Case study: Reducing reheat energy use
Design engineers for the 526,000-sq-ft, greenfield Aurora Medical Center in Grafton, Wis., were challenged to provide a U.S. Green Building Council LEED Silver hospital while demonstrating an acceptable payback of the additional first cost. Reheat energy, the largest energy use in typical health care facilities, was a primary focus of the team’s energy-saving strategies.
Typical reheat-limiting strategies such as occupancy sensors and discharge air temperature setback were analyzed and incorporated, but did not provide the substantial savings required. The solution: An energy recovery chiller, which delivered the largest savings of any energy reduction item chosen for this project. This provided a path to significant reheat energy savings while maintaining the use of a conventional variable air volume with reheat system, used throughout Aurora’s network of facilities.
The additional first cost for the energy recovery chiller was $360,000, with an annual savings of $120,000—demonstrating a first-cost payback of just 3 years. The 3-year payback, however, did not include the incentives provided by the local utility company. The energy-savings strategies resulted in Aurora receiving nearly the maximum incentive of $250,000 offered by the utility company.
Another challenge was presented by the cold climate in Wisconsin, which leads to fewer hours of typical chiller plant operation and therefore a possible increased payback period for the energy recovery chiller. To improve the payback period, several exhaust energy recovery units (EERUs) were added to transfer heat from the exhaust airstreams to the heat recovery chilled water system. This energy is then transferred through the energy recovery chillers to the heating water system. The EERUs recover heat only when the chilled water load is not adequate for the energy recovery chillers to satisfy the building’s heating needs. The EERUs replaced exhaust fans with a unit containing a fan and chilled water coil. This allowed the engineering team to economically expand the operation of the energy recovery chiller and reduce the payback period. The energy recovery chiller size was optimized with this strategy, as shown in Figure 1. The energy recovery chiller at the Aurora Grafton Medical Center was highlighted in the DOE/ASHRAE publication Advanced Energy Design Guide for Large Hospitals as an innovative and effective strategy to save energy in health care facilities.
An energy recovery chiller, consisting of six modules, was installed to transfer energy from the hospital’s chilled water loop to the heating water loop, rather than rejecting it outdoors, which can be problematic in a cold climate. Domestic hot water, via a double-wall heat exchanger, and boiler feed water were preheated by the heating water.
Providing heat from the energy recovery chiller was less expensive than boiler heat, even when cooling was not needed, and when both heating and cooling were needed the economics were extremely favorable. To take advantage of this, chilled water fan coil units were used in lieu of overhead air to cool energy-intensive spaces, such as the data center and imaging equipment rooms. In addition, chilled water coils were added to the exhaust airstream to harvest energy from the exhaust air during the winter months. By maximizing the year-round chilled water load and sizing the energy recovery chiller to match, the payback for the energy recovery chiller was optimized to be 3 years.
Another important factor in energy recovery chiller sizing is providing adequate staging to accommodate periods of low usage. This was accomplished by using a modular chiller. The six modules provided 12 stages to match the loads. Variable flow also was provided on both the chilled and condenser sides of the energy recovery chiller, and flow was controlled to use only the modules that were operating. The number of operating stages was controlled based on whichever required the smaller load (heating or cooling).
Multiple meetings were held with the facility staff to discuss the planned operation and operational savings associated with the energy recovery chiller. In addition, the system was commissioned to ensure that it would operate properly.
Maintenance over the first 4 years of operation has consisted solely of cleaning the strainers. For the first cleaning, facility staff shut down the energy recovery chiller for a day and noticed a spike in energy use during the shut-down. Since then, the staff has limited downtime as much as possible during maintenance.
To verify system operation and comply with LEED requirements, measurement and verification (M&V) was completed. The M&V process entailed tracking the energy use of the facility for 1 year to ensure that savings predicted in the energy model were being realized. The M&V showed the building was using less energy than anticipated in the energy model and consuming more than 25% less energy than allowed by ASHRAE 90.1-2004, with a recorded energy use of 190 kBtu/sq ft/year. Figures 2 and 3 show the monthly energy consumption of the building compared to the baseline energy model.
The reheat-limiting strategies at Aurora Medical Center in Grafton, led by the efficiencies of the energy recovery chiller, have provided substantial energy savings as demonstrated by the building energy use and illustrated by the temporary spike in energy use when preventive maintenance is performed on the chiller. The energy savings and minimal maintenance required indicate that the energy recovery system already has paid for itself, as expected.
The design team’s success with minimizing reheat energy use allowed it to achieve the energy goals in a cost-effective manner while delivering Wisconsin its first LEED Silver medical center.
Mike Lawless is a project executive with KJWW Engineering Consultants. He has a master’s degree in mechanical engineering design, specializing in fluid mechanics and heat transfer. He is adept at system concept and design for large and complex projects, renovations, expansions, and infrastructure upgrades.