Figure 1. The new $25 million four-story Elks Rehabilitation Hospital uses a geothermal system to cut energy costs and keep maintenance simple because the only moving parts are the control valves on the plate-and-frame heat exchangers.

By the late 1990s, it was no longer feasible to keep adding to the original structure, and it was determined that another hospital had to be built. Consequently, construction on a new hospital commenced, and the state-of-the-art facility was dedicated in May 2001. The $25 million, four-story building consists of approximately 156,000 sq ft, with each floor measuring in at around 39,000 sq ft.

The old hospital used a geothermal system to heat the facility, and it was decided that the same type of system should be used for the new building. The rationale was that the engineering staff already knew how the system worked, so it would be a simple transition. In addition, geothermal systems are incredibly energy efficient, so the nonprofit hospital would see the savings reflected in its bottom line.

Unusual City Offering

First cost wasn't really an issue for the Elks hospital, though. That's because the City of Boise operates a geothermal heating district that serves the city's downtown area. The city delivers hot geothermal water to buildings at 155°F to 165° and collects the water from customers after the heat is removed. The system currently serves 50 buildings, heating over two million square feet of floor space.

Boise's pricing structure is set to ensure an energy cost savings of 30% over natural gas, based on a comparison of energy usage of geothermal equipment taking 50° out of the geothermal water and a gas boiler with 75% boiler efficiency. If the geothermal equipment is designed to take more than 50° from the geothermal water, the savings is even greater.

Charles Paulin, P.E. with Musgrove Engineering (Boise, ID) was the project manager and principal engineer in charge of the Elks hospital design. He noted that whenever his firm does a project within the downtown vicinity, they always take a look at the opportunity to use geothermal.

He said it is very cost-efficient, because it is not necessary to bury miles of pipe somewhere. "The city brings the water to the facility and amortizes the cost over the life of the installation. So in this case, it was a no-cost option for the Elks, because they know they'll recoup that money fairly quickly."

Gregg Heyn, the hospital's director of engineering, noted that the initial cost of the equipment was a little more expensive than a set of packaged hot water boilers. "It also requires a more sophisticated control system. But we wouldn't have the energy savings, which is why we wanted to stay with the geothermal."

The water is delivered to the hospital already pressurized, so there's no need for a pumping system. As Musgrove noted, "We take their water and distribute it through several plate-and-frame heat exchangers, and from there, we just run the building systems through the other sides of the plate-and-frames. It's very cost effective to install."

Depletion of the aquifer has caused the City of Boise to place a moratorium on the use of its high-temperature geothermal district for new buildings (sidebar). Fortunately, the hospital was able to be grandfathered in, due to the fact that it was on the city's geothermal district in its previous facility.

Paulin noted that the city also provides a low-temperature geo- thermal system that runs through downtown, which uses the wastewater from the facilities using the high-temperature system. "Facil-ities typically take 50° or 60° out of the water, so a lot of times, water is being returned at 110° to 115°. The city lets us tap this water for hydronic heat pump systems, which we have been installing in a lot of the downtown office buildings."

Multipurpose System

Plate-and-frame heat exchangers are also used for the two pools located on the hospital's ground floor. The smaller 9,000-gal therapy pool is heated to a therapeutic temperature of 98° and the larger 50,000-gal therapy pool is heated to approximately 92°. These heat exchangers were constructed with titanium plates, due to the chemicals used in the pools.

With the exception of the pools, the building uses a closed loop system. Paulin noted that the water coming from the city is fairly clean, so water treatment is minimal. "What little particles and debris that come through typically will pass through the plate, so there's no clogging."

Maintaining the geothermal system is simple, because basically the only moving part is the control valve at the heat exchanger, which maintains temperature. "That's really the only item you need to look at during annual maintenance," said Paulin. "It's necessary to make sure the control valve is controlling based on water temperatures, so you're not using more than you need to. The idea is to extract as much temperature out of the water as possible to get the maximum amount of Btu out of there at the lowest flow."

Heyn agreed that maintenance is fairly easy, noting that they really keep a close eye on the modulating valves to make sure they're working correctly. "At some point we may have to change the plates or change the seals on the plates, but that shouldn't be for a long time. We operated the old facility for 10 years, and we never had to open the exchangers."

Being a hospital, having a back-up system was absolutely necessary. A central generator was installed, as well as two boilers, which were sized to back up the domestic hot water system, building heating systems, and the pool system.

Paulin said that the back-up system made the controls strategy a little trickier. "Knowing when the water flow is down and the demand is still there made some of the control sequencing a little bit challenging. The system has to measure both water flow and demand in the system before the changeover occurs. It could be that the water flow is down but there's no demand, so the back-up system isn't needed. But if there's a demand for heating and the water pressure is zero, the changeover has to happen."

The back-up system has come in handy several times since it was installed. That's because the city sometimes needs to service the system, which they usually try to do when demand is low. As Heyn noted, "The city is pretty good about trying to schedule their take-downs during the summer and giving us notice. Once in awhile, we might get an electrical storm, and they'll lose their pumps. Then we have to use the boilers."

If the need arises, the control system automatically switches the geothermal system over to the boilers, while the pool heating transition is completed manually with valves.

Figure 2. Plate-and-frame heat exchangers constructed of titanium are used for the two pools located on the hospital’s ground floor.

Rest of the System

Four air handlers averaging about 35,000 cfm each are used for the main building, supplying every room in the facility with a complete change of air every six to 10 minutes. The air handlers are equipped with economizers to further help save energy. A dedicated 6,500-cfm air handler is used for the pool area. Filtration is dictated by Joint Health Care Commission requirements, so a 30% prefilter and a 95% cartridge final filter were installed.

A Johnson Controls "Metasys" system was installed to control building functions. When the new facility was being built, the hospital still had three or four other facilities spread throughout the city. These other buildings utilized a different BAS, so the new facility was basically on its own in terms of the BAS. Now the other buildings are gone, so the Metasys system is the only BAS facility engineers need to work with.

Moving to the Metasys system meant some adjustment for the engineering staff. "We were going from a 1956 building with mostly manual controls and valves. Our learning curve has been huge, but we're gaining on it," said Heyn. "The nice part is the system gives us wonderful control."

VSDs were installed on both the heating water system and the chilled water system. "We did those in a primary/secondary system. We also put variable-speed drives on all the air handlers and on the domestic booster water pumping system," said Paulin. "We also installed energy recovery. There's a central exhaust system, and we did a runaround loop on the minimum outside air intake on each of the air handlers and in the main exhaust system."

With all these energy-saving features, it's no wonder the hospital is happy with the results. Much of the success, noted Paulin, can be attributed to the mechanical contracting firm, YMC, Inc. "They were realistic. They didn't overkill the preliminary budgets and then value engineer everything back. They took a good hard look at the design, and everything came within budget. It was ideal."

In addition, Paulin is proud of the work he and his firm did for the hospital. "We got a nice system for them that's very maintenance friendly and saves energy. That's a great feeling." ES

Popularity Challenges Sustainability

In 1987, the Idaho Department of Water Resources discontinued any new development of the resources in the designated area until the situation could be resolved. Because the city, the state energy and water departments, the federal government, and a private water district were involved, cooperation was key to solving the problem.

After the moratorium on development was established, water levels began to stabilize in the downtown area but continued to fall in the region northwest of the management area. In response, engineers began to look at the used geothermal water that was routinely being discharged into ditches and canals, eventually emptying into the Boise River. With funds from the DOE, a feasibility study was conducted to see if reinjecting the water back into the formation would recharge the aquifer.

A mathematical model of the aquifer was developed, which showed that an injection well should be beneficial. In 1999, the City of Boise completed a 3,200-ft-deep injection well. Recent monitoring results show that aquifer levels are rising, and no user has experienced a loss in temperature.