Ground-Source System Offers Advantages Over Air-Source System, Say School Building Designers
Fort River School. The likely location for a geothermal well field on the Fort River site is below the southern entrance driveway in the southwest corner of the property. Photo: Toni Cunningham
To date, the Net Zero Subcommittee of the Elementary School Building Committee has been focused on the mechanical HVAC system, that is, how to heat and cool the building. After establishing the goal of an Energy Use Intensity (EUI) of 25, discussion has centered around the choice between ground (“geothermal”) versus air-source heat pumps. Members of the DiNicso design team stated in January that “geothermal obviously is the most efficient system available; it’s going to definitely get us the lowest energy [use] possible.”
A life cycle cost analysis presented at the most recent subcommittee meeting (March 29) demonstrated that, over the life of the building, costs would be roughly the same for ground- or air-source systems. While ground-source costs more upfront, air-source has higher maintenance and operation costs and would require more solar panels. Mechanical engineer, Semoon Oh said, “in a geothermal system, everything downstream is a four-pipe system that has been around for 40 or 50 years, so it’s very familiar to all contractors. There’s nothing fancy about it; it’s a very robust system.” In contrast, Oh said that the air-source heat pump system is typically a distributed refrigerant system and is a lot more complex. Not all contractors are familiar with it yet, he said.
The design team’s analysis also showed that a ground-source system would meet the EUI goal of 25, while the air-source system could only achieve an EUI of 31, requiring nearly 25% more solar panels to meet the energy needs of the building. This would increase costs and force more panels to be located on parking lot canopies. DiNisco quoted a per-watt cost of $2.25 for roof-mounted solar and $3.00 for canopy-mounted.
DiNisco’s geotechnical analysis has indicated that a geothermal well field can more easily be accommodated on the Fort River site. In contrast, the smaller size and hilly topography of the Wildwood site would require either complicated phased construction near the existing building or an arrangement to use the neighboring Regional School District-owned middle school field. The latter would also require restoration of the athletic field, including raising the grade and installing subsurface drainage to manage groundwater.
Because all new construction and addition/renovation would be built to the same thermal envelope standards and performance, they would have similar EUIs. More solar panels would be able to be located on the larger roof area anticipated in an addition/renovation approach.
The net zero subcommittee is expected to make a recommendation to the full committee on which system to choose. The full committee will make a decision on which “preferred solution” to submit to the Massachusetts School Building Authority (MSBA) in June.
UMass seems to have come to the conclusion that widespread use of Ground Source Heat Pumps (GSHP or “Geothermal”) is the only way they are going to meet the ambitious goal of being carbon zero by 2032. From their plan at https://www.umass.edu/carbon-neutrality/about:
“The highest-impact component of our transition will be the large-scale conversion of our campus energy infrastructure. We plan to transition from fossil-fueled steam production to a modern, hot-water heating system paired with geothermal heating and cooling, and to utilize stored solar energy and energy from the rapidly greening electrical grid. As the first step in this major undertaking, we will soon test the geothermal capacity of selected campus properties, after which we will undertake a far-reaching proof-of-concept project involving 42 campus buildings from across construction eras with different HVAC systems.”
In addition, I believe the best MassSave incentives through Eversource are contingent on reaching an EUI of 25, and that does not count throwing more PV on the roof and on parking lot canopies to get there. The engineers consulting on the school building project have indicated that GSHP is the only way to reach 25 EUI. GSHP is the way to go for this project if the Town is to make progress toward meeting its ambitious climate goals.
From the MassSave Info Sheet at https://www.masssave.com/-/media/Files/PDFs/Business/New-Construction-Path-1-MOU_fillable.pdf:
“Project teams must be willing to target a 25.0 site EUI or less.”
I would caution the decision makers on their choices. I believe a full ‘ground source’ to be a life cycle one (it’s building) and the ‘solar’ to be 25 years. Panels on the roof (I’m relatively certain) could fuel the whole building and have only a 25 yr life cycle (in line w/projections for student population, other site uses). ‘Geo’ will use that space for a good while longer as the equipment needs stay in the ground or pulled up. Why not a combo of the two? Solar fluctuates through out the day and year (battery choice/cost), while the ground is steady state. This creates a smaller foot print and lower upfront coasts. One choice that must be weighed off against the costs of the 2 systems – something to consider over unintended consequences of all geo. Sending Luck to the committee~
Chad Fuller
Thank you for your comment, Chad.
The net zero consultant team working with the designers performed a life cycle costs analysis (LCCA) for a 50 year lifespan that does include replacement costs for the solar panels at 25 years. The cost for the solar panels is lower when ground source heat pumps (GSHP) are used compared to air source (ASHP) because the GSHP system is more efficient (able to reach the target EUI of 25 as opposed to 31 for ASHP). So, more panels are needed for ASHP and at a higher total cost since they will not all fit on the roof and many more will be parking-lot-canopy mounted (which are more expensive than roof-mounted). This difference in solar panel costs is also essentially doubled since they would need to be replaced halfway through the lifespan of the building.
The average useful life for the systems was provided in the table found in the packet for the 3/29 meeting and summarized here:
Geothermal well fields 50 years
GSHP system (NOT wells) 20 years
ASHP system 15 years
Photovoltaics 25 years
The net result is that if an ASHP were chosen, more frequent replacements are needed compared to GSHP so this system ends up costing at least as much over the life of the building. They also noted that there are more contractors familiar with GSHP than ASHP in terms of servicing and maintenance.
The consultants were asked about using a hybrid system (part GSHP and part ASHP). They pointed out that because much of the upfront costs for GSHP is the geothermal well fields and that drilling fewer wells does not proportionately decrease this cost, the savings of a hybrid system are also not proportional, and it adds complexity to have two different systems to operate and maintain.