By EBA Engineering
As part of its planning for sustainable infrastructure and renewable energies, the city of Whitehorse, Yukon asked EBA Engineering Consultants to map out the geoexchange potential of different areas w...
As part of its planning for sustainable infrastructure and renewable energies, the city of Whitehorse, Yukon asked EBA Engineering Consultants to map out the geoexchange potential of different areas within the city limits. The resulting maps are the first of this type to be done on a municipal scale in Canada.
Geoexchange systems rely on capturing mainly solar energy stored in the earth or its water resources, and then transferring it to buildings and facilities for space heating or cooling.
The Whitehorse Assessment of Geoexchange Potential project took into account three different types of earth energy systems. Two are well known: the closed loop systems that use a network of piping in the ground, within which a thermal exchange fluid circulates; and the open-loop systems which use water that is pumped from one source (ground or surface water) and is discharged elsewhere. A third, fairly unusual, aspect of the study included a survey of the potential for recovering heat from city sewer pipes.
Having compiled all the geological and hydrogeological subsurface data together with land use data and other information, the consultants were able to create simple graphic maps of the portions of the city that could be developed. They colour-coded the maps using red (poor), yellow (fair), and green (good) to indicate which areas would be favourable for using geoexchange energy. In October 2007, the maps were used to convey the scientific data to the public in a four-day city-sponsored charrette on sustainability held in Whitehorse.
Using engineering judgment
For the city-wide study, EBA worked with the city’s official community plan and consulted with city staff. First they defined the geographic areas to be studied, using either developed or “developable” land. Then they compiled a database of information about the subsurface geology and hydrogeology, using 121 control points.
Project leader Scott Schillereff, Ph. D., P. Geo., explains that they gathered information using a variety of sources: “paper copies of old reports, on-line bore-hole logs, and other existing sources.” Once they’d defined the control point characteristics they developed a weighted scoring system to evaluate geoexchange potential at each point. These geoexchange scores were then contoured and assigned green, yellow and red colours between established threshold values. “The contouring was relatively easy,” says Schillereff, adding: “The real engineering judgment came in weighting the scores for each of the parameters. It was not simply compiling information and then mapping. It involved thinking about what the data meant and giving it a numerical score.”
Weighting the scores
The parameters used to decide on a location’s geothermal potential included: UTM (universal transverse mercator) coordinates, lot size, depth to bedrock, dominant overburden type, bedrock type, depth to groundwater, thermal conductivity, hydraulic conductivity, transmissivity of soils and rock (i. e., its ability to transmit water through an aquifer), and ease of drilling.
Whitehorse-specific threshold values were calculated for typical “good” and “poor” scenarios using the following formula (each component scores 1-10):
Total score = log 10 [(depth to groundwater score) x (thermal conductivity score) x (ease of drilling and installation score) x (lot size score)]
A “good” area was assumed to have these attributes (or better): thermal conductivity > 2 W/m-K, depth to groundwater <20 m, drilling and completion score >3.75 and lot size >5,000 m2.
A “poor” area was assumed to have these attributes (or worse): thermal conductivity < 1 W/m-K, depth to groundwater >60 m, drilling and completion score <1.75, and lot size <1,000 m2.
For sewer waste heat, the mapping was done along sanitary sewers of 450 mm, 560 mm and 610 mm throughout the city. Facilities within several hundred metres of these sewers were considered to have a potential for using recovered heat.
One area of the city that was identified as having good to fair geoexchange potential is the site of a new planned mixed-use neighbourhood development in the northeast known as the Whistle Bend project. EBA is now conducting a feasibility study of the geoexchange, sewer heat recovery and hybrid solar thermal potential for a district energy system in the neighbourhood.
Client: City of WhitehorsePrime consultant: EBA Engineering Consultants (Scott Schillereff, Ph. D., P. Geo., Andrew Chiasson, Ph. D., P. Eng., Katherine Johnston, P. Eng., Jeff Quibell, P. Eng.)
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