Swan Lake gets it right
The basic principle of an earth energy system is to transfer solar heat from the ground into the building envelope. To ensure a successful installation, the loop must be sized correctly, with sufficient pumping capacity to circulate the transfer fluid and to extract the maximum heat possible.
At the Swan Lake First Nations School, vertical wells were drilled throughout the property, for a total borehole of nearly 2,500 m (8,000 ft). The shale overburden made drilling easier than at many sites in Canada, and each well is either 30 m or 60 m deep, with a separation of at least 3 m to ensure adequate radius to avoid 'stealing' heat from neighbouring wells.
Each hole was filled with 2-cm polyethelene pipe, as demanded by the national CSA standard, and then back-filled from bottom to top with a special grout that enhances the transfer of heat from the ground to the pipe (and then into the circulating fluid) and to minimize any chance of underground contamination. The polyethelene resin is the same plastic that is used to transport natural gas due to its flexibility, corrosion resistance, high durability, long life, and ease of fusion.
Two metres below the surface, another 130 m of 5-cm horizontal piping connects the tops of all the vertical boreholes into a configuration called a reverse return header, which ensures that fluid pressure remains equal throughout the system. In areas where the headers run under pavement, the horizontal pipe is insulated and trenched another metre deeper, to ensure adequate freeze protection.
Through this network of 6 km of piping flows 2,700 litres of heat transfer fluid (water with a 20% mixture of methanol), which is freeze-protected to -11oC.
The circulating pumps are sized to ensure optimal velocity of the fluid through the loop (called the Reynolds Number) which provides a balance between the high speed necessary to whisk heat away from the pipe wall, and the lower speed that minimizes operating costs.
In the Swan Lake school, six earth energy units are designed to provide heat for two classrooms each, while another three units are tied into a common duct to provide space conditioning for the gymnasium. The circulating pumps are designed to activate whenever an individual earth energy unit starts to run, and will pump as little as 0.88 litres per second with only one unit running, to as much as 7.5 l/sec when all nine units are operating. Although the desired turbulent flow rate is not reached at the low end, the relative over-sizing of the ground loop compensates for the heat transfer.
This facility is purposely over-sized by 20%, and during a winter cold snap (when the outside temperature dropped to -35oC for two nights in a row) the auxiliary electric baseboard heaters in the classrooms did not kick in. Another time, when a compressor in one unit stopped, no-one at the school was aware of the loss of heating capacity.
The icing on the project was that construction of this
school came in $200,000 below budget, even though air conditioning was
provided, due in large part to the decision to incorporate earth energy
"These sheets are made possible through a contribution from Natural Resources Canada."