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Ground source heat pumps in schools.

According the U.S. Department of Energy, ground source heat pump (GSHP) systems in schools reduce energy use by 25% to 50% compared to traditional systems and result in typical payback periods of two to eight years. (1) However, the U.S. General Accounting Office states that the use of GSHP systems is limited, in part, because users are unfamiliar with the technology. (2)

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This article is a cursory review of various considerations to help guide design professionals when proposing these systems to school boards, parents, and others.

The type of GSHP system used for this article is a closed-loop ground-coupled system. The concepts presented are generally applicable for other types of GSHP systems.

Educating School Boards and Others

The first cost of a GSHP system must be evaluated while also considering operating costs. When comparing a GSHP system to an HVAC system with a lower first cost, a payback analysis must be performed to assist the school district in making the best financial investment.

GSHP systems typically require less mechanical space than a conventional boiler/ chiller system. The mechanical floor space reduction can save about $100/[ft.sup.2] ($1075/ [m.sup.2]). It can be used to reduce the building footprint and resulting construction costs, or to add educational program space.

A GSHP system can be used as a teaching tool. Figure 1 is an example of one of five student interface displays from the building automation system for an Ohio middle school. The students and science teachers access real-time data from the GSHP system to learn about the energy use of the HVAC system.

Many schools are located in the center of residential neighborhoods. GSHP systems may eliminate objectionable noise from outdoor equipment used by conventional school HVAC systems. A chilled water system may require either an outdoor air-cooled chiller or cooling tower, which add to outdoor ambient noise. Even if the sound is attenuated or shielded, probably some increase in ambient noise occurs when the equipment is operating. A GSHP system can be designed so there is no outdoor equipment and no increase in the ambient outdoor noise levels.

[FIGURE 1 OMITTED]

A GSHP system has flexibility for part-load cooling and built-in safety factors regarding redundancy. When only a small part of a school building is in operation, such as the administration offices, the only equipment operating in a GSHP system is the associated heat pumps, the fluid circulating pump, and if applicable, the outdoor air ventilation systems. With a larger boiler/chiller system, the HVAC system may have to operate the chiller at a reduced part-load condition, the boiler for reheat, and an air handler that serves an area much larger than the occupied zone. All of that is only for conditioning a small part of the building. A GSHP system can operate at maximum efficiency with only one heat pump operating.

Regarding redundancy, if a single heat pump fails, only the area or classroom associated with that heat pump is affected. For a system with larger central equipment, the failure of a central piece of heating or cooling equipment, or a central air-handling unit, affects a large portion of the building.

GSHP systems have a lower environmental impact compared to traditional HVAC systems. More than 40% of carbon dioxide emissions in the U.S. are due to energy use for buildings. (3) The reduction in energy use as previously described will have a direct reduction in emissions. The use of smaller GSHPs minimizes the risk of refrigerant releases to the atmosphere that may contribute to global warming or ozone depletion. The amount of refrigerant introduced to the atmosphere due to an accidental discharge from a GSHP is small compared to the accidental discharge from larger central cooling equipment. Using GSHP systems makes a statement to students and the community regarding the importance of protecting our natural resources.

GSHP systems are a proven technology. The installation methods and materials have been tested, and guidelines have been issued to create a level of standard and accountability for the installation. The design procedures for GSHP systems have been augmented with computer software to assist the design professional. ASHRAE and the Air-Conditioning and Refrigeration Institute have performance and testing standards specific to GSHPs.

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Additionally, many case studies are available online from other school districts, state and federal government agencies, and GSHP organizations that can be used to demonstrate success stories from other school projects.

Calculations

Accurate block load HVAC calculations are critical for the proper sizing of a GSHP borefield. Each borehole adds cost to a project. If the calculations are in error on the high side, the borefield size may be too large, resulting in excess costs that affect payback calculations.

In a recent project, a new 155,000 [ft.sup.2] (14 400 [m.sup.2]) high school has a student population of approximately 825 students. The high school includes a 650-seat auditorium, a 1,200-seat gymnasium, and a field house. If the seating capacities of all rooms in the school were added together, enough seating is available for 2,500 students. The engineer must accurately determine the peak building occupancy to develop accurate calculations.

Outdoor air ventilation loads affect the total building heating and cooling loads, thus affecting the borefield size. If a system uses constant ventilation for all spaces in a school, the ventilation related loads will be unnecessarily high. A demand-controlled ventilation system is necessary in spaces with high variation in occupancy. This lowers the ventilation load on the system, and results in a more accurately sized borefield.

A thermal conductivity test is important to the sizing of a borefield. The test determines the average thermal conductivity of the assembled borehole. This information is used to size the borefield and to select the borehole grout specification. The test borehole can be used later in the final borefield installation. A drill log from the test bore can be used in the bid specifications, so bidders know what type of materials to expect for the drilling process.

Borefield Location

The borefield can be arranged as a central large borefield serving the entire system or with decentralized borefields. With decentralized borefields, smaller clusters of boreholes may serve one classroom or a group of classrooms. In a facility where a high diversity of heating and cooling loads exist, a central borefield or multiple clusters of borefields may result in fewer total boreholes than a decentralized system with boreholes dedicated to each space or a small grouping of spaces. (4)

For a large central borefield, this field can be located near the mechanical room or a substantial distance from the mechanical room. Photo 1 is an aerial view of a school building where the borefield needed to be located away from the mechanical room. To accommodate this location, a set of 8 in. (203 mm) supply and return pipes were run to an underground utility vault. A piping manifold was located in the vault to distribute the smaller piping circuits to the borefield. If the borefield is located near the mechanical room, then the piping manifold can be located in the mechanical room with the borehole circuits run directly out of the mechanical room to the borefield.

For decentralized borefields, boreholes may be located directly adjacent to the classroom or cluster of classrooms. This arrangement minimizes long runs of piping and may result in lower installation costs and possibly lower pumping power requirements.

The decision regarding the type, arrangement and location of a borefield must consider the available site for a borefield, the specific heating and cooling loads, and the architectural layout for a particular building.

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Piping/Pumping Arrangement

Many piping arrangements can be used for a GSHP system. For a large central borefield system, either a constant speed or variable speed pumping system can be used. For decentralized borefields, small constant speed circulators are generally used. For small central systems, a single set of variable speed or constant speed pumps can be used.

Variable speed pumping can be arranged either with variable speed pumps circulating through the building and borefield or as primary-secondary pumping arrangement similar to that used for a chiller system. Constant speed primary pumps provide flow to the borefield and variable speed secondary pumps provide flow to the building system. If the temperature range of the return water (or glycol solution) from the secondary loop is within a specified temperature range, then the primary pump can be turned off and the secondary system will recirculate the fluid until the secondary return temperature falls outside of the temperature range. At this time, the primary pumps are reenergized.

In general, variable speed pumping systems have been shown to have the lowest energy use for buildings with high weekly occupancy and small constant speed circulators with on-off control have been shown to have the lowest energy use for small systems with low weekly occupancy. (4)

High Occupancy Spaces

Spaces with high occupancies require special consideration. These spaces may include cafeterias, gymnasiums, field houses, or auditoriums and may require cooling during the winter and have high outdoor air demands anytime of the year.

One design concept is to use traditional air-handling units (AHU) with water cooling and heating coils. The coils are served by water-to-water heat pumps. This system design can use an airside economizer when applicable and can handle the cooling/heating loads associated with high outdoor air quantity. If a water-to-water heat pump system is used, the design of the load side (AHU side) of the heat pump must be evaluated carefully. If the heat pump is located near the AHU coil, the volume of water in the piping loop might be too low, resulting in short cycling of the heat pump compressor. The minimum volume of the load side of the piping loop must meet the manufacturer's recommendations.

If water-to-air heat pumps are used, a waterside economizer coil should be used. The waterside economizer coil allows the cool water in the GSHP system loop during the winter to be used for cooling, in lieu of operating the heat pump compressor.

Humidity control in spaces with large occupancies must be addressed. It can be achieved using cooling and reheat at the heat pump, cycling/staging of multiple heat pumps, or preconditioning of outdoor air via a dedicated outdoor air system. Simultaneous cooling and reheat should be avoided where possible due to the higher energy consumption.

Staging/Construction Coordination

During the borefield drilling and installation process, the field will be rendered unusable for other purposes until the horizontal piping has been installed, and the grade restored over the borefield. Photo 2 shows a borefield after the installation of the vertical piping and before the installation of the horizontal piping and final grading. The picture shows the condition of the borefield area during the construction process. After the borefield installation was completed, the area was used as a staging ground for masonry products and equipment.

The staging and scheduling of the borefield installation must be accounted for in the overall scheduling of the construction of the project. If the borefield installation is not properly addressed in the construction schedule, major delays and conflicts could occur.

Construction Administration

Construction administration for GSHP systems can make the difference between a successful and unsuccessful system. The following are some construction administration tasks that must be carefully considered.

* Contractor qualifications. On a large school GSHP project, the contractor must have the experience and credentials to install the project. The specifications must be written so that contractors with little or no experience in the installation of large GSHP projects are excluded.

* Confirmation of grouting. Once the borefield is covered with the final grade, it is difficult to verify that the boreholes were properly grouted. The specifications should require that the contractor report to the engineer on a regular basis about the number of holes grouted and the quantity of bags of grout used. From this information, the engineer can calculate if the correct amount of grout material is being used.

* Protection of piping system from entry of dirt and debris. During the construction process, all open ends of piping must be covered to protect the system from the entry of dirt. Small amounts of dirt and debris can create problems with strainers and heat pumps inside the building.

* Flushing and purging of underground piping. Even with the proper protection of piping from the entry of dirt, the probability that dirt will enter the system is high. Flushing and purging of the system is critical to removing contaminants from the piping system. The flushing and purging procedure also removes trapped air pockets from the piping system.

* Pressure and flow testing of piping. The 2003 International Mechanical Code (5) specifically requires pressure and flow testing of underground piping systems for GSHP systems. The intent is to identify problems with the piping system prior to the piping being covered to the final grade. Care must be taken to ensure that the pressure rating of the installed vertical borehole piping is not exceeded during pressure testing. For a deep vertical borehole, there will be a high static pressure at the bottom of the borehole u-bend, prior to the applying additional pressure for the testing.

Conclusion

Ground source heat pump systems have been proven to be a viable HVAC option for schools. The systems provide an excellent source of comfort and partial heating/cooling. As noted by the U.S. Department of Energy, the systems result in lower utility costs than traditional HVAC systems. To provide school districts with GSHP systems that meet their expectations, special consideration must be given during the design and construction process to ensure a successful conclusion to the project.

References

(1.) U.S. Department of Energy. 1998. "Geothermal Heat Pumps Score High Marks in Schools." DOE/GO-10098-650. www1.eere.energy.gov/geothermal/ pdfs/26161a.pdf.

(2.) U.S. General Accounting Office. 1994. "Geothermal Energy: Outlook Limited for Some Uses but Promising for Geothermal Heat Pumps." GAO/ RCED-94-84. http://archive.gao.gov/ t2pbat3/152033.pdf.

(3.) U.S. Department of Energy. 1998. "Environmental and Energy Benefits of Geothermal Heat Pumps." DOE/GO-10098-653. www.nd.gov/dcs/energy/ pubs/renewable/geoben.pdf.

(4.) Lambert, S.E., S.P. Kavanaugh, D. Messer. 2005. RP-1217-Development of Guidelines for the Selection and Design of the Pumping/Piping Subsystem for Ground-Coupled Heat Pump Systems. http://rp.ashrae.biz/page/rp-1217.pdf.

(5.) International Code Council. 2003 International Mechanical Code, paragraph 1208.1.1.

Jim MacMillan, P.E., is a vice president for Karpinski Engineering in Cleveland, Ohio.
COPYRIGHT 2007 American Society of Heating, Refrigerating, and Air-Conditioning Engineers, Inc.
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Author:MacMillan, Jim
Publication:ASHRAE Journal
Geographic Code:1USA
Date:Sep 1, 2007
Words:2422
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