Geothermal Heat and Graywater Storage
Systems Enable Bowdoin College to Build
Residence Halls

By Clif Greim, P.E. and David Reinheimer, P.E.

Maine 's Bowdoin College faced two stumbling blocks when it delved into adding two new residence halls to its campus in Brunswick to accommodate increased enrollment. The creative solutions that were engineered allowed the halls to go forward and resulted in a project that became a LEED award winner.

The first stumbling block came from outside the campus. The existing stormwater collection system adjacent to campus was near capacity and the town mandated that the new project not contribute any

runoff from the additional impermeable surfaces that would be created. The second stumbling block was on-campus. Bowdoin's central steam system did not have the capacity in its existing distribution system to take on the new buildings. A costly and time-consuming solution would have been to pay for upgrading the

town's stormwater infrastructure and to upgrade the capacity of the college's steam system. However, Harriman Associates of Auburn , Maine , engineering consultant on the project, working with the architect, Kyu Sung Woo of Cambridge, Mass., designed two unusual and intertwined solutions.

Rainwater Collection

To control runoff from the new buildings, a system had to be devised to dispose of any rainwater that might fall onto the residence halls. The solution was to install a collection tank and pumps in the basement mechanical spaces to collect all available rainwater and store it for use in flushing residence hall toilets.

The roof leaders from both buildings were piped down inside the buildings and then underground to a translucent plastic tank situated in the basement of the west residence hall. The tank capacity is 2,000 gallons, and if it should overfill, the overflow is piped to an underground percolation bed. Calculations of the available rainwater and the water required for toilet flushing showed that normal rainfall would fall short of flushing requirements. The new Coffin Street residence halls on the campus of Bowdoin College embody the school's enhanced commitment to sustainability.

Geothermal System

To provide the residence halls with heating and cooling and to supply the necessary domestic hot water, engineers designed a system of geothermal heat pumps to exchange energy with the earth by means of seven 1,500-ft.-deep, standing column wells. Each well has its own variable- speed, submersible well pump installed to accurately match pumping with the load of the associated heat pump. When heating is required in the halls, the heat pumps will extract heat from well water and the heat is then pumped around the building in a low-temperature, forced hot water heating system. This hot water loop is connected to fan-coil units, cabinet unit heaters and unit heaters designed to keep all spaces warm on the coldest day using only low-temperature water (110° F). When the weather dictates, the heat pumps can be switched over from heating to cooling. In this mode, the heat pumps reject heat into the well water, and it is returned to the cooler earth. Five of the heat pumps are dedicated to the heating or cooling of the living spaces, and the remaining two pumps trade off generating the first stage of domestic hot water. Since heat pumps have an upper temperature limit in the heating mode, a direct-vented condensing gas boiler covers the final stage of domestic water heating. This boiler was sized to act as a back up to the heat-pump system. Should there be a failure of any or all of the heat pumps, the boiler has the capacity to keep the buildings functioning as though nothing has happened. This boiler, along with an emergency generator that serves the vital systems in both residence halls, will ensure that students living in them will not be disturbed by a loss of power or external weather anomalies.

Making Up For Shortfalls

A byproduct of geothermal wells used for heating and cooling is an amount of water that is drawn out of the wells to ‘regenerate' them. This water is referred to as bleed . Since the water pumped from the wells to the heat pumps is then returned to that same well, there comes a time when the well has given up or received as much heat energy as it can yield/accept. To regenerate this well, water must be taken out of the well and not returned. This process is referred to as “bleeding the well.” This bleed stage requires the well to make up the shortfall from the aquifer, thereby recharging the well for continued heating or cooling.

This water, when bled from the geothermal heat pump system, is customarily dumped down the drain. In the Bowdoin design, that water was combined with the rainwater in the storage tank to ensure that the water required for toilet flushing would always be there. Calculations again showed that even with combining the standard amount of bleed water with the customary rainfall, the total would still fall short of the required flushing demand. The building management system was therefore programmed to make up the shortfall in flushing water stored in the graywater tank by requiring the heat pump wells to bleed not only as they require it, but also when the graywater tank falls below a certain level. Using these two mechanisms the rainwater is captured and reused, thereby keeping it out of the storm drain system, and the bleed water that

would have been dumped into the sanitary waste stream, requiring wastewater treatment, is also recycled.

Both solutions earned the project LEED points, and Bowdoin College was awarded Silver LEED certification for the Coffin Street Residence Halls. The two residences accommodate 208 students and were completed in August 2005, in time for the start of the fall semester.

Written for College Planning & Management,
April 2006