The Steinbach Credit Union built a new 45,000 square foot, 3 story office building in Winnipeg, MB in 2009. The building featured a number of energy efficiency features, including high insulation values, efficient lighting, a green roof and a ground coupled heat pump (GCHP) system that uses the earth under the parking lot as its energy source.

If you’ve been in Winnipeg in January, you’ll have an appreciation for winter. Our winter design temperature is -28 F (-33 C), with extreme temperatures occasionally dropping below -40. This winter, as anyone in North America knows, the Polar Vortex blessed us with the coldest winter on record. We had 25 days when the temperature didn't get above -20 F in December and January.

The last time Winnipeg experienced a winter like this one was in 1898! Water lines to homes and businesses froze even though they were buried 7-8' below ground leaving a few thousand people without water.

So what happens to a horizontal ground heat exchanger (GHX) that's buried only a few feet below that? Can it heat the building in this kind of extreme winter...or does the building freeze up?

Well, because of the extreme cold this winter, the building manager actually bumped up the building temperature a degree. He's been monitoring the temperature of the fluid circulating through the GHX. It had been designed to operate at a minimum temperature of 35 F (1.7 C) in winter and a maximum temperature of 85 F (30 C) by the end of summer during a typical year.

As one of the system designers, I was curious to see how the GHX reacted to the extreme winter. The GHX design was built on a detailed energy model using typical meteorological year (TMY) weather data…but this wasn't a typical winter!

The data showed some interesting things:

• The temperature of the fluid coming from the GHX did drop a few degrees below minimum design temperature. That wasn't surprising considering the GHX design was based on typical temperatures.

• At the beginning of December the temperature from the GHX was 49 F (9.4 C)...about 4 F warmer than the ambient ground temperature in Winnipeg. By the end of the coldest December on record the temperature had dropped to 41 F (5 C), only 4 F below normal ground temperature. By the end of January, the temperature was still 36.4 F (2.4 C)...still above the minimum design temperature.

• It wasn't till March 3 that the temperature from the GHX hit the low point of the winter…31.6 (-.2 C)...more than a month after the coldest weather.

• By April 3 it was 38.4 F (3.5 C) and by May 26 it was 45.5 F (7.4 C)…above the normal ambient temperature.
   

The coldest fluid temperature from the ground occurred more than a month after the coldest months, and even with a record winter, the actual lowest fluid temperature to the heat pumps was only 3.4 F (1.9 C) lower than the predicted minimum temperature. The average monthly temperature supplied to the heat pumps was well within the design parameters of 85 F (30 C) and 35 F (2 C) selected by the designer.

Figure 1: The average monthly temperature of fluid from the GHX to the heat pumps ranged from a high of 84 F(29 C) during one of the warmest years on record in Winnipeg (2012) and a low average temperature of 36 F (2 C) during the coldest winter since 1898 (2014).

Figure 2: The graph illustrates the lag time between the minimum and maximum GHX temperature supplied to the heat pumps and the minimum and maximum outdoor air temperatures. The efficiency and capacity of a heat pump in heating is higher when the fluid temperature it is supplied with is higher. Conversely, the efficiency and capacity in cooling is greater when the fluid temperature is lower. The lag time in temperature between the GHX and the outdoor air benefits the efficiency of the system.

This GHX for this project was designed by developing an 8,760 hour energy model for the building in Trane Trace 700. The hourly loads were imported into Ground Loop Design (GLD) software. Other inputs needed to calculate the amount of pipe required to deliver the energy to the heat pumps in the building included:

• Soil properties (thermal conductivity, thermal diffusivity and ambient soil temperature)

• Spacing between GHX piping

• Pipe size and properties

• Depth of pipe burial

• Average annual air temperature swing coldest and hottest day of the year

• Heat transfer fluid characteristics

• Fluid circulation strategy

To optimize the design of the GHX (i.e.: minimize the capital cost of building the GHX, improve the overall system efficiency and ensure the long term performance of the GHX) it’s critical the designer has the opportunity to work closely with the owner and design team to ensure the energy loads to and from the GHX are balanced on an annual basis, taking into consideration the impact of the building and mechanical system design.

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