Ground Source Heat Pump Values Rising
By PILJAE IM, XIAOBING LIU and JEFF MUNK
There’s a lot to like about Ground-Source Heat Pump (GSHP) Systems.
They’re sustainable and proven in both residential and commercial buildings. Because the earth provides a more favorable heat source and heat sink than ambient air, GSHPs are one of the most efficient technologies available for space conditioning and water heating.
Yet, recent studies reveal, less than 1 percent of U.S. houses use a GSHP system, with economics cited as the major barrier to broader use.1 Improving the economics requires that we reduce installation costs, improve system performance or both. That is the aim of a research project Oak Ridge National Laboratory (ORNL) has undertaken with several partners.
To reduce costs, we’re evaluating two technologies: (1) a foundation heat exchanger (FHX), which applies only to new construction or additions to existing homes; and (2) a ground-source integrated heat pump (GS-IHP), integrating space heating, space cooling, on-demand dehumidification and water-heating functions into one piece of equipment.
The GS-IHP, which applies to new construction and retrofits, also promises significant improvements in performance.
A little more than a year into research demonstrating the FHX, working with a state-of-the-art GSHP unit, it has maintained comfortable temperatures for household heating and cooling. Early estimates indicate that when implemented at scale in the test region of East Tennessee, the FHX approach may be feasible at $1,000 per ton — a fraction of the cost of the outdoor portion of traditional GSHP systems.
Although we’re still gathering performance data for the GS-IHP prototype, our technology partner plans to launch a product line based on the technology later this year.
Testing the FHX Concept
Why do GSHP systems cost so much more than conventional space-conditioning and water-heating systems? The cost premium is primarily associated with drilling boreholes or excavating trenches and installing vertical or horizontal ground heat exchangers (or loops) in them.
The FHX concept is based on the premise that today, in many climates, it is economically feasible to build new homes and home additions having thermal loads so modest that they can be met by a GSHP system whose loop is installed in the construction excavations — without any extra drilling or digging.
These construction excavations commonly include the overcut around the basement and below the basement floor, utility trenches (for buried water, sewer and power lines) and footer drains. If this premise is true, FHX has the potential to significantly reduce GSHP cost premiums. ORNL’s research project was designed to test this premise.
Unlike conventional horizontal ground heat exchangers, loops buried in the overcut may experience thermal interference with the basement wall, an effect not accounted for by any design tools for ground heat exchangers. This research project focused on developing what is needed to engineer loops in the overcut around the basement.
A team from Oklahoma State University (OSU), led by Dr. Jeff Spitler and including Dr. Simon Rees (De Montfort University, United Kingdom) and several post-graduate students, joined the research partnership to develop the necessary overcut-loop-design tool and a model of the entire FHX (overcut, underfloor, utility trench) suitable for integration into EnergyPlus, the U.S. Department of Energy’s flagship whole-building energy modeling software. The team uses experimental data collected from a real installation of the FHX to validate the design tool and simulation model.
Schaad Cos. (schaadcompanies.com), ORNL’s founding partner in ZEBRAlliance — a public-private collaboration to maximize cost-effective energy efficiency in buildings (zebralliance.com) — has built four energy-efficient test houses in the Crossroads at Wolf Creek Subdivision in Oak Ridge, Tenn. Houses 1 and 2 are three-level buildings with walkout basements used for the FHX research.
The side-by-side research houses have identical 3,700-square-foot floor plans. In these unoccupied research houses, human impact on energy use is simulated to match the national average, with showers, lights, ovens, washers and other energy-consuming equipment turned on and off at exactly the same times. Simulating occupancy eliminates a major source of uncertainty in whole-house research projects of this type.
Houses 1 and 2 each test different envelope strategies, but both have very low air leakage and high levels of insulation, and thus have very low heat gain and loss through the building envelope. In short, they are exactly the type of home where FHX should work. The details of each house’s envelope characteristics are described in a recent paper.2 Owing to their high-quality thermal envelopes, the 3,700-square-foot houses have been adequately served by one 2-ton GSHP system each, whereas 4 to 5 tons of space-conditioning capacity are typically installed in homes of that size in East Tennessee.
The objective of this research project was to develop and validate the FHX design tools, so these tools obviously were not available for the design of the loops for houses 1 and 2. The two houses’ cooling and heating design loads were calculated using “Manual J: Residential Load Calculation” and associated software tools developed by the Air Conditioning Contractors of America (ACCA). Our team sized the heat pumps using ACCA’s “Manual S: Residential Equipment Selection.”
The overcut loops were sized to take advantage of all the existing overcut, and several team members at ORNL and OSU estimated the remaining heat source/sink capacity needs so that conventional loop-design software could be used to size additional horizontal loops needed to maintain the entering fluid temperature of the heat pump between 35°F and 95°F (2°C to 35°C), given soil temperature and thermal conductivity at the site. All overcuts and trenches received six-pipe loops (three circuits of one-inch-diameter high-density polyethylene pipe, out and back) with a minimum 1 foot of spacing between pipes.
To obtain data for validating the overcut loop-sizing method and performance-simulation model, it is important that the overall loop be sufficiently sized so that loop operating temperatures are in the design range. To accomplish this, for the purposes of this experiment, our team used horizontal loops installed in all the utility trenches plus some additional trenching to provide adequate capacity, rather than installing loops below the basement floor. However, calculations indicate that if the loop were installed below the basement floor, no additional trenching would have been necessary.
Comparing Standard, Integrated Heat Pumps
Construction of houses 1 and 2 was completed in November 2009, and data collection began in December. Baseline data was collected during the first year, when each home used one water-to-air heat pump for space conditioning (the two-stage ClimateMaster model TTV026) and a separate water-to-water heat pump for water heating. The three-circuit (six-pipe) ground loop was “headered” into a single supply and return in the basement, allowing the two heat pumps to operate in parallel, connected to the common supply and return. The baseline data documents the performance of the FHX−GSHP system using the best water- source heat-pump equipment commercially available, from industry partner ClimateMaster; the equipment fully satisfies the space-conditioning and water-heating loads in houses 1 and 2 with national average occupancy.
In November 2010, we replaced each home’s two heat pumps with a single prototype GS-IHP that provides both space conditioning and water heating. For several years ClimateMaster has been collaborating with ORNL under a Cooperative Research and Development Agreement to develop the GS-IHP, which is expected to be significantly more energy efficient than currently available heat pumps. Comparing data from years one and two will establish the energy savings of the GS-IHP compared to the two-heat-pump configuration.
However, the most common GSHP configuration on the market is a single water-to-air heat pump with a desuperheater, which provides only a portion of the required hot water as a byproduct when the compressor operates for space heating or cooling. After the GS-IHP data is available, our team will use calibrated models to compare the performance of all three configurations (one heat pump with desuperheater, two heat pumps and the GS-IHP). Cost estimates will also be provided. ClimateMaster anticipates launching a Trilogy water-source heat-pump product line based on the GS-IHP technology in 2011.
Assessing Early Results
Preliminary analysis of the data measured at the ZEBRAlliance research houses since November 2009 indicates that space-conditioning and water-heating needs could have been provided to the houses with ground heat exchangers installed in just the excavations required for construction — the basement overcut, below the basement floor and the utility trenches. As mentioned, no extra digging or drilling would have been required except making the utility trenches slightly deeper than normal.
The data showed that the installed loops and heat pumps all performed as expected, indicating that they were adequately sized for the 3,700-square-foot homes. Temperatures of the fluid entering the heat pumps ranged from 33°F to 93°F (0.6°C to 34°C) at house 1 and 34°F to 90°F (1°C to 32°C) at house 2 — within good proximity to the design range of 35°F to 95°F (2°C to 35°C). Heating and cooling set points maintained throughout the year were 71°F and 76°F (22°C and 24°C), respectively. From January through March 2010, the supplemental electric resistance heater was never activated at house 1 and consumed only 66 kilowatt-hours at house 2.
Early estimates indicate that when implemented at scale by a production builder in this region, this FHX approach may be feasible at $1,000 per ton. That compares with traditional vertical-loop and six-pipe-per-trench horizontal-loop systems that typically are installed in this region at $3,000 per ton and $2,250 per ton, respectively. The actual cost of a particular project may vary depending on drilling/ trenching conditions, regional cost variations, underground soil thermal properties and building geometry.
This September we’re scheduled to release the FHX-sizing tool and performance-simulation model integrated with EnergyPlus, as well as a comprehensive technical report documenting the basis for the FHX-sizing tool, the performance-simulation model, the data measurements and the validation of the sizing tool and simulation model. ClimateMaster also plans to make its Trilogy line of water-source heat pumps, based on the GS-IHP technology, commercially available in 2011. These and other innovations may provide the affordability break-through for GSHP systems that homebuilders, homeowners and energy-efficiency advocates have been seeking.
Dr. Piljae Im is a staff scientist at the Building Technologies Research and Integration Center (BTRIC) of Oak Ridge National Laboratory and one of the principal investigators for the project described here.
He has performed feasibility studies for the application of GSHP systems for numerous U.S. Navy and Air Force bases.
Dr. Xiaobing Liu is a staff scientist at BTRIC. He has been working on applying and improving GSHP technology for more than 10 years.
He is the principal investigator of various GSHP-related R&D projects ongoing at Oak Ridge National Laboratory.
Jeffrey Munk is the BTRIC staff scientist responsible for the experimental GSHP systems in the ZeBralliance (zebralliance.com) research houses.
ZeBralliance’s FHX research project has been supported by BTRIC’s numerous R&D staffs, including Dr. Moonis Ally, Van Baxter, John Shonder, Anthony Gehl and Patrick Hughes, director of BTRIC.
1 Xiaobing Liu, 2010, “Assessment of National Benefits from Retrofitting Existing Single-Family Homes with Ground Source Heat Pump Systems,” ORNL/TM-2010/122.
2 Miller et al., 2010, “Advanced Residential Envelopes for Two Pair of Energy-Saver Homes,” Proceedings of 2010 American Council for an Energy-Efficiency Economy Summer Study.