ASHRAE Journal - May 2011 - 54

Optimizing Geothermal Cost

Typically, Quebec’s average winter temperature is 14°F (–10°C). Sporadically, temperatures can drop as low as –30°F (–35°C). During summer, temperatures can rise to more than 95°F (35°C). This means that designers must install powerful equipment that rarely works at full capacity. The average heating or cooling part load for a building in Quebec is generally under 50% of its peak load. The engineering team wanted to design systems that could offer good efficiency throughout the year. Knowing that heat pump efficiency is reduced considerably when used at part load and that geothermal system excavation and materials are quite costly, designers decided to evaluate a cheaper auxiliary heating system (electric boiler) to supply extra heat during periods of intense cold. Energy simulation models helped pin down the percentage of the maximum capacity that should be ensured by the geothermal system. This optimal combination of systems made it possible to reduce the size of the geothermal exchanger to 70% of the design’s peak load, consequently keeping excavation

and material costs to a minimum. Three boreholes with a depth of 500 ft (152 m) each were drilled and connected to a 14 ton (49 kW) water-to-water heat pump. Heat pump source side fluid is a mixture of water and methanol arriving from the geothermal boreholes with an inlet minimum temperature of 30°F (–1°C) while heat pump load side fluid is a mixture of water and glycol with an inlet temperature around 75°F (24°C) and a flow rate of 35 gpm (2 L/s). The heat pump can provide heating with a COP of 3.2 and cooling with an EER of 11.3. Furthermore, by reducing the size of the heat pump by 30% under peak load, designers ensured that the equipment would work more hours at full capacity, therefore maximizing heat pump efficiency.
Radiant Floor Heating

Building at a Glance
Name: St. Johns School Location: Saint-Jean-sur-Richelieu, Québec, Canada Owner: Riverside School Board Principal Use: School Includes: science rooms with laboratories and a library Employees/Occupants: 120 Gross Square Footage: 9,000 ft2 Conditioned Space: 9,000 ft2 Substantial Completion/Occupancy (date): June 2006 Occupancy: 100%

The geothermal loop is used to treat fresh air and supply an hydronic radiant floor heating system (Figure 1), installed into the concrete floor slabs of the classrooms and library. By signif icantly reducing stratif ication effects, radiant floor heating provides even, comfortable warmth because the

heated concrete floor acts as a thermal mass. Temperature sensors, located in the rooms and floor slabs, satisfy occupant demand. The radiant floor heating is comfortable and silent, and provides roomier and more aesthetically pleasing spaces since there is no apparent equipment, which was appreciated by the client.

Filter

Exhaust Solar Wall

Main Exhaust

Enthalpy Wheel Heating/Cooling Coil SCR Fresh Air Supply To Other Rooms Filter

Fresh Air

Fresh Air Bypass Water-toWater Heat Pump Fresh Air Supply

Geothermal Boreholes

To Radiant Floor Heating in Other Rooms

Electric Boiler

Radiant Floor Heating Room

Figure 1: Geothermal loop treats fresh air and supplies a hydronic radiant floor heating system.
54 ASHRAE Journal ashrae.org May 2011

ASHRAE Journal - May 2011

Table of Contents for the Digital Edition of ASHRAE Journal - May 2011