ASHRAE Journal - October 2022 - 37

FIRST PLACE | 2022 ASHRAE TECHNOLOGY AWARD CASE STUDIES
drilled prior to excavation and located entirely within the
building footprint, with each borehole carefully located
and coordinated to avoid damage during excavation.
Further, each geothermal well is individually piped to the
above ground manifold in the sub-basement mechanical
room to allow for access and isolation in the event of a
leak. A maximum of 60 boreholes could be located under
the building due to constrained land area. Therefore,
strategic hybridization was incorporated into the design,
as detailed below in the Energy Effi ciency section.
Innovation
With the project planned to occupy the entire lot, the
geothermal fi eld had to be located entirely under the
building. This posed signifi cant challenges to construction
as well as sizing of the fi eld due to a very limited site
area. Annual building loads were carefully analyzed in
energy modeling software, and parametric analysis was
used to limit uncertainty in results. Building loads at sixminute
intervals from each parametric run were then
input into a custom model, built to accurately refl ect the
geothermal fi eld, to determine fi eld performance. This
allowed for optimized sizing of the fi eld and minimized
project costs.
Energy Effi ciency
The building was modeled extensively and found to be
heating dominant. This presented the opportunity to separate
the garage heating and sidewalk snowmelt system;
serving both of these areas with a common condensing
boiler plant reduced heating load on the geothermal fi eld
while maintaining a high boiler effi ciency. An additional
connection from the boiler plant to the fi eld was provided
via a small plate and frame heat exchanger as a safety
measure, which has not been needed in the operation of
the building as of yet. Similarly, due to the uncertainty in
potential retail tenants, a small 60 ton (211 kW) cooling
tower was added to support peak fi eld loads in cooling.
The cooling tower is intelligently controlled to operate
at night during low wet-bulb conditions. Geothermal
heat pumps throughout the building incorporate variable
speed fans and multistage compressors. Ducted
vertical stack heat pumps were used in residential
spaces to balance space utilization and cost savings.
In many cases, typical construction for a building of
this type would use split-direct expansion cooling and
furnaces with exhaust and condensing units routed
to the roof. The geothermal system avoids all of the
riser and roof penetrations, reducing infi ltration and
maintenance.
Water-to-water geothermal heat pumps provide
domestic hot water preheating with fi nal tempering
by a condensing domestic hot water boiler plant in the
mechanical penthouse. This preheating confi guration
balances heat pump lift and effi ciency. Domestic heating
loads are reduced via shower drain heat recovery
heat exchangers installed in the building to recover
heat from drain water that is used to preheat incoming
cold water. A simple copper tube heat exchanger was
used at the base of typical shower drain risers to minimize
cost and maintenance. Domestic cold water for
the riser is preheated in the heat exchanger on its way
to the showers, reducing the hot water needed in the
mixing valve.
The 100% outdoor air makeup air unit (MAU) uses variable
speed supply fans to maintain building pressurization;
the internal exhaust fan modulates to maintain
duct static pressure for separate general exhaust risers,
which provide additional exhaust from each apartment
for adequate ventilation and good indoor air quality.
Energy is recovered from the exhaust via an enthalpy
wheel in the geothermal source makeup air unit in
the penthouse. The geothermal compressors are variable
speed to minimize temperature fl uctuations, and
a small electric preheat coil is used on the coldest days
for minimal preheating to 20°F (-6.7°C) as required.
Bypass dampers are incorporated into the MAU to avoid
enthalpy wheel and coil pressure drops during economizer
operation.
Common areas (a large rooftop multipurpose room,
lobbies and a fi tness center) use geothermal water to air
ducted heat pumps for space conditioning. These heat
pumps incorporate waterside economizers, variable
speed fans and two-stage compressors. Ventilation for
common areas comes from the same geothermal MAU
that serves the apartments and is delivered to each space
by variable air volume boxes controlled by carbon dioxide
(CO2) sensors.
The envelope insulation was evaluated during design
and optimized through the use of an energy model to
maximize value. A 25 kW solar photovoltaic (PV) array was
installed on the upper roof, using all the available area to
help offset some of the common area electrical use.
The result of the above summarized energy effi ciency
O CTO B E R 2 0 2 2 ashrae.o rg ASHRAE JOURNAL
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