Energy conservation at the Oak Park Public Works Center.
The Oak Park Public Works Department serves a community of more than 50,000 people in the Chicago suburb of Oak Park. In 2004, the Oak Park Public Works building burned down in a natural gas fire. At the time, all of the vehicles, as well as the street salt, were stored outside. There was minimal office space, and the fleet maintenance garage in a second building on the site. In 2004, plans began to construct a new building that would house all of the functions that the Oak Park Public Works Department needed. The new, 155,000-square-foot facility would house offices, shops, vehicle storage, a maintenance garage, and a fueling station, all under one roof. By completely enclosing all of the Public Works functions, neighbors would have no view of the industrial yard or the public works vehicles. (See Figure 1.)
[FIGURE 1 OMITTED]
The Village of Oak Park recognized the community's desire for a sustainable building that would minimize its impact on the neighbors. The Village set a goal for the building to achieve LEED Silver certification under LEED-NC version 2.1. The building would be the first public works facility to achieve LEED certification at the Silver level. (See Figure 2.)
[FIGURE 2 OMITTED]
In order to achieve a LEED rating at the Silver level, the building would need to include a restored brownfield site, recycled content and low-VOC construction materials, a reflective roof with green roof portions, and solar domestic water heating. In addition, it would have to divert more than 87% of construction waste from landfills, and it would need to achieve energy optimization at 32.7% better than ASHRAE Standard 90.1-1999 (ASHRAE 1999).
The building uses two (2) variable volume air handling units to serve the office, administrative and shop spaces of the building. These units use electric resistance preheating, split DX cooling and terminal electric resistance reheat. Due to the large volume of fresh air required for the vehicle storage and vehicle maintenance garage, two (2) 100% outside air energy recovery units were used. These units utilize air-to-air, plate-style heat exchangers and electric resistance heaters to provide tempered air to the spaces to these non-conditioned spaces.
The office, administration and shop spaces are kept at a positive pressure to the vehicle storage and maintenance areas. This is to ensure that there is no carryover of dirty air to the clean spaces. This was achieved by providing more air than is relieved from the cleaner areas of the building and exhausting more air than is supplied to the vehicle storage and maintenance areas.
The Village of Oak Park wanted to make a statement by constructing a building that would be energy-efficient. Photovoltaic power generation and ground source heat pumps were explored as possible options, but neither were found to be feasible for the site.
The vehicle storage and maintenance garage required 1.5 CFM of outside air per s.f. for ventilation, which was seen as a major hurdle for energy efficiency. The two approaches that were used to reduce the amount of energy the system uses to supply this fresh air are Contaminant-Based Variable Airflow and Heat Recovery.
Contaminant-Based Variable Airflow
ASHRAE Standard 62-2001 was used to determine the exhaust airflow rates in the building. The prescriptive airflow rate required 1.5 CFM of outside air per s.f. for over 111,500 s.f. of building space.
To reduce the volume of air that moves through the building, contaminant sensors were installed. Carbon Monoxide, Carbon Dioxide, and Nitrogen Dioxide sensors were installed in both the spaces and the exhaust ductwork. These are the prominent contaminants in the vehicle exhaust from the Oak Park vehicle fleet.
The acceptable short-term exposure limits for each of these contaminants per ASHRAE Standard 62-2001 are as follows (see Table 1):
Table 1. National Primary Ambient-Air Quality Standards for Outdoor Air as set by the U.S. EPA (Source: ASHRAE 2001) Contaminant Long Term Short Term Concentration Concentration Averaging, ppm Averaging, ppm Sulfur Dioxide 0.03 1 year 0.14 24 hours Carbon Monoxide 35 1 hour Carbon Monoxide 9 8 hours
The exhaust airflow would be controlled with butterfly damper exhaust valves. There was no minimum airflow setpoint for the exhaust air valves. The control system was left to find the best operating point for the exhaust valves. The exhaust airflow would increase as the contaminant levels increased to maintain the contaminant levels between 40% and 50% of the acceptable exposure limits for each contaminant. The exhaust valves would increase by increments of 5% of the maximum airflow setpoint once the contaminant levels reached 50%, and they would continue stepping up the exhaust volume until the sensors read 40% of the acceptable exposure limits, at which point the exhaust valve would modulate down by 5% increments to maintain the 40% level. The control of the exhaust valves is determined by wall-mounted space sensors interlocked with the exhaust valves in the general area that each sensor covers. If at any time the contaminant levels are over 75% of the acceptable exposure limit, an alarm will be triggered.
A second method of monitoring the contaminant levels was instigated to determine if a single area had higher contaminant levels that were not detected by the wall-mounted sensors. If the duct-mounted contaminant sensors indicated that there was a large difference between the wall sensors and the exhaust air stream, an alarm would sound, and all of the exhaust valves on that duct branch would go to maximum until the alarm was manually cleared.
In addition, the fleet maintenance garage has volatile organic compound (VOC) sensors to detect fuel leaks. This is to ensure that there would be no risk of an explosion during vehicle repairs, as the space is designed so that it would not require explosion-proof electrical systems per the NEC. Wall-and duct-mounted sensors are used in this area as well. The exhaust valves modulate to keep the VOC levels between 10% and 20% of the Lower Explosive Limit (LEL). If at any time the VOC levels reach 25% of the LEL, an alarm will sound, and all of the exhaust valves in the maintenance garage will go to maximum until the alarm is manually cleared. (See Figure 3.)
[FIGURE 3 OMITTED]
Air-to-air heat recovery is used to reduce the amount of heat that is required to be added to the 100% outdoor air supply air stream. Heat recovery wheels, heat pipes, water run-around loops, and plate and frame heat exchangers were considered.
Heat recovery wheels were eliminated quickly because of the cross-contamination concerns. It was important to keep in mind that the exhaust air stream would contain combustion exhaust from vehicles, which would produce a sticky film on surfaces. This film would not only reduce the heat transfer capacity of the wheel, but would also allow contaminants to offgas and enter the supply air stream as the wheel rotated to transfer energy.
Heat pipes were also eliminated quickly, due to the lack of availability of environmentally friendly refrigerants in heat pipes. At the time, the only available refrigerant as the heat transfer medium was R-22, an HCFC. Because the goal was for the building to reach LEED certification at the Silver level, Energy and Atmosphere Credit 4--Ozone Depletion was a credit that was sought. In LEED-NC version 2.1, HCFC refrigerants were not allowed.
Two methods to recover heat were left to choose from. The first was a water run-around loop and the second an air-to-air, plate-style heat exchanger. Because the project had a set budget, the initial cost of the two systems was first compared. A water run-around loop requires pumps, expansion tanks, air separators, piping, and a significant amount of labor that plate-style heat exchangers would not require. The cost to install a water run-around loop would be approximately 1.5 times that of installing a plate-style heat exchanger.
The second aspect of the two systems that was compared was the required maintenance. Water run-around loops require maintenance of pumps and the water quality. The staff would have to keep track of the conditions of the water circulating through the loop and add glycol as needed. These tasks were not in the Oak Park Public Works' interests. On the other hand, the plate-style heat exchanger had no moving parts or water to maintain. Periodic wash downs of the heat exchanger to remove the sticky film from the combustion exhaust is the only maintenance that the plate-style heat exchanger would require.
Water run-around loops were eliminated after carefully considering both the cost of the installation and the amount of maintenance the system would require.
The plate and frame heat exchangers have no possibility of cross contamination, provided they are mounted in tightly constructed, truly custom housing. Furthermore, they would not have HCFC refrigerants, and without any moving parts, they would be easy to maintain. These factors led the project team to use air-to-air, plate-style heat exchangers to transfer energy from the exhaust air stream to the supply air stream. (See Figure 4.)
[FIGURE 4 OMITTED]
A 60,000-CFM Energy Recover Unit and a 63,000-CFM Energy Recovery Unit were designed for the Oak Park Public Works Center. Each would provide supply air to maintain a 55 [degrees]F space temperature in the vehicle storage, and with additional electric heaters in the ductwork and at the perimeter, a temperature of 65[degrees]F would be maintained in the vehicle maintenance garage.
During Energy Recovery Unit design, several items became important to the team to provide the highest available amount of energy recovery given the allowed mechanical space. Air-to-air heat exchanger selections were sized to balance airflow and static pressure drop (controlling motor horsepower usage) while modifying plate spacing and plate size to maximize recovered energy. Due to the very low ambient intake temperatures in the Chicago area, frosting of the plates became a design consideration. To prevent frosting, as well as to control the amount of energy recovery, face and bypass dampers had to be designed into each Energy Recovery Unit. Unit height was important to the design team, so a relatively low profile was designed that increased unit width. Because of the wide profile, a careful analysis of the air paths in each air tunnel had to be performed. The results revealed that three (3) supply and three (3) exhaust fans would be the best arrangement to provide an even-velocity profile, resulting in each Energy Recovery Unit reducing system effect losses and increasing system efficiency. Each fan also has a dedicated variable speed drive and inlet damper to allow for some system redundancy in the event of a catastrophic motor or fan failure.
Once the units were designed, the next difficult task was to run the ductwork through the building in order to allow the supply and exhaust air streams to run parallel to each other through the units. In the initial design, both Energy Recovery Units had the supply and exhaust air streams running with counter flow through the units to maximize the energy transfer. The building did not allow for the counter flow on one of the Energy Recovery Units, so it was redesigned to allow for parallel flow through the heat exchanger. (See Figure 5.)
[FIGURE 5 OMITTED]
The resulting redesign using parallel airflow becomes very critical when trying to reduce or eliminate cross-contamination in the Energy Recovery Unit housings. Working with the custom air handling unit manufacturer allowed fan placement as needed to keep the compact footprint and maintain the even velocity profile while also keeping the exhaust fans in the draw-through position to remove the concerns of cross-contamination to the supply air stream. The resulting performance of this system exceeded the expectations of the design team by providing high rates of energy recovery without cross-contamination.
Another issue that impacted the Energy Recovery Units appeared because of the requirement to maintain 16'-6" clear vertical heights in the areas that would have large moving vehicles. In the long run, this was achieved; however, the size of the ductwork was kept to a minimum. This led to a high-velocity air stream and higher static pressures. Higher static pressures require true custom units, which are manufactured to higher-quality standards and provide very low leakage. Semi-custom or catalog-style units would allow far too much leakage, which would cause concern with cross-contamination in the system.
Fan sections of the Energy Recovery Units needed to be capable of a wide range of operations while maintaining high efficiency ratings. Fans selected on steep curves with both good system curves and more capacity in pressure and RPM allowed for flexibility and provided some redundancy. Fans are provided with NEMA premium efficient motors wired to independent-variable frequency drives, allowing the flexibility to balance and control each fan to ensure optimum airflow through each fan, thereby improving overall unit efficiency. (See Figure 6.)
[FIGURE 6 OMITTED]
ASHRAE 2001. ANSI/ASHRAE Standard 62-2001, Ventilation for Acceptable Indoor Air Quality. Atlanta: American Society of Heating, Refrigerating and Air-conditioning Engineers, Inc.
ASHRAE 1999. ANSI/ASHRAE/IESNA Standard 92.1-1999, Energy Standard for Buildings Except Low-Rise Residential Buildings. Atlanta: American Society of Heating, Refrigerating and Air-conditioning Engineers, Inc.
Jonathan Mesik, PE
Associate Member ASHRAE
Jonathan Mesik is an associate, a mechanical engineer, and manager of MEP engineering at Holabird & Root in Chicago, IL. Doug Howery is the MarCraft division manager with Arizon Companies, St. Louis, MO.
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|Author:||Mesik, Jonathan; Howery, Doug|
|Date:||Jan 1, 2009|
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