The engineering community is at the beginning of a transformation in how geothermal heating and cooling systems are applied. For many years, these systems have been used for individual buildings. The next step in the development and implementation of geothermal technology is campus systems.
There are three major factors contributing to the increased interest in campuswide geothermal systems: Geothermal has become a mainstream option for building heating and cooling, owners are aggressively exploring options to save energy, and owners are looking for new opportunities to reduce or eliminate the use of fossil fuels on their campuses. Owners who are willing to apply proven systems in new ways are the agents of change for this "geothermal revolution."
As campus geothermal systems gain momentum, it is important to note that no two campus systems are the same. However, the approach for how a system is designed and evaluated for any campus is similar.
Accurately Identifying Campus Thermal Profile
Accurately identifying a campus' thermal profile is the first and one of most important steps in the design process. It is the foundation of the entire system and helps guarantee the system will operate as expected without thermally saturating the well field or increasing first cost because of excessive well-field sizing.
To correctly classify campus thermal profile, it is important to analyze campus utility data over a minimum of three years. Rarely do all campus buildings have individual meters on cooling and heating loads. If campus utility data are not available, the entire campus can be modeled with energy modeling software, although this can be a time-consuming and costly exercise.
In addition to existing campus utility information, the campus master plan for the next 20 years must be examined to ensure a system will have capacity for future expansions. Most institutions are adept at identifying what their future building needs will be and where new buildings will be located on campus.
After utility data and master-plan information are gathered, monthly campus thermal-load characteristics will determine:
- Base simultaneous load: The minimum simultaneous heating and cooling load required year round on campus.
- Instantaneous simultaneous load: The amount of simultaneous heating and cooling load required above the base simultaneous load at different times throughout the year. Instantaneous simultaneous load is largest in the shoulder seasons (i.e., the transition from summer to fall or spring to summer).
- Unbalanced heating load: The amount of heating load required on campus in addition to the base simultaneous and instantaneous simultaneous heating and cooling load. The energy for the unbalanced heating load on campus is supplied from the well field.
- Unbalanced cooling load: The amount of cooling load required on campus in addition to the base simultaneous and instantaneous simultaneous heating and cooling load. The energy for the unbalanced cooling load on campus is deposited in the well field.
Regardless of whether a campus proves to be compatible with geothermal technologies, the evaluation of its thermal profile and future growth can start the journey to improved energy performance and the reduction of its carbon footprint.
Sizing the Well Field
After the load characteristics and future expansion of a campus are determined, the well field needs to be sized. A test well will need to be drilled at a potential well-field location on campus. The data gained from the test well will determine the thermal conductivity, diffusivity, ground temperature, and geological conditions of the site where the well field will be located. Once the information from the campus loads and the test well are known, they are used in geothermal heat-exchanger ground-loop design software to model the depth and amount of bores required for the campus. Ground-loop sizing parameters include the ground temperature, increasing approximately 20˚F above normal ground temperature in the summer and decreasing approximately 10˚F below normal ground temperature in the winter. The ground-loop design software will predict how the ground loop will affect the earth throughout the next 20 years.
Evaluating the Building Systems
Large campuses will have an array of building-conversion approaches because of the various vintages of building systems across the campus. Typically, the design approach should maximize the existing building assets. This means the design hot-water temperature for the heat pump will be as low as possible to save energy, but not so low that every heat-transfer device (air-handling-unit coil, finned-tube radiation, cabinet unit heater, unit heater, etc.) must be replaced.
One recommended test for any building that has a hot-water distribution system is to set the water supply temperature down to the anticipated level of the future campus geothermal system. This test helps identify buildings that are short of heat-transfer area prior to the start of construction.
Determining the Distribution System
There are many options to consider when converting a campus to a geothermal system. The design engineer and owner need to evaluate many factors that affect the decision. Several of the important considerations are:
Existing infrastructure available for reuse. An example of campus infrastructure that can be reused is chilled-water supply and return piping. If a campus chilled-water system is in place, it easily can be reused in a four-pipe distribution system. If a campus chilled-water system does not exist, installing one probably will be cost-prohibitive.
Space for equipment in existing buildings. Equipment can be located in central-plant buildings or campus buildings. If the existing buildings do not have space for more equipment, a new energy station may be required for a centralized system.
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Temperature of hot-water supply. Conversion of an existing campus includes converting existing buildings to a new hot-water temperature. After a building analysis is done to evaluate which heat-transfer media are affected by various hot-water supply temperatures, a higher hot-water supply temperature may be selected because replacing a large amount of heat-transfer media could be cost-prohibitive. Electrical consumption is the penalty for a higher hot-water supply temperature. The amount of electricity needed to generate a 150¢ªF hot-water supply temperature is significantly greater than the electricity consumed to generate 140¢ªF hot-water supply temperatures.
Equipment selection. It is important to determine the right equipment to meet design criteria. Some types of heat pumps and their design parameters include:
- Centrifugal chillers: 600 to 2,500 tons, up to 155¢ªF hot-water temperature across all tonnage ranges, up to 170¢ªF hot-water temperature with chillers between 1,500 and 4,000 tons.
- Screw chillers: 50 to 600 tons with single compressor, up to 140¢ªF hot-water temperature.
- Scroll chillers: Up to 150 tons, up to 130¢ªF hot-water temperature but 120¢ªF recommended.
Space available for well fields. The space available on a campus for well fields and where they tie into the geothermal distribution loop are important considerations for determining the geothermal distribution system type. For example, a single-pipe geothermal-distribution loop is the least expensive, but it is dependent on well fields spread throughout the campus and connected into the geothermal distribution loop in various locations to add or remove energy being transferred to and from the buildings.
Phasing and transformation of an existing campus to the geothermal system while maintaining use. Maintaining operation of the campus is very important to the owner. The system type may be limited to the amount of construction that can be completed during short, intense construction periods. For example, it is difficult to phase a single-pipe geothermal distribution system. If the entire loop is not installed at one time, none of it will be able to be used. Decisions regarding the type of system to implement should be made on a per-campus basis with input from the owner and evaluation of the existing systems and campus geographical constraints.
Campus Basis of Design
After decisions about the campus system have been determined and a direction for the campus system has been agreed upon, a campus utilities basis of design can be written. If a campus is a good candidate for geothermal, the importance of the basis of design is undeniable. It shapes the future operation of the campus system and acts as the starting point for any future building projects. The basis of design will standardize the building systems' parameters during and after the geothermal conversion.
Michael Luster, PE, LEED AP, is senior mechanical engineer/project manager for MEP Associates LLC. He is a registered professional mechanical engineer in Minnesota and South Dakota and has a bachelor's degree in mechanical engineering from Iowa State University. He can be reached at mailto:[email protected]
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By JEFF URLAUB, PE,
and AMBER BETTINGER
MEP Associates LLC
Eau Claire, Wis.
More than a decade ago, Ball State University in Muncie, Ind., was already looking into options to reduce its overall energy costs while remaining committed to providing a sustainable, healthy campus and community. So it was no surprise when the university decided to take its four aging coal-fired boilers offline and replace them with a geothermal heating and cooling system to serve 47 buildings on the 731-acre campus. This effort will save the university $2 million in annual energy costs and cut its carbon footprint in half.
Ever since the Ball brothers came to Muncie and founded Ball State, the university has been a pioneer in positive environmental ethics. Today, Ball State is recognized as a top green university and has received a number of honors, including the Lugar Energy Patriot Award in 2007. The Energy Patriot Award is part of an ongoing effort by U.S. Sen. Dick Lugar to recognize professionals, scholars, students, or businesses that demonstrate leadership and initiative in taking concrete action to reduce America's dependence on foreign energy sources.
Ball State not only ventured into the world of geothermal, it sets the standard for other campuses. This conversion of an entire campus to a geothermal system is the first installation of its kind by any university in the United States, and Ball State soon will be home to the largest ground-source, closed-loop district geothermal heating and cooling system in the nation.
When it came to the design and engineering of the new geothermal system, the university partnered with MEP Associates. MEP has been involved in more than 150 geothermal installations over the past 10 years. The project was designed to be installed in two major phases with several subphases. On May 9, 2009, the first phase began the conversion to geothermal on half the campus. This phase was completed in the fall of 2011. Phase two is expected to be completed by the end of 2014.
The campuswide geothermal system will allow energy to move flexibly from one building to another with a common loop connecting approximately 5.6 million sq ft. One thousand miles of vertical closed-loop pipes located within two borehole fields will act as heat exchangers by sinking or pulling heat from the ground. The heat is transferred to "energy stations," where it is exchanged and dispatched throughout the campus.
Once complete, the cost of the new geothermal system, when compared with traditional campus heating and cooling systems, is projected to pay for itself within seven to 10 years. In addition, by taking its aging boilers offline and not burning 36,000 tons of coal annually, the university eliminated approximately 85,000 tons of carbon emissions.
Jeff Urlaub, PE, is president of MEP Associates LLC, Eau Claire, Wisc. He is a registered professional mechanical engineer in 20 states and has a bachelor's degree in mechanical engineering from the University of Florida, Gainesville. He can be reached at [email protected]. Amber Bettinger is the marketing coordinator for MEP Associates LLC.