From summer roof to winter living room: the DLSC heat cycle

Step 1: collecting heat

Eight hundred flat-plate collectors faced south on the garage roofs. On a typical summer day they produced about 1.5 megawatts of thermal power. A glycol and water mix ran through them, travelled along a buried pipe to the Energy Centre and gave up its heat to tank water through a heat exchanger. The cooled glycol went back up to the roofs. More detail is on the solar collection page.

Step 2: storing it for months

In summer, hot water from the tank was pumped into the middle of a field of 144 boreholes and spiralled outward through U-shaped pipes, leaving its heat in the soil and rock around them. The field is about 35 m across and the holes reach roughly 35 to 37 m deep. By late summer the core was designed to approach 80 C. On top sit sand, high-density insulation, a waterproof membrane, clay and a landscaped park. See borehole storage.

Step 3: heat for winter

When houses called for heat, cool water entered at the edge of the field, picked up warmth on its way to the centre and went to the tanks. From there a loop of insulated plastic pipe, normally at 40 to 50 C, carried it to the houses. In each basement a coil in an air-handler unit passed the heat to household air, and a valve shut off flow once the thermostat was satisfied. The pipe network has its own page, district heating.

The control logic

Heat only moves one way down the chain, and only when the upstream part is warmer than the next. Collectors switch on when the glycol is warmer than the tank. The ground pump runs when the tank is warmer than the ground. The collectors could warm the tanks about twice as fast as the ground could absorb heat, so the ground pump kept going well into the night.

In winter, collector heat went straight to the loop instead of the ground. If that fell short, the field topped it up, and if the field could not keep up either, a gas boiler in the Energy Centre covered the difference.

Site conditions and what came of it

Okotoks sits near 51.1 N at about 1,084 m elevation. The design winter temperature was -33 C and the design summer dry-bulb temperature 28.3 C. The store needed roughly three years of charging before it could carry most of a heating season. Once warm, it delivered: see the monitoring results for the solar share by year, or the 2010 article on the 80 percent milestone for how early success was received.

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