A Hospital on the Lake
A lake-coupled geothermal system powers a medical center, cutting down on energy and patient-care costs
When hospital administrators at Great River Medical Center in West Burlington, Iowa, look out over the 15-acre lake alongside their new medical campus, they know it’s no ordinary lake. It will never be used for boating, fishing or water skiing. Instead, it provides energy-efficient heating, cooling and ventilation to more than 700,000 square feet of building space in Iowa’s largest hospital complex.
The $120 million medical campus was built on 82 acres—a few miles from the site of an older hospital—along the banks of the Mississippi River. The campus opened in April 2000 following a fast-track 18-month construction schedule.
Project engineers designed the artificial lake to serve as the thermal source and heat sink for a lake-coupled geothermal system—one of the nation’s largest. The system pumps more than 5,000 gallons of water per minute through a 100-mile-long piping system, and in the process, ensures virtually constant temperatures for the hospital as well as for two adjoining office buildings and the hospital’s Center for Rehabilitation and Cancer Care Center.
System designers faced challenges in designing a geothermal system for a hospital application: complying with hospital officials’ concern for patient and staff safety and comfort, meeting state codes for indoor-air quality and producing enough capacity to meet the large cooling demands of a hospital.
The first hurdle was determining the size of the lake required to provide enough surface area to collect and dissipate energy. Extensive modeling and engineering calculations were required to determine the appropriate size. It was determined that a 15-acre lake with a depth of 12 feet would be required to produce 1,500 tons of cooling capacity, enough energy to serve the equivalent of 500 single-family homes. Configuring the system
Geothermal systems work by using the Earth’s constant 55e closed-loop piping system to the ground beneath the lake. When heating, the system is reversed and the water in the pipes absorbs heat from the lake and carries it to heat-pump units, which compress the extracted heat to a high temperature and deliver it to the buildings.
The system’s 1,500-ton capacity was achieved by locating 105 “grids” at the bottom of the lake to act as heat exchangers. Each grid comprises 14 coils of 3/4-inch pipe, 300 feet long. Each coil is capable of producing one ton of cooling for a total of 14 tons per grid. Two-inch supply and return lines connect each grid to a manifold building at the lake’s edge. A total of 210 pipes, totaling nearly a hundred miles in length, lay along the bottom of the lake.
The manifold building serves two purposes: It is the mechanical room for the system, housing three sets of 14-inch stainless steel headers; in addition, its roof is an observation deck overlooking the lake. All the equipment is constructed of stainless steel to prevent rust. The pipes from the lake are set over a metal grate that comes up through the floor, directing water toward the hospital and offices. The water level is about four feet below the grate with the pipes entering through the bottom of the manifold building.
The 14-inch supply and return lines are routed underground from the manifold building to the hospital to prevent freezing. The lines connect to three 150-horsepower circulating pumps. Each pump is sized at approximately 50 percent of total system capacity to allow the system to remain operational in the event a pump fails. Each pump is equipped with variable-frequency drives to match load and maintain a set system pressure, providing additional energy savings. Building operating needs
From there, the recirculated water passes through more than 800 heat pumps throughout the buildings. The pumps help regulate temperatures in offices and patient rooms and allow individual room temperature control for increased staff and patient comfort.
The hospital uses more than 450 ceiling-mounted water-to-air heat-pump units, with two office buildings each using more than 150 units. The entire medical center has 22 dedicated 30-ton water-to-water heat pumps for ventilation air. Due to stringent air-quality requirements, rather than a cooling tower, the surgical and intensive care rooms use conventional variable-air-volume (VAV) units with high-efficiency particle arrestor (HEPA) filtration and water-cooled chillers that tie into the geothermal system. The chilled-water air-handling system is capable of delivering 62
Designers found that total system performance was actually better when they added the water-to-water units to the mix, because it helped moderate loop temperatures. Often, the system’s internal zones are in cooling mode and the ventilation air requires heating. The water-to-water units are able to use the heat rejected from inside the building. Hospital requirements
The application and size of Great River’s system posed a unique and complex set of challenges. Typical geothermal systems produce 200 to 300 tons of cooling, but in the case of Great River, 1,500 tons of cooling were required.
As hospital administrators were mapping out plans for the new campus, they kept three goals in mind: patient comfort, safety and care cost. Their vision was to build one of the most energy-efficient health-care facilities in the country—both to reduce operating costs and to minimize patient-care costs.
What was exciting for design engineers was the hospital’s openness to new technologies and willingness to evaluate alternate systems. The decision to pursue geothermal technology was based on the hospital’s experience using heat pumps in its old facility in downtown Burlington. Hospital administrators didn’t want a large central utility plant that needed to be manned 24 hours a day, seven days a week.
Another factor was the cost-benefit ratio. While geothermal is more energy-efficient than a conventional heating, ventilating and air-conditioning system, on average it costs 15 percent more to install. It was estimated that the geothermal system, including lake construction, cost $14 million. Design engineers worked with the local utility to obtain an energy-efficiency rebate to offset the incremental cost between a conventional and geothermal system. As a result, the utility rewarded the hospital with an unprecedented $2 million rebate.
Steve Leavitt, Great River Medical Center’s director of development, chose the geothermal system for its flexibility, after considering a conventional air-to-air system. “The medical center is always changing,” says Leavitt. “The new building will change substantially over its lifetime. The nice thing about the system is the ability to tie into the loop and add additional capacity in locations where it’s needed.”
For example, the radiology department may acquire new equipment with a higher heat load than its older machines, or the department may need to increase its space. With a geothermal system, the loop connection is right there and the tonnage can be adjusted.
The system’s zoning capability, energy efficiency and expected operating costs were other deciding factors. “We can easily provide temperature control in small zones, and make our users comfortable and happy,” Leavitt says.
Heat pumps provide individual room-temperature control, increasing patient and staff comfort and matching heating and cooling needs to occupancy. The system also provides greater variance in heating and cooling individual areas without conditioning the entire space. Maintenance can be done on individual heat pumps, eliminating the shutdown and restart of the entire system.
The chilled-water unit is tied into the geothermal system, eliminating the need for a cooling tower or chemicals for treating the water, which could be harmful to the environment.
Additional energy-saving features include the use of heat recovery. Heat from various equipment, including kitchen coolers and freezers, mainframe computers and chillers, is recovered and pumped back into the system. All pumps and fan motors use variable-frequency drives to match load with energy usage.
Geothermal technology reduces energy consumption and the earth’s dependence on natural resources, providing Great River with a more cost-effective system operation and the means to minimize patient-care costs. The project made use of an existing, yet underutilized technology on a larger scale than previously undertaken.
Cooling a medical center is not an easy feat under any circumstances. The Great River Medical Center has become a showcase project closely observed by other medical centers interested in the geothermal approach.
The Grid at the Bottom of the Lake
According to Grant Schmidt, lead mechanical engineer, KJWW Engineering, Rock Island, Ill., a major hurdle to installing the geothermal system at Great River Medical Center was placing more than a hundred coil grids at the bottom of the lake. “Each grid contained more than 4,200 feet of pipe,” says Schmidt. “The enormous weight made placement difficult. We had to take a different approach than is typical for a lake-coupled geothermal system. A fabrication shop was set up off-site to construct the grids. Tractors were used to pull the grids into position. The lake was then filled to capacity. This allowed us to position the grids correctly, preventing crossover of pipes.”
Another challenge was gaining access to the grids at the bottom of the lake for maintenance. The system was design so that each grid could be filled with air and floated to the surface.
Besides serving as a geothermal system, the lake is a therapeutic healing environment for patients to observe and walk around; it is also a storm-water retention area. In addition, the hospital’s computerized irrigation system uses water from the lake to automatically waters trees and grass. It also detects when it has rained. Rainwater from the building’s roof and parking lots is diverted into the lake to help maintain the required twelve-foot depth.
A Socioeconomic Success, Too
Hospital administrators saw geothermal technology as a means for making Great River Medical Center one of the most pleasant, reliable, energy-efficient and cost-effective health care facilities in the country. Operational since February 2000, first-year energy data shows a nearly 30-percent reduction in energy costs for the 700,000 square feet of building served by the geothermal system, compared with the old hospital, which totals roughly 400,000 square feet and uses a conventional boiler-and-chiller plant. Geothermal technology minimizes the use of valuable natural resources such as oil and natural gas.