For a nonprofit located on the shores of Lake Michigan and an adolescent treatment center in Minnesota, geothermal heat pump systems have been an upfront investment that pays off in terms of maintenance, ROI, and effectiveness within an integrated sustainable design.
A handful of new buildings designed and constructed in the Midwest are utilizing geothermal heat pump systems - sometimes called ground source heat pump systems - for heating and cooling. This sustainable low-maintenance system requires a large, relatively undisturbed site (such as a parking lot or green space) under which the simple, closed-loop system of pipes and boreholes (often called “wells”) can be installed and connected to the heat pumps. While expensive to install, the heat pump system offers significant reductions in building heating and cooling costs over the long term, when compared to traditional HVAC systems.
The heat pump system uses the earth as a heat source during cold weather, and as a heat sink during hot weather. The system’s design takes advantage of the moderate, constant temperatures in the ground - a steady 55°F - to boost energy efficiency and reduce the operational costs of a heating and cooling system. Geothermal heat pumps are also known as water source heat pumps, which helps avoid confusing this system with traditional geothermal power, which utilizes a heat source at high temperatures to generate electricity.
For the Milwaukee Community Sailing Center, (MCSC) a 6,000-sq-ft facility on Lake Michigan’s waterfront in Wisconsin, the investment in a geothermal heat pump system protects the nonprofit organization’s limited financial resources through its longevity, energy efficiency, and 11-yr lifecycle payback. For United Hospital District’s 19,450-sq-ft Adolescent Treatment Center (ATC) in Winnebago, MN, the heat pump system performs approximately 30% better than the energy efficiency required in the ASHRAE 90.1 Energy Standard.
In both structures, green building design, technologies, and construction materials enhance the efficiency of the heating and cooling system, comprised of closed-loop water piping connected to a series of heat pump units. While MCSC’s geothermal well system cost about $60,000 more upfront than a traditional gas-fired heating and A/C setup, the $223,000 heat-pump system is saving the Milwaukee Community Sailing Center $4,000/yr in annual operating costs, which is a huge factor for a small nonprofit organization. ATC’s heat pump system was an $115,000 premium over a conventional system, but it is saving United Hospital District $13,000/yr in utility costs, which results in a payback of less than 10 years.
THE GEOTHERMAL HEAT PUMP SYSTEMThe geothermal heat-pump system is extremely efficient, as it uses energy in the ground to both cool and heat the building via a simple structure of geothermal boreholes, water pipes, and heat pumps. As these systems need to be installed at sites that remain relatively undisturbed, both the MCSC and ATC systems were buried under green spaces.
In each case, a geothermal technician performed a series of conductivity tests on the ground to determine depths for the wells. Those heat-transfer rates helped dictate the depth and quantity of wells. The rule of thumb for geothermal heat pump systems is 150 ft deep per well, dug on a 20- by 20-ft grid to prevent oversaturation of the earth; this produces one ton of cooling per well.
ATC’s system has 60 boreholes, each one 155 ft deep. Bedrock found in the ground prevented the wells from being drilled deeper. MCSC’s system was able to use 14 bore holes at 300 ft deep, on a 15- by 15-ft grid. This depth resulted in 1.5 tons of cooling per well and a cost savings in drill-rig setup time.
All of the piping connected to the boreholes is high-density, polyethylene (HDPE) pipe specifically manufactured for this application. The piping is buried below the frost line, 5 to 6 ft below grade, where it connects up with the wells. Welded together by heat fusion, the pipes are maintenance free, since they don’t rot or rust. The pipes don’t use ground water, but rather a closed loop of hydronic water.
The HDPE piping from the wells is connected to a series of heat-pump units. At MCSC, each of the 12 pump units are a dedicated heating/cooling unit (heats or cools) with a thermostat serving a particular zone or space in the building. Similarly, ATC uses a high level of zoning so that each sleeping room in the treatment center, as well as the group rooms and the classrooms, can be individually controlled with their own thermostats.
Heat pumps are extremely efficient in that they only consume energy for operation of the compressor and fan motor. At both MCSC and ATC, water-to-air heat pump units were used. The actual heating and cooling of the air supplied to a building’s spaces is accomplished by changing the refrigerant state in the airside and waterside coils in the heat pump. Depending on the need for heating or cooling, the refrigerant either absorbs or rejects heat to the geothermal hydronic water loop.
The airside coil takes room air, heats or cools it, and conditions the space. The waterside coil takes water from the geothermal loop and rejects or absorbs heat. The heat pumps have a valve that can flip-flop or reverse the coil functions relative to the refrigeration circuit. So when a unit is in heating mode, the airside coil becomes the condenser, heating the supply air, and the waterside coil becomes the evaporator, cooling the loop water. When the unit is in cooling, the airside coil becomes the evaporator, cooling the supply air and the waterside coil becomes the condenser, heating the loop water.
The geothermal system takes advantage of the stable ground temperature for rejecting or absorbing heat. Hydronic circulating pumps circulate the loop water between the buried geothermal wells and the heat pump units in the building. The supply and return piping from the wells are piped to common supply and return manifolds after entering the building.
The flow rates to each well are self-balancing, with a reverse-return piping configuration. Two pumps (one is redundant, in case of failure) in the mechanical room are connected to the loop piping, and constantly circulate water between the heat-pump units and the well field.
At MSCS, one heat pump unit is a water-to-water unit serving a dedicated closed water loop used in winter to provide hot water to heat a radiant slab in the first floor of the building. All other heat pump units are water-to-air. In hot summer weather, 55° water from the water loop is used to cool the building. To reduce upfront costs, a DDC system was not installed, requiring the loop water pumps to run at constant speed. But variable-speed controllers were installed on the pumps allowing MCSC to switch to automatic modulating when a DDC system is added in the future.
ATC’s heat pump system is decentralized and thus fits well within the building’s residential-style construction. A centralized system requires a large mechanical room with air handlers, boilers, and outside chillers, as well as large ductwork in the ceiling cavities. The decentralized system provided by the heat pumps only needs smaller ductwork distributing out from a central point off of the energy-recovery ventilators.
MAINTENANCE REQUIREMENTSThe geothermal heat pump system requires minimal upkeep. Routine maintenance involves changing air filters on the heat pumps and energy recovery ventilators (ERVs) on a quarterly basis. Several times a year, the condensate drain on the heat pumps should be checked; the air coils looked over once a year and vacuumed clean if necessary; and amperage draw on the fan compressors check annually to verify they’re operating within 10% of serial plate data.
Heat pumps have lubricated fan motors and typically do not need any additional lubrication. ERV maintenance includes lubrication of moving components as needed. On a periodic basis, damper operation, fan belt condition, and energy recovery media cleanliness should be checked. While hydronic pumps are permanently lubricated, inspect the seals on a periodic basis and lubricate the pump motor as needed.
SUSTAINABLE DESIGN THAT ENHANCES HEAT PUMP STRATEGIESBecause natural gas was not available at MCSC’s lakefront site, the ground-source heat pump system became the reasonable alternative for an on-site energy resource. The geothermal heat pump system was combined with additional sustainable-design strategies to ensure the MCSC facility remains as energy efficient as possible.
ERVs provide outside air to the building. The outside air is ducted directly from the ERVs to the individual heat pumps. The ERVs reduce energy consumption by taking the building’s exhaust air through the energy recovery wheel and using it to transfer sensible and latent energy to the incoming outside air and either pre-cool or pre-heat it, depending on the season.
Early on, the overall size of the heat pump system was reduced after the client decided on operable windows, through which lake breezes cool the facility during the summer. The architects sited the building to the south. Twenty-foot roof overhangs on the south side keep solar gain off the building’s high-performance, low-e windows in the summer, but allow in low winter light. In addition, 7- to 8-ft overhangs protect the structure’s other walls and windows.
The facility is also super-insulated to retain heat (in winter) and keep the building cool (in summer). The walls of the structure’s wood frame (a low embodied-energy material) are packed with blown-in rigid insulation. The super-insulated walls minimize energy use to allow the geothermal system to provide all of the building’s heating and cooling.
The structure’s unfinished galvanized roof of Galvalume was also super-insulated and reflects sun to help keep the building cooler in summer. The building’s exterior cladding is pre-finished fiber-cement panels. This low-cost, durable, maintenance-free option also has a high content of recycled materials and the panels are fully recyclable.
ATC also has wood-frame construction that was super-insulated with spray foam insulation. Occupancy sensors and above-code low-energy lighting help reduce cooling loads, which reduces the cooling required from the heat pump mechanical system. The low-e, argon-filled windows boast a low U value of .30 and low solar heat gain coefficient of .37, which is better than dictated by code. Low-flow plumbing fixtures reduce water use.
Throughout the building, cost-effective, low VOC and sustainable materials were used, including stained concrete floors in the dining room and rubber flooring in the recreation room. Wheat board for paneling and bookcases was one of the innovative materials used for sustainability and durability. Wood-frame joists in common areas were left exposed and unfinished to reduce material use. Acoustical wall panels modulate sound in the classrooms, recreation room, and game room. Throughout the interior, abundant natural light flows through large windows.
For its integrated sustainable design strategies, the ATC building is applying for LEED® certification. The new facility, located in the rural community of Winnebago, MN, will be the first LEED certified treatment center in the nation.
MCSC’s sustainable design and low-energy, high-conservation profile communicates the organization’s commitment to fiscal responsibility and environmental citizenship. According to one newspaper reporter, MCSC sets a “high standard for environmental stewardship on the shores of Lake Michigan.”
Through the use of high-efficiency, low-maintenance geothermal heat pump systems, which are enhanced through a variety of sustainable design and construction strategies, the Milwaukee Community Sailing Center and the Adolescent Treatment Center buildings serve as models of environmental stewardship and energy independence, fiscal responsibility, and civic pride for similar organizations around the country. ES