Heat recovery for school.
The original 1980 HVAC system was heat-only with classroom induction units fed from underfloor primary air tunnels. The classroom induction units were built into bookshelves along the perimeter wall. Gas-fired hot water boilers provided heat. In the mid-1990s, an air-conditioning project was undertaken, and direct-expansion (DX) cooling coils were added to the existing primary air handlers. The classroom induction units were replaced by hot water (HW) fan coil units retrofitted into the existing bookshelves. The existing primary air tunnel supplied summer tempered air at 58[degrees]F to 65[degrees]F (14[degrees]C to 18[degrees]C).
However, unresolved issues from the 1990s renovation remained including temperature degradation of primary air; noise from classroom fan coils; lack of access to the fan coils units for maintenance; lack of adequate cooling; lack of temperature control in summer; and inadequate classroom lighting.
An HVAC upgrade project in 2010 was initiated to improve the classroom environment for children and teachers. The owner's primary objective was an "optimal learning environment" defined as thermal comfort, indoor air quality, humidity control, low noise levels and appropriate lighting. Energy saving was a secondary objective as was delivering the optimal learning environment in the most cost-effective way.
The solution included a dedicated outdoor air system (DOAS) for ventilation, new classroom fan coils with ducted supply, low temperature (130[degrees]F [54[degrees]C] maximum) boilers, an air-cooled main chiller and a dedicated heat recovery chiller (1) (DHRC) to address the need for summer reheat for humidity control and winter cooling for interior spaces (Figure 1). The existing underground air tunnels were reused for relief air rather than supply air, and new ventilation air supply ducts were run overhead. The DOAS delivers room-neutral (72[degrees]F [22[degrees]C] and dehumidified) air year-round (Figure 2).
Design primary airflows for the renovation were 10,000 cfm (4719 L/s) makeup (calculated per ASHRAE Standard 62.1-2007) and 8,000 cfm (3776 L/s) exhaust/relief at each of two units serving the classroom wings. The balance of makeup air allows for building pressurization and general exhaust. Other areas of the facility (gym, media center, cafeteria and offices) are fed from conventional VAV air handlers that were not connected to the makeup/relief arrangement serving the classrooms.
The mechanical room spaces and the configuration of the building dictated the size and location of the air handlers. There was not room to install conventional energy recovery devices such as total-energy wheels or air-to-air exchangers. Preliminary layout of an air handler with a conventional runaround coil (RAC) per 2012 ASHRAE Handbook--HVAC Systems, Chapter 26.13, would have four coils: runaround, heating, cooling and reheating. Given the room dimensions, it was not possible to fit the four-coil unit. This dictated a two-coil dedicated outdoor air system (DOAS) unit with a primary coil that would switch from heating to cooling, and a reheat coil.
This plan reused the tunnels but changed their use from supply to relief air. Since the relief air ducts (existing tunnels) terminated in the location of the old primary air handlers, building relief was accomplished by relief air handlers (RAH), one for the north end of the school and another for the south. An energy recovery coil was installed in the RAH similar to a runaround coil but connected to both the building hot water loop (summer) and chilled water loop (winter). This makes the RAH coil a heat rejection coil (summer) and a heat recovery coil (winter) (Figure 3).
This configuration does not make operating sense without a heat recovery chiller. Dedicated heat recovery chiller enhanced runaround coils (DERAC) increase the heat recovery chiller operating window and create additional opportunities for the dedicated heat recovery chiller (DHRC) to contribute to energy savings.
Two three-way diverting valves are at each DOAS and RAH, and a two-way control valve is at each coil. In summer, the DOAS primary coil cools outdoor air to 53[degrees]F (12[degrees]C), and a reheat coil will raise the temperature to room neutral conditions of 72[degrees]F (22[degrees]C). The RAH coil heats up the relief airstream (heat rejection) when the efficiency of the DHRC is better than the efficiency of the air-cooled main chiller. The boilers are locked out when the RAH is rejecting heat.
In cooler temperatures, the three-way valves reverse position, and the DOAS primary coil becomes a heating coil. The coil is large enough to do all of the heating duty from 0[degrees]F to 72[degrees]F (-18[degrees]C to 22[degrees]C) using low temperature (130[degrees]F [54[degrees]C] maximum, reset versus outdoor air temperature) heating water. The RAH coil is now a heat recovery (cooling) coil lowering the relief air temperature. The school circulates an engineered heat transfer fluid for freeze protection, and the fluid is common to both heating and cooling loops.
The reheat coil in the makeup air unit is always a heating coil, and it can provide supplemental heating in winter, allowing the boiler supply water temperatures to be lowered below the usual hot water reset schedule for additional energy savings. Dual-purpose coils make the DOAS unit smaller, and they decrease the fan static that otherwise would be seen from an additional large coil.
Table 1 shows the cost to deliver 10,000 cfm (4719 L/s) of room neutral air (72[degrees]F [22[degrees]C] and 50% maximum humidity) with no energy recovery, with a traditional runaround coil and with a dedicated heat recovery chiller enhanced runaround coil. The relief airstream is 8,000 cfm (3776 L/s). Gas is calculated at $1 per therm ($0.009 per MJ) and a boiler efficiency of 90%. The electric rate is calculated at $0.085 per kWh ($0.024 per MJ). The heat recovery chiller is calculated to be operating at 120[degrees]F (49[degrees]C) leaving condenser water and 1.02 kW/ton (0.30 kW/kW). DHRC power consumption includes pump power.
In these calculations, the capacity of the DHRC is limited to the amount of cooling that can be done to the relief airstream (winter) and the amount of heat that can be rejected into the relief airstream in the summer. Annualized for one school year, the DOAS energy intensity (kBtu/[ft.sup.2]-yr [MJ/([m.sup.2] x yr)]) is:
* No Energy Recovery = 16.545 (187.893);
* RAC = 12.653 (143.694); and
* DERAC = 6.109 (69.377).
Any winter cooling needs in the building, such as computer rooms and data centers, would further expand the DHRC operating window (Note 3, Table 1). Likewise, any other summer heating requirements, such as domestic water heating, natatoriums, or VAV reheat for humidity control, would also expand the DHRC operating window (Note 2, Table 1) Both would result in additional savings.
The DHRC has a heating COP of 4.4 and a combined heating/cooling COP of 7.7. It is noteworthy that the DHRC operating as specified will be more efficient (1.02 kW/ton [0.30 kW/kW]) than a conventional air cooled chiller (1.25 kW/ton [(0.36 kW/kW)] at 95[degrees]F) most of the cooling hours. When the outdoor air temperatures are below 75[degrees]F (24[degrees]C), the heat rejection coil at the relief air handler is off, and the DHRC is running only to satisfy DOAS and building reheat requirements. The boiler HW reset schedule is coordinated with the DHRC so that the DHRC can run 12 months per year. Unlike most energy recovery systems, the DERAC can recover energy during all occupied hours.
The school was awarded a 94 ENERGY STAR rating in spring 2012 (Table 2). In addition to the dedicated heat recovery chiller enhanced runaround coils, efficiency improvements include: controls upgrade, condensing boilers, eliminating envelope leakage, sealing of existing ducts, occupancy sensors and ventilation control strategies. Prior to the project, the school's lighting levels were low, an average classroom was at 38 footcandles (409 lux). Lighting throughout the school has been up-graded to T-5 and T-8 technology and now meets ASHRAE/IESNA Standard 90.1-2007 (post-renovation classroom levels are 55 footcandles [592 lux]). The gym, the cafeteria and the corridors are now air conditioned (they were not prior).
Detailed cost estimates of alternates to the dedicated heat recovery chiller enhanced runaround coils were not done prior to design. Table 3 qualitative analysis indicates that the most efficient option, dedicated heat recovery chiller enhanced runaround coils, would also be least expensive to build, even less expensive than a "no energy recovery" option. Both the RAC and the DHRC enhanced version allow for size decrease in the heating plant. Also, the DHRC enhanced system allows for tonnage decrease at the main chiller. The DERAC results in less total tonnage since the heat recovery chiller can always contribute, while, in the other two scenarios, the DHRC can only run to meet the building reheat requirements.
Sometimes It's Not the Engineer's Fault
When high utility bills occur or when systems don't perform as anticipated or when there are a lot of comfort complaints, the reason may be envelope issues rather than HVAC system issues. Early in the design process, a visible difference in winter frost formations was observed from one end of the roof to the other. This dictated appropriate corrective measure is included in the project scope. The "after" thermoscan (Figure 4) showed only the windows and doors as identifiable thermo-leaks, and they were all judged to be in good condition given their age.
The mechanical system at Hamilton Heights Elementary School (DOAS with room neutral delivery) requires a summer reheat source for humidity control, and, in that situation, a DHRC is the energy-efficient solution. System selections that do not require summer reheat will not find the dedicated heat recovery chiller enhanced runaround coils solution as beneficial. And, of course, comparative costs of gas and electricity will affect the savings.
The dedicated heat recovery chiller enhanced runaround coils arrangement offers many benefits in the school's design. It is even more beneficial when the main chiller is air cooled. Where the lack of space limited the heat recovery options, the DHRC enhanced runaround coils took less room than a conventional runaround system and than a "no energy recovery" system.
The dedicated heat recovery chiller enhanced runaround coils saved money in all operating hours, including the moderate outdoor temperatures when small differences between inside and outdoor airstream make air-to-air energy recovery devices ineffective. The DRHC enhanced version saved 70% more energy than a conventional runaround coil, although it was not as efficient in the coldest weather. This would dictate that future versions have the ability to function as conventional runaround coils in winter, and DHRC enhanced runaround coils the rest of the time.
Building at a Glance
Hamilton Heights Elementary School
Location: Arcadia, Ind.
Owner: Hamilton Heights School Corporation
Principal Use: Grades 3 through 6 school
Includes: Classrooms, administrative offices
Employees/Occupants: 55 staff, 515 students
Gross Square Footage: 107,000
Conditioned Space: 105,000 [ft.sup.2]
Substantial Completion/Occupancy: August 2010
(1.) Durkin, T.H., J.B. Rishel. 2003. "Dedicated heat recovery." ASHRAE Journal 45(10).
By Thomas H. Durkin, P.E., Fellow ASHRAE, Alvaro Villalta, P.E.
About the Authors Thomas H. Durkin, P.E., is senior partner and principal mechanical engineer, and Alvaro Villalta, P.E., is the president and principal electrical engineer at Durkin & Villalta Partners Engineering, Indianapolis. Durkin is a member of ASHRAE's Central Indiana chapter.
Table 1: Energy and cost to recondition 10,000 cfm (4719 L/s) to room neutral conditions. No Energy Recovery OAT, HTG (1) CLG (1) $/hr [degrees]F 5 727 0 8.08 20 564 0 6.27 35 401 0 4.46 50 239 0 2.65 55 184 0 2.05 65/57 71 0 0.79 75/63 208 289 4.38 85/69 208 515 6.43 95/76 208 789 9.58 Runaround Coil (RAC) OAT, HTG (1) CLG (1) kWh $/hr [degrees]F 5 483 0 1.5 5.49 20 329 0 1.5 3.79 35 234 0 1.5 2.73 50 139 0 1.5 1.68 55 108 0 1.5 1.32 65/57 71 0 0 0.79 75/63 208 289 0 4.38 85/69 208 515 0 6.43 95/76 208 789 0 9.58 DHRC Enhanced (DERAC) OAT, HTG (1) CLG (1) kWh $/hr [degrees]F 5 413 0 (3) 22.8 6.53 20 251 0 (3) 22.8 4.72 35 88 0 (3) 22.8 2.91 50 0 (2) 0 (3) 17.4 1.48 55 0 (2) 0 (3) 13.4 1.14 65/57 0 (2) 0 (3) 5.2 0.44 75/63 0 (2) 32 23.9 2.26 85/69 0 (2) 150 23.9 3.11 95/76 0 (2) 424 23.9 5.07 (1) MBH required from boiler/chiller plant; (2) Hot water is available for heating; (3) Chilled water is available for cooling. Table 2: Before and after energy performance. Hamilton Energy Intensity $/[ft.sup.2] x yr ENERGY Heights (Source) kBtu/ STAR Rating Elementary [ft.sup.2] x yr School 2008-09 175.2 0.99 40 2011-12 91.2 0.64 94 Table 3: Equipment comparisons. Equipment No Energy RAC DERAC Recovery Main Chiller 250 ton 250 ton 200 ton DHRC 15 ton 15 ton 50 ton Boilers 4.6 MMBH 4.2 MMBH 4.0 MMBH DOAS 3 Coils, 4 Coils, 2 Coils, 9.4 bhp 13.1 bhp 7.2 bhp Relief Fan No Coil, HR coil, HR Coil, 4.0 bhp 5.05 bhp 5.05 bhp Runaround Loop No Yes No Pumps 6 8 6
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|Title Annotation:||Honorable Mention: Educational Facilities, Existing: 2013 ASHRAE Technology Award Case Studies|
|Author:||Durkin, Thomas H.; Villalta, Alvaro|
|Date:||Mar 1, 2013|
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