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Reheat for Medical Center

©2013 This excerpt taken from the article of the same name which appeared in ASHRAE Journal, vol. 55, no. 6, June 2013.

By Mike Lawless, P.E., Associate Member ASHRAE

About the Author
Mike Lawless, P.E., is a project executive at KJWW in St. Louis. He is a member of ASHRAE’s St. Louis chapter.

The challenge for the design team of the new Aurora Medical Center in Grafton, Wis., was providing a LEED Silver hospital while demonstrating a payback of the additional first cost. Energy efficiency was the main driver of the cost savings that would demonstrate the payback. The design team targeted areas of highest energy use in standard health-care facilities, including fan energy, reheat, heating/cooling plant energy and lighting and then optimized the savings compared to first cost for a series of energy-efficient options. The goal was not to save as much energy as possible regardless of the cost, but to save a significant amount of energy in a cost effective manner. The result is a building that can be optimized to provide more than $340,000 in annual savings with a simple payback of less than four years.

 

Energy Efficiency

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 one year to ensure 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 25% less energy than allowed by ASHRAE Standard 90.1-2004, with a recorded energy use of 207 kBtu/ft2•yr (2 350 899 kJ/m2•yr). Figures 1 and 2 show the monthly energy consumption of the building compared to the baseline energy model.

To achieve the savings, the team focused on reducing fan and reheat energy, designing an efficient heating/cooling plant, specifying high efficiency lighting and optimizing use of the energy recovery chiller.

 

Fan Energy

Custom air-handling units, coils, and filters were sized to maintain a maximum face velocity of 350 fpm (1.8 m/s). This reduces pressure drop and fan horsepower consumption for the life of the building, also reducing noise levels to provide a healthier acoustical environment.
Innovative filters within the air-handling units use advanced pleated technology that allow air to travel through larger filter surface areas in smaller housings, while lowering overall pressure loss.

Air-handling unit fan system effect was reduced by properly designing an evase at each supply fan discharge. This gradual transition to the supply duct system accounted for static pressure reductions and overall fan size reduction.

In addition, much of the facility’s ductwork was sized at no more than 0.1 in. w.c. (24.9 Pa) friction per 100 ft (30 m) of duct to facilitate lower pressure drop within the entire duct system.

The reduced fan horsepower showed significant savings when compared to typical hospital designs and also showed savings when compared to the Standard 90.1-2004 baseline system. When comparing the fan horsepower to a conventional design, a savings of more than 200 hp (149 kW) was calculated.

 

Reheat Energy

Energy recovery chillers were installed to transfer energy from the hospital’s chilled water loop to the heating water loop, rather than rejecting it outdoors. Domestic hot water and boiler feed water, in turn, were preheated by the heating water via a double-wall heat exchanger.

Providing heat from the energy recovery chiller was less expensive than boiler heat, even when cooling was not needed. When both heat 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 (data centers, imaging equipment rooms, etc.). In addition, chilled water coils were added to the exhaust airstream to harvest energy from the exhaust air during the prolonged cold periods in Wisconsin. By maximizing the year-round chilled water load and sizing the energy recovery chillers to match, the payback for the energy recovery chillers was optimized to three years.

 

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