The future of ground-source heat pumps (GSHPs) may well live under a 2,772-square-foot, two-story suburban home near Pittsburgh. Between the home’s concrete slab and the depth of its footings are the closed-loop piping of a GSHP, one of several building systems installed and monitored in the Best Practices Research Alliance’s Energy Efficiency Lab Home, a full-scale, occupant-simulated prototype that celebrated its one-year anniversary in March.
Conventionally, pipes carrying the liquid medium that facilitates heat transfer with the ground’s temperature are buried across a wide swath of nearby ground or are set vertically when available land is at a premium.
In both cases, the cost to excavate for the closed-loop system is often prohibitive (estimates range from $5,000 to far more) and therefore inhibits a reasonable return on investment and, in turn, geothermal’s market share—despite the fact that the ongoing energy demand for a GSHP system is far more efficient than air-source heat pumps, especially in climate extremes.
The idea to install the loop within an already excavated area under the Lab Home’s slab is inspired and—if it works—would basically eliminate the cost premium for GSHP and enable better access into the mainstream.
After a year of testing and analysis, the jury’s still out. “In the first cooling season (summer 2011), we found that the system is maintaining the indoor temperatures we want, but its efficiency is not exactly what we expected or modeled,” says Anthony Grisolia, services manager at IBACOS, which is spearheading the Lab Home project funded by the DOE’s Building America program.
One possible reason for the discrepancy, Grisolia speculates, is that the fill under the slab is too dry to transfer thermal energy with the pipes as effectively as the team anticipated, causing the heat pump to run longer to maintain desired indoor conditions. “The more you can know about the conductivity of the soil throughout the year is really important to the system’s performance,” he says.
Grisolia also senses that the Lab Home’s exemplary rainwater and snowmelt drainage system may have unwittingly kept the soil drier than normal. “Maybe we did too good of a job directing water away from the slab.”
Going forward, the IBACOS team will analyze the system’s performance from the recent heating season and perhaps tweak the loop for the upcoming summer cooling cycle.
“We need to address some nuances before we can declare it market viable,” says Dave Stecher, a building performance specialist at IBACOS, who is guiding the direction of the Lab Home research effort. “This is a prototype, the first step of many.”
To learn more about the IBACOS Best Practices Research Alliance Energy Efficiency Lab Home, go to www.theresearchalliance.org/lab-home.aspx.