EQUIPMENT: THINK RER, NOT ROI

Energy recovery with desiccant wheels should not be specified purely on efficiency ratings and capital cost, without regard to static pressure. This advice comes from the Air-conditioning Heating and Refrigeration Institute (AHRI) Guideline V, Calculating the Efficiency of Energy Recovery Ventilation and its Effect on Efficiency and Sizing of Building. Doing so may in fact defeat a building owner’s attempt to maximize long-term energy savings. Instead of choosing desiccant wheel equipment for return-on-investment (ROI), a wheel should be reviewed for its recovery efficiency ratio (RER), according to AHRI Guideline V. The RER takes into consideration the efficiency and the static pressure of a desiccant wheel. Not calculating the RER could result in tens of thousands of dollars in lost energy savings over the course of the desiccant wheel’s lifecycle. Intended for service contractors, engineers, and building owners, AHRI Guideline V provides a means for calculating the impact of applied energy recovery equipment on the energy efficiency of the HVAC system at a single selected operating condition. More simply, the calculations comprehensively take a host of factors into consideration, such as geographical climate, fan/motor efficiency, exhaust air transfer ratio (EATR), pressure drop, and energy recovery methodology. Guideline V also allows specifying engineers (manufacturer representatives can also perform the service) to compare energy recovery ventilation (ERV) wheels and arrive at a comprehensive savings for heating and cooling, rather than just a wheel manufacturer’s efficiency rating that doesn’t consider all variables.

Thomas Rice is director of sales at SEMCO LLC, a Fläkt Woods Company specializing in energy recovery equipment, chilled beams, spiral metal ductwork, and acoustical products. He has 15 years of experience in HVAC/R and has assisted, consulted, and participated in the specification and installation in dozens of high-profile CB and dedicated outdoor air system projects. Rice serves on American Society of Heating, Refrigerating, and Air-conditioning Engineers (ASHRAE) technical committees for room air distribution, air-to-air energy recovery, and desiccant dehumidification components. 
He can be reached at thomas.rice@flaktwoods.com[7].

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